The present investigation examined the effect of inflammation produced by intravesical zymosan on spinal dorsal horn neuronal responses to urinary bladder distension (UBD).
Extracellular single-unit recordings of neurons excited by UBD were obtained in spinalized female Sprague-Dawley rats. Neurons were classified as Type I - inhibited by heterotopic noxious conditioning stimuli (HNCS) or as Type II - not inhibited by a HNCS. In Experiment 1 - following neuronal characterization, 1% zymosan was infused into the bladder and after two hours spinal units were recharacterized. Control rats received intravesical saline or subcutaneous zymosan. In Experiment 2 – rats were pretreated with intravesical zymosan 24 hours prior to surgical preparation. Control rats only received anesthesia.
137 spinal dorsal horn neurons excited by UBD were characterized. In comparison with controls, Type II neurons demonstrated increased spontaneous and UBD-evoked activity following intravesical zymosan treatment (both Experiments 1 & 2) whereas Type I neurons demonstrated either no change (Experiment 1) or decreased activity (Experiment 2) following bladder inflammation. No significant changes were noted in neuronal activity in control experiments.
Inflammation differentially affects subpopulations of spinal dorsal horn neurons excited by UBD that can be differentiated according to the effect of HNCS. This results in an altered pattern of spinal sensory transmission that may serve as the mechanism for the generation of visceral nociception.
visceral; urinary bladder; cystitis; zymosan; spinal
This investigation examined the effect of inflammation produced by intravesical zymosan during the neonatal period on spinal dorsal horn neuronal responses to urinary bladder distension (UBD) as adults.
Female rat pups (P14-P16) were treated with intravesical zymosan or with anesthesia-only. These groups of rats were subdivided forming four groups: half received intravesical zymosan as adults and half received anesthesia-only. One day later, rats were anesthetized, the spinal cord transected at a cervical level and extracellular single-unit recordings of L6-S1 dorsal horn neurons obtained. Neurons were classified as Type I - inhibited by heterotopic noxious conditioning stimuli (HNCS) or as Type II - not inhibited by HNCS - and were characterized for spontaneous activity and responses to graded UBD (20–60 mm Hg).
227 spinal dorsal horn neurons excited by UBD were characterized. In rats treated as neonates with anesthesia-only, Type II neurons demonstrated increased spontaneous and UBD-evoked activity following adult intravesical zymosan treatment whereas Type I neurons demonstrated decreased spontaneous and UBD-evoked activity relative to controls. In rats treated as neonates with intravesical zymosan, the spontaneous and UBD-evoked activity of both Type I and Type II neurons increased following adult intravesical zymosan treatment relative to controls.
Neonatal bladder inflammation alters subsequent effects of acute bladder inflammation on spinal dorsal horn neurons excited by UBD such that overall there is greater sensory neuron activation. This may explain the visceral hypersensitivity noted in this model system and suggest that impaired inhibitory systems may be responsible.
visceral; urinary bladder; cystitis; zymosan; spinal
Stress-induced hyperalgesia (SIH), a common clinical observation associated with multiple painful diseases including functional urinary disorders, presently has no mechanistic explanation. Using a Footshock treatment, a classical stressor, to magnify physiological responses in a model of urinary bladder pain, we examined one potential group of mediators of SIH, the corticotropin-releasing factor (CRF)-related neuropeptides. Exposure to a Footshock treatment produced bladder hypersensitivity in female Sprague Dawley rats, manifested as significantly more vigorous visceromotor responses (VMRs) to urinary bladder distension (UBD) compared to rats that were exposed to a Non-footshock treatment. This bladder hypersensitivity was significantly attenuated by blocking spinal CRF2 receptors but not CRF1 receptors. Furthermore, spinal administration of urocortin 2, a CRF2 receptor agonist, augmented UBD-evoked VMRs in a way similar to what was observed following exposure to Footshock, an effect significantly attenuated by pretreatment with spinal aSVG30, a CRF2 receptor antagonist. Surprisingly, neither spinal administration of CRF nor the CRF1 receptor antagonist, antalarmin, had an effect on bladder nociceptive responses. The results of the present study not only provide further support for a role of stress in the exacerbation of bladder pain, but also implicate spinal urocortins and their endogenous receptor, the CRF2 receptor, as potential mediators of this effect.
This study presents evidence that spinal urocortins and CRF2 receptors are involved in stress-induced hypersensitivity related to the urinary bladder. This provides a basis for investigating how urocortins mediate SIH, ultimately leading to more effective treatment options for patients suffering from painful bladder syndromes as well as stress-exacerbated chronic pain.
