The inhibitory action of noradrenaline and other monoamines on spinal neurones

Abstract

SUMMARY1. L-Noradrenaline (NA), 5-hydroxytryptamine (5-HT) and acetylcholine (ACh) were administered micro-electrophoretically to feline lumbar neurones while recording their spike potentials extracellularly.2. There was no evidence to suggest that NA acts as an excitatory transmitter in the spinal cord.3. NA had potent inhibitory effects on some interneurones as revealed by a depression of spontaneous and synaptic firing and on the firing to a local application of an excitant amino acid. The effects on Renshaw cells and motoneurones were less marked.4. The depressant actions of 5-HT were less marked than those of NA. ACh and carbamylcholine had depressant effects on some NA-sensitive interneurones but were invariably far less potent and on other NA-sensitive cells were completely inactive.5. NA had no detectable effect on the normal spike amplitude but when the action potentials were reduced by excessive depolarization then both NA and synaptic inhibition increased the spike amplitude; this effect could be due to a hyperpolarization of the cell membrane.6. There was a correlation between the distribution of NA-sensitive cells and the relative densities of NA-containing terminals in various layers of the grey matter.7. It was postulated that NA acts as an inhibitory transmitter released from the terminals of descending pathways in the spinal cord. Other possible mechanisms were discussed but lacked experimental support.

Excitation and depression of cortical neurones by 5‐hydroxytryptamine

“…In particular, the number of cells responding to particular potential transmitter substances may be markedly reduced and the pattern of responses to them distorted by anaesthetics. Engberg & Ryall (1966) and Weight & Salmoiraghi (1966) observed that 5-HT and noradrenaline did have effects on spinal cord neurones in decerebrate cats and they suggested that it was the use of anaesthetics in the experiments of and Curtis (1962) which had probably been responsible for their failure to observe similar effects. The number of 5-HT sensitive cells in spinal cord was reduced threefold by the use of ether (Weight & Salmoiraghi, 1966).…”

“…The application of the balancing current to the control barrel when it contained sodium bimaleate also meant that there was no change in the rate of ejection of maleate anions when 5-HT was 5-HT AND CORTICAL NEURONES 273 ejected (cf. Engberg & Ryall, 1966). This excluded the possibility that 5-HT effects were an artifact resulting from the removal of the backing current which had been expelling the potentially potent anion, maleate.…”

The Modification of Lumbar Motoneurone Excitability by Stimulation of a Putative 5‐hydroxytryptamine Pathway

“…The direct application of 5-HT onto spinal interneurones or motoneurones by microiontophoresis has usually caused either the depression of cell excitability or has been without effect (Curtis, Phillis & Watkins, 1961;Engberg & Ryall, 1966). Two studies, however, have noted the occurrence of infrequent excitatory responses (Weight & Salmoiraghi, 1966;Phillis, Tebecis & York, 1968).…”

ANALYSIS OF THE EFFECTS OF p‐METHOXY‐PHENYLETHYLAMINE ON SPINAL CORD NEURONES

“…The interneurone population depressed by microiontophoretically applied PMPEA, so far as it has been identified, coincides well with the population influenced by NA (Engberg & Ryall, 1966) and by intravenous PMPEA (Jordan & Willis, 1973). The depressant effects of NA, 5-HT and PMPEA, when they were observed on identified neurones (orthodromically activated neurones, Table 1), were limited to interneurones activated by flexion reflex afferents.…”