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Growth hormone releasing hormone (GHRH) promotes non-rapid eye movement sleep (NREMS), in part via a well-characterized hypothalamic sleep-promoting site. However, GHRH may also act in the cortex to influence sleep. Application of GHRH to the surface of the cortex changes electroencephalographic (EEG) delta power. GHRH and the GHRH receptor (GHRHR) mRNAs are detectable in the rat cortex and the expression of cortical GHRHR is activity-dependent. Here we microinjected a GHRH antagonist or GHRHR siRNA (siGHRHR) onto the somatosensory cortex surface in rats. The unilateral application of the GHRH antagonist ipsilaterally decreased EEG delta wave power during NREMS, but not wakefulness, during the initial 40 min after injection. Similarly, the injection of siGHRHR reduced cortical expression of GHRHR and suppressed NREMS EEG delta wave power during 20- to 24-h post-injection. Using the Fura-2 calcium imaging technique, cultured cortical cells responded to GHRH by increasing intracellular calcium. Approximately 18% of the GHRH-responsive cells were GABAergic as illustrated by GAD67 immunostaining. Double labeling for GAD67 and GHRHR in vitro and in vivo indicated that only a minorityof cortical GHRHR-containing cells were GABAergic. Our data suggest that endogenous cortical GHRH activates local cortical cells to affect EEG delta wave power state-specifically. Results are also consistent with the hypothesis that GHRH contributes to local network state regulation.
Growth hormone releasing hormone (GHRH) is involved in sleep regulation (Obal et al., 1986, 1988, 2003; Steiger et al., 1992). Specifically, central or systemic administration of GHRH enhances non-rapid eye movement sleep (NREMS) while inhibition of endogenous GHRH suppresses spontaneous NREMS (Obal et al., 1991, 1992). The chronic disruption of GHRH mechanisms in several mutant animal models is associated with disturbed sleep. For instance, mice lacking functional GHRH receptors have less spontaneous NREMS, rapid eye movement sleep (REMS) and circulating growth hormone (GH); GH-replacement rescues REMS but not NREMS (Obal et al., 2001, 2003). The hypothalamus is a site of action for GHRH-mediated NREMS. Microinjection of GHRH into the anterior hypothalamus enhances NREMS while hypothalamic injection of a GHRH antagonist decreases NREMS (Zhang et al., 1999). Further, hypothalamic GHRH mRNA levels and GHRH release vary with daily sleep rhythms and are enhanced during sleep deprivation (Toppila et al., 1997; Zhang et al., 1998).
Hypothalamic or intraventricular injections of GHRH also enhance another sleep phenotype, electroencephalographic (EEG) slow wave activity (SWA) (0.5–4 Hz). EEG SWA is considered an index of NREMS intensity because it is enhanced during the NREMS that occurs after sleep deprivation and it is a useful quantitative parameter in the 2-process model of sleep regulation (Pappenheimer et al., 1975; Borbély, 1982). EEG SWA is regulated independently from sleep duration (reviewed Kapás et al., 2008). EEG SWA also seems to reflect prior neuronal activity within local circuits (reviewed Krueger et al.,2008). Thus, targeted sensory activation of the somatosensory cortex is followed by enhanced EEG SWA during subsequent NREMS (Kattler et al., 1994; Huber et al., 2004). Conversely, targeted reduction of afferent activity during waking is followed by reduced EEG SWA in subsequent NREMS (Huber et al., 2006). Further, EEG SWA is regulated, in part, via intrinsic cortical mechanisms (Steriade, 2003; Gerashchenko et al., 2008).
Cortical GHRH receptors (GHRHR)s are functional and may play a role in EEG SWA regulation. GHRH- and GHRHR-immunoreactive cells and GHRH and GHRHR mRNAs exist in the cortex (Matsubara et al., 1995; Peterfi et al., 2006; Szentirmai et al., 2007). The cortical GHRHRs are functional. Unilateral microinjection of GHRH locally onto the surface of the cortex changes subsequent EEG SWA ipsilaterally during NREMS, but not during REMS or wakefulness (Szentirmai et al., 2007). The expression of cortical GHRH is enhanced by sleep deprivation (Szentirmai et al., 2007) and neuronal GHRHR expression is associated with cortical column neuronal activity (De et al., 2006). However, it remains unknown whether endogenous cortical GHRH contributes to these NREMS-specific EEG SWA effects, or how GHRH-receptive cells respond to GHRH or if cortical GHRH-receptive neurons, like hypothalamic GHRH-receptive cells, have a GABAergic phenotype. We report herein that unilateral blockade of cortical GHRH state-specifically decreased ipsilateral EEG SWA. Further, GHRH enhanced intracellular Ca2+ but most of these cells lacked the GABAergic phenotype. Results reinforce the concept of a GHRH-contribution to local network state regulation.
