Effects of dopaminergic agonists on somato-autonomic reflexes

In cats lightly anesthetized with urethane (600 mg/kg, i.p.) reflexes of the blood pressure (BP) and of the nictitating membrane (NM) were elicited by stimulation of the sciatic nerve (16 V, 0.3 ms, 1-128 Hz, 2 s or 2 min) prior to and after the administration of apomorphine (0.05-0.2 mg/kg, i.v.) or piribedil (0.4-1.0 mg/kg, i.v.). In case of short-train (2 s) stimulation, both dopaminergic agonists shifted the frequency-response curves of NM contractions to the right, i.e. depressed NM reflexes in the entire range of the stimulation frequencies applied. At the same time, BP reflexes were depressed only in the range of lower frequencies (1-4 Hz). At higher rates (32-128 Hz) BP reflexes were potentiated. The reactions of BP to sustained (2 min) stimulations displayed a flat pressor plateau in response to lower-frequency stimulation, and a two-component pattern (an initial pressor peak followed by a plateau) to the higher-frequency one. Compatibility with the effects seen to short-train stimulations, the dopaminergic agonists prolonged the rise-time and augmented the amplitude of the initial pressor peak to sustained stimulations with lower and higher frequencies, respectively. The plateau of the pressor response to higher frequencies was depressed by higher doses (greater than 0.4 mg/kg) of piribedil. Administration of haloperidol (0.05-0.2 mg/kg, i.v.) resulted only in a partial restoration of the reflexes of BP and NM. The manifold effects of dopaminergic agonists on the somato-autonomic reflexes studied support the thought than NM and BP reflexes are organized, at least partially, in different ways.