F. Santangelo

Effects of 5-hydroxytryptamine on the firing rates of neurons of the lateral vestibular nucleus in the rat

Summary5-hydroxytryptamine (5-HT) was delivered microiontophoretically (20–80 nA) to cells of the lateral vestibular nucleus of anaesthetized rats to test its influence on the spontaneous activity of single neurons. 5-HT increased the rate of firing of 94% of the units tested. The enhancement persisted for up to 700 s after the end of the 5-HT ejection and the maximum magnitude of the excitation (10–3400%) showed a hyperbolic correlation (ϱ=0.86) with background firing. In 43% of units the enhancement was preceded by a short-lasting (less than 105 s) depression of the neuronal firing rate, the magnitude of which was unrelated to the background mean firing rate. Both components of the 5-HT response were dose-dependent. Only the excitatory responses were antagonized by metergoline, methysergide and ketanserin. The putative 5-HT agonist, 5-methoxy-N,N-dimethyltryptamine, applied microiontophoretically, depressed the background firing rate and was not antagonized by methysergide. These results demonstrate that 5-HT modifies the responsiveness of vestibular neurons and suggest that at least two mechanisms and maybe two types of receptors are activated by 5-HT in this nucleus.

Serotonin-evoked modifications of the neuronal firing rate in the superior vestibular nucleus: A microiontophoretic study in the rat

Microiontophoretic ejection (10-100 nA) of serotonin (5-hydroxytryptamine) into the superior vestibular nucleus induced modifications of the mean firing rate in 87% of the neurons examined. The responses to 5-hydroxytryptamine application were excitatory in 48% of the cells, inhibitory in 29%, and biphasic (inhibitory/excitatory) in the remaining 10%. The excited neurons were scattered throughout the nucleus; the units inhibited or characterized by biphasic responses were distinctly more numerous in the ventrolateral sector of the nucleus. The magnitude of both excitatory and inhibitory effects was dose-dependent. The excitatory responses to 5-hydroxytryptamine were blocked or greatly reduced by two 5-hydroxytryptamine antagonists, methysergide and ketanserin, or even reversed in many cases. Inhibitory responses were enhanced by simultaneous application of 5-hydroxytryptamine antagonists in half of the units studied. In the remaining units, ketanserin left the response unmodified, whereas methysergide reduced but never quite blocked it. The application of 5-methoxy-N,N- dimethyltryptamine, a 5-hydroxytryptamine agonist more effective on 5-hydroxytryptamine1 than on 5-hydroxytryptamine2 receptors, and of 8-hydroxy-2(di-n-propyl-amino) tetralin, a 5-hydroxytryptamine1A-specific agonist, induced a decrease in the firing rate which was unaffected by methysergide. These results support the hypothesis that 5-hydroxytryptamine exerts various functions throughout the superior vestibular nucleus by various receptors and that the inhibitory action is limited to an area of it.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory and inhibitory effects of 5-hydroxytryptamine on the firing rate of medial vestibular nucleus neurons in the rat

The effects of microiontophoretic application of 5-hydroxytryptamine (5-HT) on the neuronal firing rate of the medial vestibular nucleus (MVN) were studied in anaesthetized rats. Ninety-three % of the units modified their background activity following 5-HT iontophoresis, enhancements of the firing rate being recorded in 42%, decreases in 38% and biphasic effects in 13%. 5-HT antagonists methysergide and ketanserin blocked the excitatory but not the inhibitory responses to 5-HT. These latter were, however, mimicked by 5-HT agonists 5-methoxy-N,N-dimethyltryptamine (5MeODMT) and 8-hydroxy-2(di-n-propyl-amino)tetralin (8-OH-DPAT). It is concluded that 5-HT can variously influence the background activity of MVN neurons and that 5-HT2 and probably 5-HT1A receptors are involved in the responses.

