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Behav Brain Res. 2014 Aug 1;269:20-7. doi: 10.1016/j.bbr.2014.04.005. Epub 2014 Apr 13.

Delta-opioid receptor blockade in the ventral pallidum increases perceived palatability and consumption of saccharin solution in rats.

Author information

1
Division of Behavioral Physiology, Department of Behavioral Sciences, Graduate School of Human Sciences, Osaka University, 1-2 Yamadaoka, Suita, Osaka 565-0871, Japan. Electronic address: kenken@hus.osaka-u.ac.jp.
2
Division of Behavioral Physiology, Department of Behavioral Sciences, Graduate School of Human Sciences, Osaka University, 1-2 Yamadaoka, Suita, Osaka 565-0871, Japan.

Abstract

The ventral pallidum (VP) is involved in ingestive behaviour. It receives dense GABAergic projections from the nucleus accumbens. GABAergic terminals in the VP co-express enkephalin, an endogenous ligand of delta-opioid receptors. The role of the delta-opioid receptors in the VP in the context of ingestive behaviour remains unclear, in contrast to the well-understood involvement of the mu-opioid receptors. We used the single-bottle test to examine the effects of VP microinjections of the delta-opioid receptor antagonist naltrindole on consumption of a saccharin solution. Naltrindole injections significantly increased the intake of saccharin, but not water, during a 2-h test session. We also investigated perceived palatability of saccharin using a taste reactivity test. The drug treatments increased ingestive responses to intraorally infused saccharin. Further experimentation explored the role of VP delta-opioid receptors in behavioural responses to saccharin that were previously paired with malaise upon the retrieval of conditioned taste aversion (CTA). Naltrindole-injected rats exhibited longer latency for the first occurrence of aversive responses than vehicle-injected control rats. However, there was no between-group difference in total aversive responses. These results suggest that naltrindole injections into the VP induce an enhancement of perceived palatability of a normally preferred saccharin solution, and thereby facilitate consumption of the solution. On the other hand, delayed aversive responses to the conditioned aversive saccharin suggest that the delta-opioid receptors in the VP mediate the initiation of aversive taste reactivity responses to the conditioned stimulus upon CTA retrieval.

KEYWORDS:

Conditioned taste aversion; Consumption behavior; Delta-opioid receptor; Naltrindole; Taste palatability; Ventral pallidum

PMID:
24739358
DOI:
10.1016/j.bbr.2014.04.005
[Indexed for MEDLINE]

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