Differential short-term changes in GABAergic or glycinergic synaptic efficacy on rat hypoglossal motoneurons

Serotoninergic control of glycinergic inhibitory postsynaptic currents in rat hypoglossal motoneurons

This report presents the results of a study of the frequency potentiation of inhibitory postsynaptic currents (IPSCs) in hypoglossal motoneurons and its modulation by serotonin. A release-site model of synaptic plasticity was used to characterize the frequency-related potentiation of evoked IPSCs. Data were obtained to determine if the frequency potentiation of IPSCs occurs as a consequence of a low baseline quantal content of evoked IPSCs using whole cell patch-clamp recordings from hypoglossal motoneurons in the neonatal rat brainstem slice preparation. In these motoneurons, EPSCs and GABAergic IPSCs were blocked by the application of CNQX, AP-5 and bicuculline. Glycinergic IPSCs were evoked by threshold stimulation of inhibitory neurons in the nucleus of Roller, which is located ventro-lateral to the hypoglossal nucleus. IPSC responses to trains of stimuli were recorded in control solutions and in solutions containing serotonin, which is known to reduce IPSPs in this preparation. The amplitude of non-potentiated IPSCs was reduced and their frequency potentiation was enhanced when serotonin was added to the bath. These data were examined using a release-site model of synaptic plasticity in which facilitation is attributed to a time-dependent increase in the probability of transmitter release; depression is attributed to a time-dependent decrease in the number of sites available for release. Using this model, the effect of serotonin on frequency potentiation was explained by a combination of a reduction in the baseline probability of transmitter release and an increase in the time constant of decay of the increase in probability of release that follows a stimulus.

Frequency-dependent modulation of glycine receptor activation recorded from the zebrafish larvae hindbrain

In vertebrates, most glycinergic inhibitory neurons discharge phasically at a relatively low frequency. Such a pattern of glycine liberation from presynaptic terminals may affect the kinetics of post-synaptic glycine receptors. To examine this influence, we have analyzed the behavior of glycine receptors in response to repetitive stimulation at frequencies at which consecutive outside-out currents did not superimpose (0.5-4 Hz). Neurotransmitter release was mimicked on outside-out patches from zebrafish hindbrain Mauthner cells using fast flow application techniques. The amplitude of outside-out currents evoked by short (1 ms) repetitive applications of a saturating concentration (3 mM) of glycine remained unchanged for application frequencies<or=1 Hz. When the application frequency was increased from 1 to 4 Hz, the amplitude of the outside-out currents decreased with time to reach a steady state level. This decrease in current amplitude was larger and occurred faster with increasing application frequencies. Recovery occurred when the stimulation frequency was decreased back to 1 Hz. The recovery time constant was independent on the application frequency. This frequency-dependent inhibition was also observed for non-saturating glycine concentrations. Our results indicate that glycine receptor activity is down-regulated when the stimulation frequency increases to values>1 Hz. Glycine-evoked current simulations using a simple Markov model describing zebrafish glycine receptor kinetic behavior, indicates that this down-regulation of glycine receptor efficacy is due to a progressive accumulation of the receptors in a long lasting desensitization state. Our simulations suggest that this down-regulation can occur even when spontaneous inhibitory currents were generated randomly at a frequency>1 Hz.

Vasopressin facilitates glycinergic and GABAergic synaptic transmission in developing hypoglossal motoneurons

The hypoglossal nucleus of young rats contains vasopressin binding sites and vasopressin can directly excite hypoglossal motoneurons. In addition, indirect evidence suggests that vasopressin can enhance the synaptic input to motoneurons. We have characterized this latter effect by using brainstem slices and whole-cell recordings. We found that, in the presence of blockers of fast glutamatergic transmission, vasopressin strongly facilitated inhibitory synaptic activity. On average, vasopressin caused a six-fold increase in the frequency and a 1.5-fold increase in the amplitude of GABAergic postsynaptic currents. The effect of vasopressin on glycinergic postsynaptic currents was similar in magnitude. Vasopressin did not affect the frequency of GABAergic or glycinergic miniature postsynaptic currents, indicating that the peptide-induced facilitation of inhibitory transmission was mediated by receptors located on the somatodendritic region rather than on axon terminals of presynaptic neurons. The pharmacological profile of these receptors was determined by using d[Cha4]AVP and dVDAVP, selective agonists of V1b and V2 vasopressin receptors, respectively, and Phaa-D-Tyr-(Et)-Phe-Gln-Pro-Arg-Arg-NH2, a selective antagonist of V1a vasopressin receptors. The two agonists had no effect on the frequency of inhibitory postsynaptic currents. By contrast, the antagonist suppressed the vasopressin-induced facilitation of these currents, indicating that the receptors involved were exclusively of the V1a type. Thus, vasopressin exerts a dual action on hypoglossal motoneurons: a direct excitatory action and an indirect action mediated by GABAergic and glycinergic synapses. By virtue of this dual effect, vasopressin could alter the input-output properties of these motoneurons. Alternatively, it could play a role in generating or modulating specific motor patterns.

