Stress hormone exposure reduces mGluR5 expression in the nucleus accumbens: functional implications for interoceptive sensitivity to alcohol

Abstract

Escalations in alcohol drinking associated with experiencing stressful life events and chronic life stressors may be related to altered sensitivity to the interoceptive/subjective effects of alcohol. Indeed, through the use of drug discrimination methods, rats show decreased sensitivity to the discriminative stimulus (interoceptive) effects of alcohol following exposure to the stress hormone corticosterone (CORT). This exposure produces heightened elevations in plasma CORT levels (eg, as may be experienced by an individual during stressful episodes). We hypothesized that decreased sensitivity to alcohol may be related, in part, to changes in metabotropic glutamate receptors-subtype 5 (mGluR5) in the nucleus accumbens, as these receptors in this brain region are known to regulate the discriminative stimulus effects of alcohol. In the accumbens, we found reduced mGluR5 expression (immunohistochemistry and Western blot) and decreased neural activation (as measured by c-Fos immunohistochemistry) in response to a moderate alcohol dose (1 g/kg) following CORT exposure (7 days). The functional role of these CORT-induced adaptations in relation to the discriminative stimulus effects of alcohol was confirmed, as both the systemic administration of 3-Cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB) an mGluR5 positive allosteric modulator and the intra-accumbens administration of (R,S)-2-Amino-2-(2-chloro-5-hydroxyphenyl)acetic acid sodium salt (CHPG) an mGluR5 agonist restored sensitivity to alcohol in discrimination-trained rats. These results suggest that activation of mGluR5 may alleviate the functional impact of the CORT-induced downregulation of mGluR5 in relation to sensitivity to alcohol. Understanding the contribution of such neuroadaptations to the interoceptive effects of alcohol may enrich our understanding of potential changes in subjective sensitivity to alcohol during stressful episodes.

Figure 1

Significant reductions in mGluR5 expression in the nucleus accumbens following CORT exposure (7 days). (a) Following CORT exposure (7 days) a significant reduction in mGluR5 immunoreactivity (IR) is evident in the core, but not the shell of the nucleus accumbens. No change in mGluR5 IR in the dorsomedial (DM) caudate putamen was observed (CPu). (b, c) Representative photomicrographs ( × 40) showing cytological expression of mGlu5 receptors in the core for the Water and CORT groups. Arrows identify areas of dense mGlu5 receptor immunoreactivity. Scale bar represents 50 microns. (d) Decreased mGluR5 dimer protein expression (250 kDa) was observed in the nucleus accumbens core as confirmed by western blot analysis following CORT exposure (7 days). Monomer (130 kDa) expression was unaltered. (e) Representative western blots showing decreased mGluR5 expression in the mGluR5 dimer (right) in the CORT group. *Signifies differences from Water. Values on graphs represent mean±SEM.

Figure 2

Blunted neural activity in the nucleus accumbens core in response to a moderate alcohol dose (1 g/kg, IG) following CORT exposure (7 days). (a) Vehicle (water, IG) administration did not change c-Fos immunoreactivity (IR) between the Water and CORT-exposed groups. Alcohol (1 g/kg, IG) induced a significant increase in c-Fos immunoreactivity (IR) in the nucleus accumbens core in the Water group, which was not evident in CORT-exposed group (n=8). (b) Representative pictographs ( × 20) showing c-Fos IR in the nucleus accumbens core in the Water and CORT groups following vehicle (water, IG) and alcohol (1 g/kg, IG). Scale bar represents 100 microns. (c) No significant CORT- or alcohol-induced changes in c-Fos IR in the nucleus accumbens shell (n=6–8) were observed. (d) Representative pictographs ( × 20) showing c-Fos IR in the medial nucleus accumbens shell in the Water and CORT groups following vehicle (water, IG) and alcohol (1 g/kg, IG). Scale bar represents 100 microns. *Signifies difference from Vehicle.⁺Signifies difference from Water. Values on graphs represent mean±SEM.

Figure 3

Positive allosteric modulation of mGluR5 does not alter the interoceptive effects of alcohol. (a) Pretreatment with CDPPB a positive allosteric modulator of mGluR5 administered IP did not alter the discriminative stimulus effects of alcohol, but (b) produced an overall reduction in response rate. These graphs represent the data from two separate assessments in which one group of rats was tested on 0, 1, 10 mg/kg CDPPB (n=13) and the other was tested on 0, 20, 40 mg/kg (n=7). Therefore, the data points for the vehicle condition on the graph represent a combination of the two groups (n=20). Data from the two groups were analyzed separately but are graphically depicted together for ease of presentation. Horizontal dashed line (>80%) represents full expression of the discriminative stimulus effects of alcohol. Values on graphs represent mean±SEM.

Figure 4

Following CORT exposure, positive allosteric modulation of mGluR5 restores sensitivity to the interoceptive effects of alcohol. (a) CORT exposure (7 days) prevented the full expression of the discriminative stimulus effects of alcohol (1 g/kg) following vehicle (0) pretreatment. In contrast, pretreatment with CDPPB a positive allosteric modulator of mGluR5 restored sensitivity to alcohol (n=7–12/group/CDPPB dose). (b) Response rate was not affected by CORT exposure or CDPPB pretreatment. *Signifies differences from Water; ⁺signifies difference from 0 (vehicle-CORT group; Tukey, p<0.05). Values on graphs represent mean±SEM.

Figure 5

mGluR5 activation in the nucleus accumbens restores sensitivity to alcohol following CORT exposure. (a) Site specific activation of mGluR5 in the nucleus accumbens core by CHPG (5 and 10 μg/side) restored sensitivity to alcohol (1 g/kg, IG) following CORT exposure, as indicated by similar levels of alcohol-appropriate responses following CHPG pretreatment between the CORT and Water groups (n=7–11/group/CHPG dose), and significantly greater alcohol-appropriate responses in the CHPG-treated CORT groups than the vehicle-treated CORT group. *Signifies significant difference from Water. ⁺signifies significant difference from 0 (aCSF-CORT group) Tukey, p<0.05). (b) There was a significant overall reduction in response rate following CORT exposure. (c) Nucleus accumbens (core) bilateral injector tip placements from individual rats with accurate placements. The groups are represented by the following symbols: Circles (Vehicle), triangles (5 μg CHPG), squares (10 μg CHPG); filled and unfilled symbols are placements from the Water and CORT groups, respectively. (d) A corresponding photomicrograph showing an injector tip (arrow). Values on graphs represent mean±SEM.