Blockade of muscarinic acetylcholine receptors in the ventral tegmental area disrupts food-related learning in rats.

RATIONALE

Stimulation of ventral tegmental area (VTA) muscarinic acetylcholine receptors (mAChRs) is implicated in feeding.

OBJECTIVE

To investigate the effects of mAChR blockade in the VTA on food-related learning.

METHODS

In experiment 1, rats (N=12) were placed in chambers containing food and received microinjections of 0 or 5 microg/0.5 microl scopolamine prior to the first four feeding sessions and the alternate dose prior to the tenth feeding session. In experiment 2 (N=9), the effects of daily microinjections of scopolamine on lever pressing under a progressive ratio schedule of food reinforcement were tested. In experiment 3 (N=34), the effects of daily microinjections of scopolamine on lever pressing maintained by conditioned reward were investigated.

RESULTS

In experiment 1, all rats demonstrated low consumption during session 1. However, pellet consumption for rats initially pretreated with the 0-microg dose rose to and stayed at maximal levels for the remaining sessions, even when pretreated with the 5-microg dose during the tenth session. Pellet consumption for rats initially pretreated with the 5-microg dose remained low, even for the first two sessions following the cessation of scopolamine pretreatment, and gradually rose to maximal levels by the eighth session. In experiment 2, scopolamine significantly decreased break points. In experiment 3, scopolamine failed to significantly decrease responding specifically on the lever producing the conditioned reward.

CONCLUSIONS

Altogether, these data suggest that VTA mAChR stimulation is involved in feeding and food-related learning but may not be involved in responding maintained by conditioned reward.