The surgical procedures were performed as described previously (Szentirmai et al., 2007). Institutional guidelines for the care and use of research animals were followed, and the experimental protocols were approved by the Institutional Animal Care and Use Committee. Using ketamine (87 mg/kg) and xylazine (13 mg/kg) for anesthesia, rats (Male; Sprague Dawley; 300–330 g kept at 24°C on a 12:12 h L/D cycle) were provided two cortical EEG electrodes over the somatosensory cortex (Sctx, Bregma coordinates: 2.5 mm AP, 5.5 mm ML) on each brain hemisphere and one electromyographic (EMG) electrode in the nuchal muscles. In addition a common reference EEG electrode was implanted 10 mm posterior to bregma on the midline over the cerebellum. Lead wires were fitted to a miniature plug and attached to the skull with dental cement (Duz-All; Coralite Dental Products, Skokie, IL). Two injection guide cannulae, one for each hemisphere, were implanted between the surface of the Sctx and the dura with their tips positioned under the EEG electrodes for injections. To verify the location of the microinjection cannulae, immediately after surgery while under ketamine-xylazine anesthesia, 1 μl of 20% lidocaine was injected via the cannulae guide through an injection needle that pierced the dura (lidocaine induced decreases in EEG power). Only animals with correctly placed cannulae were included in the data analysis. The rats were placed in their home cages inside environmental chambers (Hotpack 352600, Philadelphia, PA) and given 10 days to recover from the surgery.
A flexible tether (Plastics One) connected the EEG and EMG electrodes to an electronic swivel as described previously (Taishi et al., 2007). After the 10 day recovery from surgery, rats were given three days to acclimate to this experimental condition. During each of the acclimation days, the rats were handled for 10 min during the 40 min period just prior to dark onset. Recordings began at dark onset (1900 h). The vigilance states NREMS, REMS and wake were determined off line in 10 sec epochs using previously published criteria (Zhang et al., 1999). For power spectra analyses, only the pure-state epochs without state transitions or any artifact within them were included.
Two-way analysis of variance (ANOVA) for repeated measures was performed to analyze the sleep duration, EEG SWA and EEG power spectra. Time blocks of 40-min (antagonist treatment) and 2-h (siGHRHR treatment) were used for sleep duration. Treatment (between baseline recording day and treatment day) and time were the two factors of the two-way ANOVA. For EEG SWA during each time block, treatment effect (between the reagent injection side and vehicle injection side) and time were the two factors of the two-way ANOVA. Then the EEG power spectra for individual time blocks were analyzed, using treatment (between the reagent injection side and vehicle injection side) and frequency as the two factors of the two-way ANOVA. When significant differences were detected by ANOVA, Fisher’s LSD multiple comparison test was used to detect what time block or frequency was changed.
On the baseline recording day, 1 μl saline was injected onto each side of the brain at dark onset and the EEG was recorded. On the following day, one side of the brain received 5 nmol of the GHRH antagonist [Phenylac-Tyr1,D-Arg2,p-chloro-Phe6,Arg9,Abu15,Nle27,D-Arg28,Homoarg29 -GRF (1–29), Bachem, Torrance, CA] while the opposite control side received the same volumeof saline (n=9). For the injections, each rat was disconnected from its tether and removed from its environmental chamber for about 5 min. Each injection took about 2 min. Experiments were performed using 4 rats in each series to minimize the time between treatment and dark onset (1820 and 1900 h). After the injection, EEG and EMG were continuously recorded for 1 day. EEG SWA (0.5–4Hz) during various states was normalized by dividing by the baseline values of the same side of brain at the same time periods after saline injection to minimize the impact of the injections (Taishi et al., 2007). Only the data from the first 2-h post- antagonist injection period were analyzed because it was previously reported that the antagonist has a limited duration effect (Varga et al., 1999). Only the first two 40-min periods were used for statistical analyses. During the first 40-min after GHRH antagonist injection, only 5 of the 9 rats had REMS.