Urinary bladder; hypersensitivity; visceral; urocortin; corticotropin-releasing factor
Anecdotal evidence suggests that chronic bladder pain improves while breastfeeding. The present study sought to identify potential mechanisms for such a phenomenon by investigating the effects of the lactogenic hormones prolactin (PL) and oxytocin (OXY) in a rat model of bladder nociception. Lactating rats were less sensitive to urinary bladder distension (UBD) than controls. In investigating potential antinociceptive and anxiolytic roles for these hormones, we found exposure to a footshock paradigm (STRESS groups) produced bladder hypersensitivity in saline-treated rats, manifested as significantly higher electromyographical (EMG) responses to UBD, compared to rats exposed to a non-footshock paradigm (SHAM groups). This hypersensitivity was attenuated by the intraperitoneal administration of OXY prior to footshock in the STRESS-OXY group. The administration of PL augmented EMG responses in the SHAM-PL group but had no effect on the responses of the STRESS-PL group. In the absence of behavioral pretreatment, OXY attenuated UBD-evoked responses while PL had no effect. Moreover, OXY-treated rats spent more time in the open arm of an elevated plus maze compared to saline-treated rats suggesting anxiolysis. These studies suggest the potential for systemic OXY, but not PL, as an analgesic and anxiolytic treatment for painful bladder disorders such as interstitial cystitis.
oxytocin; prolactin; nociception; pain; stress; anxiety; bladder
Background and Objectives
Oxytocin (OXY) is a neuropeptide that has recently been recognized as an important component of descending analgesic systems. The present study sought to determine if OXY produces antinociception to noxious visceral stimulation.
Urethane-anesthetized female rats had intrathecal catheters placed acutely, and the effect of intrathecal OXY on visceromotor reflexes (VMRs; abdominal muscular contractions quantified using electromyograms) to urinary bladder distension (UBD; 10-60 mm Hg, 20 s; transurethral intravesical catheter) was determined. The effect of OXY applied to the surface of exposed spinal cord was determined in lumbosacral dorsal horn neurons excited by UBD using extracellular recordings.
OXY doses of 0.15 μg or 1.5 μg inhibited VMRs to UBD by 37 ± 8% and 68 ± 10%, respectively. Peak inhibition occurred within 30 minutes and was sustained for at least 60 minutes. The effect of OXY was both reversed and prevented by the intrathecal administration of an OXY receptor antagonist. Application of 0.5 mM OXY to the dorsum of the spinal cord inhibited UBD-evoked action potentials by 76 ± 12%. Consistent with the VMR studies, peak inhibition occurred within 30 minutes and was sustained for greater than 60 minutes.
These results argue that intrathecal OXY produces an OXY receptor specific antinociception to noxious UBD, with part of this effect due to inhibition of spinal dorsal horn neurons. To our knowledge, these studies provide the first evidence that intrathecal OXY may be an effective pharmacological treatment for visceral pain.
A minority of patients who experience awareness and/or pain during surgery subsequently develop posttraumatic stress disorder. In a rodent model of posttraumatic stress disorder, stress-enhanced fear learning (SEFL), rats are pre-exposed to a stressor of 15 footshocks. Subsequent exposure to a single footshock produces an enhanced fear response. This effect is akin to sensitized reactions shown by some posttraumatic stress disorder patients to cues previously associated with the traumatic event.
We studied the effect of isoflurane and nitrous oxide on SEFL. Rats were exposed to the inhaled anesthetic during or after a 15-footshock stressor. Then rats were given a single footshock in a different environment. Their fear response was quantified in response to the 15-footshock and single-footshock environments. SEFL longevity was tested by placing a 90-day period between the 15 footshocks and the single footshock. In addition, the intensity of the footshock was increased to evaluate treatment effectiveness.
Increasing isoflurane concentrations decreased SEFL when given during, but not after, the stressor. At 0.40 minimum alveolar concentration, isoflurane given during the stressor blocked SEFL 90 days later. A three-fold increase in the stressor intensity increased the isoflurane concentration required to block SEFL to no more than 0.67 minimum alveolar concentration. As with isoflurane, nitrous oxide suppressed SEFL at a similar minimum alveolar concentration fraction.
These results suggest that sufficient concentrations (perhaps 0.67 minimum alveolar concentration or less) of an inhaled anesthetic may prevent SEFL.
