The effects of beta-carbolines on responses to acetylcholine, noradrenaline, 5-hydroxytryptamine and amino acids in the rat spinal cord.

Various drugs have been applied electrophoretically to Renshaw cells and to unidentified spinal neurones in pentabarbitone anaesthetized or decerebrated rats. Responses to noradrenaline (NA) and 5-hydroxytryptamine (5-HT) have not previously been described at this site and were of two types; either monophasic depression or biphasic depression-excitation. The effect of harmine on these responses was examined. Harmine and harmaline were also tested on the excitant responses to acetylcholine (ACh) and DL-homocysteate (DLH), and on the depressant responses to glycine and gamma-aminobutyric acid (GABA). On some cells harmaline antagonized ACh, but not DLH, and glycine, but not GABA, responses. Harmine caused only non-specific depression and spike configuration changes. The effects of harmine on NA and 5-HT responses were usually non-specific, and any anatagonism was usually accompanied by, or soon followed by spike changes. LSD was also tested on the amine responses. LSD itself had a clear depressant effect on neuronal firing rates. It could either antagonize or potentiate NA and 5-HT depressant responses, but the antagonism in particular was closely followed by spike changes. Somewhat more specific antagonism of the late excitation was seen.