Central effects of 5-HT on respiratory rhythm in newborn rats in vivo

The role of 5-HT in inducing apnoeas (a major element in sudden infant death syndrome) is controversial because while 5-HT is a respiratory depressant in vivo, it evokes respiratory analeptic effects when applied to the isolated brainstem of the newborn rat. In decerebrate newborn rats, the electrical activity of the diaphragm and that of a hypoglossally innervated tongue muscle, as well as the cardiac frequency (Fc), were recorded before and after the application of 5-HT and related agents to the floor of the IVth ventricle. To assess the spontaneous variability in inspiratory frequency (Fi) and Fc, a sham group was studied. A decrease in Fi was observed in response to 5-HT. This respiratory depressant effect was associated with an activation of the tongue muscle, but there was no change in Fc. Application of agonists elicited a small increase in Fi linked to activation of 5-HT1A receptors, and decreases in both Fi and the activity of the tongue muscle resulting predominantly from activation of 5-HT2 receptors. The decrease in Fi was much smaller in newborn rats than that reported in newborn kittens. Indeed, in newborn rats, we did not observe long-lasting apnoeas. Our results differ from those obtained from the newborn rat in vitro, inasmuch as in vivo 5-HT essentially depressed the respiratory rhythm generator. The role of the afferent system appears to be crucial in modulating the action of 5-HT.

Premotor input to hypoglossal motoneurons from Kölliker-Fuse neurons in decerebrate cats

Experiments were performed in 18 paralysed, ventilated, decerebrate adult cats to characterise projections from the Kölliker-Fuse nucleus(KFN) to hypoglossal(HG) motoneurons. Efferent neural activity was recorded from the medial branch of both HG nerves, right recurrent laryngeal(RL) nerve, and C3 branch of the left phrenic nerve. Electrical stimulation(1 Hz, 1 msec pulse duration) of discrete areas within the KFN coordinates elicited a preferential, predominantly ipsilateral burst of HG action potentials with an average stimulus-response latency of 8 msec. Tonic stimulation(5-20 Hz) frequently produced considerable HG temporal summation and the appearance of phasic inspiratory HG activity. Injection of 40-100 nl kainic acid(6.37 mM) into the rostral pontine site with the lowest electrical threshold for HG activation elicited a prolonged tonic HG activation. Following kainate injection, the electrical stimulation threshold for HG excitation increased. Pressure injections of 5-100 nl of 2 mM N-methyl-D-aspartic acid (NMDA)and 2 mM(+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide(AMPA) into the KFN were associated with activation and/or suppression of HG motor output. The results indicate that HG motoneurons innervating protrusor tongue muscles receive a selective projection from the KFN that can be activated by glutamate receptors on KF neurons.

Gamma-aminobutyric acid-induced responses in acutely dissociated neurons from the rat sacral dorsal commissural nucleus

The electrophysiological and pharmacological properties of GABA-activated Cl- currents (IGABA) were investigated in enzymatically dissociated rat sacral dorsal commissural nucleus (SDCN) neurons using the nystatin perforated patch recording configuration under voltage-clamp conditions. Exogenous application of GABA to SDCN neurons induced Cl- currents which increased in a concentration-dependent manner. Bicuculline (BIC) and strychnine (STR) antagonized the IGABA in a concentration-dependent manner. Zn2+ suppressed the IGABA with an IC50 of 2.8 X 10(-5) M. Muscimol mimicked the IGABA, while baclofen evoked no response. Pentobarbital (PB) and 5beta-pregnan-3alpha-ol-20-one (pregnanolone, PGN) also induced GABAA-mimic Cl- currents. Diazepam (DZP), PB and PGN all enhanced the IGABA by increasing the apparent affinity of the GABAA receptors to GABA. Moreover, spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) were observed in mechanically dissociated SDCN neurons attached with synaptic boutons, so called 'synaptic bouton preparation'. These results indicate that SDCN neurons express GABAA receptors with relatively low sensitivity to Zn2+ inhibition, and that GABA may have a functional role as an inhibitory transmitter in the SDCN regulating nociceptive, analgesic, and autonomic functions.

Voltage-activated K+ currents of hypoglossal motoneurons in a brain stem slice preparation from the neonatal rat

Whole cell, patch-clamp recordings were performed on motoneurons of the hypoglossus nucleus in a brain stem slice preparation from the neonatal rat brain. The aim was to investigate transient outward currents activated by membrane depolarization under voltage clamp conditions. In a Ca2+-free medium containing tetrodotoxin and Cs+, depolarizing voltage commands from a holding potential of -50 mV induced slow outward currents (Islow) with 34 +/- 6 ms (SE) onset time constant at 0 mV and minimal decline during a 1 s pulse depolarization. When the depolarizing command was preceded by a prepulse to -110 mV, the outward current became biphasic as it comprised a faster component (Ifast), which could be investigated in isolation by subtracting the two sets of records. Ifast showed rapid kinetics (9 +/- 4 ms 10-90% rise time and 70 +/- 20 ms decay time constant at 0 mV) and strong voltage-dependent inactivation (half inactivation was at -92.9 +/- 0.2 mV) from which it readily recovered with a biexponential timecourse (4.4 +/- 0.6 and 17 +/- 2 ms time constants at -110 mV membrane potential). Islow was selectively blocked by TEA (10-30 mM) while Ifast was preferentially depressed by 2-3 mM 4-aminopyridine. Analysis of tail current reversal indicated that both Islow and Ifast were predominantly due to K+ with minor permeability to Na+ (92/1 and 50/1, respectively). These results suggest that membrane depolarization activated distinct K+ conductances that, in view of their largely dissimilar kinetics, are likely to play a differential role in regulating the firing behavior of hypoglossal motoneurons.

Modulation of motoneuron N-methyl-D-aspartate receptors by the inhibitory neurotransmitter glycine

Previous studies in the central nervous system have shown that glycine is a co-agonist with glutamate at central N-methyl-D-aspartate receptors (NMDA-Rs). However, there is considerable controversy as to whether the glycine site on NMDA-Rs is saturated. If this site were not saturated then glycine released from glycinergic synaptic terminals might 'spill-over' and activate NMDA-Rs. Since motoneurons have both NMDA and glycine synapses these neurons present an optimal substrate for testing whether the glycine binding site of NMDA-Rs is activated by transmitter released from glycine synaptic terminals. Using an in vitro brainstem slice preparation we report on initial experiments to investigate whether the glycine binding site of NMDA-Rs is saturated in motoneurons. Specifically, we investigated the question of whether the response of neonatal rat hypoglossal motoneurons (HMs) to a brief application of NMDA is enhanced by the presence of exogenous glycine. We found that exogenously applied glycine (1 mM) enhanced the NMDA activated membrane current. We conclude that in brainstem slices the glycine site at motoneuronal NMDA-Rs is not saturated, and that synaptically-released glycine may modulate NMDA-Rs mediated responses.

