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Infusion of gamma 2-MSH produce a conditioned taste aversion in morphine-dependent rats
1Biobehavioral Research Department, Addiction Research Foundation, Toronto, Ontario, Canada.
Abstract:
A two flavour, unbiased, taste preference conditioning procedure was used to test for possible motivating effects of gamma 2-MSH. Three training trials failed to produce any significant effect with doses ranging from 2.4 to 40 micrograms/ICV infusion in drug-naive, non-operated or placebo-implanted rats. However, in rats made dependent by SC implantation of a morphine pellet 4 days earlier 15 micrograms gamma 2-MSH/infusion produced a taste aversion that was comparable to that produced by infusion of a low dose of the competitive opioid receptor antagonist naloxone (0.32 micrograms). The findings confirm with a conditioning procedure and with opiate-dependent animals the naloxone-like effects of gamma 2-MSH. They also suggest that this endogenously-located peptide may acquire an aversive property as a result of chronic morphine treatment.
Insights
Gamma-2-melanocyte-stimulating hormone (gamma-2-MSH) did not show motivating effects in drug-naive rats. However, in morphine-dependent rats, gamma-2-MSH induced a taste aversion, similar to naloxone.
Area of Science:
- Neuroscience
- Pharmacology
- Behavioral Science
Background:
- The neuropeptide gamma-2-melanocyte-stimulating hormone (gamma-2-MSH) is endogenously located.
- Its potential motivating or aversive effects, particularly in the context of opioid dependence, require investigation.
Purpose of the Study:
- To investigate the motivating effects of gamma-2-MSH using a taste preference conditioning procedure.
- To determine if gamma-2-MSH exhibits naloxone-like effects in opioid-dependent rats.
Main Methods:
- A two-flavour, unbiased taste preference conditioning procedure was employed.
- Rats were tested under drug-naive conditions and after developing morphine dependence via subcutaneous pellet implantation.
- Intracerebroventricular (ICV) infusions of varying doses of gamma-2-MSH and naloxone were administered.
Main Results:
- Gamma-2-MSH did not produce significant effects in drug-naive rats across tested doses (2.4-40 micrograms).
- In morphine-dependent rats, a 15 microgram infusion of gamma-2-MSH induced a taste aversion.
- This aversion was comparable to the effect of a low dose of naloxone (0.32 micrograms).
Conclusions:
- The findings confirm naloxone-like effects of gamma-2-MSH in opioid-dependent rats using a conditioning procedure.
- Chronic morphine treatment may induce an aversive property in the endogenously located peptide gamma-2-MSH.