Non-reciprocal postsynaptic inhibition of digastric 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.

Role of the parvicellular reticular formation in facilitating the jaw-opening reflex in rats by stimulation of the red nucleus

In a previous study, we have shown that electrical and chemical stimulation of the red nucleus (RN) facilitates the jaw-opening reflex (JOR). The RN sends projection fibers bilaterally, with contralateral dominance, to the part of the parvicellular reticular formation (RFp) containing premotor neurons projecting to the trigeminal motor nucleus. This implies that RN-induced facilitation of the JOR might be mediated via last-order neurons in the RFp. Here, we address this issue by investigating whether microinjection of lidocaine or l-glutamate into the RFp affects RN-induced modulation of the JOR. Experiments were performed on rats anesthetized with urethane-chloralose. The JOR was evoked by electrical stimulation of the inferior alveolar (IA) nerve and was recorded as an electromyographic response from the anterior belly of the digastric muscle. Conditioning stimulation was delivered unilaterally to the RN 12 ms before the IA test stimulation. We found that local injections of 2% lidocaine (0.5 microl) into the RFp, contralateral to the RN, significantly (P < 0.05) reduce RN-induced facilitation of the JOR, whereas corresponding injections of 0.1 mM l-glutamate (0.5 microl) significantly (P < 0.05) increase it. These results suggest that the facilitatory effect of RN stimulation on the JOR is mediated partly by a relay in the RFp.

The unique inhibitory potentials in motoneurons that occur during active sleep are comprised of minimal unitary potentials

Loss of muscle tone during active (rapid-eye-movement, REM) sleep is due to the inhibition of motoneurons. This inhibition is manifest in high-gain intracellular electrophysiological records as hyperpolarizing synaptic noise, which includes large amplitude active sleep-specific inhibitory postsynaptic potentials (IPSPs). We report here evidence that the large active sleep-specific IPSPs are comprised of a small number of minimal unitary potentials that are characterized by fast rise-times (10-90% rise-times < or = 0.75 ms); they are present in high-gain records during quiet sleep or during active sleep where they are intermingled with larger active sleep-specific IPSPs with 10-90% rise-times > or = 1.00 ms and amplitudes that are integer multiples of the minimal unitary potentials. In hypoglossal motoneurons, the amplitude of these minimal unitary potentials averaged 0.33 +/- 0.04 mV (mean +/- S.D., n = 6). It is concluded that the large IPSPs with slow rise-times that are observed in motoneurons during active sleep are due to the nearly simultaneous arrival of multiple (< or = 5) minimal unitary potentials. We hypothesize that the same inhibitory interneurons that produce small IPSPs with fast rise-times during quiet sleep are also responsible for the large amplitude active sleep-specific IPSPs.

c-fos expression in brainstem premotor interneurons during cholinergically induced active sleep in the cat

The present study was undertaken to identify trigeminal premotor interneurons that become activated during carbachol-induced active sleep (c-AS). Their identification is a critical step in determining the neural circuits responsible for the atonia of active sleep. Accordingly, the retrograde tracer cholera toxin subunit B (CTb) was injected into the trigeminal motor nuclei complex to label trigeminal interneurons. To identify retrograde-labeled activated neurons, immunocytochemical techniques, designed to label the Fos protein, were used. Double-labeled (i.e., CTb(+), Fos(+)) neurons were found exclusively in the ventral portion of the medullary reticular formation, medial to the facial motor nucleus and lateral to the inferior olive. This region, which encompasses the ventral portion of the nucleus reticularis gigantocellularis and the nucleus magnocellularis, corresponds to the rostral portion of the classic inhibitory region of. This region contained a mean of 606 +/- 41.5 ipsilateral and 90 +/- 32.0 contralateral, CTb-labeled neurons. These cells were of medium-size with an average soma diameter of 20-35 micrometer. Approximately 55% of the retrogradely labeled cells expressed c-fos during a prolonged episode of c-AS. We propose that these neurons are the interneurons responsible for the nonreciprocal postsynaptic inhibition of trigeminal motoneurons that occurs during active sleep.