Multiple ionic mechanisms mediate inhibition of rat motoneurones by inhalation anaesthetics

1. We studied the effects of inhalation anaesthetics on the membrane properties of hypoglossal motoneurones in a neonatal rat brainstem slice preparation. 2. In current clamp, halothane caused a membrane hyperpolarization that was invariably associated with decreased input resistance; in voltage clamp, halothane induced an outward current and increased input conductance. Qualitatively similar results were obtained with isoflurane and sevoflurane. 3. The halothane current reversed near the predicted K+ equilibrium potential (EK) and was reduced in elevated extracellular K+ and in the presence of Ba2+ (2 mM). Moreover, the Ba2+-sensitive component of halothane current was linear and reversed near EK. The halothane current was not sensitive to glibenclamide or thyrotropin-releasing hormone (TRH). Therefore, the halothane current was mediated, in part, by activation of a Ba2+-sensitive K+ current distinct from the ATP- and neurotransmitter-sensitive K+ currents in hypoglossal motoneurones. 4. Halothane also inhibited Ih, a hyperpolarization-activated cationic current; this was primarily due to a decrease in the absolute amount of current, although halothane also caused a small, but statistically significant, shift in the voltage dependence of Ih activation. Extracellular Cs+ (3 mM) blocked Ih and a component of halothane-sensitive current with properties reminiscent of Ih. 5. A small component of halothane current, resistant to Ba2+ and Cs+, was observed in TTX-containing solutions at potentials depolarized to approximately -70 mV. Partial Na+ substitution by N-methyl-D-glucamine completely abolished this residual current, indicating that halothane also inhibited a TTX-resistant Na+ current active near rest potentials. 6. Thus, halothane activates a Ba2+-sensitive, relatively voltage-independent K+ current and inhibits both Ih and a TTX-insensitive persistent Na+ current in hypoglossal motoneurones. These effects of halothane decrease motoneuronal excitability and may contribute to the immobilization that accompanies inhalation anaesthesia.

Differential suppression of upper airway motor activity during carbachol-induced, REM sleep-like atonia

Microinjections of carbachol into the pontine tegmentum of decerebrate cats have been used to study the mechanisms underlying the suppression of postural and respiratory motoneuronal activity during the resulting rapid eye movement (REM) sleep-like atonia. During REM sleep, distinct respiratory muscles are differentially affected; e.g., the activity of the diaphragm shows little suppression, whereas the activity of some upper airway muscles is quite strong. To determine the pattern of the carbachol-induced changes in the activity of different groups of upper airway motoneurons, we simultaneously recorded the efferent activity of the recurrent laryngeal nerve (RL), pharyngeal branch of the vagus nerve (Phar), and genioglossal branch of the hypoglossal (XII) and phrenic (Phr) nerves in 12 decerebrate, paralyzed, vagotomized, and artificially ventilated cats. Pontine carbachol caused a stereotyped suppression of the spontaneous activity that was significantly larger in Phar expiratory (to 8.3% of control) and XII inspiratory motoneurons (to 15%) than in Phr inspiratory (to 87%), RL inspiratory (to 79%), or RL expiratory motoneurons (to 72%). The suppression in upper airway motor output was significantly greater than the depression caused by a level of hypocapnia that reduced Phr activity as much as carbachol. We conclude that pontine carbachol evokes a stereotyped pattern of suppression of upper airway motor activity. Because carbachol evokes a state having many neurophysiological characteristics similar to those of REM sleep, it is likely that pontine cholinoceptive neurons have similar effects on the activity of upper airway motoneurons during both states.

Catecholaminergic modulation of respiratory rhythm in an in vitro turtle brain stem preparation

An in vitro brain stem preparation from adult turtles was used to determine effects of dopamine (DA) and norepinephrine (NE) on the pattern of respiratory motor output recorded from hypoglossal nerve roots (XII). Bath-applied DA (10-200 microM) increased the frequency of respiratory bursts (peaks) from 0.9 +/- 0.2 to 2.4 +/- 0.3 (SE) peaks/min, resulting in a 99 +/- 9% increase in neural minute activity. R[+]-SCH-23390 (10 microM, D1 antagonist) and eticlopride (20 microM, D2 antagonist) attenuated the DA-mediated increase in peak frequency by 52 and 59%, respectively. On the other hand, the DA-receptor agonists apomorphine (D1, D2), quinelorane (D2), and SKF-38393 (D1) had no effect on peak frequency. Prazosin, an alpha1-adrenergic antagonist (250 nM) abolished the DA-mediated frequency increase. Although NE (10-200 microM) and phenylephrine (10-200 microM, alpha1-adrenergic agonist) increased peak frequency from 0.5 +/- 0.1 to 1.2 +/- 0.3 peaks/min and from 0.6 +/- 0.1 to 1. 0 +/- 0.2 peaks/min, respectively, these effects were not as large as that with DA alone. The data suggest that both dopaminergic and adrenergic receptor activation in the brain stem increase respiratory frequency in turtles, but the DA receptor-mediated increase is dependent on coactivation of alpha1-adrenergic receptors.

Hypercapnia-induced long-term depression of respiratory activity requires alpha2-adrenergic receptors

We investigated the effects of repeated hypercapnic episodes (inspired CO2 fraction = 0.10) on posthypercapnic respiratory nerve discharge. Anesthetized (urethan), vagotomized, and artificially ventilated rats were presented with three consecutive 5-min episodes of hyperoxic hypercapnia, separated by 5 min of hyperoxic normocapnia (inspired O2 fraction = 0.5). Respiratory nerve discharge and blood gases were recorded before and 30 and 60 min after the final hypercapnic episode. Posthypercapnia, arterial PCO2 was maintained within 1 Torr of initial baseline values. Integrated phrenic and hypoglossal burst amplitudes decreased posthypercapnia by up to 46 +/- 17 and 55 +/- 13% of baseline values, respectively, and remained reduced for at least 1 h [long-term depression (LTD)]. The protocol was repeated in rats pretreated with the alpha2-adrenergic antagonists yohimbine HCl (0.5 mg/kg; n = 7) or 2-[2-(2-methoxy-1,4-benzodioanyl)]imidazoline (RX-821002) HCl (0.25 mg/kg; n = 3). Both drugs attenuated LTD in the phrenic and hypoglossal neurograms. Results indicate that episodic hypercapnia elicits a yohimbine- and RX-821002-sensitive LTD of respiratory nerve activity in rats, suggesting that LTD requires alpha2-receptor activation.

