A comparison of the effects of monoamines on neurones of the bulbar reticular formation

Characteristicas and response differences to iontophoretically applied norepinephrine, D-amphetamine and acetylcholine on neurons in the medial and lateral vestibular nuclei of the cat

Midcollicular decerebrate cats, with their cerebellum removes, were tested with controlled acceleratory motion in order to identify neurons in the medial vestibular nucleus (MVN) and lateral vestibular nucleus (LVN) which responded to a motion stimulus. Five-barredled micropipettes were used to record single neuron activity and to apply norepinephrine (NE), d-amphetamine and acetylcholine (ACh). These agents were studied on spontaneously firing cells which responded to a motion stimulus and others which were in the MVN were inhibited by NE and d-amphetamine but were unaffected by iontophoresis of the alpha-adrenergic blocking agent phentolamine or the beta-antagonists, MJ-1999 or propranolol. In the LVN a majority of the cells tested were excited by NE and d-amphetamine. NE excitation in the LVN was antagonized by phentolamine but not by MJ-1999 or propranolo. Cats pretreated with reserpine to deplete brain catechlamines showed typical responses to NE BUT IONTOPHORESIS OF D-AMPHETAMINE WAS WITHOUT EFFECT. Unlike the differential sensitivity observed for NE, ACh excited most cells in both the MVN and LVN. NE and ACh produced similar responsed on vestibular neurons modulated by motion and those not responsive to motion. These observations suggest that NE-containing terminals are in close proximity to the vestibular neurons which were tested and further implicate both NE and ACh as neurotransmitters in afferent pathways to the vestibular nuclei.

The modification of lumbar motoneurone excitability by stimulation of a putative 5-hydroxytryptamine pathway

1 Changes in lumbar motoneurone excitability were monitored by recording spinal reflex activity from the ventral roots of rats anaesthetized with fluothane.2 Electrical stimulation of nucleus raphes medianus increased the amplitude of the monosynaptic reflex via a pathway having a slow conduction velocity. Stimulation elsewhere in the lower brain stem was less effective. This increase in motoneurone excitability was potentiated by the intravenous injection of L-tryptophan and reduced by intravenous injections of lysergic acid diethylamide (LSD), methysergide or Cinanserin.3 Extracellular field potential responses to stimulation of dorsal or ventral roots were recorded with six barrelled microiontophoresis electrodes. Stimulation of nucleus raphes medianus and iontophoretic application of 5-hydroxytryptamine (5-HT) both increased the excitability of lumbar motoneurones as reflected by an increase in field potential amplitude.4 Responses to both stimulation of raphe nuclei and iontophoretic application of 5-HT were reduced by iontophoretic application of Cinanserin and methysergide.5 The similarities of the responses of lumbar motoneurones to applied 5-HT and activity within the raphe-spinal pathway are discussed. It is suggested that activity within the raphe-spinal pathway can increase lumbar motoneurone excitability via the release of 5-HT in the ventral horn of the spinal cord.

Modification of the responses of brain stem neurones to transmitter substances by anaesthetic agents

1. The effects of microiontophoretic applications of acetylcholine (ACh), (-)-noradrenaline ((-)-NA) and 5-hydroxytryptamine (5-HT) have been investigated on spontaneously active brain stem neurones in decerebrate unanaesthetized rats and in rats anaesthetized with either tribromoethanol, urethane or pentobarbitone.2. Four types of responses to both (-)-NA and 5-HT were seen. These were: simple excitation; excitation preceded by a short-lasting inhibition; short-lasting inhibition and prolonged inhibition. Three types of responses to ACh were seen: an excitation with long latency of onset; excitation with short latency of onset, resembling the response to an excitant amino acid, and a short-lasting inhibitory response.3. The types of responses to microiontophoretically applied ACh, (-)-NA or 5-HT in anaesthetized and unanaesthetized animals were similar.4. The number of ACh excitatory responses with short latency of onset were significantly reduced in the pentobarbitone-anaesthetized group and a small but significant increase in the number of 5-HT inhibitory effects were observed in each anaesthetized group of animals.5. A significantly greater proportion of slower firing neurones (less than 10 spikes/s) were found in the pentobarbitone-anaesthetized animals.6. The effects of microiontophoretically applied and i.v. administered pentobarbitone were studied on spontaneously active neurones which responded consistently to ACh and a control agonist.7. Pentobarbitone administered by either route reduced the firing rate of most neurones studied and was shown to antagonize specifically the excitation of neurones by exogenously applied ACh.8. It is suggested that postsynaptic antagonism of endogenously released ACh may be a contributing factor in the mechanism of action of pentobarbitone.

The excitation of neurons by noradrenaline

Responses of single cortical neurons to microelectrophoretically applied noradrenaline at pH 3.1 and 5.0 and to hydrogen ions were compared in the halothane-anaesthetized cat. Of 16 neurons sensitive to noradrenaline, 13 were excited and 3 were depressed by noradrenaline at both pH values, whereas hydrogen ions released from an HCl solution did not affect the firing rate. Dose-response studies showed that noradrenaline at pH 3.1 was more potent than at pH 5.0. It is concluded that excitatory responses to noradrenaline are not artifacts and that the adjustment of the pH of noradrenaline solutions with NaOH should be avoided.

Synergy of ethanol and putative neurotransmitters: glycine and serine

The putative neurotransmitters, glycine and serine, significantly enhanced the sleeping time (loss of the righting reflex) that was induced by ethanol in mice. The observed synergistic effect between ethanol and the amino acids is probably not related to an alteration of ethanol metabolism, but rather to an interaction of these compounds in the central nervous system.