title: Delta-opioid receptor blockade in the ventral pallidum increases perceived palatability and consumption of saccharin solution in rats.
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.
authors.affiliation: 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.
authors.forename: Tadashi
authors.initials: T
authors.lastname: Inui
authors.affiliation: Division of Behavioral Physiology, Department of Behavioral Sciences, Graduate School of Human Sciences, Osaka University, 1-2 Yamadaoka, Suita, Osaka 565-0871, Japan.
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Shimura
mesh_terms: D000818:Animals; D001362:Avoidance Learning / Q000187:drug effects / Q000502:physiology; D066187:Basal Forebrain / Q000187:drug effects* / Q000502:physiology; D003213:Conditioning, Psychological / Q000187:drug effects / Q000502:physiology; D004305:Dose-Response Relationship, Drug; D004326:Drinking / Q000187:drug effects / Q000502:physiology; D004435:Eating / Q000187:drug effects* / Q000502:physiology; D008297:Male; D009043:Motor Activity / Q000187:drug effects / Q000502:physiology; D009271:Naltrexone / Q000031:analogs & derivatives* / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology*; D017208:Rats, Wistar; D017465:Receptors, Opioid, delta / Q000037:antagonists & inhibitors* / Q000378:metabolism; D012439:Saccharin / Q000008:administration & dosage; D055697:Taste Perception / Q000187:drug effects* / Q000502:physiology
chemical_list: D009292:Narcotic Antagonists; D017465:Receptors, Opioid, delta; D009271:Naltrexone; D012439:Saccharin; C055382:naltrindole
keywords: Conditioned taste aversion; Consumption behavior; Delta-opioid receptor; Naltrindole; Taste palatability; Ventral pallidum