AMPA receptor activation and phosphatase inhibition affect neonatal rat respiratory rhythm generation

1. We investigated the role of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors and their regulation in affecting respiratory-related neurones in a neonatal rat medullary slice that spontaneously generates respiratory-related rhythm and motor output in the hypoglossal (XII) nerve. 2. Bath application of the AMPA receptor antagonist 1-(4-aminophenyl)-4-methyl-7,8-methylenedioxy-5H-2, 3-benzodiazepine (GYKI) completely blocked XII nerve activity, as well as respiratory-related synaptic drives in neurones within the preBötzinger Complex (preBotC), site of rhythm generation in the slice. 3. Local application of GYKI to the preBötC blocked respiratory rhythm. Local application of AMPA to the preBötC increased rhythm frequency and depolarized respiratory-related neurones. 4. In the presence of tetrodotoxin (TTX), GYKI completely blocked the inward current induced by local application of AMPA, but not that induced by kainate. 5. Local application of okadaic acid, a membrane-permeable inhibitor of phosphatase 1 and 2A, to the preBotC increased the frequency of respiratory motor discharge. 6. Intracellular application of microcystin, a membrane-impermeable inhibitor of phosphatase 1 and 2A, enhanced endogenous inspiratory drive and exogenous AMPA-induced current (in the presence of TTX) in preBotC inspiratory neurones. Both the enhanced inspiratory drive and the increased AMPA-induced current were completely blocked by GYKI. 7. We suggest that AMPA receptor activation and AMPA receptor modulation by phosphorylation are crucial for the rhythm generation within the preBötC.

Taurine-activated chloride currents in the rat sacral dorsal commissural neurons

The electrophysiological and pharmacological properties of taurine (Tau)-activated Cl- currents (ITau) were investigated in the dissociated rat sacral dorsal commissural nucleus (SDCN) neurons using the nystatin perforated patch recording configuration under voltage-clamp conditions. The reversal potential of ITau was close to the Cl- equilibrium potential. The ITau was not affected by a preceding GABA response but cross-desensitized by a preceding glycine (Gly) response. Strychnine (STR), picrotoxin (PIC), bicuculline (BIC) and Zn2+ suppressed the ITau in a concentration-dependent manner. The pharmacology of the ITau and Gly-induced response (IGly) was similar, though Zn2+ inhibition on ITau differed from that on IGly in being much slower in recovery. Serotonin potentiated the ITau via protein kinase C. The results indicate that both Tau and Gly act on a strychnine-sensitive site to open the same Cl- channels in the SDCN neurons, and suggest that Tau may act as a functional neurotransmitter in the mammalian SDCN.

Evidence for reduced nitric oxide synthase (NOS) activity in the ipsilateral medial vestibular nucleus and bilateral prepositus hypoglossi following unilateral vestibular deafferentation in the guinea pig

The aim of the present study was to examine, using a radioenzymatic assay technique, nitric oxide synthase (NOS) activity in the bilateral medial vestibular nuclei (MVN) and prepositus hypoglossi (PH), during the development of vestibular compensation for unilateral vestibular deafferentation (UVD) in the guinea pig. In the MVN ipsilateral to the UVD, and bilaterally in PH, NOS activity decreased following UVD compared to sham controls and did not recover significantly up to 50 h later, when a substantial degree of behavioural vestibular compensation had occurred. These results suggest that UVD causes a decrease in NOS activity in the ipsilateral MVN and the bilateral PH, and that a consequent decrease in NO may be responsible for some of the ocular motor and postural symptoms of UVD.

Differential sensitivity of laryngeal and pharyngeal motoneurons to iontophoretic application of serotonin

Serotonergic neurons decrease their activity during sleep, especially rapid eye movement sleep, thereby reducing their facilitatory effect on upper airway motoneurons. The magnitude of teh sleep-related loss of tone varies among upper airway muscles (e.g., pharyngeal dilator motoneurons are more suppressed than laryngeal motoneurons). We hypothesized that these differences may be related to the sensitivity of different groups of upper airway motoneurons to serotonin. Experiments were done on decerebrate, vagotomized, paralysed and artificially-ventilated cats. Hypoglossal and laryngeal motoneurons were recorded extracellularly using five-barrel pipettes filled with: serotonin, glutamate and methysergide (serotonergic antagonist) for iontophoresis, and NaCl for recording and current balancing. All but two of the 65 hypoglossal motoneurons (45 inspiratory, 10 expiratory, 10 tonic) and 27 out of 32 laryngeal motoneurons (14 inspiratory, 18 expiratory) were excited by serotonin, and the excitation was abolished by methysergide. To compare the magnitude of the excitatory effect among distinct motoneuronal groups, we applied small ejection currents in a standardized manner (+15 nA for 3 min; 10 mM serotonin in 150 NaCl) onto spontaneously active motoneurons (13 inspiratory hypoglossal, 11 inspiratory laryngeal and 11 expiratory laryngeal). Serotonin increased the number of spikes per respiratory burst of inspiratory hypoglossal motoneurons from 19 +/- 4.0 (S.E.M.) to 35 +/- 4.8, of inspiratory laryngeal motoneurons from 44 +/- 8.3 to 55 +/- 8.8, and of expiratory laryngeal motoneurons from 23 +/- 4.8 to 33 +/- 6.2. The relative increases in activity (to 220% +/- 24, 147% +/- 23 and 148% +/- 9 of control, respectively) were significantly higher in hypoglossal than in laryngeal motoneurons. In addition, the excitatory effect developed significantly faster in hypoglossal than in laryngeal motoneurons. Methysergide reduced the spontaneous activity of about half the hypoglossal and laryngeal motoneurons to 66% +/- 5 of control. Thus, the sensitivity to the excitatory effects of serotonin varies among different pools of upper airway motoneurons. These differences correlate with the pattern of airway muscle hypotonia seen during sleep.

Eye movement deficits after ibotenic acid lesions of the nucleus prepositus hypoglossi in monkeys. I. Saccades and fixation

It has been suggested that the function of the nucleus prepositus hypoglossi (nph) is the mathematical integration of velocity-coded signals to produce position-coded commands that drive abducens motoneurons and generate horizontal eye movements. In early models of the saccadic system, a single integrator provided not only the signal that maintained steady gaze after a saccade but also an efference copy of eye position, which provided a feedback signal to control the dynamics of the saccade. In this study, permanent, serial ibotenic acid lesions were made in the nph of three rhesus macaques, and their effects were studied while the alert monkeys performed a visual tracking task. Localized damage to the nph was confirmed in both Nissl and immunohistochemically stained material. The lesions clearly were correlated with long-lasting deficits in eye movement. The animals' ability to fixate in the dark was compromised quickly and uniformly so that saccades to peripheral locations were followed by postsaccadic centripetal drift. The time constant of the drift decreased to approximately one-tenth of its normal values but remained 10 times longer than that attributable to the mechanics of the eye. In contrast, saccades were affected minimally. The results are more consistent with models of the neural saccade generator that use separate feedback and position integrators than with the classical models, which use a single multipurpose element. Likewise, the data contradict models that rely on feedback from the nph. In addition, they show that the oculomotor neural integrator is not a single neural entity but is most likely distributed among a number of nuclei.

