Single-unit responses to cholinergic agents in the rat inferior colliculus

Single units were recorded from the inferior colliculi of adult male rats anesthetized with urethane. Units were driven with tonal stimuli, and changes in unit responses to the tones were monitored during iontophoretic application of cholinergic drugs. The cholinergic agonists acetylcholine and carbamylcholine potentiated responses in about 50% and suppressed responses in about 35% of units tested. Cholinergic antagonists typically produced effects when delivered alone. Both d-tubocurarine and atropine methyl nitrate excited over 80% of units tested, while mecamylamine and scopolamine inhibited the majority of tested units. Dihydro-beta-erythroidine was generally ineffective. Alpha-bungarotoxin was generally ineffective when delivered alone, but blocked agonist effects. Post-stimulus time histogram (PSTH) patterns, response-intensity functions and response areas were examined for changes during drug delivery. Cholinergic agents did not differentially affect either time periods within the PSTH or frequency bands of response areas, but were especially effective for those intensities producing larger response rates. Taken together with evidence from biochemical studies, our results suggest the presence of a functional cholinergic input into the inferior colliculus which acts to modulate acoustic processing.