The Mechanisms by which chronic nicotine self-administration augments hypothalamo-pituitary-adrenal (HPA) responses to stress are only partially understood. Nicotine self-administration alters neuropeptide expression in corticotropin-releasing factor (CRF) neurons within paraventricular nucleus (PVN) and increases PVN responsiveness to norepinephrine during mild footshock stress. Glutamate and GABA also modulate CRF neurons, but their roles in enhanced HPA responsiveness to footshock during chronic self-administration are unknown. We show that nicotine self-administration augmented footshock-induced PVN glutamate release, but further decreased GABA release. In these rats, intra-PVN kynurenic acid, a glutamate receptor antagonist, blocked enhanced adrenocorticotropic hormone and corticosterone responses to footshock. In contrast, peri-PVN kynurenic acid, which decreases activity of GABA afferents to PVN, enhanced footshock-induced corticosterone secretion only in control rats self-administering saline. Additionally, in rats self-administering nicotine, footshock-induced elevation of corticosterone was significantly less than in controls after intra-PVN saclofen (GABA-B receptor antagonist). Therefore, the exaggerated reduction in GABA release by footshock during nicotine self-administration disinhibits CRF neurons. This disinhibition combined with enhanced glutamate input provides a new mechanism for HPA sensitization to stress by chronic nicotine self-administration. This mechanism, which does not preserve homeostatic plasticity, supports the concept that smoking functions as a chronic stressor that sensitizes the HPA to stress.
nicotine self-administration; adrenocorticotropic hormone; corticosterone; footshock stress; homeostatic plasticity; rat
Chronic nicotine self-administration augments the thalamo-pituitary-adrenal (HPA) responses to stress. Altered neuropeptide expression within corticotropin-releasing factor (CRF) neurons in the hypothalamic paraventricular nucleus (PVN) contributes to this enhanced HPA response to stress. Herein, we determined the role of norepinephrine, a primary regulator of CRF neurons, in the responses to footshock during nicotine self-administration. On day 12-15 of self-administration, microdialysis showed nicotine reduced PVN norepinephrine release by footshock (<50% of saline). Yet, the reduction in footshock-induced adrenocorticotropic hormone (ACTH) and corticosterone secretion due to intra-PVN prazosin (α1 adrenergic antagonist) was significantly greater in rats self-administering nicotine (2-fold) than saline. Additionally, PVN phenylephrine (α1 agonist) stimulated ACTH and corticosterone release to a similar extent in unstressed rats self-administering nicotine or saline. Nicotine self-administration also decreased footshock-induced c-Fos expression in the nucleus of the solitary tract (NTS)-A2/C2 catecholaminergic neurons that project to the PVN. Therefore, footshock-induced NTS activation and PVN norepinephrine input are both attenuated by nicotine self-administration, yet PVN CRF neurons are more responsive to α1 stimulation, but only during stress. This plasticity in noradrenergic regulation of PVN CRF neurons provides a new mechanism contributing to the HPA sensitization to stress by nicotine self-administration and smoking.
nicotine; hypothalamic paraventricular nucleus; corticotrophin-releasing factor; norepinephrine; adrenocorticotropic hormone; stress
Many dopaminergic neurons exhibit a short-latency response to noxious stimuli, the source of which is unknown. Here we report that the nociceptive-recipient parabrachial nucleus appears to be a critical link in the transmission of pain related information to dopaminergic neurons. Injections of retrograde tracer into the substantia nigra pars compacta of the rat labelled neurons in both the lateral and medial parts of the parabrachial nucleus, and intra-parabrachial injections of anterograde tracers revealed robust projections to the pars compacta and ventral tegmental area. Axonal boutons were seen in close association with tyrosine hydroxylase-positive (presumed dopaminergic) and negative elements in these regions. Simultaneous extracellular recordings were made from parabrachial and dopaminergic neurons in the anaesthetized rat, during the application of noxious footshock. Parabrachial neurons exhibited a short-latency, short duration excitation to footshock while dopaminergic neurons exhibited a short-latency inhibition. Response latencies of dopaminergic neurons were reliably longer than those of parabrachial neurons. Intra-parabrachial injections of the local anasethetic lidocaine or the GABAA receptor antagonist muscimol reduced tonic parabrachial activity and the amplitude (and in the case of lidocaine, duration) of the phasic response to footshock. Suppression of parabrachial activity with lidocaine reduced the baseline firing rate of dopaminergic neurons, while both lidocaine and muscimol reduced the amplitude of the phasic inhibitory response to footshock, in the case of lidocaine sometimes abolishing it altogether. Considered together, these results suggest that the parabrachial nucleus is an important source of short-latency nociceptive input to the dopaminergic neurons.
nociception; extracellular recording; tract tracing; rat; ANOVA, analysis of variance; BDA, biotinylated dextran amine; BSA, bovine serum albumin; DA, dopaminergic; DAB, diaminobenzidine; FLI, fos-like immunoreactivity; NHS, normal horse serum; PB, phosphate buffer; PBN, parabrachial nucleus; PBS, phosphate buffered saline; PHA-L, Phaseolus vulgaris leukoagglutinin; PPTg, pedunculoponting tegmental nucleus; PSTH, peri-stimulus time interval histogram; RMTg, rostromedial tegmental nucleus; SD, standard deviation; SNPc, substantia nigra pars compacta; SNPr, substantia nigra pars reticulata; TH, tyrosine hydroxylase; TX, Triton X 100; VTA, ventral tegmental area
Gamma-aminobutyric acid type A receptor potentiation and/or N-methyl-d-aspartate (NMDA) receptor inhibition might explain the anesthetic properties of fluorinated aromatic compounds. We hypothesized that depression of dorsal horn neuronal responses to noxious stimulation would correlate with the magnitude of effect of benzene (BNZ), o-difluorobenzene, and hexafluorobenzene (HFB) on NMDA receptors.
