The effect of intraspinal microinjections of 6-hydroxydopamine on the inhibitory influence exerted on spinal sympathetic activity by the baroreceptors

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Abstract
The inhibition of spinal somato-sympathetic reflex activity, following electrical stimulation of the carotid sinus nerve in cats has been studied before and after chronic intraspinal microinjections of 6-hydroxydopamine into the cervical spinal cord. The effect of these microinjections on bulbospinal catecholaminergic neurones was assessed using the formaldehyde fluorescence technique for the demonstration of intraneuronal monoamines. In 5 6-hydroxydopamine-treated animals, inhibition of the spinal sympathetic reflex response following stimulation of the sinus nerve was reduced compared to control untreated animals. When damage to the bulbo-spinal catecholamine system in the dorsolateral funiculi was extensive, the inhibition was almost completely abolished; with more limited destruction a lesser effect was observed. These effects were not seen in a group of control animals, which received intraspinal microinjections of the 5-hydroxytrytamine, neurotoxin 5,6-dihydroxytryptamine and which showed damage to the indoleaminergic neuronal system but no damage to the bulbo-spinal catecholaminergic system. It is concluded that the inhibitory effects of the baroreceptors on spinal sympathetic activity are mediated by bulbo-spinal catecholaminergic neurones.