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

1. 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. 2. The effect of these microinjections on bulbospinal catecholaminergic neurones was assessed using the formaldehyde fluorescence technique for the demonstration of intraneuronal monoamines. 3. 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 dorso-lateral funiculi was extensive, the inhibition was almost completely abolished; with more limited destruction a lesser effect was observed. 4. These effects were not seen in a group of control animals, which received intraspinal microinjections of the 5-hydroxytryptamine neurotoxin 5,6-dihydroxytryptamine and which showed damage to the indoleaminergic neuronal system but no damage to the bulbo-spinal catecholaminergic system. 5. It is concluded that the inhibitory effects of the baroreceptors on spinal sympathetic activity are mediated by bulbo-spinal catecholaminergic neurones.