Rats were anesthetized with desflurane. A T13-L1 laminectomy allowed extracellular recording of neuronal activity from the lumbar spinal cord. After discontinuing desflurane administration, MAC for each aromatic anesthetic was determined. A 5-s noxious mechanical stimulus was then applied to the hindpaw receptive field of nociceptive dorsal horn neurons, and single-neuron responses were recorded at 0.8 and 1.2 MAC. These responses were also recorded in decerebrate rats receiving BNZ and HFB at 0–1.2 MAC.
In intact rats, depression of responses of dorsal horn neurons to noxious stimulation by peri-MAC increases in BZN, o-difluorobenzene, and HFB correlated directly with their in vitro capacity to block NMDA receptors. In decerebrate rats, 1.2 MAC BNZ depressed nociceptive responses by 60%, with a further percentage decrease continuing from 0.8 to 1.2 MAC approximately equal to that found in intact rats. In decerebrate rats, HFB caused a progressive dose-related decrease in MAC (maximum 25%), but in intact rats, an increase from 0.8 to 1.2 neuronal response caused an (insignificant) increase in neuronal response.
The findings in intact rats suggest that NMDA blockade contributes to the depression of dorsal horn neurons to nociceptive stimulation by fluorinated aromatic anesthetics. These results, combined with the additional findings in decerebrate rats, suggest that supraspinal effects (perhaps on γ-aminobutyric acid type A receptors) may have a supraspinal facilitatory effect on nociception for HFB.
The neuropeptides, arginine vasopressin (AVP) and vasoactive intestinal polypeptide (VIP) are synthesized by neurons of the suprachiasmatic nucleus (SCN) of the hypothalamus and are important regulators of SCN function. Previous studies have demonstrated that acute exposure to stressors can disrupt circadian activity rhythms, suggesting the possibility of stress-related alterations in the expression of these neuropeptides within SCN neurons. In this study, we examined the effect of intermittent footshock stress on AVP mRNA and heterogeneous nuclear RNA (hnRNA) and VIP mRNA expression in neurons of the SCN. Young adult male Sprague/Dawley rats were subjected to 15 seconds of scrambled intermittent footshock (0.50 mA pulses, 1 pulse/second, 300 msec) every 5 minutes for 30 minutes. Animals were sacrificed 75 or 135 min after the onset of stress and brains examined for AVP mRNA and hnRNA, and VIP mRNA using in situ hybridization. Footshock stress increased AVP hnRNA levels at the 75 min timepoint whereas AVP mRNA was elevated at both the 75 and 135 min. timepoints. In contrast, footshock stress decreased the number of cells expressing VIP mRNA in the SCN without changing hybridization level per cell. These data indicate that the disruptive effect of stress on activity rhythms correlate with alterations in the expression of regulatory peptides within the SCN.
Drug-associated cues and stress increase craving and lead to greater risk of relapse in abstinent drug addicts. This risk may be increased when these factors occur simultaneously. The current study examined whether the presentation of three different levels of intermittent footshock would trigger reinstatement or potentiate reinstatement of cocaine-seeking caused by conditioned cues. Male, Long Evans rats underwent daily i.v. cocaine self-administration, followed by extinction of lever responding in the absence of previously cocaine-paired cues. Reinstatement of cocaine-seeking was measured during presentation of cocaine-paired cues, following pretreatment with three levels of intermittent footshock (0.25, 0.5, and 0.75 mA), or after the combination of footshock and cues. Footshock at the 0.5 and 0.75 mA levels led to significant reinstatement when presented alone, and also potentiated the reinstatement triggered by the presentation of conditioned cues. These results demonstrate that while stress and drug-paired cues reinstate drug-seeking when presented in isolation, their interaction leads to potentiated reinstatement. Dual targeting of stress and cues is thus a critical consideration for treatment intervention in abstinent drug users.