For the GHRHR siRNA (siGHRHR) injection (n=7), the baseline EEG was recorded for 1 day prior to the injection. On the experimental day, 0.1 nmol GHRHR siRNA (sense: 5′-CGCUGUUUGGAAUUCAUUAtt-3′, antisense: 5′-UAAUGAAUUCCAAACAGCGgg-3′, s129475, Ambion, Austin, TX) was injected on one side of the Sctx while the opposite side received 0.1 nmol scrambled siRNA (Ambion) as a negative control. After the injection, EEG and EMG were recorded for 2 days because previous studies indicated that another siRNA begins to have sleep effects about 24-h post-injection (Taishi et al., 2007). Because we did not know exactly when and how long the GHRHR siRNA would act, there were no injections on the baseline day. Therefore, the average power of EEG SWA during NREMS throughout the 23-h baseline-recording period was normalized to 100% for each animal.
At least 10 days after completion of the EEG recordings described above, the same rats (n=7) in experiment 2 were again microinjected at dark onset with the same doses of siGHRHR or scrambled siRNA as previously used in the EEG studies. Rats were sacrificed 20 h after the injection and the area around the tip of the microinjection cannulae was quickly dissected and immediately placed in RNAlater (Ambion) and stored at −20°C until preparation for real-time PCR analyses.
The RNA extraction, cDNA preparation, PCR amplification and gel electrophoresis were performed as previously described (Bredow et al., 1996). Total RNA was extracted by the acid guanidinium-phenol-chloroform method using TRIzol reagent according to the manufacturer’s instructions (Invitrogen, Carlsbad, CA). At the end of the isolation procedure, the RNA pellets were suspended in DNase I buffer that contained 1 U SUPERase· In and 2 U DNA-free (Ambion) and were then incubated at 37°C for 90 min to degrade contaminating DNA. The RNA concentration was estimated by UV absorbance at 260/280 and the quality of the RNA was verified by 1% agarose gel electrophoresis.
cDNA was synthesized using Superscript III (Invitrogen). Two micrograms of total RNA were heated together with 0.5 μg oligo (dT)12–18 and 1 μl of 10 mM dNTPs at 65°C for 5 min, then chilled on ice. 0.1 M DTT, RNaseOUT (Invitrogen), 200 U of Superscript III, and 5× first-strand buffer were added, and the mixture was incubated at 55°C for 60 min. The reaction was stopped by incubating at 70°C for 15 min and then cooled to 4°C. The RNA template was degraded for 20 min at 37°C with RNase H. Samples were diluted with sterile DNase-free water, aliquoted, and stored at −20°C. Real-time PCR reactions were performed as previously described (Taishi et al., 2004). Briefly, the PCR reaction mixture (25 μl) contained 5 μl diluted cDNA (100 ng total RNA equivalents for GHRHR; 10 ng for cyclophilin A), 12.5 μl 2X PLATINUM Quantitative PCR SuperMix-UDG (Invitrogen), 0.25 μl of 1:1,000 dilutions of SYBR Green (Invitrogen) and flourescein (Bio-Rad, Hercules, CA), and 0.5 μl of 10 μM primers for GHRHR (5′-CTCCATTGTAGCCCTCTGCGTG-3′; 5′-AGGAACACAGCACTGGCCTTGA-3′) and cyclophilin A (5′-AAATGCTGGACCAAACACAAA-3′; 5′-CTCATGCCTTCTTTCACCTTC-3′); these primers recognize all three isoforms of the GHRHR. Reactions were performed in triplicate, and threshold cycles (Ct) values were averaged. Gene expression was analyzed by the comparative Ct method using the formula: 2 −ΔΔCt (Szentirmai et al., 2007). The fold-changes between the mRNA expression levels of the siGHRHR and the scrambled siRNA treated samples were calculated. Results are expressed as relative change compared with cyclophilin. Paired t-tests were performed to compare GHRHR mRNA values obtained from the scrambled siRNA and siGHRHR treated sides of the same rat brains. The dissociation curves of used primer pairs showed a single peak, and PCR products had a single expected DNA band when run on an agarose gel (data not shown).
Rats (n=8) were implanted with EEG and guide cannulae as in experiment 2. At dark onset, rats were injected with the same doses of siGHRHR on one hemisphere and scrambled siRNA on the other hemisphere as described in experiment 2. Twenty-two h after the injection, the rats were anesthesized using isoflurane and then perfused with warm saline followed by 4% paraformadehyde. Brains were removed and processed for GHRHR immunohistochemistry.
