Catecholamines are known to influence sympathetic function through regulation of the activity of sympathetic preganglionic neurones at the spinal level. Neuropharmacological, neurophysiological and autoradiographic techniques have been used to study the possible involvement of α1-adrenoceptors in the intermediolateral cell column of the spinal cord in regulating the activity of sympathetic preganglionic neurones in the intact animal. The location of α1-adrenoceptor ligand binding sites in the cat spinal cord was investigated autoradiographically using [3H]prazosin. High levels of [3H]prazosin binding sites were found in the intermediolateral cell column. Non-specific binding was very low, indicating that the binding was to α1-adrenoceptors. In anaesthetised rats extracellular recordings were made from antidromically identified sympathetic preganglionic neurones and effects of iontophoretic applications of α1-adrenoceptor agonists in the vicinity of these neurones studied. Iontophoresis of phenylephrine and methoxamine (both highly selective α1-adrenoceplor agonists) caused increases in the firing rate of the majority of neurones studied. SPNs were also analyzed for respiratory- and ECG-related activity, and the axonal conduction velocities of the neurones were calculated to determine whether there was relationship between the physiological characteristics of a neurone and its response to α1-adrenoceptor agonists. The results indicated that the effect of α1-adrenoceptor agonists on an SPN is not related to its physiological characteristics. Recordings were made from the inferior cardiac nerve of anaesthetised cats to determine whether sympathetic preganglionic neurones innervating this nerve are excited by α1-adrenoceptor agonists microinjected into the intermediolateral cell column. Microinjection of 100 nl of phenylephrine caused increases in inferior cardiac nerve activity, an effect which could be antagonised by microinjection of the α1-adrenoceptor antagonist alfuzosin. This antagonism appeared selective for phenylephrine since alfuzosin did not block the excitatory effects of microinjections of glutamate and 5-HT. It is concluded that α1-adrenoceptors in the intermediolateral cell column may be involved in mediating excitatory input onto sympathetic preganglionic neurones in the intact animal. The possible roles and supraspinal sources of this input are discussed
