Effects of serotonin on neuronal response induced in the sensorimotor cortex by tactile and conditioned acoustic stimuli

Neuronal firing response in the sensorimotor cortex to tactile (non-conditioned) and acoustic (conditioned) stimuli was investigated in trained cats before and after iontophoretic application of serotonin and lysergide. Three functionally distinct groups of neurons were identified from the response produced by presenting tactile and acoustic stimuli. Applying serotonin was found to facilitate preliminary and residual spike response induced by tactile stimulation; it also facilitates and modulates response in many cortical neurons to conditioned stimuli. Facilitation takes the form of reduced latency of response and increased numbers of spikes in response to conditioned stimulus presentation, especially at the initial phase of response to sound and immediately after the onset of conditioned reflex motion. Additional neurons formerly unresponsive to acoustic stimuli joined in the reaction under the effects of serotonin. Changed response patterns often evolve following minor fluctuations in background activity level. It is suggested that facilitation of response following iontophoretic serotonin application in the neocortex is associated with activation of excitatory serotonin receptors (S2). The lysergide-induced increase in background and evoked activity noted during experimentation can apparently be put down to blockade of inhibitory serotonon (S1B) receptors.