Effects of noradrenaline on the firing rate of vestibular neurons

The effects of microiontophoretic noradrenaline on the firing rate of neurons located in the vestibular complex have been studied in anaesthetized rats. Eighty-five per cent of the neurons tested in all the vestibular nuclei modified their background firing rate upon noradrenaline application, generally by reducing it (86% of them). In few cases inhibitions were followed by a rebound. Responses were dose-dependent. No significant difference was found between vestibular neurons projecting to the spinal cord and those delivering their fibres to the oculomotor complex. Phentolamine, an alpha-adrenergic antagonist, blocked the noradrenaline-evoked inhibitions, whereas beta-adrenergic antagonist timolol was ineffective or enhanced them. Furthermore, responses were blocked by yohimbine, an alpha 2-adrenergic antagonist, and mimicked by clonidine, an alpha 2-adrenergic agonist, in the majority of neurons. In few cases prazosin, an alpha 1-adrenergic antagonist, was able to antagonize weak inhibitions and phenylephrine, an alpha 1-adrenergic agonist, to evoke an inhibitory effect blocked by prazosin. Isoproterenol, a beta-adrenergic agonist was totally ineffective on the neuronal firing rate. It is concluded that noradrenaline can modify the level of neuronal activity in the vestibular complex by acting mostly, but not exclusively, through alpha 2-adrenergic receptors. An influence of noradrenergic systems on the vestibular function by a direct action of noradrenaline inside the vestibular nuclei is proposed.

Modulation of AMPA receptor‐mediated ion current by pituitary adenylate cyclase‐activating polypeptide (PACAP) in CA1 pyramidal neurons from rat hippocampus

Pituitary adenylate cyclase‐activating polypeptide (PACAP), a neurotrophic and neuromodulatory peptide, was recently shown to enhance NMDA receptor‐mediated currents in the hippocampus (Macdonald, et al. 2005. J Neurosci 25:11374–11384). To check if PACAP might also modulate AMPA receptor function, we tested its effects on AMPA receptor‐mediated synaptic currents on CA1 pyramidal neurons, using the patch clamp technique on hippocampal slices. In the presence of the NMDA antagonist D‐AP5, PACAP (10 nM) reduced the amplitude of excitatory postsynaptic currents (EPSCs) evoked in CA1 pyramidal neurons by stimulation of Schaffer collaterals. Following a paired‐pulse stimulation protocol, the paired‐pulse ratio was unaffected in most neurons, suggesting that the AMPA‐mediated EPSC was modulated by PACAP mainly at a postsynaptic level. PACAP also modulated the currents induced on CA1 pyramidal neurons by applications of either glutamate or AMPA. The effects of PACAP were dose‐dependent: at a 0.5 nM dose, PACAP increased AMPA‐mediated current; such effect was blocked by PACAP 6–38, a selective antagonist of PAC1 receptors. The enhancement of AMPA‐mediated current by PACAP 0.5 nM was abolished when cAMPS‐Rp, a PKA inhibitor, was added to the intracellular solution. At a 10 nM concentration, PACAP reduced AMPA‐mediated current; such effect was not blocked by PACAP 6–38. The inhibitory effect of 10 nM PACAP was mimicked by Bay 55–9837 (a selective agonist of VPAC2 receptors), persisted in the presence of intracellular BAPTA and was abolished by intracellular cAMPS‐Rp. Stimulation‐evoked EPSCs in CA1 neurons were significantly reduced following application of the PAC1 antagonist PACAP 6–38; this result indicates that PAC1 receptors in the CA1 region are tonically activated by endogenous PACAP and enhance CA3‐CA1 synaptic transmission. Our results show that PACAP differentially modulates AMPA receptor‐mediated current in CA1 pyramidal neurons by activation of PAC1 and VPAC2 receptors, both involving the cAMP/PKA pathway; the functional significance will be discussed in light of the multiple effects exerted by PACAP on the CA3‐CA1 synapse at different levels.

5-Hydroxytryptamine modifies neuronal responses to glutamate in the red nucleus of the rat