Clonidine reduces hyperpolarization-activated inward current (Ih) in rat hypoglossal motoneurons

We used intracellular recording techniques to investigate the actions of clonidine on hypoglossal motoneurons (HMs) in rat brainstem slices. Clonidine (10-100 microM) produced a small (2-6 mV), dose-dependent hyperpolarization in HMs, accompanied by an increase in peak input resistance (RN). It also slowed the time course of the depolarizing 'sag' of the voltage response to constant hyperpolarizing current steps. These effects were mimicked by the alpha2-adrenoceptor (alpha2-AR) agonist guanabenz, but not by the Ih-imidazoline receptor agonists moxonidine or rilmenidine. Recorded in single-electrode voltage clamp mode, clonidine decreased input conductance of HMs and reduced the amplitude of a hyperpolarization-activated inward current (Ih). Clonidine's effect on Ih was three-fold: it shifted the half-activation voltage (V1/2) in the hyperpolarizing direction (by 4.4 +/- 0.7 mV at a dose of 10 microM), decreased the maximal current (by approximately 20%), and slowed the time course of Ih activation at all voltage steps. At the most hyperpolarized potential steps, clonidine slowed activation of Ih dramatically, yielding a striking increase in the activation time constant. The alpha2-AR antagonists yohimbine and idazoxan reduced clonidine's effect on V1/2 and on the Ih activation time course, but neither blocked clonidine's reduction of the maximal current, nor its strong slowing of Ih activation at the most hyperpolarized steps. We were unable to mimic or occlude clonidine's actions with the adenylate cyclase inhibitor SQ 22536 nor with the non-specific protein kinase inhibitor H-7. We conclude that clonidine hyperpolarizes HMs via a reduction of the amount of Ih that is active at rest, and that the response is mediated in part by alpha2-ARs. Some effects of clonidine on these neurons do not appear to be receptor-mediated, and may be due to physical block by clonidine of Ih channels.

Evidence that perihypoglossal neurons involved in vestibular-auditory and gaze control functions respond to nerve growth factor

Nerve growth factor (NGF), which has long been considered to be a trophic factor for peripheral sensory and sympathetic neurons, has been found recently to influence cholinergic neurons in the basal forebrain and neostriatum. In the present study, we provide evidence that brainstem neurons in the perihypoglossal area that relay information from the inner ear and vestibular apparatus to the cerebellum and tectum are responsive to NGF. These neurons, which are located in the nucleus prepositus hypoglossi (NPH), spinal vestibular nucleus, cochlear complex, and gigantocellular and paragigantocellular nuclei of the reticular formation, express functional receptors for NGF and up-regulate the expression of trkA receptors after injection of NGF into targets. In addition, the developmental up-regulation of NGF in the cerebellum coincides with the differentiation of the perihypoglossal nuclei. These results suggest that neurons representing the principal brain relays for auditory and vestibular pathways and perihypoglossal neurons involved in gaze coordination are a novel group of central neurons (besides cholinergic neurons in the basal forebrain and neostriatum) that respond to NGF.

Response of glial cells and activation of complement following motorneuron degeneration induced by toxic ricin

Motor nerve transection in adult rats induce a series of metabolic and structural changes in the injured neurons as well as in surrounding glial cells; however, without substantial neuronal degeneration. In the present study we found, in contrast with axotomy, a massive neuronal death in the ipsilateral hypoglossal nucleus following injection of toxic ricin (RCA) into the hypoglossal nerve, which is in line with previous observations. Injection of RCA enables examination of the glial reaction in a situation where neuronal degeneration is profound, which has been the approach in the present study. We found an increase in OX42-, GFAP-, and transferrin-immunoreactivity in microglial, astroglial, and oligodendroglial cells respectively, in the ipsilateral hypoglossal nucleus three to seven days following injection of toxic ricin in the hypoglossal nerve. Proliferation was found in astrocytes as well as in microglial cells, as shown by uptake of bromodeoxyuridine. In addition, the complement cascade was activated locally in the ipsilateral hypoglossal nucleus, as demonstrated by immunohistochemical detection of complement components C3d and C9. Complement activation may serve several effects in the glial-neuronal interactions. Stimulation of phagocytosis by reactive microglia is probably the most important one. Furthermore, the degenerative neuronal somata showed increased immunoreactivity for clusterin, which is a known complement inhibitor, but a decrease in clusterin-mRNA. In conclusion, the glial cell response was in several aspects principally different following massive motorneuron degeneration induced by toxic ricin in comparison to previous findings reported after axotomy.

Strychnine eliminates reciprocation and augmentation of respiratory bursts of the in vitro frog brainstem

We have recorded rhythmic bursts of efferent action potentials from nerves of respiratory muscles in the frog (Rana pipiens), using a modified in vitro preparation, in which the brainstem lies in situ in the ventral half of the skull. The burst in the sternohyoid branch of the hypoglossal nerve (Hsh) was augmenting, and alternated with a relatively brief augmenting burst in the main branch of the hypoglossal nerve (Hm). The laryngeal branch of the vagus nerve (XI) displayed a biphasic burst, beginning before peak activity of Hsh and spanning the Hm burst. The spatio-temporal patterns of these bursts closely resemble those recorded from the same nerves in intact and in decerebrate frogs, indicating that the bursting rhythm of this in situ preparation constitutes fictive breathing. The nature of neurotransmission responsible for burst reciprocity and augmentation was investigated by applying the glycine receptor blocker, strychnine. Low levels of strychnine (1 and 5 M) increased the frequency of fictive breathing without changing the shape or timing of Hsh, Hm and XI bursts; at higher doses (10 and 20 M) the bursts in all nerves abruptly changed shape and timing to become synchronous and decrementing. The strychnine-induced changes were associated with the appearance of a prominent peak (10-20 Hz) on the spectral analysis of the nerve discharge, possibly indicating a fundamental change in neurogenesis of the respiratory pattern. We conclude that the burst augmentation and reciprocation discharge characteristics of fictive breathing in the frog require strychnine-sensitive inhibitory networks.

Presynaptic depression of excitatory synaptic inputs to rat hypoglossal motoneurons by muscarinic M2 receptors

1. Whole cell recordings of glutamatergic excitatory postsynaptic currents (EPSCs) evoked by electrical stimulation in the reticular formation were made from visualized hypoglossal motoneurons (HMs) in rat brain stem slices. 2. Carbachol, muscarine, or physostigmine reduced EPSC amplitude to 50 +/- 3%, 37 +/- 3%, and 54 +/- 7% (mean +/- SE) of control, respectively; effects of carbachol and physostigmine were antagonized by atropine (1-2 microM). EPSC depression was most effectively antagonized by methoctramine, an M2 muscarinic acetylcholine receptor (mAChR) antagonist with a high affinity constant (pKB) of 8.07 for the receptor mediating this response, whereas pirenzepine, an M1 mAChR antagonist, had a pKB of < 7.0, showing that EPSC depression was mediated by the M2 mAChR. 3. Postsynaptic properties of HMs (holding current and input resistance), EPSCs (reversal potential, rise time, half-width, and decay time constant), and postsynaptic glutamate-gated currents (amplitude and waveform) were not altered by carbachol or muscarine. 4. Muscarine did not decrease presynaptic neuron excitability, because the frequency of spontaneous EPSCs in HMs in the absence of tetrodotoxin (TTX) was either unchanged or increased. Leak and action currents of reticular formation neurons were not significantly altered by muscarine. In contrast, with TTX present, the frequency of spontaneous miniature glutamatergic EPSCs in HMs was decreased by both carbachol (mean change = 203 +/- 46%) and muscarine (mean change = 185 +/- 26%), with no change in miniature EPSC amplitude distribution. 5. Muscarinic depression of excitatory transmission to HMs thus occurs at the presynaptic terminal, most probably affecting release mechanisms downstream from calcium entry, and is likely to be significant during rapid eye movement sleep, possibly underlying the loss of tongue tone and inspiratory activity during this state.

