Influence of dopamine on ventrolateral thalamic inputs in cat motor cortex

Neuronal activity in several brain regions is modulated by dopaminergic inputs. When single neuronal activity/20 trials of single-pulse ventrolateral thalamic (VL) stimulation was extracellularly recorded in the in vivo, anesthetized cat motor cortex, iontophoretic application of dopamine (DA) elicited either suppression or, in a fewer instances, facilitation of evoked unitary responses. The predominant inhibition exerted by DA appeared to be consistent for successive trials, and a D(1), D(2), and D(1)/D(2) receptor antagonist restored the effect, thereby reflecting a possible coexistence of two DA receptors. By contrast, only a fewer neurons' response to DA displayed facilitation, which was not attenuated by DA antagonists. Moreover, subsequent trials with receptor agonist and antagonists induced inconsistent effects. Except for the jitters, single unit spikes showed invariant latency, which was constant during all recording parameters, and the mean latency remained unchanged. The modulatory effects mediated by DA did not reveal any substantial difference between short- and long-latency responses. Both pyramidal tract neurons and non-pyramidal tract neurons, determined on the basis of antidromic potentials from the pyramidal tract, responded to DA essentially in a similar manner. It appears that DA overall inhibits cat motor cortical neuronal activity in response to VL inputs. We propose that such DAergic inhibition of thalamocortical excitation in the motor cortex could be critical for ongoing sensorimotor transformation.