Inhibitory and indirect excitatory effects of dopamine on sympathetic preganglionic neurones in the neonatal rat spinal cord in vitro

Abstract

Regions of the thoraco-lumbar spinal cord containing sympathetic preganglionic neurones are rich in dopamine terminals. To determine the influence of this innervation intracellular recordings were made from antidromically identified sympathetic preganglionic neurones in (400 μm) transverse neonatal rat spinal cord slices. Dopamine applied by superfusion caused a slow monophasic hyperpolarisation in 46% of sympathetic preganglionic neurones, a slow monophasic depolarisation in 28% of sympathetic preganglionic neurones and a biphasic effect consisting of a slow depolarisation followed by a slow hyperpolarisation or vice-versa in 23% of sympathetic preganglionic neurones. Three percent of sympathetic preganglionic neurones did not respond to the application of dopamine. Low Ca 2+/high Mg 2+ Krebs solution or TTX did not change the resting membrane potential but abolished the slow depolarisation elicited by dopamine, indicating this was synaptic and did not prevent the dopamine induced hyperpolarisation. The dopamine induced slow hyperpolarisation was mimicked by the selective D 1 agonists SKF 38393 or SKF 81297-C and blocked by superfusion with the D 1 antagonist SCH 23390. It was not prevented by superfusion of the slices with alpha 1 or alpha 2 or beta-adrenoceptor antagonists, whereas the inhibitory or excitatory actions of adrenaline were prevented by alpha 1 or alpha 2 antagonists, respectively. The dopamine induced slow depolarisation occurring in a sub-population of sympathetic preganglionic neurones was mimicked by quinpirole, a D 2 agonist, and blocked by haloperidol, a D 2 antagonist. Haloperidol did not block the dopamine induced hyperpolarisations. Dopamine also induced fast synaptic activity which was mimicked by a D 2 agonist and blocked by haloperidol. D 1 agonists did not elicit fast synaptic activity.