Pre- and postsynaptic modulation of glycinergic and gabaergic transmission by muscarinic receptors on rat hypoglossal motoneurons in vitro

The motor output of hypoglossal motoneurons to tongue muscles takes place in concert with the respiratory rhythm and is determined by the balance between excitatory glutamatergic transmission and inhibitory transmission mediated by glycine or GABA. The relative contribution by these transmitters is a phasic phenomenon modulated by other transmitters. We examined how metabotropic muscarinic receptors, widely expressed in the brainstem where they excite cranial motor nuclei, might influence synaptic activity mediated by GABA or glycine. For this purpose, using thin slices of the neonatal rat brainstem, we recorded (under whole-cell patch clamp) glycinergic or GABAergic responses from visually identified hypoglossal motoneurons after pharmacological block of glutamatergic transmission. Muscarine inhibited spontaneous and electrically induced events mediated by GABA or glycine. The amplitude of glycinergic miniature inhibitory postsynaptic currents was slightly reduced by muscarine, while GABAergic miniature inhibitory postsynaptic currents were unaffected. Motoneuron currents induced by focally applied GABA and glycine were depressed by muscarine with stronger reduction in glycine-mediated responses. Histochemical observations indicated the presence of M1, M2 and M5 subtypes of muscarinic receptors in the neonatal hypoglossal nucleus. These results suggest that muscarine potently depressed inhibitory neurotransmission on brainstem motoneurons, and that this action was exerted via preterminal and extrasynaptic receptors. Since the large reduction in inhibitory neurotransmission may contribute to overall excitation of brainstem motoneurons by muscarinic receptors, these data might help to understand the central components of action of antimuscarinic agents in preanesthetic medication or against motion sickness.

Metabotropic glutamate receptor activity induces a novel oscillatory pattern in neonatal rat hypoglossal motoneurones

Tongue muscles innervated by the hypoglossal nerves play a crucial role to ensure airway patency and milk suckling in the neonate. Using a slice preparation of the neonatal rat brain, we investigated the electrophysiological characteristics of hypoglossal motoneurones in the attempt to identify certain properties potentially capable of synchronizing motor commands to the tongue. Bath-applied DHPG, a selective agonist of group I metabotropic glutamate receptors (mGluRs), generated persistent, regular electrical oscillations (4-8 Hz) recorded from patch-clamped motoneurones. Under voltage clamp, oscillations were biphasic events, comprising large outward slow currents alternated with fast, repeated inward currents. Electrical oscillations had amplitude and period insensitive to cell membrane potential, and required intact glutamatergic transmission via AMPA receptors. Oscillations were mediated by subtype 1 receptors of group I mGluRs (mGluR1s), and were routinely observed during pharmacological block of glycinergic and GABAergic inhibition, although they could also be recorded in standard saline. Simultaneous recordings from pairs of motoneurones within the same hypoglossal nucleus demonstrated that oscillations were due to their strong electrical coupling and were blocked by the gap junction blocker carbenoxolone. Pacing of slow oscillations apparently depended on the operation of K(ATP) channels in view of the block by tolbutamide or glibenclamide. Under current clamp, oscillations generated more regular spike firing of motoneurones and facilitated glutamatergic excitatory inputs. These data suggest that neonatal motoneurones of the nucleus hypoglossus possess a formerly undisclosed ability to express synchronous electrical oscillations, unveiled by activation of mGluR1s.

Dynamic interactions of excitatory and inhibitory inputs in hypoglossal motoneurones: respiratory phasing and modulation by PKA

The balance of excitation and inhibition converging upon a neurone is a principal determinant of neuronal output. We investigated the role of inhibition in shaping and gating inspiratory drive to hypoglossal (XII) motoneuronal activity. In neonatal rat medullary slices that generate a spontaneous respiratory rhythm, patch-clamp recordings were made from XII motoneurones, which were divided into three populations according to their inhibitory inputs: non-inhibited, inspiratory-inhibited and late-inspiratory-inhibited. In late-inspiratory-inhibited motoneurones, blockade of GABA(A) receptors with bicuculline abolished inspiratory-phased inhibition and increased the duration of inspiratory drive currents. In inspiratory-inhibited motoneurones, bicuculline abolished phasic inhibition, which frequently revealed excitatory inspiratory drive currents. In non-inhibited motoneurones, neither bicuculline nor strychnine markedly changed inspiratory drive currents. Inhibitory currents in XII motoneurones were potentiated by protein kinase A (PKA) activity. Intracellular dialysis of the catalytic subunit of PKA or bath application of the PKA activator Sp-cAMP significantly increased the amplitude of expiratory-phased IPSCs without any change in IPSP frequency. Inspiratory-phased inhibition in inspiratory-inhibited motoneurones was potentiated by Sp-cAMP. We conclude that inspiratory-phased inhibition is prevalent in neonatal XII motoneurones and plays an important role in shaping motoneuronal output. These inhibitory inputs are modulated by PKA, which also modulates excitatory inputs.

