Evidence that the firing pattern of sympathetic preganglionic neurones is determined by an interaction between amines and an excitatory amino acid

Extracellular recordings were made from identified sympathetic preganglionic neurones (SPN) located in the second and third thoracic segments of the adult rat spinal cord, in-vivo. Iontophoretic application of the amines, 5-hydroxytryptamine (5-HT) and noradrenaline (NAdr) in the vicinity of these neurones evoked either long lasting excitations, inhibitions or bi-phasic changes (inhibition followed by excitation, 5-HT only) in neuronal firing rate. The excitatory response to both 5-HT or NAdr could only be obtained in spontaneously active neurones, or silent neurones, in which a subliminal level of an excitatory amino acid (EAA) was also present. In 4 neurones, the response evoked by NAdr was dependent on neuronal firing rate and hence the level of EAA present. At low basal firing rates, inhibitions were observed whereas at higher firing rates, excitations were observed. Subliminal levels of either 5-HT or NAdr markedly potentiated the increases in neuronal firing rate evoked by an EAA. In a sub-population of SPN, both 5-HT (18/88 neurones) and NAdr (21/51 neurones) induced a repeated bursting pattern of action potential discharge during the amine evoked changes in neuronal firing rate in otherwise irregularly discharging neurones. We conclude that these actions of the monoamines, that is excitations or inhibitions, gain enhancement, prolongation of action, burst firing and oscillations are likely to be due to the modulation by these agents of intrinsic membrane conductances. The differing responses evoked in SPN will produce marked differences in the efficacy of synaptic transmission in the sympathetic ganglia. As a result of these mechanisms, there will be a greater versatility in the sympathetic control systems than would otherwise exist.