The control of jaw-opener motoneurons during active sleep

Brainstem and spinal cord motoneurons that innervate somatic musculature serving antigravity functions are postsynaptically inhibited during active sleep. However, it has been reported that hypoglossal motoneurons (which do not innervate antigravity muscles) are not postsynaptically inhibited during active sleep, but are disfacilitated. In the present report we describe changes, during active sleep, in the excitability and membrane potential of digastric and synergistic motoneurons of the trigeminal motor pool; these neurons do not perform antigravity functions. The experiments described in the present report were performed in chronic cats that were prepared for intracellular recording. The motoneurons hyperpolarized an average of 11 mV (S.D. +/- 1.29, n = 8, P < 0.005) during active sleep compared to quiet sleep. Hyperpolarization was accompanied by a reduction in the excitability of the somadendritic regions of the neurons, as indicated by an increase in the delay of propagation of antidromic spikes from the initial segment to the somadendritic portion of the cell. High gain membrane potential recordings from these motoneurons revealed the occurrence of a remarkably large number of hyperpolarizing potentials during active sleep. When K-chloride-filled microelectrodes were utilized and chloride ions were injected intracellularly, the polarity of these potentials was reversed. During phasic episodes of active sleep, there was a clear increase in hyperpolarizing potential activity, a blockade of somadendritic spikes and phasic reductions in the amplitude of the initial segment spikes. Hyperpolarizing potentials occurred in conjunction with ponto-geniculo-occipital waves.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibitory commissural connections of neurones in the trigeminal motor nucleus of the rat

Physiological evidence is presented for the existence of commissural fibres that cross the midsagittal plane in the medulla of the rat at the level of the trigeminal motor nucleus (Mo5). These fibres, which have their origin in the Mo5, terminated in the contralateral Mo5. Small inhibitory postsynaptic potentials were recorded in jaw-closing motoneurones by electrical activation of the commissural fibres; jaw-opening and fusimotor neurones as well as the jaw-closing and jaw-opening reflex were not affected. Electromyographic recordings from jaw-closing and jaw-opening muscles in the unrestrained rat showed that masseter activity was inhibited by the commissural fibres. These trigeminal commissural connections might play a part in the co-ordination of bilateral activity of the jaw-closing musculature during unilateral chewing.

Synaptic potentials produced in jaw-closer and jaw-opener motoneurons by palatal stimulation

Excitation and inhibition of temporal and digastric motoneurons (Temp. and Dig. Mns) during transient jaw closing, the so-called jaw-closing reflex, were studied in cats. Application of diffuse pressure stimulation to the posterior palatal surface produced the jaw-closing reflex and it was found that mechanosensory inputs from the posterior palatal mucosa produce depolarizing potentials on the Temp. Mns responsible for jaw closure during the jaw-closing reflex. We have demonstrated that in one-third of 27 explored Temp. Mns the initial bursts of spikes were elicited before the onset of jaw closure, suggesting that these cells contribute to initiate jaw closure during the jaw-closing reflex. The remaining cells probably contributed to maintain the occlusal phase. Furthermore, it was found that mechanosensory inputs from the posterior palatal mucosa produce a hyperpolarization-depolarization sequence in the Dig. Mns responsible for the jaw-closing reflex. In addition, when pressure stimulation was applied to the anterior palatal mucosa, sustained jaw opening was elicited and an increase of firing frequency of Dig. Mns occurred 40 ms before the onset of jaw opening and continued for 80 ms.

Strychnine blockade of the non-reciprocal inhibition of trigeminal motoneurons induced by stimulation of the parvocellular reticular formation

Stimulation of a region within the parvocellular medullary reticular formation (PcRF) that contains somas of premotor interneurons produces short latency inhibitory synaptic potentials (IPSPs) in cat trigeminal motoneurons. The present study was undertaken to determine whether glycinergic synapses are responsible for these IPSPs. The intravenous administration of strychnine, an established glycine antagonist, abolished these PcRF-IPSPs. This effect appears to be specific for glycinergic inhibitory synapses because the short lasting component of the IPSP produced by inferior alveolar nerve (IAN) stimulation was also abolished, whereas, in contrast, the long lasting non-glycinergic component of this IPSP was not suppressed. These results indicate that a glycinergic system in the reticular formation is responsible for the non-reciprocal postsynaptic inhibition of trigeminal motoneurons.

A medullary inhibitory region for trigeminal motoneurons in the cat

The present report describes the effects on trigeminal motoneurons of stimulation of a circumscribed site within the parvocellular region of the medullary reticular formation. This medullary site was selected because anatomical studies have shown that premotor interneurons project from this site to the trigeminal motorpool. Electrical stimulation of this site induced IPSPs (PcRF-IPSPs) in jaw-closer motoneurons. A population of these IPSPs, recorded contralateral to the site of stimulation, exhibited latencies shorter than 1.5 ms (mean 1.16 +/- 0.08 SD). Their mean amplitude was 1.72 mV +/- 1.13 SD and their mean duration was 3.52 ms +/- 2.15 SD. We believe that these PcRF-IPSPs arose as the result of activation of a monosynaptic pathway. A comparable inhibitory input from this site to ipsilateral jaw-closer motoneurons and to both contra and ipsilateral digastric motoneurons was also observed. We therefore conclude that this medullary PcRF site contains premotor interneurons that are capable of postsynaptically inhibiting motoneurons that innervate antagonistic jaw muscles.