The responses of cortical neurones to monoamines under differing anaesthetic conditions

Abstract

SUMMARY1. Noradrenaline, isoprenaline, 5-hydroxytryptamine and acetylcholine have been applied into the environment of single neurones in the cat cerebral cortex by micro-iontophoresis. The influence of anaesthesia on the neuronal responses to these drugs was studied.2. In general, excitation of neurones by noradrenaline occurred commonly in both unanaesthetized encephale isole (48 %) and N20-halothane anaesthetized preparations (57 %) while depressions were less frequent (24 and 20 % respectively). The picture differed markedly in barbiturate anaesthetized animals where excitation was uncommon (12 %) and the majority of cells (59 %) was depressed by noradrenaline. Although fewer cells were studied, similar differences were obtained with isoprenaline and 5-hydroxytryptamine for the three types of preparation. In sharp contrast, the vast majority of cells was excited by acetylcholine in each of the preparations: encephale isole 84 %; N20-halothane 92 %; barbiturate anaesthetized preparations 85 %.3. The differing neuronal responses observed in these experimental situations were not the result of variations in the depth of anaesthesia.4. Although the depths at which neurones were encountered within the cortex did not differ in N20-halothane and encephale isole preparations, significantly more cells were found in deeper layers of cortex in barbiturate preparations. The proportion of cells excited or depressed by noradrenaline was generally similar at each depth in the three preparations used.5. The distribution of rates of cell firing was strikingly similar in each preparation, and most of the cells studied had frequencies below 10/sec. * Rothschild Fellow. IPhy. 203 9 E. S. JOHNSON AND OTHERS 6. The direction in which a cell responds to noradrenaline is mainly determined by the type of anaesthetic used, and not by the depth of anaesthesia, the rate of cell firing, or cell depth within the cortex. This suggests important differences in the central pharmacology of halothane and barbiturates.

Effects of 5‐hydroxytryptamine agonists and antagonists on the responses of rat spinal motoneurones to raphe obscurus stimulation

“…Lipski (1981) in a review of antidromic invasion of motoneurones and other cells, concluded that the technique was of value. This leaves the possibility that the barbiturate anaesthesia used by Engberg et al (1979) significantly affected the response of motoneurones to 5-HT in much the same way as barbiturates affect the responses of cortical neurones to 5-HT (Johnson et al, 1969).…”

Amino‐acid induced depression of cortical neurones

“…Details of the techniques used for the extracellular iontophoresis of drugs onto spontaneously firing neurones in the cat posterior sigmoid gyrus have been presented elsewhere (Roberts & Straughan, 1967;Johnson, Roberts & Straughan, 1969b). Anaesthesia was induced by N20-halothane and was maintained either on 75% nitrous oxide in oxygen and 10% halothane, or on intravenous pentobarbitone sodium (25 mg initially, supplemented as necessary).…”

“…Barbiturate anaesthetized cats were used in experiments with 5-HT to achieve mainly depressant responses (Johnson et al, 1969b) The following drugs were used in aqueous solution in the micropipettes: Glycine and y-aminobutyric acid (GABA) 0-2 M, acidified to pH 3-5 with HCl; 5-hydroxytryptamine hydrogen maleate (5-HT) 0-2 M, pH 3 5; acetylcholine chloride (ACh) 0-2 M, pH 3 5-4 0 and strychnine sulphate 0-01 M, pH 4 0.…”

Cardiovascular responses of rats to intrahypothalamic injection of carbachol and noradrenaline

“…The opposite effects of noradrenaline were not due to injection at different sites since these were frequently indistinguishable from subsequent inspection of cannulae positions. It is possible that the depth of anaesthesia determined the direction of the response to noradrenaline, a possibility that is consistant with the finding that responses of cortical neurones to iontophoretic application of monoamines (but not acetylcholine) were modified by the depth of anaesthesia (Johnson, Roberts & Straughan, 1969). This work was supported by a grant from the Wellcome Trust.…”