cocaine; cues; footshock; reinstatement; relapse; stress
Heroin and cocaine have very different unconditioned receptor-mediated actions; however, in the brain circuitry of drug-reward and motivation, the two drugs establish common conditioned consequences. A single experience with either drug can change the sensitivity of ventral tegmental area (VTA) dopamine neurons to glutamatergic input. In the case of cocaine, repeated intravenous self-administration establishes de novo VTA glutamate release and dopaminergic activation in response to conditioned stimuli and mild footshock stress. Here we determined whether repeated self-administration of heroin would establish similar glutamate release and dopaminergic activation. Although self-administration of heroin itself did not cause VTA glutamate release, conditioned glutamate release was seen when rats expecting rewarding heroin were given nonrewarding saline in its place. Mild footshock stress also caused glutamate release in heroin-trained animals. In each case, the VTA glutamate release was accompanied by elevations in VTA dopamine levels, indicative of dopaminergic activation. In each case, infusion of the ionotropic glutamate antagonist kynurenic acid blocked the VTA dopamine release associated with VTA glutamate elevation. Although glutamate levels in the extinction and reinstatement tests were similar to those reported in cocaine studies, the effects of heroin self-administration itself were quite different from what has been seen during cocaine self-administration.
heroin; self-administration; extinction; reinstatement; ventral tegmental area; glutamate; Addiction & Substance Abuse; Glutamate; Catecholamines; Opioids; VTA; reinstatement; footshock
Stress is a physiological, adaptive response to changes in the environment, but can also lead to pathological alterations, such as relapse in psychiatric disorders and drug abuse. Evidence demonstrates that the dopamine system plays a role in stress; however, the nature of the effects of sustained stressors on dopamine neuron physiology has not been adequately addressed. By employing a combined electrophysiological, immunohistochemical and behavioral approach, we examined the response of ventral tegmental area (VTA) dopamine neurons in rats to acute as well as repeated stressful events using noxious (footshock) and psychological (restraint) stress. We found that aversive stimuli induced a pronounced activation of the dopamine system both electrophysiologically (population activity; i.e., number of dopamine neurons firing spontaneously) and behaviorally (response to psychostimulants). Moreover, infusion of TTX into the ventral hippocampus (vHPC) reversed both behavioral and electrophysiological effects of stress, indicating that the hyperdopaminergic condition associated with stress is driven by hyperactivity within the vHPC. Therefore, the stress-induced activation of the dopamine system may underlie the propensity of stress to exacerbate psychotic disorders or predispose an individual to drug-seeking behavior. Furthermore, the vHPC represents a critical link between context-dependent DA sensitization, stress-induced potentiation of amphetamine responsivity, and the increase in DA associated with stressors.
dopamine; stress; ventral hippocampus; amphetamine sensitization; in vivo electrophysiology
Stress is suggested to exacerbate symptoms and contribute to relapse in patients with schizophrenia and several other psychiatric disorders. A prominent feature of many of these illnesses is an impaired ability to filter information through sensorimotor gating processes. Prepulse inhibition (PPI) is a functional measure of sensorimotor gating, and known to be deficient in schizophrenia and sometimes in post-traumatic stress disorder (PTSD), both of which are also sensitive to stress-induced symptom deterioration. We previously found that a psychological stressor (exposure to a ferret without physical contact), but not footshock, disrupted PPI in rats, suggesting that intense psychological stress/trauma may uniquely model stress-induced sensorimotor gating abnormalities. In the present experiment, we sought to recreate the conditions where we found this behavioral difference, and to explore possible underlying neural substrates. Rats were exposed acutely to ferret stress, footshock, or no stress (control). 90 minutes later, tissue was obtained for Fos immunohistochemistry to assess neuronal activation. Several brain regions (prelimbic, infralimbic, and cingulate cortices, the paraventricular hypothalamic nucleus, the paraventricular thalamic nucleus, and the lateral periaqueductal gray) were equally activated following exposure to either stressor. Interestingly, the medial amygdala and dorsomedial periaqueductal gray had nearly twice as much Fos activation in the ferret-exposed rats as in the footshock-exposed rats, suggesting that higher activation within these structures may contribute to the unique behavioral effects induced by predator stress. These results may have implications for understanding the neural substrates that could participate in sensorimotor gating abnormalities seen in several psychiatric disorders after psychogenic stress.
c-fos; predator; immediate early gene; trauma; psychogenic
The medial prefrontal cortex (mPFC) has been implicated in the processing of emotionally significant stimuli, particularly the inhibition of inappropriate responses. We examined the role of the mPFC in regulation of fear responses using a differential fear conditioning procedure in which the excitatory conditioned stimulus (CS+) was paired with an aversive footshock and intermixed with the inhibitory conditioned stimulus (CS-). In the first experiment, using rats as subjects, muscimol, a gamma-amino-butyric acid type A (GABAA) receptor agonist, or artificial cerebrospinal fluid (aCSF) was infused intracranially into the mPFC across three conditioning sessions. Twenty-four hours after the last conditioning session, freezing response of the rats was tested in a drug-free state. Neither the muscimol nor the aCSF infusion had any effect on differential responding. In the second experiment, the same experimental procedure was used except that the infusion was made before the testing session rather than the conditioning sessions. The results showed that muscimol infusion impaired differential responding: the level of freezing to CS- was indiscriminable from that to CS+. Taken together, these results suggest that the mPFC is responsible for the regulation of fear response by inhibiting inappropriate fear expressions.