For GHRHR immunohistochemistry, nonspecific binding sites were blocked with 3% normal goat serum for 1 h at room temperature. The brain slices were incubated with 1:5000 polyclonal rabbit anti-GHRHR (Biosynthesis, Lewisville, TX) made against a peptide sequence on the third extracellular loop: NH2-GCDSAGLGIRLPLE-OH for 3 days at 4 °C followed by a 2-h incubation of 1:500 secondary biotinyated goat anti-rabbit IgG (Vector Laboratories, Burlingame, CA). Then the brain slices were incubated with ABC complex (Vector Laboratories) for 90 min and then developed with diaminobenzidine (DAB)/NiCl2 (Vector Laboratories). Photomicrographs of Sctx sections 0.5 mm lateral to the injection sites were prepared using a 40× objective on a Leica DMLB microscope with a Spot digital camera. The number of GHRHR immunoreactive layer V pyramidal neurons within each figure (660μm ×540μm) was counted and normalized by dividing with the average number of cells in the same area on the control side of the same animal. The numbers of GHRHR immunoreactive pyramidal cells in siGHRHR and scrambled siRNA-injected sides of the same rat brains were compared statisticallyusing a paired t-test (n=8).
The specificityof the GHRHR antibody was previously described (Peterfi et al., 2006). Western blots showed the detection of a 47 kD band. There was no signal when the primary antibody was absent or preincubated with a GHRHR peptide.
Rats were implanted with EEG and guide cannulae using the surgical procedures as in experiment 2. At dark onset, rats (n=8) were injected with the same doses of siGHRHR on one side and scrambled siRNA on the other side as in experiment 2. Between 20–22 h post-injection, rats were sacrificed and the area around the tip of the microinjection cannulae was quickly dissected and immediately frozen in liquid nitrogen. All samples were stored at −70°C until western blot analyses. Western blots of GHRHR were performed as previously described (Szentirmai et al., 2007). The levels of GHRHR were normalized using GAPDH values from the same sample. A paired t-test was used to compare the siGHRHR and scrambled siRNA injected sides of the same rats.
Primary cortical cells were dissected from Sprague-Dawley rat embryonic day 18 fetuses and cultured on glass coverslips for 10–12 days. One culture was performed each week using one pregnant rat. Cells from 31 coverslips from 8 cultures were challenged with 100 nM GHRH and the cytosolic Ca2+ levels were monitored by Ca2+ imaging.
Primary cultures of fetal cerebral cortex cells were prepared as previously described (Simasko et al., 1999) with slight modifications. Briefly, cortices were placed in ice cold Hank’s Balanced Salt Solution (HBSS) containing 1% Fungi-bact, 0.1% BSA, and 200 mM ascorbic acid. The cortices were washed 3 times in HBSS and the tissue was incubated at 37°C for 15 min. Then the cortices were resuspended in Dulbecco’s Modified Eagle’s Medium (DMEM) and mechanically dissociated by gently passing 3 times through a 20 gauge needle fixed to a 20 ml syringe followed by a 22 gauge needle. After centrifugation at 1200 rpm for 5 min, cells were resuspended in DMEM containing 10% fetal bovine serum and filtered though a 70 μm-pore cell strainer. Cells were then plated at a density of 2 ×106 cells coverslip. The coverslips had previously been coated with 100 μg/ml polyornithine in 0.15 M borate buffer, pH 8.4. After 2 days the medium was replaced with a serum-free medium containing DMEM supplemented with 30 nM selenium, 20 nM progesterone, 1 μM iron-free human transferrin, 100 μM putrescine, and 5 μg/ml insulin. Medium was then replaced every 2 days until the cells were used for experiments. All buffer components were obtained from Sigma (St. Louis, MO) except fetal bovine serum (Hyclone, Logan, UT).
Cytoplasmic Ca2+ levels were measured with Fura-2 using dual-wavelength fluorescent measurements with an imaging-based detection system (MetaFluor, Universal Imaging, West Chester, PA) as described (De et al., 2002) with slight modification. The average ratio of fluorescence intensities (340/380) for an area of the image over a cell was used to estimate cytoplasmic Ca2+ levels. For data analyses, only cells in which the basal level of Ca2+ was between 0 and 200 nM were included. It was reasoned that cells with over 200 nM basal Ca2+ may represent cells that were dying or stressed. A positive response to a GHRH challenge was defined as a change of greater than 20 nM or 5% of the basal value, whichever was greater. For GHRH challenge, the number of cells from all the coverslips was summarized and the ratio of GHRH-responsive cells to all monitored cells was calculated.
To find out the source of GHRH-induced Ca2+ spike, primary cortical cells were cultured and Ca2+ imaging was performed as experiment 4a. Cells from 10 coverslips from 5 independent cultures were challenged with GHRH followed by GHRH in Ca2+ free buffer (containing 140 mM NaCl, 5 mM KCl, 3 mM MgCl2, 10 mM HEPES and 6 mM glucose). One minute later, the cells were reperfused with standard buffer for 2 min then exposed to GHRH again. Only the cells which responded to both the initial and third GHRH exposure were included in the data analysis. The responses to the second GHRH exposure were compared to the second GHRH exposure in experiment 4a using Student’s t-test.