Abstract The effects of 5-hydroxytryptamine (5-HT) on the responses of red nucleus (RN) neurones to glutamate (glu) and its agonists were studied using a microiontophoretic technique in anaesthetised rats. Extracellular unitary recordings of RN neuronal activity showed that 5-HT application induced a significant and reversible depression of glu-evoked excitations in 85% of the RN units tested. This effect was independent of the action of the amine on background firing, which appeared enhanced in the majority of cases but was either depressed or uninfluenced in other cases. Microiontophoretic 5-HT also depressed the excitatory responses evoked in RN neurones by electrical stimulation of sensorimotor cortex. Methysergide application, which prevented the enhancing effects of 5-HT on the background firing, was scarcely effective in antagonising the depression of glu responses. In contrast, the serotonergic effects on the glu responses were reduced by the iontophoretically applied antagonist of 5-HT1A receptors, NAN-190. Microiontophoretic 5-HT was also able to influence the neuronal responses evoked by glu agonists quisqualate (quis) and N-methyl-d-aspartate (NMDA), acting on non-NMDA and NMDA receptors respectively. In fact 5-HT depressed quis-evoked excitations and induced mixed effects on NMDA responses, which were reduced in 45%, enhanced in 34% and unmodified in 21% of the units tested. These results suggest that 5-HT is able to modulate the motor glutamatergic input to RN by acting mostly on non-NMDA receptors. The modulation of non-NMDA and NMDA receptors by 5-HT in the RN appears significant and its functional meaning is discussed.

Motor responses evoked by microstimulation of restiform body in the cat

SummaryMotor effects produced by microstimulation of restiform body (RB) were studied in acute unanesthetized cats, using tungsten electrodes for stimulating the peduncle and bipolar steel electrodes for recording muscular activity (EMG). The main results were the following.1.Threshold microstimulation (18.24 μA±8.77 S.D.) of effective foci within RB elicited single muscle contractions of ipsilateral limbs, primarily of forelimb; overthreshold activation (32.83 μA±9.25 S.D.) of the same points produced complex movements in 61.54% of cases that involved muscles of shoulder, neck, and trunk.2.Single muscle contractions exhibited a mean latency (20.09 msec ± 2.04 S.D.) which was significantly longer than that shown by complex movements (10.00 msec±3.10 S.D.). Furthermore, a decrease in frequency of stimulating train below 300 Hz and a reduction in duration below 30 msec caused a steep rise of threshold for single muscle responses that was not observed when studying complex movements.3.Acute RB interruption between stimulating electrode and cerebellum abolished single muscle contractions; conversely, complex movements remained unmodified even when the RB was lesioned in cats chronically submitted to interruption of brachium conjunctivum (BC).4.The pathway involved in promoting RB induced single muscle activation includes interpositus nucleus, BC and rubrospinal tract. Possible modalities of RB afferent participation to the motor control are briefly discussed.

Influence of Serotonin on the Glutamate-Induced Excitations of Secondary Vestibular Neurons in the Rat

The excitatory responses evoked by glutamate and its agonists in secondary vestibular neurons of the rat were studied during microiontophoretic application of 5-hydroxytryptamine (5-HT). Ejection of 5-HT modified neuronal responsiveness to glutamate in 86% of the studied units, the effect being a depression of the excitatory responses in two-thirds of cases and an enhancement in the remaining third. 5-HT was also effective in modifying 94% of the responses evoked by N-methyl-d-aspartate (NMDA), inducing a depressive effect in 76% of cases and an enhancement in the remaining ones. Quisqualate-evoked effects were depressed and enhanced by 5-HT in about the same number of cases; in contrast, kainate-evoked responses were enhanced. The depressive action of 5-HT was mimicked by application of alpha-methyl-5-hydroxytryptamine (alpha-Me-5-HT), a 5-HT(2) receptor agonist, whereas the enhancing effect could be evoked by application of 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT(1A) receptor agonist. The 5-HT(2) receptor antagonist ketanserin was able to reduce, but not to block totally, the depressive action of 5-HT on glutamate- or NMDA-evoked responses. No significant difference was detected between neuronal responses in the lateral and the superior vestibular nucleus. These results indicate that 5-HT is able to modulate the responsiveness of secondary vestibular neurons to excitatory amino acids. Its action is mostly depressive, involves 5-HT(2) receptors, and is exerted on NMDA receptors. A minor involvement of other 5-HT receptors (at least 5-HT(1A)) and other glutamate receptors (for quisqualate and kainate) in the modulatory action of 5-HT is plausible.

