Microinjection of procaine or GABA into the nucleus basalis magnocellularis affects cue-elicited unit responses in the rat frontal cortex.

Male rats were chronically implanted for recording of single units in the frontal cortex during a cue-event paradigm. The rats were sedated and restrained during the experiments. Units were selected which had large-amplitude, clearly isolated action potentials. The animals were first trained to associated a 2-s tone cue with rewarding medial forebrain bundle stimulation. After training, units responded to the cue by an increase or decrease in discharge rate. Cumulative histograms of the unit response to the cue were obtained and then either procaine hydrochloride or GABA was microinjected into the nucleus basalis magnocellularis (nBM). Immediately after drug administration another histogram was obtained to ascertain the drug effect. Procaine microinjections to the nBM suppressed the frontal cortex unit responses in 9 of 10 units that had previously responded with an increase in firing rate and 10 of 12 units that had decreased their firing rate before drug administration. GABA microinjections antagonized the response in 15 of 19 excited units and 2 of 2 inhibited units. Recovery was obtained in 23 units. Other units did not remain isolated long enough to obtain complete recovery. The nBM supplies the frontal cortex with as much as 70% of its cholinergic innervation. Lesions of the region do not significantly alter the amounts of neurotransmitters other than acetylcholine in the frontal cortex. These results indicate that neurons in the nucleus basalis magnocellularis are involved in the cue-elicited changes in the rate of discharge of units in the rat frontal cortex.