Evidence for a dopaminergic innervation of cat primary visual cortex

Experiments have been conducted to determine whether dopamine fulfills the criteria to be considered as a neurotransmitter in cat primary visual cortex. N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine, a neurotoxin with high selectivity for noradrenergic terminals, was administered into kitten cerebral ventricles. Two weeks later, the concentration of norepinephrine in visual cortex was reduced to 15% of control while dopamine and serotonin were not depleted. Receptor binding assays with [3H]SCH 23390 showed that membranes prepared from cat primary visual cortex contain a binding site that has the properties of a D1 receptor. This site was localized by autoradiography to two bands, one in layer VI and the second in upper layers of visual cortex. A dopamine-stimulated adenylate cyclase activity was demonstrated that was inhibited by SCH 23390 but not by alprenolol. Norepinephrine was shown to stimulate adenylate cyclase activity through both a beta-noradrenergic receptor and a D1 receptor. Binding assays with [3H]spiperone indicated that D2 dopamine receptors are absent from cat visual cortex or present in very low amounts. Taken together these results strongly suggest the existence of a dopamine innervation of cat primary visual cortex. The neurotoxin experiments show that some of the dopamine in cat visual cortex is not in noradrenergic terminals while the receptor assays demonstrate the presence of D1 receptors functionally linked to the synthesis of cyclic 3',5'-adenosine monophosphate. The demonstration of a dopaminergic innervation in cat primary visual cortex is also relevant to the interpretation of data on the involvement of catecholamines in developmental plastic phenomena.