The organisation of cardiovascular neurons in the spinal cord

Preganglionic sympathetic neurones innervating the rat adrenal medulla: immunocytochemical evidence of synaptic input from nerve terminals containing substance P, GABA or 5-hydroxytryptamine

Sympathetic preganglionic neurones that innervate the adrenal medulla were identified for subsequent light and electron microscopic study by the retrograde transport of horseradish peroxidase (HRP) or a conjugate of HRP and cholera B-chain. Most labelled neurones were found in the intermediolateral column, but some occurred in the intercalated nucleus and in the lateral funiculus of the thoracic spinal cord. Three morphologically distinct types of neurone were retrogradely labelled, two of which had dendrites that extended medially towards the central canal and laterally across the entire lateral funiculus. A combination of retrograde labelling with pre-embedding immunocytochemistry allowed us to demonstrate synaptic contacts between boutons immunoreactive for substance P or 5-hydroxytryptamine (5-HT) and the cell bodies or proximal dendrites of sympathoadrenal neurones. The 5-HT-immunoreactive boutons appeared to be of two morphologically distinct types. Postembedding immunocytochemistry enabled us to show that sympathoadrenal neurones receive a heavy synaptic innervation from GABA-immunoreactive boutons: 32% of a random series of boutons in synaptic contact with cell bodies were GABA-immunoreactive. Proximal dendrites and also distal dendrites within the white matter were ensheathed in synaptic boutons, 37% of which were GABA-immunoreactive. It is concluded that sympathoadrenal neurones receive at least 4 distinct types of afferent synaptic input: from neurones containing substance P, or GABA and from two types of neurones containing 5-HT. The presence of synaptic inputs on distal dendrites that extend across the white matter adds further complexities to the control of the activity of sympathetic preganglionic neurones.

A comparison of the effects of intrathecally administered 5-hydroxytryptamine and thyrotropin-releasing hormone on renal and muscle sympathetic nerve activity

Thyrotropin-releasing hormone (TRH) has been shown to coexist with 5-hydroxytryptamine (5-HT) in spinally projecting raphe neurones, some of which terminate in the sympathetic nuclei of the spinal cord. In an attempt to mimic the actions of these neurones, the effects of intrathecal administration of 5-HT was compared to that of TRH on activity in two sympathetic postganglionic nerves, the renal nerve and sympathetic fibres to skeletal muscle of the hind limb. In chloralose-urethane anaesthetised rats intrathecal infusion of TRH at the T9 level in doses of 5-30 micrograms increased activity in renal nerve as did low doses (20-100 micrograms) of 5-HT. In contrast, intrathecal infusion of TRH in doses of 5-30 micrograms and 5-HT in doses of 20-800 micrograms at L4 level decreased activity in sympathetic fibres to muscle. The results show that TRH, unlike 5-HT, has an excitatory but not an inhibitory action on renal sympathetic activity whereas both substances have only an inhibitory action on muscle sympathetic activity.

Effects of activating spinal alpha-adrenoreceptors on sympathetic nerve activity in the rat

The effects of several alpha-adrenoreceptor agonists and antagonists administered intrathecally at T10 level on renal sympathetic nerve activity (RSNA) were examined, in chloralose-urethane anaesthetised rats. Intrathecal noradrenaline (NA, 0.5-500 micrograms) produced one of 3 responses depending on dose, an inhibition of RSNA at low doses, an excitation of RSNA at high doses, or a biphasic effect. Intrathecal adrenaline (5-200 micrograms) was inhibitory in the main but some doses elicited poorly repeatable brief excitation followed by prolonged inhibition. Intrathecal methoxamine (ME; 2.5 ng-25 micrograms) caused a dose-dependent increase in RSNA (mean maximum response 27 +/- 0.5%). The excitatory effects of NA and ME were blocked (72% +/- 12%) by pretreatment with intrathecal prazosin (PRA, 20-200 ng) but not by yohimbine (YOH, 200 ng). Intrathecal guanabenz (GUA 3-15 micrograms) caused a dose-dependent inhibitory effect on RSNA (mean maximum 32% +/- 5%). The inhibitory effects of NA, adrenaline and GUA were blocked by pretreatment with intrathecal YOH (200 ng-2 micrograms). Intrathecal PRA (200 ng) had no effect on the inhibitory effects of NA and GUA. Intravenous administration of each of the adrenoreceptor agonists (apart from adrenaline), at similar doses to those given intrathecally, in most cases had no significant effect on RSNA; in a few cases the opposite effects to those produced by intrathecal administration were seen.