To identify the phenotype of the GHRH-responsive cells, GAD67 immunohistochemistry was performed after the Ca2+ measurement as previously described with minor alterations (De et al., 2002). Photomicrographs were taken at the end of Ca2+ imaging. Then the coverslips were processed for GAD67 staining using mouse anti-rat GAD67 (Millipore#MAB5406, Billerica, MA). The anti-GAD67 has no cross-reactivity with GAD65 as determined by western blot (technical information was provided by the manufacturer) and it was used previously for immunohistochemistry on rat and mouse tissues (Gritti et al., 2006, Gvilia et al., 2006; Chen et al., 2009; Sakata et al., 2009). The specific labeling was detected by DAB/NiCl2 (Vector laboratories) staining. The GHRH-responsive cells were relocated by matching the GAD67 staining photomicrographs with the ones taken after Ca2+ imaging. Then their immunoreactivity to GAD67 was determined. Control experiments without primary antibody did not produce any labeled cells.
Brain sections from the rats in experiment 3b were used. Nonspecific binding sites were blocked with 5% normal goat serum and 5% normal chicken serum for 1 h at room temperature. Then the brain sections were incubated in 1: 2000 mouse anti-rat GAD67 and 1: 4000 rabbit anti-rat GHRHR over 2 nights at 4°C. Specific binding was detected by incubated in Alexa Fluor 488 chicken anti-mouse IgG (1:500, Invitrogen) and Alexa Fluor 568 goat anti-rabbit IgG (1:500, Invitrogen) for 2 h at room temperature. For each brain section, at least 2 fields (225 μm × 225 μm) in the Sctx on the scrambled siRNA receiving side were scanned by the confocal laser scanning. GHRHR- and GAD67-immunoreactive cells within these fields were counted and the average was calculated. For each animal, at least 6 brain sections were scanned and the averages of all the sections were calculated. Two rats were excluded due to the poor quality of double labeling. The data are expressed as means±S.E. (n=6).
Primary cortical cells were cultured for 12 days as in experiment 4a. Cells from 8 cultures were fixed with 4% paraformaldehyde for 30 min at room temperature (n=8). Immunoreactions to GHRHR and GAD67 were performed as in experiment 5a. The cell nuclei were stained with bisBenzimide H33258 (5 μg/ml, Sigma). Photomicroscopy of cells was performed using a 40× objective on a fluorescence microscope (Leica). For each culture, photographs of 3 fields were taken and the number of cells labeled with bisBenzimide H33258, or that were GHRHR-immunoreactive, GAD67-immunoreactive or GHRH/GAD67 double labeled cells were determined. The ratios of GHRHR immunoreactive cells to cells labeled by bisBenzimide H33258 or the ratios of double labeled cells to GHRHR-immunoreactive cells were calculated. The data are expressed as means ±S.E.
Local injection of the GHRH antagonist did not alter whole animal sleep or wakefulness durations (data not shown). In contrast, the GHRH antagonist unilaterally decreased EEG delta wave power during NREMS (two-way ANOVA repeated measures, n=9, p<0.05), but not during wakefulness or REMS (there were few REMS epochs) during the initial 40 min after injection (Figure 1A). EEG power spectra analyses showed that GHRH antagonist selectively inhibited 1–1.5 Hz slow waves (two-way ANOVA repeated measures, n=9, p<0.05) during NREMS while the REMS and wakefulness power spectra were not affected (Figure 1B). During the 40–80 min period post-injection, the ipsilateral EEG delta power (0.5–4 Hz) was lower than that occurring on the control side but this effect was not significant (Figure 1A). During 80–120 min post-injection, there were no differences in EEG slow wave power between the two sides of the cortex during NREMS, REMS or wakefulness (data not shown).
