THE EXCITABILITY AND RHYTHM OF MEDULLARY RESPIRATORY NEURONS IN THE CAT ARE ALTERED BY THE SEROTONIN RECEPTOR AGONIST 5-METHOXY-N,N,DIMETHYLTRYPTAMINE

5-Methoxy=N,N-dimethyltryptamine (5-MeODMT) is an indolealkylamine whi ch has agonist activity at 5HT receptors. In the present investigation , 5-MeODMT had two types of effects on medullary respiratory neurons o f the cat. Iontophoretic administration or i.v. doses (43 +/- 8.9 mu g /kg) of 5-MeODMT hyperpolarized respiratory neurons and severely reduc ed action potential discharges. Cinanserin, a 5HT-2/1 c receptor antag onist, when injected i.v. reduced the inhibition produced by i.v. inje ction of 5-MeODMT. Iontophoresis of cinanserin did not antagonize inhi bition produced by iontophoresis of 5-MeODMT or 5-HT. The depression o f respiratory discharge by i.v. injection of 5-MeODMT is attributed to presynaptic effects (network depression) and post-synaptic activation of 5HT-1A receptors on respiratory neurons. 5-MeODMT (27 +/- 2.78 mu g/kg i.v.) also increased discharge frequency of inspiratory and expir atory neurons. Inspiratory neuron discharges were briefer and expirato ry neuron discharges occurred earlier in relation to phrenic nerve act ivity. It is suggested that the effects of the smaller doses are due t o binding of 5-MeODMT to SHT-1A receptors on early inspiratory neurons of the medulla.