Inactivation of the thalamic paraventricular nucleus promotes place preference and sucrose seeking in male rats

Abstract

Rationale: The experience of reward entails both positive affect and motivation. While the brain regions responsible for these distinct aspects of reward are dissociable from each other, the paraventricular nucleus of the thalamus (PVT) may play a role in both.

Objectives: To investigate the role of the PVT in both affect and motivation, and to identify neuropeptides that might mediate these effects.

Methods: Male rats were tested for conditioned place preference following temporary inactivation of the anterior or posterior PVT with local injections of the GABA_B and GABA_A agonists, baclofen + muscimol. They were tested for sucrose seeking under a fixed ratio 3 (FR3) schedule of reinforcement and after extinction, following injection into the posterior PVT of baclofen + muscimol or saline vehicle. Finally, quantitative real-time PCR was used to examine local neuropeptide gene expression following injection into the posterior PVT of baclofen + muscimol or saline vehicle.

Results: Conditioned place preference was induced by temporary inactivation of the posterior but not anterior PVT. While sucrose seeking under an FR3 schedule of reinforcement was unaffected by inactivation of the posterior PVT, reinstatement of sucrose seeking was promoted by posterior PVT inactivation. Local gene expression of pituitary adenylate cyclase-activating polypeptide (PACAP), but not enkephalin or neurotensin, was reduced following inactivation of the posterior PVT.

Conclusions: Temporary inactivation of the posterior PVT affects both affect and motivation as well as local gene expression of PACAP. These results suggest that the posterior PVT is one brain region that may participate in both major aspects of reward.

Keywords: Anterior; Baclofen; Conditioned place preference; Fixed ratio; Muscimol; Operant; PACAP; Posterior; Reinstatement; mRNA.

Fig. 1

Experimental timelines. (a) In Experiment 1, after one week of acclimation, rats were cannulated in the anterior or posterior PVT. After one week of recovery, they underwent conditioned place preference testing: a baseline pretest, three test days involving injection with 0.3 μl of saline vehicle or 0.3 nmol baclofen and 0.03 nmol muscimol, and a posttest. (b) In Experiment 2, rats were trained for fifteen days under a fixed ratio 1 (FR1) schedule for a sucrose reward, then for three days under an FR2 schedule, then for two days under an FR3 schedule, and they were subsequently cannulated in the posterior PVT. Following one week of recovery, they had five more FR3 sessions and were then tested for FR3 responding following injection with saline vehicle or baclofen+muscimol. After that, they underwent extinction training for eight days and were then tested again following saline vehicle or baclofen+muscimol. (c) In Experiment 3, rats after one week of acclimation, were cannulated in the posterior PVT and then given one week of recovery or were given one week of recovery after conditioned place preference testing (Experiment 1). They were then injected with saline vehicle or baclofen+muscimol and sacrificed 30 min later for analysis using qRT-PCR.

Fig. 2

Effects of thalamic paraventricular nucleus (PVT) subregion inactivation on conditioned place preference. (a) Injection locations (indicated by black dots) in the anterior and posterior PVT (n = 5/subregion). Coordinates are relative to Bregma. Adapted from The Rat Brain, 5th edition, G. Paxinos and C. Watson, Copyright 2005, with permission from Elsevier. (b) Time spent in the baclofen+muscimol-paired chamber was significantly increased after injections in the posterior but not anterior PVT. The conditioned place preference (CPP) score is the difference in seconds between the time spent in the drug-paired chamber during the posttest and the pretest. ⁺p < 0.05 vs. pretest; *p < 0.05 vs. Anterior PVT. (c) Distance traveled during the test was significantly decreased following injections in the PVT. *p < 0.05 vs. Acclimation (pretest). (d) Representative traces of posttest locomotor activity from rats injected in the anterior or posterior PVT. Injection of baclofen+muscimol was paired with the left chamber for this anterior PVT subject and with the right chamber for the posterior PVT subject. Values are Mean ± S.E.M.

Fig. 3

Effects of posterior thalamic paraventricular nucleus (PVT) inactivation on sucrose seeking behavior. (a) Injection locations (indicated by black dots) in the posterior PVT (N = 7). Coordinates are relative to Bregma. Adapted from The Rat Brain, 5th edition, G. Paxinos and C. Watson, Copyright 2005, with permission from Elsevier. (b) Self-administration of sucrose under a fixed ratio 1 schedule of reinforcement. *p < 0.05 vs. Session 1. (c) Self-administration under a fixed ratio 2 schedule of reinforcement. *p < 0.05 vs. Session 1. (d) Self-administration under a fixed ratio 3 schedule of reinforcement. *p < 0.05 vs. Session 1. Dotted line indicates the time away from fixed ratio testing due to cannulation and recovery. (e) Self-administration under a fixed ratio 3 schedule of reinforcement was not significantly altered by injection with baclofen+muscimol into the posterior PVT. (f) Self-administration during extinction training. *p < 0.05 vs. Session 1. (g) Reinstatement of extinguished sucrose seeking was promoted by injection with baclofen+muscimol into the posterior PVT. *p < 0.05 vs. last extinction and saline vehicle. Values are Mean ± S.E.M.

Fig. 4

Effects of posterior thalamic paraventricular nucleus (PVT) inactivation on local neuropeptide gene expression. (a) Injection locations in the posterior PVT (N = 10, n = 5/injection). Red dots indicate locations from “drug-naïve” rats; black dots indicate locations from “drug-experienced” rats from Experiment 1. Coordinates are relative to Bregma. Adapted from The Rat Brain, 5th edition, G. Paxinos and C. Watson, Copyright 2005, with permission from Elsevier. (b) Example of a fresh brain slice from a rat that had previously received an injection in the PVT. Note that both the tract from the guide cannula and tissue damage from the injector can be seen in this image. (c) Local injection of baclofen+muscimol significantly reduced gene expression of pituitary adenylate cyclase-activating polypeptide (PACAP) but not enkephalin or neurotensin in the posterior PVT. *p < 0.05 vs. saline vehicle. (d) Injection of baclofen+muscimol in the posterior PVT did not significantly affect gene expression of PACAP, enkephalin, or neurotensin in the anterior PVT. Values are Mean ± S.E.M.