The localized microinjection of the siGHRHR into the Sctx did not alter whole animal NREMS or REMS durations (data not shown). In contrast, the unilateral injection of siGHRHR reduced ipsilateral EEG delta wave power during NREMS (Figure 2A). After the injections during the dark period of day 1 (first 12 h post-injection) the rats exhibited their normal EEG SWA during NREMS and there were no significant differences between the two sides of the brain. During the light period of day 1 (13–24 h post-injection), there was a significant decrease in EEG SWA during NREMS on the side that received the siGHRHR compared to the side that received scrambled siRNA treatment (two-way ANOVA repeated measures, n=7, p<0.05). Fisher’s LSD multiple comparison tests illustrated that the differences in EEG SWA between the two sides were significant 20–24 h post-injection. On post-injection day 2, siGHRHR failed to induce significant differences in EEG SWA between the two sides of the brain (Figure 2A). There were no differences in EEG SWA during wakefulness or REMS at any time after siGHRHR treatment (Figure 2A). Finally, EEG power spectra illustrated that while slow wave power in 0.5–2.5 Hz was significantly reduced by siGHRHR at 20–22 h post-injection (two-way ANOVA repeated measures, n=7, p<0.01), EEG power in higher frequencies (> 4Hz) was not altered by the siGHRHR (Figure 2B).
The siGHRHR induced a 65.9% decrease (paired t-test, n=7, p<0.001) in GHRHR mRNA in the ipsilateral Sctx as compared to that observed after administration of the scrambled siRNA to the contralateral Sctx at 20 h post-injection. Western blots illustrated that after treatment with siGHRHR for 20 h, the GHRHR bands (47kD) were less dense in comparison with the control bands (Figure 3A). Quantitative evaluation of these GHRHR bands indicated a significant 16% decrease as compared to the scrambled siRNA treated side (paired t-test, n=8, p<0.05). siGHRHR treatment of the Sctx reduced the number of darkly stained GHRHR-immunoreactive cells in layer V at 22 h post-injection. The GHRHR-immunoreactive cells illustrated in Figure 3B, C were about 0.5 mm from the siGHRHR-injected or scrambled siRNA-injected site. Quantitative evaluation of these GHRHR-immunoreactive cells indicated a significant 28.4% decrease (paired t-test, n=8, p <0.01) in cell number on the siGHRHR treatment side as compared to scrambled siRNA treatment side.
The average basal Ca2+ level for cortical neurons was 30.1±0.5 nM (n=1531). GHRH (100 nM) increased intracellular Ca2+ in 10.0 % of the neurons naively exposed to GHRH (153 of 1531 cells tested; average response over baseline 91.9±5.4 nM, n=153; maximal response 302.1 nM; results from 31 coverslips from 8 cultures). When neurons were exposed to a second challenge of the GHRH 2–3 min after the first exposure, only 35.3% (54 of 153) GHRH-responsive neurons responded (Figure 4A).
The GHRH-responsive cells were further tested by using GHRH in a bath in which Ca2+ had been replaced with Mg2+ (results from 14 cells, 10 coverslips from 5 cultures) about 2 to 3 min after an exposure to GHRH. The cells responded to the initial GHRH exposure with an increase of68.6±12.4 nM (n=14) of Ca2+. When exposed to GHRH in Ca2+ free buffer, the Ca2+ spikes were −2.0±1.2 nM (n=14), which was significantly lower than the second GHRH exposure (35.9±4.2 nM, n=153) in experiment 4a (p<0.001, Student’s t-test). When the cells were again perfused with standard buffer, the Ca2+ increase (65.6±13.0 nM, n=14) in response to GHRH was restored. These data suggest that the GHRH-induced Ca2+ response was dependent on extracellular Ca2+.
To study the phenotype of the cortical GHRH-responsive cells, we examined immunoreactivity for GAD67, a GABAergic cell marker in the GHRH-responsive cells detected by Ca2+ imaging. Seven out of the 38 neurons (18.4 %) that we were able to relocate were GABAergic (Figure 5A, B).
To verify the results in experiment 4c, double labeling immunocytochemistry for GHRHR and GAD67 (Figure 5C–F) was performed. In the cell culture 10.8±3.4% (mean±S.E., n=8) of total cells expressed GHRHR. 32.3±4.7% (mean±S.E., n=8) of these GHRHR containing cells are GAD-immunopositive. We further examined the GHRHR cortical cells in the adult rat Sctx in vivo. Of the GHRHR-immunoreactive cells 19.7±1.55% (mean±S.E., n=6) (Figure 5G-I) were also GAD67-immunopositive.
Our primary finding is that unilateral blockade of endogenous cortical GHRH state-specifically reduces ipsilateral EEG SWA. The GHRH antagonist-induced reductions in EEG SWA during NREMS were greatest within the first 40 min. Others also describe a rapid onset and short duration of action of GHRH peptide antagonists. Thus after intravenous injection of the GHRH antagonist used herein, GH secretion is maximally inhibited for about 30 min and disappears by 60 min (Varga et al., 1999). Intrahypothalamic injection of another GHRH antagonist delayed the latency to the first NREMS epoch from 15 to 30 min, decreased NREMS duration maximally within the first post-injection h and suppressed EEG SWA about 20% during NREMS for the first 2 h (Zhang et al., 1999). siGHRHR-induced unilateral down-regulation of GHRHR correlated with ipsilateral EEG SWA suppression during 20- to 24-h post-injection. The timing of these siGHRH-induced effects is consistent with previous studies showing that siRNAs effects on sleep first manifest about 24 h post-injection (Taishi et al., 2007; Chen et al., 2006).