medial prefrontal cortex; differential fear conditioning; extinction; inhibitory learning
Adolescence is a vulnerable period in that stress experienced during this time can affect the incidence of psychiatric disorders later, during adulthood. Neurogenesis is known to be involved in the postnatal development of the brain, but its role in determining an individual's biological vulnerability to the onset of psychiatric disorders has not been addressed.
We examined the role of postnatal neurogenesis during adolescence, a period between 3 to 8 weeks of age in rodents. Mice were X-irradiated at 4 weeks of age, to inhibit postnatal neurogenesis in the sub-granule cell layer of the hippocampus. Electrical footshock stress (FSS) was administered at 8 weeks old, the time at which neurons being recruited to granule cell layer were those that had begun their differentiation at 4 weeks of age, during X-irradiation. X-irradiated mice subjected to FSS during adolescence exhibited decreased locomotor activity in the novel open field, and showed prepulse inhibition deficits in adulthood. X-irradiation or FSS alone exerted no effects on these behaviors.
These results suggest that mice with decreased postnatal neurogenesis during adolescence exhibit vulnerability to stress, and that persistence of this condition may result in decreased activity, and cognitive deficits in adulthood.
While Pavlovian conditioning alters stimulus-evoked metabolic activity in the cerebral cortex, less is known about the effects of Pavlovian conditioning on neuronal metabolic capacity. Pavlovian conditioning may increase prefrontal cortical metabolic capacity, as suggested by evidence of changes in cortical synaptic strengths, and evidence for a shift in memory initially processed in subcortical regions to more distributed prefrontal cortical circuits. Quantitative cytochrome oxidase histochemistry was used to measure cumulative changes in brain metabolic capacity associated with both cued and contextual Pavlovian conditioning in rats. The cued conditioned group received tone-footshock pairings to elicit a conditioned freezing response to the tone conditioned stimulus, while the contextually conditioned group received pseudorandom tone-footshock pairings in an excitatory context. Untrained control group was handled daily, but did not receive any tone presentations or footshocks. The cued conditioned group had higher cytochrome oxidase activity in the infralimbic and anterior cingulate cortex, and lower cytochrome oxidase activity in dorsal hippocampus than the other two groups. A significant increase in cytochrome oxidase activity was found in anterior cortical areas (medial, dorsal and lateral frontal cortex; agranular insular cortex; lateral and medial orbital cortex and prelimbic cortex) in both conditioned groups, as compared to the untrained control group. In addition, no differences in cytochrome oxidase activity in the somatosensory regions and the amygdala were detected among all groups. The findings indicate that cued and contextual Pavlovian conditioning induces sustained increases in frontal cortical neuronal metabolic demand resulting in regional enhancement in the metabolic capacity of anterior cortical regions. Enhanced metabolic capacity of these anterior cortical areas after Pavlovian conditioning suggests that the frontal cortex may play a role in the retention and regulation of learned associations.
Cytochrome oxidase; hippocampus; amygdala; context
Rats can acquire the cognitive component of CS-US associations between sensory and aversive stimuli without a functional basolateral amygdala (BLA). Thus, other brain regions should support such associations. Some septal/dorsal CA1 (dCA1) neurons respond to both spatial stimuli and footshock, suggesting that dCA1 could be one such region. We report that, in both dorsal and ventral hippocampus, different neuronal ensembles express immediate-early genes (IEGs) when a place is experienced alone vs. when it is associated with foot shock. We assessed changes in the size and overlap of hippocampal neuronal ensembles activated by two behavioral events using a cellular imaging method, Arc/Homer1a catFISH. The control group (A-A) experienced the same place twice, while the experimental group (A-CFC) received the same training plus two foot shocks during the second event. During fear conditioning, A-CFC, compared to A-A, rats had a smaller ensemble size in dCA3, dCA1, and vCA3, but not vCA1. Additionally, A-CFC rats had a lower overlap score in dCA1 and vCA3. Locomotion did not correlate with ensemble size. Importantly, foot shocks delivered in a training paradigm that prevents establishing shock-context associations, did not induce significant Arc expression, rejecting the possibility that the observed changes in ensemble size and composition simply reflect experiencing a foot shock. Combined with data that Arc is necessary for lasting synaptic plasticity and long-term memory, the data suggests that Arc/H1a+ hippocampal neuronal ensembles encode aspects of fear conditioning beyond space and time. Rats, like humans, may use the hippocampus to create integrated episodic-like memory during fear conditioning.