Suppression of genioglossus muscle tone and activity during reflex hypercapnic stimulation by GABA(A) mechanisms at the hypoglossal motor nucleus in vivo

The genioglossus muscle is involved in the maintenance of an open airway for effective breathing. Inhibitory neurotransmitters may be responsible for the major suppression of hypoglossal motor output to genioglossus muscle that occurs in certain behaviours such as rapid-eye-movement sleep. There is evidence for GABA(A) receptor-mediated inhibition of hypoglossal motoneurons in vitro. However, comparable studies have not been performed in vivo and the interactions of such mechanisms with integrative reflex respiratory control have also not been determined. Urethane-anaesthetised, tracheotomized and vagotomized rats were studied whilst diaphragm and genioglossus muscle activities, blood pressure and the electroencephalogram were recorded. Microdialysis probes were implanted into the hypoglossal motor nucleus, with sites verified by histology. Genioglossus responses to microdialysis perfusion of muscimol (GABA(A) agonist: 0, 0.1, 1 and 10 microM in artificial cerebrospinal fluid) were recorded at inspired CO(2)s of 0, 5 and 7.5% in six rats. Responses to bicuculline (GABA(A) antagonist, 0, 1, 10, 100 and 1000 microM) were also studied in six rats with and without CO(2) stimulation. Genioglossus activity decreased with muscimol (P<0.0001), with major suppression at 1 and 10 microM during air breathing (decreases=70.2% and 92.8%, P<0.005). Genioglossus activity increased with CO(2) (P=0.003), but genioglossus activation with 5 and 7.5% CO(2) were almost abolished with 10-microM muscimol. Responses were specific to genioglossus muscle as there were no changes in diaphragm, respiratory rate or blood pressure with muscimol (P>0.144). Antagonism of GABA(A) receptors increased genioglossus activity (P<0.001). These results show that GABA(A) receptor stimulation at the hypoglossal motor nucleus suppresses both genioglossus muscle tone and activity in the presence of reflex stimulation produced by hypercapnia. Recruitment of such mechanisms may contribute to the major suppression of genioglossus activity observed with and without CO(2) stimulation in behaviours such as rapid-eye-movement sleep.

Presence of functional vasopressin receptors in spinal ventral horn neurons of young rats: a morphological and electrophysiological study

The objective of the present work was double. (i) Light microscopic autoradiography was used to determine the distribution of vasopressin and oxytocin binding sites in the spinal cord of rats. (ii) Whole-cell recordings were performed in lumbar spinal cord slices in order to assess whether these receptors are functional, whether they are located pre- or postsynaptically and whether they are present in motoneurons. In newborns, vasopressin binding sites of the V1a type were present in all laminae of the central gray at all segmental levels, whereas oxytocin binding sites were found only in the superficial layers of the dorsal horn. In adults, binding sites for both neuropeptides were also present, but were less dense. The dissociation constants for vasopressin were similar in newborns and adults. Whole-cell recordings showed that in identified motoneurons vasopressin exerted a direct effect, by inducing a membrane depolarization or by generating a sustained inward current, and an indirect effect, by enhancing glycinergic and GABAergic inhibitory transmission. Vasopressin-induced facilitation of inhibitory transmission could also be demonstrated in unidentified ventral horn neurons. All these effects were mediated by V1a but not V1b receptors. In some neurons, glycinergic transmission was also facilitated by a selective oxytocin receptor agonist. Our data, together with data obtained previously in brainstem motor nuclei, suggest that vasopressin of hypothalamic origin could play a role in motricity. The neuropeptide could act as a neuromodulator, because it would not directly activate motoneurons, but rather render them more responsive to incoming excitatory inputs. Vasopressin may thus act as a regulator of muscular force.

