PHASIC FIRING TIME LOCKED TO COCAINE SELF-INFUSION AND LOCOMOTION - DISSOCIABLE FIRING PATTERNS OF SINGLE NUCLEUS-ACCUMBENS NEURONS IN THE RAT

The activity of single nucleus accumbens (NAcc) neurons of rats was ex tracellularly recorded during intravenous cocaine self-administration sessions (0.7 mg/kg per infusion, fixed ratio 1). We reported previous ly that NAcc neurons showed a change, usually a decrease, in firing ra te during the first 1 min after the cocaine-reinforced lever press. Th is postpress change was followed by a progressive reversal of that cha nge, which began within the first 2 min after the press and was not co mplete until the last 1 min before the next lever press (termed the ch ange + progressive reversal firing pattern). In the present study we d ocumented a regular pattern of locomotion that occurred in parallel wi th the change + progressive reversal firing pattern. This observation suggested that discharges time locked to locomotion may determine the change + progressive reversal firing pattern. However, 55% of the neur ons failed to show firing time locked to locomotion that could have co ntributed to the change + progressive reversal firing pattern. Moreove r, for all neurons, the change + progressive reversal firing pattern w as apparent even if the calculation of firing rate excluded all period s of locomotion. The present data showed that the change + progressive reversal firing pattern is not solely attributable to phasic changes in firing time locked to the execution of locomotion. The change + pro gressive reversal firing pattern closely mirrors changes in drug level and dopamine overflow observed by previous researchers and may thus b e a component of the neurophysiological mechanism by which drug level regulates drug-taking behavior during an ongoing self-administration s ession.