Blockade of spinal pathways decreases pre- and postganglionic discharge differentially

Abstract

Although discharge of many sympathetic nerves decreases substantially on transection of the spinal cord in cats, firing of splenic and mesenteric postganglionic nerves continues unabated (R. L. Meckler and L. C. Weaver, Brain Res. 338: 123-135, 1985; R. D. Stein and L. C. Weaver. J. Physiol. Lond. 396: 155-172, 1988). Therefore, ongoing sympathetic outflow directed to the splanchnic circulation was proposed to be less dependent on supraspinal excitatory drive than that directed to other vascular beds. Blockade or transection of cervical spinal pathways in chloralose-anesthetized cats significantly decreased firing of gastric, hepatic, adrenal, and lumbar chain sympathetic nerves. Discharge of gastric and hepatic nerves decreased as much as that of adrenal and lumbar chain nerves; therefore, sustained discharge in the absence of bulbospinal excitation is not a characteristic of all sympathetic nerves innervating the splanchnic circulation. In contrast, discharge of the preganglionic greater splanchnic nerves was not decreased after spinal transection, although it changed from a rhythmic to an asynchronous pattern. This provocative finding suggests that rhythmicity of preganglionic discharge may be important for effective synaptic transmission to some postganglionic neurons, since postganglionic gastric, hepatic, and adrenal nerve firing decreased in the presence of sustained but asynchronous preganglionic input.

Evidence that different regional sympathetic outflows vary in their sensitivity to the sympathoinhibitory actions of putative 5‐HT1A and α2‐adrenoceptor agonists in anaesthetized cats

“…The finding that splanchnic nerve activity seems to be affected equally by all the drugs studied may be explained by the observation that activity in this nerve is only slightly influenced by supra-spinal pathways, as transection of the spinal cord at C1 only slightly reduced the activity in this nerve (Qu et al, 1988). However, the differential sensitivity of different sympathetic outflows observed in this study may just reflect the accessibility of various receptor sites within the CNS to these drugs, when given i.v.…”

Submandibular responses to stimulation of the parasympathetic innervation in bursts in the anaesthetized ferret.

“…Evidence in support of this suggestion is provided by the finding that blocking spinal pathways in cats anaesthetized with chloralose affects sympathetic pre-and postsynaptic discharges differentially. Thus, the discharge in preganglionic sympathetic fibres was changed from a regular bursting pattern to an irregular pattern and this was associated with a significant decrease in the activity in the postganglionic splanchnic fibres in spite of the fact that overall activity in the preganglionic fibres had not decreased (Qu, Sherebrin & Weaver, 1988). Furthermore, inhibition of ganglionic transmission at higher frequencies might explain why responses were not potentiated by stimulating intermittently above the equivalent of 5 Hz continuously, but there is no direct evidence which bears on this point.…”

Selective control of sympathetic pathways to the kidney, spleen and intestine by the ventrolateral medulla in rats.

“…Therefore, a loss of tonic activity (by blockade of the RVLM) in a small number of preganglionic axons could cause the loss of activity in many postganglionic neurones. Loss of rhythmicity in preganglionic firing after elimination of supraspinal input (Qu et al 1988) also could decrease summation of postsynaptic potentials leading to greater decreases in postganglionic firing.…”

“…For example, spinal cord transection in cats reduces efferent activity in some but not all sympathetic nerves. Discharge of postganglionic renal, gastric, hepatic, adrenal and lumbar chain sympathetic nerves is substantially decreased in spinal cats while firing of splenic, mesenteric and greater splanchnic nerves is maintained (Meckler & Weaver, 1985Qu, Sherebrin & Weaver, 1988;Stein & Weaver, 1988). Furthermore, inhibiting the discharge of RVLM neurones by bilateral microinjections of the inhibitory amino acid glycine, or by application of glycine to the surface of the ventral medulla, elicits greater reductions in renal than in mesenteric nerve activity (Stein, WVeaver Yardley, Stein & Weaver, 1989).…”