Effects of serotonin and substance P on bulbar respiratory neurons in vivo.

We studied the effects of separate or co-applications by microiontophoresis of serotonin (5-HT) and substance P(4-11) onto brainstem respiratory neurons in anesthetized or decerebrate cats. 5-HT either produced an excitation (36%, n = 10) or an inhibition (43%, n = 12) or had no effect (21%, n = 6). SP(4-11) had predominantly an excitatory effect (84%, n = 26) or no effect. Fifteen respiratory neurons responded to both 5-HT and SP(4-11). Test applications of 5-HT were made during a long application of SP(4-11). We obtained 'additive effects' when the inhibitory effect of 5-HT was superimposed to the excitation of SP(4-11) with slight modification (n = 1) or without any modification (n = 2). In other cases, called 'non-additive effects', we observed a great modification of the responsiveness of the neuron to the inhibitory effect of 5-HT (n = 2) or a complete blockade of the excitatory effect of 5-HT (n = 2) during co-application. The remaining results presented a potentiation of 5-HT effect by SP(4-11) or a biphasic response to 5-HT during SP(4-11) application. The results indicate that both 5-HT and SP receptors coexist on the membrane of the same respiratory-related neurons in the brainstem of cat and suggest an interaction between both substances in vivo in the central respiratory system.