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Self-administration of methionine enkephalin into the nucleus accumbens
Abstract:
Microinfusions of the endogenous opiate neurohumor, methionine enkephalin, into the nucleus accumbens initiated a reinforcing stimulus in a dose-related manner. The reinforcing nature of this intracranial self-administration was evaluated with intermittent schedules of reinforcement and a two-lever discrimination procedure. Opiate receptors are likely responsible for the initiation of this reinforcing stimulus since naloxone effectively blocked self-administration. These data suggest the mediation of opiate reinforcement through interactions with opiate receptors in brain regions outside the ventral tegmental area, questioning the current dopamine hypothesis for the initiation of these reinforcement processes.
Insights
Methionine enkephalin microinfusions into the nucleus accumbens caused reinforcement, blocked by naloxone. This suggests opiate receptors, not dopamine, may initiate reinforcement outside the ventral tegmental area.
Area of Science:
- Neuroscience
- Neuropharmacology
- Behavioral Neuroscience
Background:
- The nucleus accumbens is a key brain region involved in reward and motivation.
- The role of endogenous opiates in reinforcement is an area of ongoing research.
- Current hypotheses often emphasize dopamine's role in initiating reinforcement processes.
Purpose of the Study:
- To investigate the reinforcing effects of methionine enkephalin microinfusions into the nucleus accumbens.
- To determine the involvement of opiate receptors in this reinforcing effect.
- To explore the neural pathways mediating opiate-induced reinforcement.
Main Methods:
- Microinfusions of methionine enkephalin into the nucleus accumbens of subjects.
- Evaluation of intracranial self-administration using intermittent reinforcement schedules.
- Assessment of naloxone's effect on self-administration behavior using a two-lever discrimination procedure.
Main Results:
- Methionine enkephalin induced a dose-related reinforcing stimulus.
- Naloxone administration effectively blocked the self-administration behavior.
- Reinforcement was observed even when targeting brain regions outside the ventral tegmental area.
Conclusions:
- Endogenous opiates, like methionine enkephalin, can initiate reinforcing stimuli.
- Opiate receptors in the nucleus accumbens are crucial for mediating this reinforcement.
- These findings challenge the exclusive dopamine hypothesis for reinforcement initiation, suggesting a significant role for the opiate system in extra-VTA regions.