The changes in GHRH-GHRHR-induced EEG SWA are state-specific occurring only during NREMS. Mechanistically, the GHRH-mediated effects likely involve extracellular Ca2+. At a cellular level, cortical pyramidal cells characteristically oscillate between a bursting (up) state and a silent (down) state during sleep (Steriade, 2003). During NREMS, the extracelluar Ca2+ fluctuates with slow oscillations with lower levels during the depolarizing phase and higher levels during the hyperpolarizing phase (Massimini and Amzica, 2001). Thus the high extracellular Ca2+ is expected to influence GHRH exocytosis and post-synaptically Ca2+ entry into the GHRH-receptive cells. Because the hyperpolarizing down state occurs primarily during sleep, state-specificity can arise. The actions of other sleep regulatory substances on EEG SWA are also state-specific (reviewed Krueger et al., 2008). For instance application of interleukin-1 onto the Sctx enhances EEG SWA only during NREMS (Yasuda et al., 2005); Interleukin-1 affects sleep in part via GHRH (Obal et al., 1995). Finally, that the effects of the GHRH antagonist and the siGHRHR are state-specific is consistent with the hypothesis that the induced decreases in EEG SWA reflect a sleep phenotype.
The cortical origin of EEG SWA is suggested by previous studies. EEG slow oscillations (0.5–1.5 Hz) are generated intracortically, survive thalamectomy (Steriade et al., 1993), and are absent in the thalamus of decorticated animals (Timofeev and Steriade, 1996). Slow oscillatory field potentials occur in single cortical columns in cortical slices (Sanchez-Vives and McCormick, 2000). In addition, cortical islands, disconnected from their thalamic inputs but with blood flow maintained, exhibit local field potentials that oscillate over periods of 10–20 min between episodes of high-amplitude EEG delta waves and low EEG delta power (Kristiansen and Courtois, 1949). Those data coupled with the current studies, suggest that GHRH plays a role in NREMS-associated slow waves of local cortical origin.
The localized EEG effects caused by the disturbance of cortical GHRH/GHRHR mechanisms observed herein are consistent with prior work. Unilateral application of GHRH to the Sctx changes ipsilateral EEG delta power during NREMS. However, high doses of GHRH enhance EEG SWA, while low doses decrease unilateral delta power (Szentirmai et al., 2007). The mechanisms of the bidirectional effects induced by exogenous GHRH on EEG SWA are unknown but could involve the different forms of GHRHRs expressed in the cortex (Szentirmai et al., 2007). Further, a dominant-negative GHRHR splice variant that lacks the ability to stimulate cAMP has been described (McElvaine and Mayo, 2006). The different splice variants are differentially expressed (Szentirmai et al., 2007) and likely have different affinities for GHRH and different rates of GHRH/GHRHR internalization. Such differences could lead to bidirectional dose-response effects. GHRH also impacts GHRHR expression. For example, in spontaneous dwarf rats, the higher cortical GHRH levels and corresponding lower numbers of GHRHR-immunopositive cells compared to controls suggests the active cortical regulation of GHRHRs (Peterfi et al., 2006). Cortical and subcortical GHRH sources may also be differentially regulated and the GHRH-responsive cortical cells could potentially integrate the effects of GHRH from these different origins. This could provide a mechanism for differential EEG slow wave changes in response to the neurochemical changes of other brain regions or for the declines in EEG SWA observed with aging or some mental disorders (reviewed Steiger 2003). These GHRH-GHRHR interactions and the various GHRHR splice variants may offer a partial explanation for the GHRH-EEG SWA dose response effects. Regardless of such speculation, current data suggest that the cortical GHRH/GHRHR interaction is needed to maintain normal EEG SWA during NREMS.