Arc; Homer 1a; fear conditioning; emotional memory; ventral CA1; ventral CA3; catFISH; dorsal hippocampus
Numerous studies have implicated neurogenesis in the hippocampus in animal models of depression, especially those related to controllability and learned helplessness. Here, we tested the hypothesis that uncontrollable, but not controllable stress would reduce cell proliferation in the hippocampus of male and female rats, and would relate to the expression of helplessness behavior.
To manipulate controllability, groups of male and female rats were trained in one session (acute stress) or over seven sessions (repeated stress) to escape a footshock, while yoked controls could not escape, but were exposed to the same amount of stress. Cell proliferation was assessed with immunohistochemistry of BrdU and immunofluorescence of BrdU and NeuN. Separate groups were exposed to either controllable or uncontrollable stress and their ability to learn to escape during training on a more difficult task was used as a behavioral measure of helplessness.
Acute stress reduced cell proliferation in males, but did not affect proliferation in the female hippocampus. When animals were given the opportunity to learn to control the stress over days, males produced more cells than the yoked males without control. Repeated training with controllable stress did not influence proliferation in females. Under all conditions, males were more likely than females to express helplessness behavior, even males that were not previously stressed.
The modulation of neurogenesis by controllability was evident in males but not in females, as was the expression of helplessness behavior, despite the fact that men are less likely than women to experience depression.
dentate gyrus; depression; sex differences; neurogenesis; controllability; stress; learned helplessness
This study sought to determine whether acute and/or chronic psychological stress produce changes in urinary bladder nociception. Female Sprague-Dawley (SD; low/moderate anxiety) or Wistar-Kyoto (WK; high-anxiety) rats were exposed to either an acute (1 day) or a chronic (10 days) water avoidance stress paradigm or a sham stress paradigm. Paw withdrawal thresholds to mechanical and thermal stimuli and fecal pellet output, were quantified at baseline and after the final stress or sham stress exposure. Rats were then sedated, and visceromotor responses (VMRs) to urinary bladder distension (UBD) were recorded. While acute stress exposure did not significantly alter bladder nociceptive responses in either strain of rats, WK rats exposed to a chronic stress paradigm exhibited enhanced responses to UBD. These high-anxiety rats also exhibited somatic analgesia following acute, but not chronic, stress. Furthermore, WK rats had greater fecal pellet output than SD rats when stressed. Significant stress-induced changes in nociceptive responses to mechanical stimuli were observed in SD rats. That chronic psychological stress significantly enhanced bladder nociceptive responses only in high-anxiety rats provides further support for a critical role of genetics, stress and anxiety as exacerbating factors in painful urogenital disorders such as interstitial cystitis (IC).
Bladder; Stress; Hyperalgesia; Visceral
Stressful experiences in humans can result in elevated alcohol drinking, as exemplified in many individuals with post-traumatic stress disorder. However, how stress history, rather than acute stressors, influences alcohol intake remains uncertain. To model the protracted effects of past stress, male Wistar rats were subjected to light-cued footshock stress (Stress History) or light cues alone (Control) prior to their acquisition of alcohol self-administration (1-h sessions, fixed ratio1–3, 100 µl of 10% v/v alcohol as reinforcer). Stress history did not alter mean alcohol intake during acquisition of self-administration, but it increased preference for the alcohol-paired lever over the inactive lever. Following an extinction period, rats with a history of stress exposure and low baseline alcohol intake showed a 2-fold elevation in alcohol self-administration, as compared to low-drinking rats with no stress history. Similar effects were not seen in rats self-administering 0.1% sucrose. Analysis of mRNA levels of phosphodiesterase 10A (PDE10A), a dual-specificity cAMP and cGMP hydrolyzing enzyme, showed that stress history increased Pde10a mRNA levels in the basolateral amygdala and, in low drinking rats, the prelimbic prefrontal cortex (plPFC). Pde10a mRNA levels in the plPFC correlated directly with greater alcohol self-administration during the relapse-like phase, and greater BLA Pde10a mRNA levels correlated with increased ethanol preference after acquisition. The data demonstrate that stress history sensitizes otherwise low alcohol drinkers to consume more alcohol in a relapse-like situation, and identify stress-induced neuroadaptations in amygdala and prefrontal cortical Pde10a expression as changes that may drive heightened alcohol intake and preference in susceptible individuals.