Presynaptic inhibition by serotonin: a possible mechanism for switching motor output of the hypoglossal nucleus

We examined the influence of serotonin (5-HT), an important state-dependent neuromodulator, on synaptic transmission to hypoglossal motoneurons (HMs). Our data demonstrate that 5-HT acts presynaptically to inhibit excitatory (glutamatergic) synaptic currents recorded in HMs. We discuss this result in relation to the role of 5-HT in state-dependent modulation of respiration-related activity in HMs.

Hypoglossal and phrenic motoneuron responses to serotonergic active agents in rats

5-Hydroxytryptamine (serotonin, 5-HT) affects upper airway and chest wall inspiratory muscle control. The purpose of this study was to investigate the relative interaction of serotonergic agents on these two muscle groups. We measured the responses of the hypoglossal and phrenic nerves to the systemic administration of serotonergic-active agents and determined the receptor types through which these agents act in anesthetized, vagotomized, paralyzed and artificially ventilated rats. The serotonin precursor, L-5-hydroxytryptophan (L-5-HTP) produced equivalent stimulation of phasic inspiratory activity of the hypoglossal and phrenic nerves. General serotonin antagonists produced significant and equivalent diminution of both motoneuron pools. Specific 5-HT1A stimulation and 5-HT1C/2 antagonism enhanced ventilatory activity. We conclude: (1) a baseline level of serotonergic input to hypoglossal and phrenic motoneuron pools was present, (2) different 5-HT receptors had different effects on ventilatory neural activity, and (3) hypoglossal and phrenic motoneuron pools responded similarly to the serotonergic agents given.

Nitric oxide production by brain stem neurons is required for normal performance of eye movements in alert animals

Although nitric oxide (NO) is produced by discrete groups of neurons in the brain, participation of NO in premotor structures directly involved in reflexively evoked, sensory-motor functions has not been demonstrated so far. We now show that NO is a physiological mediator in the generation of a specific motor response in alert behaving animals. In the oculomotor system, numerous neurons expressing nitric oxide synthase (NOS) are located in the prepositus hypoglossi, a nucleus involved in the control of horizontal eye movements. Unilateral inhibition of NOS within this nucleus results in severe ocular nystagmus with slow phases directed to the contralateral side. Accordingly, local increases of NO or cyclic GMP produced a nystagmus in the opposite direction. It is concluded that a balanced production of NO by prepositus hypoglossi neurons is a necessary condition for the normal performance of eye movements in alert animals.

Central effects of 5-HT on activity of respiratory and hypoglossally innervated muscles in newborn kittens

1. In decerebrate kittens (n = 29), electrical activity was studied in the 3rd intercartilaginous (inspiratory), the 9th internal intercostal (expiratory) and the hypoglossally innervated muscles (geniohyoid m. and sternohyoid m.) evoked by the application of 5-HT (n = 16) or related agents (5-HT1A agonist, 8-OH-DPAT (n = 6) and 5-HT2 agonist, DOI floor of the IVth ventricle. 2. The application of a control solution (n = 2) produced no significant changes either in minute inspiratory frequency (Fi) or in the electrical activity of the muscles studied. Except for these controls, only one trial with one dose of one drug was performed in a given kitten. 3. A dose-related decrease in Fi was observed in response to 5-HT. Low doses (50-500 nmol, n1 = 8) induced a long-lasting bradypnoea; high doses (5000-10,000 nmol, n2 = 8) induced prolonged periods of apnoea. 4. The apnoeas observed in tracheotomized (n = 3) or non-tracheotomized (n2 = 8) kittens were mainly of central origin and linked to the lengthening of expiratory time. The expiratory muscle activation came on with the reinforcement of the activity of hypoglossally innervated muscles. 5. Application of agonists showed that both the 5-HT-dependent modulation of Fi and the effects of 5-HT on the activity of the muscles studied resulted predominantly from activation of 5-HT2 receptors.

The effects of methylenedioxymethamphetamine (MDMA, "Ecstasy") on monoaminergic neurotransmission in the central nervous system

Methylenedioxymethamphetamine (MDMA, Ecstasy) is a popular recreationally used drug among young people in Europe and North America. The recent surge in use of MDMA and increasing concerns about possible toxic effects of the drug have inspired a great deal of research into the mechanisms by which the drug may affect the central nervous system. This paper reviews studies on the neurochemical, behavioral and neurophysiological effects of MDMA, with emphasis on MDMA effects in regions of the brain that have been implicated in reward. Experiments in awake, behaving laboratory animals have demonstrated that single injections of MDMA increase extracellular levels of the neurotransmitters dopamine (DA) and serotonin (5HT) in the nucleus accumbens and in several other brain regions that are important for reward. Most of the behavioral and electrophysiological changes that have been reported to date for single doses of MDMA appear to be mediated by this MDMA-induced increase in extracellular DA and 5HT. As an example, MDMA-induced hyperthermia and locomotor hyperactivity in laboratory animals can be blocked by administering drugs that prevent MDMA-induced 5HT release and can be attenuated by administering 5HT receptor antagonists, whereas effects of MDMA on delayed reinforcement tasks appear to be mediated by MDMA-induced increases in extracellular DA. Similarly, the effects of MDMA on neuronal excitability in the nucleus accumbens and in several other brain regions can be prevented by administering drugs that block MDMA-induced 5HT release and can be attenuated by depleting brain DA levels or by administering either DA D1 receptor antagonists or 5HT receptor antagonists. In addition to the acute effects of MDMA, it is now well established that repeated or high-dose administration of MDMA is neurotoxic to a subpopulation of 5HT-containing axons that project to the forebrain in laboratory animals. Recent studies have shown that this neurotoxic effect of MDMA is associated with long-duration changes in both DA and 5HT neurotransmission in the nucleus accumbens. Whether these long-duration changes in neurotransmission might be related to reports of depression and other psychopathologies by some frequent users of MDMA remains to be determined. Methylene-dioxymethamphetamine has been found to increase extracellular levels of norepinephrine and to alter brain levels of several neuropeptides as well as altering levels of DA and 5HT. Much additional research is required to understand the multiple ways in which this complex drug may alter neurotransmission in the brain, both acutely and in the long term.

