The responses of cortical neurones to monoamines under differing anaesthetic conditions.

1. 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 encéphale isolé (48%) and N(2)O-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: encéphale isolé 84%; N(2)O-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 N(2)O-halothane and encéphale isolé 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.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.