The main finding of this study was that administration of the mGlu5 receptor antagonist MPEP directly into the NAcc shell dose-dependently decreased nicotine self-administration without affecting food self-administration. In contrast, MPEP microinfusion into the VTA decreased responding for both nicotine and food at the two lower doses tested but had no effect on responding for either reinforcer at the highest tested dose.
In the present study, low MPEP doses (0.5-2 μg/0.5μl/side) did not decrease nicotine-self-administration when administered into the NAcc shell (). In a previously published study, MPEP (3-10 μg/0.5μl/side) microinfused into the NAcc shell attenuated ethanol self-administration (Besheer et al., 2009
). Neuroadaptations in mGlu5 receptor expression have been reported after chronic exposure to psychostimulant drugs, such cocaine and nicotine (Ghasemzadeh et al., 1999
; Kenny et al., 2003
). Thus, it is possible that neuroadaptations in mGlu5 receptors after chronic nicotine exposure contributed to the lack of effects of low doses (0.5-2 μg/0.5μl/side) of MPEP microinfusions in the NAcc shell on nicotine self-administration in rats with chronic nicotine self-administration experience. Furthermore, the highest effective MPEP dose (10 μg/0.5μl/side) in the Besheer and colleagues (2009)
study, was not effective in attenuating nicotine self-administration when infused into the NAcc shell of rats with chronic nicotine self-administration experience in the present study (Experiment 2; ). Therefore, subsequent experiments in this study used higher MPEP doses (10-40 μg/0.5μl/side).
The selective and dose-dependent decrease in nicotine self-administration after microinfusion of higher MPEP doses (10-40 μg/0.5μl/side) into the NAcc shell is consistent with similar effects seen with systemic administration of MPEP on nicotine self-administration (Liechti and Markou, 2007
; Paterson and Markou, 2005
; Paterson et al., 2003
). These findings also extend our previous findings, which showed attenuation of nicotine self-administration after a decrease of glutamatergic transmission in the NAcc shell via activation of predominantly presynaptic inhibitory mGlu2/3 receptors (Liechti et al., 2007
). Importantly, anatomical control injections 2 mm above the NAcc shell into the lateral septal nucleus using the MPEP dose that was most effective in decreasing nicotine self-administration when administered into the NAcc shell (40 μg/0.5μl/side) did not result in attenuation of nicotine self-administration compared to the vehicle condition.
MPEP microinfusions into the NAcc shell did not affect food-maintained responding under the FR5 TO20 s schedule of reinforcement at doses that affected nicotine self-administration. Systemically administered MPEP (Paterson et al., 2003
) or microinfusions of another mGlu5 receptor antagonist, 3-(2-methyl-1,3-thiazol-4-yl)ethynyl)pyridine (MTEP; Gass and Olive, 2009
) into the NAcc shell also did not affect food-maintained responding under the same FR5 TO20 s schedule of reinforcement. Finally, consistent with our present findings, a decrease in glutamatergic transmission in the NAcc shell by blocking presynaptic mGlu2/3 receptors using the mGlu2/3 receptor agonist LY379268 also had no impact on food self-administration at doses that decreased nicotine self-administration (Liechti et al., 2007
This reinforcer-selective decrease in nicotine self-administration suggests that the attenuation of nicotine self-administration was not attributable to an anhedonia- or dysphoria-like state or nonspecific motor- or cognitive-impairing effects induced by MPEP administration. Interestingly, few studies have reported that MPEP may have reinforcing effects on its own (van der Kam et al., 2009a
). Thus, the decrease in nicotine self-administration could be attributable to a substitution of the reinforcing effects of nicotine by MPEP. However, this interpretation is unlikely because MPEP microinfusions into the NAcc shell did not affect food self-administration. Furthermore, the majority of the currently available literature shows that MPEP has no reinforcing effect on its own when tested using the conditioned place preference procedure (Aoki et al., 2004
; Herzig et al., 2005
; Herzig and Schmidt, 2004
; Popik and Wrobel, 2002
). In addition, systemic administration of MPEP did not lower brain reward thresholds, suggesting that MPEP does not have reward-enhancing properties (Harrison et al., 2002
) that are commonly induced by drugs that have intrinsic reinforcing properties of their own (Kornetsky and Esposito, 1979
). In fact, at the highest doses tested, MPEP elevated brain reward thresholds, suggesting a mild dysphoric effect (Harrison et al., 2002
). Finally, the aforementioned reinforcing effect of MPEP in the conditioned place preference procedure has only been seen after intravenous MPEP administration and not after intraperitoneal or intracranial administration into discrete brain sites.
