Bulbospinal catecholamine neurones and sympathetic pattern generation.

Populations of neurones containing noradrenaline, dopamine and possibly adrenaline project to the spinal cord where they innervate sympathetic preganglionic neurones (SPN). Studies are described which illustrate the actions of catecholamines on SPN and which suggest ways in which the catecholamine neurones could regulate the cardiovascular system. Experiments on rats using, intrathecal application of drugs to the spinal cord, iontophoresis, or superfusion of drugs whilst recording either postganglionic nerve activity, extracellularly or intracellularly from SPN respectively, reveal that catecholamines may excite or inhibit SPN. A slow depolarisation is mediated by alpha 1 adrenoceptors whereas alpha 2 adrenoceptors mediate a slow hyperpolarisation. Catecholamines may also excite glycinergic interneurones which elicit fast IPSPs in SPN. By regulating different ionic conductances in the membrane of SPN catecholamines are able to induce SPN to discharge tonically or to oscillate with bursts of action potentials. Furthermore these actions may be modified in the presence of an excitatory amino acid. It is suggested that via these mechanisms differential responses in the sympathetic outflow could be produced.