Serotonergic effects on hypoglossal neural activity and reflex responses

We determined the effects of serotonin (5HT; 6 concentrations ranging from 0.005 to 500 microM) pressure microinjection into the hypoglossal (XII) motor nucleus (100-500 nl; pH = 7.2-7.4) on XII whole nerve activity and reflex response to upper airway negative pressure in 15 decerebrated, vagotomized, paralyzed and artificially ventilated cats. Increasing 5HT concentration resulted in a concentration dependent increase in ipsilateral tonic XII activity, with no change in phasic XII activity. Threshold concentrations ranged from 0.005 to 0.5 microM, with the maximal response reached at 5 microM. Increasing 5HT concentration also increased the duration of the XII response. This ranged from 50 s with 0.5 microM, to over 10 min with 500 microM 5HT. However, 5HT did not significantly change the XII whole nerve reflex response to upper airway negative pressure (-20 cm H2O) at any 5HT concentration (n = 5). All 5HT effects were reversed by microinjection of 1.0 mM methysergide. We conclude that XII responses to 5HT are elicited at low concentrations of 5HT, which have a relatively short duration of effect, but that 5HT at the XII motor nucleus has no effect on the XII reflex response to upper airway negative pressure.

Serotonergic and noradrenergic effects on respiratory neural discharge in the medullary slice preparation of neonatal rats

Rhythmically active medullary slice preparations isolated from neonatal rats (postnatal days 0-3, P0-P3) were used to study the modulation of respiraory rhythmogenesis and hypoglossal (XII) nerve discharge by serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline (NA). 5-HT, NA and their respective receptor agonists and antagonists were applied either to the bathing medium or focally via pressure injection into regions encompassing the pre-Botzinger complex or XII motoneurons. The effects of endogenously released 5-HT were also studied by chemical stimulation of neurons within the raphe obscurus. The frequency of respiratory burst discharge was increased when 5-HT was applied: (1) to the bathing medium (37+/-16%; 30 "mu"M; P < 0.05); (2) via pressure injection into the region of the pre-Botzinger complex (22 +/- 14%; < 25 pmol; P < 0. 05); or (3) endogenously released in response to activation of neurons within the raphe obscurus via pressure injection of (R,S)- "alpha"-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA, 34 +/- 15%; P < 0.05) or 5-HT (33 +/- 5%; P < 0. 05). All of these effects were antagonized by bath application of methysergide (30-40 "mu"M). NA caused a reduction of respiratory burst frequency when applied to the bathing medium (40 +/- 15%; 100 "mu"M; P < 0.05) or when pressure injected into the region of the pre-Botzinger complex (22 +/- 11%; < 25 pmol; P < 0.05). These effects were blocked by the bath application of the "alpha"2-receptor antagonist idazoxan (2 "mu"M). 5-HT and NA both caused an augmentation of tonic discharge of XII nerves when applied either to the bathing medium or via pressure injection into the XII motoneuron pool. The 5-HT-induced XII nerve tonic discharge was mimicked by the 5-HT2 receptor agonist R(-)2-(2, 5-dimethoxy-4-iodophenyl) (DOI.HCl, 5 "mu"M) and blocked by the 5-HT2 receptor antagonist ketanserine tartrate (30-40 "mu"M). The NA-induced XII nerve tonic discharge was mimicked by the "alpha"1-receptor agonist phenylephrine HCl (500 nM) and blocked by the "alpha"1-receptor antagonist prozasin HCl (1 "mu"M).

Central infusions of brain-derived neurotrophic factor and neurotrophin-4/5, but not nerve growth factor and neurotrophin-3, prevent loss of the cholinergic phenotype in injured adult motor neurons

Neurotrophic factors are molecules that prevent neuronal degeneration and regulate neuronal phenotype during either development or adulthood. Relatively little is known about the comparative responsiveness of injured adult central nervous system motor neurons to various neurotrophic factors. In the present study we examined the effects of four members of the neurotrophin family on injured adult motor neurons. Nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3 or neurotrophin-4/5 were infused intracerebroventricularly into adult rats following transection of the motor hypoglossal nerve. Two weeks after axotomy, brain-derived neurotrophic factor and neurotrophin-4/5 completely prevented the loss of the cholinergic phenotype in hypoglossal motor neurons (97 +/- 11% and 99 +/- 5%, respectively) as assessed by choline acetyltransferase immunolabeling. In contrast, nerve growth factor and neurotrophin-3 exerted no protective effect. The low-affinity p75 neurotrophin receptor, capable of binding all four neurotrophins, was re-expressed in injured hypoglossal neurons; the majority of injured hypoglossal neurons also express trkB receptors but not trkA or trkC receptors. Thus, injury-induced responses to neurotrophins in adult motor neurons are mediated by trk receptors and their agonists, but may or may not also require low-affinity p75 neurotrophin receptors. Intracerebroventricular infusions of trkB agonists may be a useful means of targeting multiple and distantly separated populations of motor neurons for neurotrophic factor therapy.

Interaction of serotonergic excitatory drive to hypoglossal motoneurons with carbachol-induced, REM sleep-like atonia

The facilitatory effect of serotonin (5HT) on hypoglossal (XII) motoneurons is likely to be reduced during rapid eye movement (REM) sleep, when the activity of the brainstem serotonergic system reaches its nadir. Therefore, we assessed the hypothesis that application of exogenous 5HT will attenuate the REM sleep-like suppression of XII motoneurons produced in decerebrate cats by pontine microinjections of a cholinergic agonist, carbachol. Microinjections of 5HT or 5-carboxamidotryptamine into the XII nucleus increased XII nerve activity to 182 +/- 53% (standard deviation; SD) of control. Subsequent pontine microinjections of carbachol reduced XII nerve activity by 55 +/- 21% of the pre-5HT level (n = 12). Microinjections of methysergide (a 5HT antagonist) into the XII nucleus reduced XII nerve activity to 54 +/- 17% of the pre-methysergide control (n = 6). Pontine carbachol injections after methysergide further reduced XII nerve activity by 49 +/- 20% of the pre-methysergide level. Treatments with both agonists and the antagonist attenuated the carbachol-induced decrease when compared to two previous studies using the same model: 1) In experiments with no injections of serotonergic agents, pontine carbachol injections decreased XII nerve activity by 90 +/- 6% of control. 2) After enhancement of XII nerve activity by inhibitory amino acid antagonists (to 135 +/- 60%), the subsequent carbachol-induced decrease was even larger, 112 +/- 62% of control. We propose that serotonergic excitation can significantly attenuate the REM sleep-like suppression of XII nerve activity, and that this is achieved, in part, by substituting for the decreased endogenous 5HT in the XII nucleus. The study also demonstrates that other, non-serotonergic, mechanisms also contribute to the carbachol-induced suppression.