Sleep seems to be a fundamental property of local neuronal networks such as cortical columns (Krueger et al., 2008). For example, cortical columns, characterized using evoked response potentials, oscillate between states (Rector et al., 2005). One of the states is sleep-like in that it has homeostatic qualities, e.g. if it is in a wake-like state for prolonged periods it has a higher probability of transitioning into the sleep-like state. Further, during whole animal sleep, most cortical columns are in a sleep-like state, but there are individual cortical columns which are in a wake-like state. Conversely, during the whole animal wakefulness, there are individual cortical columns in a sleep-like state. As monkeys enter sleep but are still performing a visual discrimination task, some neurons in the visual cortex begin to display the characteristic burst-pause sleep firing patterns. Such data also suggest that sleep develops locally (Pigarev et al., 1997). Much other data support this conclusion ranging from the NREMS in dolphins that only occurs unilaterally (Mukhametov et al., 1977), to state separations in humans such as sleep walking (Mahowald and Schenck, 2005). Current data are consistent with the hypothesis that sleep, as evidenced by differential EEG SWA occurring simultaneously in both cerebral hemispheres, is initiated within local neuronal networks.
The current studies are the first to report that cortical cells respond to acute GHRH exposure with an increase of cytosolic Ca2+. GHRH enhanced cytosolic Ca2+ in 10% of the cultured cortical cells. This value is very close to the 10.8±3.4% of the cultured cells that expressed the GHRHR determined by immunocytochemistry, suggesting that those GHRHR present are functional. These GHRH-receptive cortical cells may share the same signaling pathway with somatotropes and hypothalamic neurons (De et al., 2002). GHRH elevates cytosolic Ca2+ by activating an influx of Ca2+ through voltage-dependent Ca2+ channels in somatotropes (Kato et al., 1992). Similarly, we showed that GHRH-enhanced cortical intracellular Ca2+ is a result of Ca2+ influx.
The desensitization of the GHRH signaling reported here was previously observed in hypothalamic cells (De et al., 2002). Down-regulation of GHRHR mRNA and internalization of GHRHR are involved in desensitization of GHRH signaling in kidney cells and somatotropes (Boisvert et al., 2002; Veyrat-Durebex et al., 2005). In our studies, the desensitization happened within min of the first exposure to GHRH. Inactivation of the GHRHR down-stream signaling molecules which are necessary to trigger Ca2+ influx may be another desensitization mechanism. Thus, preincubation of hamster kidney cells and rat somatotropes with GHRH peptide greatly reduces GHRH-induced adenylate cyclase activity, cAMP production and GH release (Hansen et al., 2001). The maximum reduction of adenylate cyclase was observed within 2–5 min after GHRH pre-exposure. Although we do not know whether the GHRH mediated Ca2+ increase in the cortical cells depends on the adenylate cyclase/cAMP system, our finding that a 2–3 min exposure to GHRH silenced the GHRH-induced Ca2+ spike in more than 60% of the cells is another example of rapid desensitization after short exposure of GHRH. In addition, the fact that some GHRH-responsive cells are not desensitized by pre-exposure of GHRH implies the diversity of the GHRH-responsive cortical cells.
The role of cortical GABAergic neurons in sleep regulation is incompletely characterized although sleep modifies GAD mRNA in cortical neurons (Churchill et al., 2001) and a subset of cortical GABAergic interneurons, identified by anatomical shape and that express neuronal nitric oxide synthase, is activated during sleep (Gerashchenko et al., 2008). Most evidence relating GABAergic neurons to GHRH sleep regulatory functions is from studies of subcortical mechanisms (reviewed Szymusiak and McGinty, 2008). In hypothalamus 96% of the GHRH-responsive neurons are GABAergic (De et al., 2002). Intracerebroventricular injection of GHRH activates GABAergic neurons in the rat preoptic hypothalamus, an area involved in NREMS regulation (Peterfi et al., 2010). The number of activated GABAergic neurons correlates with the subsequent duration of NREMS evoked by GHRH. The GABAergic neurons in the reticular thalamus are posited to play a role in thalamocortical synchronizations (Steriade, 2001). Although it remains unknown whether cortical GHRH influences those thalamic GABAergic neurons, current work clearly implicates a cortical GHRH mechanism in EEG synchronization. However, in the cortex the fraction of GHRHR-containing cells that are GABAergic is similar to that in total neurons (Beaulieu, 1993).
In summary, our results suggest that GHRH activates cortical cells and endogenous cortical GHRH plays a role in localized EEG delta power. Our data clearly illustrate that at least some sleep phenotypes such as EEG SWA are local phenomena. Consequently, our results are important to hypotheses concerning the brain organization of sleep and what exactly it is that sleeps.
This work was supported by the National Institutes of Health, Grants NS27250, NS31453. We thank Dr. Parijat Sengupta for help with confocal microscopy and Dr. Alok De for advice on Ca2+ imaging procedures.