alcohol or ethanol; phosphodiesterase 10A; post-traumatic stress disorder; relapse; stress; trauma
Chronic stress facilitates fear conditioning in rats with hippocampal neuronal atrophy and in rats in which the atrophy is prevented with tianeptine, a serotonin re-uptake enhancer. The purpose of this study was to determine whether the lack of dissociation between fear conditioning performance and hippocampal integrity was masked by the presence of endogenous corticosteroids during training. As in previous studies, rats were stressed by daily restraint (6 h/day for 21 days), trained in the conditioning chamber (day 23), and then assessed for conditioned fear (day 25) at a time when hippocampal dendritic atrophy persists. On the training day, half of the control and stressed rats were injected with metyrapone to reduce corticosterone release. Two hours later, two paired or unpaired presentations of tone and footshock were delivered. Although metyrapone reduced conditioned fear in all rats, only stressed rats showed dissociated fear conditioning (i.e. tone conditioning was reduced while contextual conditioning was eliminated). Chronically stressed rats, regardless of metyrapone treatment displayed more rearing in the open field when tested immediately after the completion of fear conditioning. These data support the hypothesis that increased emotionality and enhanced fear conditioning exhibited by chronically stressed rats may be due to endogenous corticosterone secretion at the time of fear conditioned training. Moreover, these data suggest that chronic stress impairs hippocampal-dependent processes more robustly than hippocampal-independent processes after metyrapone to reduce corticosterone secretion during aversive training.
Amygdala; Hippocampus; Fear conditioning; Glucocorticoid; Learning
The lateral nucleus of the amygdala (LA) is a site of convergence for auditory (conditioned stimulus) and footshock (unconditioned stimulus) inputs during fear conditioning. The auditory pathways to LA are well characterized, but less is known about the pathways through which footshock is transmitted. Anatomical tracing and physiological recording studies suggest that the posterior intralaminar thalamic nucleus, which projects to LA, receives both auditory and somatosensory inputs. In the present study we examined the expression of the immediate-early gene c-fos in the LA in rats in response to footshock stimulation. We then determined the effects of posterior intralaminar thalamic lesions on footshock-induced c-Fos expression in the LA. Footshock stimulation led to an increase in the density of c-Fos-positive cells in all LA subnuclei in comparison to controls exposed to the conditioning box but not shocked. However, some differences among the dorsolateral, ventrolateral and ventromedial subnuclei were observed. The ventrolateral subnucleus showed a homogeneous activation throughout its antero-posterior extension. In contrast, only the rostral aspect of the ventromedial subnucleus and the central aspect of the dorsolateral subnucleus showed a significant increment in c-Fos expression. The density of c-Fos-labeled cells in all LA subnuclei was also increased in animals placed in the box in comparison to untreated animals. Unilateral electrolytic lesions of the posterior intralaminar thalamic nucleus and the medial division of the medial geniculate body reduced footshock-induced c-Fos activation in the LA ipsilateral to the lesion. The number of c-Fos labeled cells on the lesioned side was reduced to the levels observed in the animals exposed only to the box. These results indicate that the LA is involved in processing information about the footshock unconditioned stimulus and receives this kind of somatosensory information from the posterior intralaminar thalamic nucleus and the medial division of the medial geniculate body.
Emotional learning; fear conditioning; pain; freezing; electrolytic lesions; somatosensory thalamus
Stress produces significant alterations in sleep that appear to vary with the type, intensity and duration of the stressor. Brief manual restraint may be stressful in rodents, but is often required for experimental procedures. We examined the effects of brief manual restraint on sleep and its possible influence on sleep induced after footshock and after the opportunity to explore a neutral enclosure. Sleep was recorded during non-interrupted baseline and during 8-h light and 12-h dark periods after three sessions of 5-min manual restraint (M1-3), after 30-min in neutral enclosure alone (NE) or with previous manual restraint (mNE), and after 20 footshocks presented over the course of 30-min alone (FS) or with previous manual restraint (mFS). Compared to baseline, M1-3 increased total sleep and NREM during both light and dark periods, and significantly increased dark period REM. Both NE and mNE increased dark-period total sleep, NREM and REM; however, mNE also increased light-period total sleep and NREM, but not REM. FS and mFS increased total sleep, NREM and REM during the dark period and total sleep and NREM during light period. FS also significantly decreased light-period REM whereas mFS did not. M1, mNE and mFS significantly increased EEG delta power during NREM, but M2-3, NE and FS alone did not. The results revealed that manual restraint can increase sleep and EEG delta power and that increases in sleep may persist across repeated sessions whereas the magnitude of EEG delta power may vary across sessions. In addition, prior manual restraint may significantly alter the changes in sleep and EEG induced by footshock and by the opportunity to explore a neutral enclosure. The results suggest that mild stressors may interact in their effects on sleep.
Manual restraint; Sleep; Activity; EEG power; Footshock; Stress