SINGLE NEURONAL RESPONSES IN MEDIAL PREFRONTAL CORTEX DURING COCAINE SELF-ADMINISTRATION IN FREELY MOVING RATS

Chronic single neuronal recording techniques were applied to investiga te the involvement of the medial prefrontal cortex (mPFC) during cocai ne self-administration in the rat. Rats were trained to press a lever for cocaine under continuous reinforcement and fixed ratio schedules. Different patterns of phasic neuronal activity changes were found to b e associated with lever-pressing for cocaine. The neuronal responses c ould be classified into five categories: 1) increases in neuronal firi ng before the lever press (15 out of 121 neurons, 12.4%); 2) decreases in neuronal firing before the lever press (13 neurons, 10.7%); 3) inc reases in neuronal firing after cocaine infusion (4 neurons, 3.3%); 4) decreases in neuronal firing after cocaine infusion (32 neurons, 26.4 %); and 5) no alteration of neuronal activity throughout the self-admi nistration session (67 neurons, 55.4%). The anticipatory responses, i. e., neuronal activity appearing before the lever press, were observed for both the continuous reinforcement and fixed ratio schedules. In a few cases, alteration of firing rate was not observed for the first le ver press but appeared before subsequent lever presses in fixed ratio schedules. Eliminating cocaine abolished the inhibitory neuronal respo nses observed after lever press, suggesting that these inhibitory resp onses after cocaine self-administration were attributable to the pharm acologic effect of cocaine. The data provide initial electrophysiologi cal evidence that the mPFC may play a role in mediating the task seque ncing which leads to cocaine self-administration. (C) 1997 Wiley-Liss, Inc.