Thyrotropin-releasing hormone (TRH) depolarizes a subset of inspiratory neurons in the newborn mouse brain stem in vitro

1. To extend the classification of respiratory neurons based on active membrane properties and discharge patterns to include responses to respiratory modulators, we have studied the effect of thyrotropin-releasing hormone (TRH, 1-5 microM) on the spontaneous respiratory-related neural activity in a thick brain stem slice preparation from the newborn mouse. The action of TRH on the respiratory output from the slice was investigated by recordings from the XII nerve. Cellular responses to TRH were investigated using whole cell recordings from hypoglossal motoneurons and three types of inspiratory neurons located in the rostral ventrolateral part of the slice. 2. Bath-applied TRH (1 microM) decreased the time between inspiratory discharges recorded on the XII nerve from 12.3 +/- 3.3 s to 4.9 +/- 1.1 s (n = 28; means +/- SD), i.e., caused an approximate threefold increase in the respiratory frequency. The coefficient of variation of the time between the inspiratory discharges decreased by one-half. Thus the respiratory output became more stable in response to TRH. The duration of the inspiratory discharges increased from 474 +/- 108 ms to 679 +/- 114 ms, and the amplitude decreased by 24%. An increase in the interdischarge noise on the XII nerve was recorded in the early phase of the TRH application. 3. Anatomically identified hypoglossal motoneurons (7 cells) responded to bath applied TRH with a depolarization eliciting spikes between the inspiratory potentials. The depolarization was accompanied by an increase in spontaneous excitatory synaptic activity that disappeared late during the TRH application. The duration of the inspiratory potentials was increased, indicating that the hypoglossal motoneurons received a longer duration synaptic input from the respiratory rhythm generator. 4. Type-1 inspiratory neurons showed a prolonged depolarization (3 cells), a transient depolarization (2 cells), or no change in membrane potential (2 cells) during 10 min of continued superfusion with a TRH-containing solution. The duration of the inspiratory potentials was increased during the TRH superfusion. With tetrodoxin (TTX, 1 microM) present in the superfusing solution TRH induced a prolonged depolarization (3 cells) or a transient depolarization (1 cell), demonstrating that type-1 inspiratory neurons are depolarized postsynaptically by TRH. The input resistance was not changed during the depolarizing response to TRH. 5. Type-2 inspiratory neurons showed a transient depolarization (7 cells) in response to bath-applied TRH. The duration of the inspiratory potentials was increased markedly during TRH. The transient depolarization was not the result of a postsynaptic action of TRH, because type-2 neurons (9 cells) showed no depolarization to TRH with TTX present in the superfusing solution. 6. Type-3 inspiratory neurons showed a transient depolarization (4 cells) with a partial recovery of the membrane potential late during the TRH application. The duration of the inspiratory potentials increased markedly during TRH. Four cells showed a transient depolarization with an increase in input resistance during TRH with TTX present in the superfusing solution. Thus type-3 neurons are depolarized postsynaptically by TRH. 7. We conclude that TRH increases the frequency of the respiratory rhythm in newborn mice through an action at the level of the brain stem.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by riluzole of glycinergic postsynaptic currents in rat hypoglossal motoneurones

1. Riluzole has been shown to have beneficial effects in motoneurone disease, yet its effect on motoneurones is not known. To address this question, we investigated synaptic modulation by riluzole in hypoglossal motoneurones by recording glycinergic inhibitory postsynaptic currents evoked by stimulation of nearby single interneurones. 2. Glycinergic inhibitory postsynaptic currents were evoked by electrical stimulation of single interneurones and were recorded from visually identified hypoglossal motoneurones. Riluzole (10 microM) inhibited mean amplitude of evoked glycinergic inhibitory postsynaptic currents by 87%. 3. We found that riluzole suppressed sodium currents in brainstem interneurones by 23.8%. Riluzole did not modulate barium currents through voltage-activated calcium channels (98% of control). Therefore, the effect of riluzole on synaptic transmission may be mediated, in part, by stabilizing presynaptic neurones through inhibition of voltage-activated sodium currents. 4. In the presence of tetrodotoxin (0.5 microM), riluzole reduced the frequency (1.2 Hz in control to 0.6 Hz in riluzole) of spontaneous transmitter release recorded in motoneurones. 5. Riluzole was found to have no effect on mean miniature inhibitory postsynaptic current amplitude, therefore the reduction in spontaneous transmitter release cannot be due to an action on postsynaptic glycine receptors. 6. We conclude that riluzole inhibits synaptic transmission presynaptically, independent of a reduction in the excitation of presynaptic neurones.

Actions of norepinephrine on rat hypoglossal motoneurons

1. We used conventional intracellular recording techniques in 400-microns-thick slices from the brain stems of juvenile rats to investigate the action of norepinephrine (NE) on subthreshold and firing properties of hypoglossal motoneurons (HMs). 2. In recordings in current-clamp mode, 50 or 100 microM NE elicited a reversible depolarization accompanied by an increase in input resistance (RN) in all HMs tested (n = 74). In recordings in single-electrode voltage-clamp mode, NE induced a reversible inward current (INE) accompanied by a reduction in input conductance. The average reversal potential for INE was -104 mV. The NE responses could be elicited in a Ca(2+)-free solution containing tetrodotoxin, indicating that they were postsynaptic. 3. The NE response could be blocked by the alpha-adrenoceptor antagonist prazosin, but not by the beta-adrenoceptor antagonist propranolol, and could be mimicked by the alpha 1-adrenoceptor agonist phenylephrine but not by the alpha 2-adrenoceptor agonist UK 14,304 or by the beta-adrenoceptor agonist isoproterenol when alpha-adrenoceptors were blocked. 4. Substitution of barium for calcium in the perfusion solution blocked the increase in RN in response to NE without completely blocking the depolarization. Replacement of sodium chloride with choline chloride in the barium-substituted perfusion solution blocked the remaining depolarization. 5. The neuropeptide thyrotropin-releasing hormone (TRH), which also depolarizes and increases the RN of HMs, occluded the response of HMs to NE. 6. NE altered HM firing properties in three ways: it always lowered the minimum amount of injected current needed to elicit repetitive firing, it increased the slope of the firing frequency versus injected current relation in 8 of 14 cells tested, and it increased the delay from the onset of the depolarizing current pulse to the first evoked spike in all cells tested. 7. We conclude that NE acts directly on alpha 1-adrenoceptors to increase the excitability of HMs. It does this by reducing a barium-sensitive resting potassium current and activating a barium-insensitive inward current carried primarily by sodium ions. A portion of the intracellular pathway for these actions is shared by TRH. In addition, there is evidence that NE alters HM firing patterns by affecting currents that are activated following depolarization.