Neurotransmitter-Mediated Control of Neuronal Firing in the Red Nucleus of the Rat: Reciprocal Modulation between Noradrenaline and GABA

The electrical activity of neurons from the red nucleus, a mesencephalic structure involved in motor control, is under the influence of several neurotransmitters released from afferent fibers and/or from local interneurons. We have investigated the combined effects of gamma-aminobutyric acid (GABA) and noradrenaline (NA), both present at high levels in the red nucleus, on the firing activity of single rubral neurons recorded extracellularly in vivo on anesthetized adult rats. NA inhibited the firing activity of a large part of rubral neurons and induced excitatory or biphasic inhibitory/excitatory effects in a smaller group of cells. Neuronal firing was also inhibited by GABA in all the cells studied. When the effect of GABA was tested during continuous applications of NA, the magnitude of GABA response was modified in 58% of the cells: the effect of GABA was potentiated by NA in half of the responding neurons and was decreased in the remaining half. NA-induced potentiation of GABA response was mimicked by the alpha(2)-adrenoceptor agonist clonidine and was abolished by the alpha(2)-adrenoceptor antagonist yohimbine. On the other side, the decrease of GABA response was reproduced by the beta-adrenoceptor agonist isoprenaline and was blocked by timolol, an antagonist of beta-adrenoceptors. Neuronal firing activity was reduced by nipecotic acid, an inhibitor of GABA reuptake mechanism, and was instead increased during application of the GABA(A) receptor antagonist bicuculline, suggesting that rubral neurons in vivo were under tonic control by endogenous GABA. Both the inhibitory and the excitatory effects of NA were reduced in the presence of nipecotic acid and were instead potentiated during application of bicuculline, suggesting that NA responses were modified by endogenous GABA. Taken together, our results indicate a reciprocal modulation between the effects of GABA and NA on neuronal firing activity in the red nucleus of the rat: GABA depresses the responsiveness of rubral neurons to NA, whereas NA is able either to potentiate or to decrease the effects of GABA by activation of alpha(2)- and beta-adrenoceptors, respectively. The functional significance of such interaction, as well as the possible implication in diseases affecting motor control, will be discussed.

Effects of motor cortex and single muscle stimulation on neurons of the lateral vestibular nucleus in the rat

The neuronal responses to stimulation of motor cortical sites and of forelimb single muscles were studied in the lateral vestibular nucleus of anaesthetized rats. Of the 228 neurons tested for response to stimulation of contralateral motor cortex, 63% responded to cortical sites controlling extensor muscles and 30% to those controlling flexors. The corresponding figures for responders to ipsilateral stimulation were 34 and 21%. Vestibulospinal units responded to cortical sites controlling extensor and flexor muscles whereas the remaining lateral vestibular nucleus neurons, very reactive to cortical sites controlling extensor muscles, responded little to contralateral and not at all to ipsilateral cortical sites controlling flexor muscles. The effects evoked by contralateral cortical sites controlling extensors varied, those induced by cortical sites controlling flexors were inhibitory in 77% of cases. The responses to ipsilateral motor cortex stimulation differed not so much by cortical sites controlling extensor or flexor muscles as by whether the neuron was in the dorsal or ventral zone of the lateral vestibular nucleus: mixed in the former, all inhibitory in the latter. Of the lateral vestibular nucleus units tested for response to stimulation of ipsilateral or contralateral forelimb distal muscles, only 11% responded. All the vestibulospinal units responsive to muscle stimulation lay in the dorsal zone of the nucleus. The remainder, dorsal or ventral, were not responsive to contralateral muscles. Single lateral vestibular nucleus cells influenced both by ipsilateral muscle and by contralateral motor cortex made up 24% of the pool, vestibulospinal and non-vestibulospinal. They fell into three groups: responsive to one or both structures but responding more strongly to combined stimulation; responsive to each of the two structures but showing a response to combined stimulation not significantly different from that evoked by the cortex alone; responsive only to combined stimulation. The lateral vestibular nucleus units included in these three groups accounted for 29% of those tested for response to extensor muscles and cortical sites controlling extensors and 15% of those tested for response to flexor muscles and cortical sites controlling flexors. Twenty-five per cent of the vestibulospinal neurons responded both to contralateral muscles and to ipsilateral motor cortex stimulation but none of the non-vestibulospinal neurons responded to both. All the responders to both were in the dorsal zone of the lateral vestibular nucleus and responded to extensor stimuli, always in the same way. These results indicate that motor cortex output exerts a major influence on lateral vestibular nucleus discharges, while the muscle afferents have a modulatory influence on the lateral vestibular nucleus responses to cortex.(ABSTRACT TRUNCATED AT 400 WORDS)