GABAA receptor antagonism at the hypoglossal motor nucleus increases genioglossus muscle activity in NREM but not REM sleep

The pharyngeal muscles, such as the genioglossus (GG) muscle of the tongue, are important for effective lung ventilation since they maintain an open airspace. Rapid-eye-movement (REM) sleep, however, recruits powerful neural mechanisms that can abolish GG activity, even during strong reflex respiratory stimulation by elevated CO2. In vitro studies have demonstrated the presence of GABAA receptors on hypoglossal motoneurons, and these and other data have led to the speculation that GABAA mechanisms may contribute to the suppression of hypoglossal motor outflow to the GG muscle in REM sleep. We have developed an animal model that allows us to chronically manipulate neurotransmission at the hypoglossal motor nucleus using microdialysis across natural sleep-wake states in rats. The present study tests the hypothesis that microdialysis perfusion of the GABAA receptor antagonist bicuculline into the hypoglossal motor nucleus will prevent the suppression of GG muscle activity in REM sleep during both room-air and CO2-stimulated breathing. Ten rats were implanted with electroencephalogram and neck muscle electrodes to record sleep-wake states, and GG and diaphragm electrodes for respiratory muscle recording. Microdialysis probes were implanted into the hypoglossal motor nucleus for perfusion of artificial cerebrospinal fluid (ACSF) or 100 microM bicuculline during room-air and CO2-stimulated breathing (7 % inspired CO2). GABAA receptor antagonism at the hypoglossal motor nucleus increased respiratory-related GG activity during both room-air (P = 0.01) and CO2-stimulated breathing (P = 0.007), indicating a background inhibitory GABA tone. However, the effects of bicuculline on GG activity depended on the prevailing sleep-wake state (P < 0.005), with bicuculline increasing GG activity in non-REM (NREM) sleep and wakefulness both in room air and hypercapnia (P < 0.01), but GG activity was effectively abolished in those REM periods without phasic twitches in the GG muscle. This abolition of GG activity in REM sleep occurred regardless of ACSF or bicuculline at the hypoglossal motor nucleus, or room-air or CO2-stimulated breathing (P > 0.63). We conclude that these data in freely behaving rats confirm previous in vitro studies that GABAA receptor mechanisms are present at the hypoglossal motor nucleus and are tonically active, but the data also show that GABAA receptor antagonism at the hypoglossal motor nucleus does not increase GG muscle activity in natural REM sleep.

Glycine at hypoglossal motor nucleus: genioglossus activity, CO(2) responses, and the additive effects of GABA

There is evidence for glycine and GABA(A)-receptor-mediated inhibition of hypoglossal motoneurons in vitro. However, comparable studies have not been performed in vivo, and the interactions of such mechanisms with integrative reflex respiratory control have also not been determined. This study tests the hypotheses that glycine at the hypoglossal motor nucleus (HMN) will suppress genioglossus (GG) muscle activity, even in the presence of hypercapnic respiratory stimulation, and the effects of glycine will be blocked by strychnine. We also determined whether coapplication of glycine and muscimol (GABA(A)- receptor agonist) to the HMN is additive in suppressing GG activity. Twenty-four urethane-anesthetized, tracheotomized, and vagotomized rats were studied. Diaphragm and GG activities, the electroencephalogram, and blood pressure were recorded. Microdialysis probes were implanted into the HMN for delivery of artificial cerebrospinal fluid (control), glycine (0.0001-10 mM), or muscimol (0.1 microM). Increasing glycine at the HMN produced graded suppression of GG activity (P < 0.001), although the GG still responded to stimulation with 7% inspired CO(2) (P = 0.002). Strychnine (0.1 mM) reversed the glycine-mediated suppression of GG activity, whereas combined glycine and muscimol were additive in GG muscle suppression. It remains to be determined whether the recruitment of such glycine and GABA mechanisms explains the periods of major GG suppression in behaviors such as rapid eye movement sleep.

Inhibition of spinal or hypoglossal motoneurons of the newborn rat by glycine or GABA

The function of GABA or glycine during early postnatal development remains controversial as their action is reported as either excitatory or inhibitory. The present study addressed the question of the functional role of GABA or glycine on rat motoneurons shortly after birth. For this purpose, using in vitro preparations from immature rats (postnatal age, P0-P4 days), we recorded from lumbar spinal motoneurons and hypoglossal motoneurons. All data were obtained under current clamp conditions (recording with potassium methylsulphate containing electrodes) from cells at about -70 mV resting potential. On spinal motoneurons we used the glycinergic and GABAergic recurrent postsysnaptic potential (PSP) mediated by Renshaw cells to assess its impact on excitatory synaptic inputs from dorsal afferent fibres. Despite its depolarizing nature, the recurrent PSP consistently inhibited synaptic excitation of lumbar motoneurons. On hypoglossal motoneurons, exogenously applied GABA or glycine produced depolarization with decreased input resistance. This response was always associated with inhibition of cell firing induced by intracellular current pulses. Even when the membrane potential was repolarized to resting level in the presence of GABA or glycine, hypoglossal motoneurons failed to generate spikes. Conversely, similar depolarization produced by glutamate consistently facilitated spike firing. GABAergic and glycinergic synaptic potentials evoked by focal stimulation of the reticular formation inhibited firing and/or increased firing latency in the majority of hypoglossal motoneurons. These results indicate that, immediately after birth, rat motoneurons were inhibited by synaptically released or exogenously applied GABA or glycine.