The effects of drugs in relation to the afferent collateral system of the brain-stem

Abstract

The effects of chlorpromazine, d-lysergic acid diethylamide (LSD 25) and pentobarbitone have been studied on arousal responses, produced by electrical stimulation at different levels in the specific auditory pathway. Two types of response were noted; firstly, widespread electrocortical desynchronisation (diffuse response) associated with behavioural arousal and, secondly, a phasic response restricted to alterations in electrocortical activity over the auditory cortices (local response). Pentobarbitone (2.5–10 mg/kg i.v.) brought about a progressive rise in the threshold voltages necessary to evoke both types of response and the increases were approximately the same for each stimulating position. Chlorpromazine produced a differential effect depending upon the position of the stimulating electrode. Doses of 2.5 or 5.0 mg/kg i.v. induced a 50–60% rise in threshold voltages for stimulating positions in the inferior colliculus, brachium of the inferior colliculus and the medial geniculate body, but a marked rise of 250–350% for positions in the auditory pathway within, and caudal to, the nucleus of the lateral lemniscus. LSD 25 (5–15 μg/kg i.v.) induced a decrease in the thresholds for diffuse arousal responses providing the stimulating electrodes were located in the lateral lemniscus, trapezoid body or cochlear nuclei, but failed to produce any change in responses evoked by stimulation of the inferior colliculus, brachium of the inferior colliculus or the medial geniculate body. Moreover, thresholds for local responses remained unaffected irrespective of the stimulating position. The concept of a differential afferent collateral input to the brain-stem reticular activating system is discussed in relation to the effects of these drugs.