The VTA is a heterogeneous structure and can be broadly divided into anterior and posterior regions (Ikemoto, 2007
). In the present study, MPEP microinfusions were made into the posterior VTA because rats have been shown to self-administer nicotine in the posterior VTA and not anterior VTA (Ikemoto et al., 2006
). Furthermore, blockade of glutamatergic transmission in the posterior VTA by either activating inhibitory presynaptic mGlu2/3 receptors (Liechti et al. 2007
) or blocking postsynaptic N
-aspartate (NMDA) receptors (Kenny et al. 2009
) attenuated nicotine self-administration. Importantly, fibers from the posterior VTA project to the NAcc shell (Ikemoto, 2007
), which is the other brain target assessed in the present study. Nevertheless, the anterior VTA may also be involved in nicotine dependence. Animals chronically exposed to nicotine showed decreased sensitivity of dopamine D1
-like, but not dopamine D2-like, receptors in the anterior VTA (Bruijnzeel and Markou, 2005
In the present study, MPEP microinfusions into the posterior VTA had similar effects on nicotine and food self-administration. MPEP decreased nicotine and food self-administration after bilateral administration of either 10 or 20 μg/0.5μl/side, whereas no effect was observed on responding for either reinforcer after administration of 40 μg/0.5μl/side. These data suggest that mGlu5 receptors in the VTA are critical for maintaining reward-dependent instrumental responding. Consistent with these data, a decrease in glutamatergic transmission in the VTA by administration of the amino-3-hydroxy-5-methyl-4-isoxazolepropionate/kainate (AMPA) receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoylbenzo(f
)quinoxaline (NBQX) decreased both nicotine and sucrose self-administration in rats (Wang et al., 2008
). In contrast, blockade of glutamatergic activity via activation of predominantly presynaptic mGlu2/3 receptors in the VTA selectively attenuated nicotine self-administration and did not affect food self-administration (Liechti et al., 2007
). Furthermore, blockade of glutamatergic transmission by blockade of ionotropic NMDA receptors in the VTA, similar to blockade of mGlu5 receptors in the same area, dose-dependently attenuated nicotine self-administration (D'Souza and Markou, unpublished observations; Kenny et al., 2009
). These previous studies, however, did not assess the effects of NMDA antagonist microinfusions into the VTA on food self-administration. Importantly, anatomical control injections 2 mm above the VTA into the red nucleus using the most effective MPEP dose in decreasing nicotine self-administration when administered into the VTA (20 μg/0.5μl/side) did not result in attenuation of nicotine self-administration compared to the vehicle condition. Altogether, these data suggest that glutamatergic neurotransmission in the VTA is critical for the reinforcing effects of nicotine. However, the decrease in food self-administration seen after blockade of postsynaptic glutamatergic receptors suggests that glutamatergic activity via these postsynaptic receptors may be required for the maintenance of operant-seeking regardless of the type of reinforcer.
The lack of effects of the highest tested MPEP dose (40 μg/0.5μl/side) on nicotine and food self-administration when microinjected into the VTA could be due to activity of MPEP at off-site targets or an interaction between mGlu5 receptors and other receptors. MPEP, although relatively selective for mGlu5 receptors, also binds to other targets, such as the mGlu4 and NMDA receptors and the norepinephrine transporter (Heidbreder et al., 2003
; Mathiesen et al., 2003
; O'Leary et al., 2000
). Furthermore, interactions between mGlu5 receptors and other receptor subtypes, including NMDA (glutamate), D1
(dopamine), and CB1
(endocannabinoids), are also known (Martin et al., 1997
; Robbe et al., 2002
; Schotanus and Chergui, 2008
). These interactions of MPEP with receptors/transporters other than mGlu5 receptors may have neutralized the attenuating effects of MPEP on nicotine and food self-administration. Another possible reason for the lack of effects of MPEP infusions (40 ug/0.5 ul/side) into the VTA on nicotine and food reinforcement could be due to action of MPEP at presynaptic mGlu5 receptors in the VTA. Although mGlu5 receptors are predominantly postsynaptic, some evidence suggests that mGlu5 receptors may be located on axon terminals and therefore could act as presynaptic autoreceptors also (Romano et al., 1995
). Blockade of presynaptic mGlu5 receptors by MPEP could theoretically have opposite effects to those induced by blockade of postsynaptic mGlu5 receptors.
Nicotine (0.03 mg/kg/infusion) self-administration in rats is attenuated after intraperitoneal administration of 9 mg/kg MPEP administered 30 min prior to a nicotine self-administration session (Paterson et al., 2003
). The precise concentrations of MPEP after systemic administration of this dose in the NAcc and VTA have not been determined. One study has reported MPEP concentrations in the hippocampus 1 h after systemic intraperitoneal administration of 3 mg/kg MPEP to be in the range of 0.8 ± 0.05 μM (Cosford et al., 2003
). As mentioned above, in the present study, infusions of 3-7 μM MPEP concentration in the NAcc did not have any effect on nicotine self-administration; MPEP was effective in decreasing nicotine self-administration only when infused at higher concentration (70-300 μM) in the NAcc shell or VTA. Thus, the concentrations of MPEP required to decrease nicotine self-administration appear to be higher than those obtained in the brain after systemic MPEP injections. However, it must be pointed out that systemically administered MPEP blocks mGlu5 receptor activity in several brain regions at the same time, and the aggregate of these multisite effects likely leads to the observed behavioral effects. In contrast, localized microinjections of MPEP blocks mGlu5 receptor activity in only a specific brain region, such as the NAcc shell or VTA, and therefore complete blockade of all mGlu5 receptors and/or off target activity of MPEP at this specific brain site may be required to induce the behavioral effect. Furthermore, it should be noted that injection concentrations may be much higher than concentrations achieved at the synapse. Taken together, the findings of the present study therefore suggest that while the NAcc shell and VTA play an important and sufficient role in mediating the effects of systemically administered MPEP, other brain regions are also possibly involved in mediating the effects of systemically administered MPEP.
In summary, MPEP microinfusions in the NAcc shell selectively attenuated nicotine self-administration without affecting responding for a food reinforcer, whereas MPEP microinfusions in the VTA had similar effects on both nicotine and food self-administration. These effects of MPEP are possibly due to blockade of mGlu5 receptor activity in mesolimbic brain sites, such as the VTA and NAcc shell, although offsite effects of MPEP cannot be precluded. Thus, our study extends the existing body of literature on the role of mGlu5 receptors in the reinforcing effects of nicotine by identifying brain sites that mediate the actions of the mGlu5 receptor antagonist MPEP. The present data also suggest that mGlu5 receptors in the VTA possibly mediate the reinforcing effects of both drug and natural reinforcers and may be critical for the maintenance of motivated operant responding.