Multiple- and single-unit activity in area 32 (prelimbic region) of the medial prefrontal cortex during Pavlovian heart rate conditioning in rabbits.

Data from both brain lesion and brain stimulation experiments suggest that area 32 of the medial prefrontal cortex (prelimbic area) participates in the acquisition and/or expression of conditioned bradycardia. This report describes experiments designed to determine whether cells in this area of the brain exhibit changes in neural activity during classical conditioning that can be related to these learned heart rate changes. Thus, multiple- and single-unit activity was recorded from area 32 in rabbits during Pavlovian heart rate (HR) conditioning. In the first experiment, neuronal discharge recorded from chronically implanted multiple-unit electrodes in the superficial and deep layers of area 32 increased systematically in response to the presentation of tone conditioned stimuli (CS) paired with paraorbital electric shock as the unconditioned stimulus (US). This tone-evoked increase in multiple-unit activity (MUA) closely paralleled the acquisition of the decelerative HR conditioned response in animals that received paired CS/US presentations, but was of smaller amplitude in animals given unpaired CS/US presentations. Tone-evoked increases in MUA also occurred during tone-alone presentations prior to training, but this activity declined over trials. During extinction, CS-evoked MUA increases also declined over trials. These findings suggest that the CS-evoked increase in neuronal activity in area 32 was associatively produced. A second experiment examined the CS-evoked response of single units (n = 98) in area 32 during differential Pavlovian conditioning, in which one tone (CS+) was consistently followed by the paraorbital shock US and a second tone (CS-) was not. Four types of cells were found: (1) 32% of the cells studied showed no change in response to either CS+ or CS- presentation, (2) 48% showed CS-evoked increases to either CS+ or CS-, (3) 14% showed CS-evoked decreases to CS+ or CS-, and (4) 7% showed a biphasic response, in which an increase was followed by a decrease in activity. These cell types are similar to those previously found in the more dorsal anterior cingulate cortex (area 24) and frontal eyefields (area 8). However, a major difference between the present findings and those of our previous studies of areas 8 and 24 is that in several instances changes in CS-evoked activity in area 32 were significantly greater in response to the CS- than to the CS+, suggesting that some cells in the prelimbic region may code the absence of aversive stimulation, that is, a period of relative "safety."