Effects of ethanol on respiratory activity in the neonatal rat brainstem-spinal cord preparation

Ethanol (1-12 mM) added to the superfusion medium of the isolated brainstem-spinal cords of newborn rats did not affect phrenic activity but significantly reduced hypoglossal activity by 54%, 67% and 55% at 3, 6 and 12 mM, respectively. Although the reasons for the suppression of hypoglossal activity remain unknown, this preparation may be a useful model for determining why cranial motoneurons are more vulnerable than phrenic motoneurons to various agents and, more generally, how ethanol impairs neural function.

Effects of the pyrethroid insecticide, deltamethrin, on respiratory modulated hypoglossal motoneurons in a brain stem slice from newborn mice

We have studied the action of deltamethrin on respiratory modulated hypoglossal motoneurons in a brain stem slice from newborn mice. Deltamethrin depolarized the hypoglossal motoneurons, increased the background synaptic noise and reduced the frequency and amplitude of current elicited action potentials. Deltamethrin transiently increased the frequency of the respiratory rhythm. Inspiratory potentials in hypoglossal motoneurons were decreased in amplitude and increased in duration. In conclusion, deltamethrin perturbs the respiratory output from the hypoglossal nucleus through postsynaptic actions on hypoglossal motoneurons and by affecting the inspiratory synaptic drive.

Effect of I1-imidazoline receptor activation on responses of hypoglossal and phrenic nerve to chemical stimulation

Sedation elicited by some centrally acting antihypertensive agents may interfere with respiratory control, and by selectively inhibiting upper airway dilating muscle activity it may facilitate obstructive sleep apnea. Autoradiographic studies with [125I]p-iodoclonidine in the presence of 10 microM epinephrine to block alpha 2-adrenergic sites or 100 nM moxonidine to mask I1-imidazoline sites show that both I1- as well as alpha 2-sites are localized in putative chemosensory areas of the rostral ventrolateral medulla in the cat. We sought to determine the effect of activating I1 and alpha 2-receptors on central chemosensitivity by using moxonidine as a selective I1 agonist, clonidine as a mixed I1/alpha 2 agonist, SK&F-86466 as a specific alpha 2-antagonist, and efaroxan as a mixed I1/alpha 2 antagonist. We recorded responses of phrenic, hypoglossal, and cervical sympathetic nerve activities to progressive hypercapnia after hyperventilation to apnea. Moxonidine (3-100 micrograms/kg i.v.) caused dose-dependent decreases in tonic cervical sympathetic nerve activity and blood pressure, but had no effect on the CO2 threshold (after 30 or 100 micrograms/kg moxonidine, phrenic nerve activity reappeared at 5.8 +/- 0.2% CO2 versus 5.6 +/- 0.3% CO2 in control). Following moxonidine, the slope of the steep portion of the CO2 response tended to increase (10.3 +/- 1.8 versus 7.3 +/- 0.9). Peak phrenic nerve activity was comparable to control at 7.5% CO2 (20 +/- 2 U in control) and at 9.5% CO2 (30 +/- 3 versus 27. +/- 2 U). Similarly, the response of hypoglossal and inspiratory phasic cervical sympathetic nerve activity to a progressive CO2 rise was not affected by moxonidine. By contrast, clonidine in the same doses decreased CO2 sensitivity, because the CO2 threshold was elevated from 5.3 +/- 0.5% to 6.7 +/- 0.4% (p < 0.001). The slope of the CO2 response was decreased from 9.7 +/- 1.9 to 7.4 +/- 1.3 (p = 0.05). Peak phrenic nerve activity was reduced at 7.5% CO2 (11 +/- 5 versus 25 +/- 2 U; p < 0.05) and at 9.5% CO2 (21 +/- 4 versus 33 +/- 2 U; p = 0.06). Clonidine selectively inhibited the response of hypoglossal nerve activity to CO2. The depressive effects of clonidine were reversed by alpha 2-blockade with SK&F-86466 (0.5 or 1 mg/kg). Inspiratory phasic cervical sympathetic nerve activity increased after SK&F-86466 in parallel with phrenic and hypoglossal nerve activity, but the tonic component of cervical sympathetic nerve activity and blood pressure increased only transiently.(ABSTRACT TRUNCATED AT 400 WORDS)

Central effects of 5-HT on respiratory and hypoglossal activities in the adult cat

The activities of the diaphragmatic, internal intercostal and hypoglossal-innervated muscles were studied in adult decerebrate cats in response to 5-HT and related agents (8-OH-DPAT and DOI). The drugs were placed on the floor of the IVth ventricle. The mean respiratory frequency (Fi) increased (124-193% of the control value) within 3 min of the 5-HT application, and decreased thereafter (30-90%). The mean Ti and Te changed similarly, but opposite to Fi. With some delay, the hypoglossal-innervated muscles were tonically activated or exhibited increased activities. Methysergide pretreatment completely blocked the effect of 5-HT on all the respiratory parameters and the hypoglossal-innervated muscles activities. The responses to 8-OH-DPAT and DOI indicate that 5-HT modulates the respiratory frequency via activation of both 5-HT1A and 5-HT2 receptors. Nevertheless, the effect of 5-HT on both the expiratory and hypoglossal-innervated muscles seems to depend on 5-HT2 receptors activation only.

Localization of chemosensitive structures in the isolated brainstem of adult guinea-pig

1. Central respiratory chemosensitivity has been intensively examined but some questions remain unsolved; namely, what is the nature of the stimulus (fixed acid and/or CO2) and where is the site of brainstem chemosensitivity (near the ventral medullary surface or structures deeper within the brainstem)? To examine these questions, we used the in vitro isolated brainstem of adult guinea-pig perfused independently through the basilar artery and the bath. 2. Respiratory motor output was recorded with a suction electrode from cranial hypoglossal (XII) roots. Changes in pH and CO2 in the Krebs perfusate were made by changing either the bicarbonate concentration or the PCO2 saturating the Krebs solution. 3. Changes in basilar artery perfusate consisting of (i) an acidifying increase in PCO2 (hypercapnic acidic Krebs solution), (ii) an increase in PCO2 with no change in pH (hypercapnic Krebs solution), or (iii) a decrease in pH with no change in PCO2 (acidic Krebs solution) evoked increases in respiratory frequency and a concomitant decrease in inspiratory burst amplitude. 4. Bath superfusion with hypercapnic acidic Krebs solution increased the inspiratory burst amplitude with no effect on respiratory burst frequency. 5. Bath superfusion with hypercapnic non-acidic Krebs solution increased the inspiratory burst amplitude and decreased the respiratory frequency, while normocapnic acidic Krebs solution increased the respiratory frequency with no change in burst amplitude. 6. These results show that respiratory responses to changes in CO2 and pH depend upon the sites of action. While a CO2 increase or a pH decrease affected the respiratory frequency in the deep brainstem structures (perfused through the basilar artery), CO2 respiratory chemosensitivity at the ventral surface could be differentiated from the hydrogen ion chemosensitivity. This suggests that different mechanisms mediated respiratory responses when deep versus superficial brainstem structures were stimulated.

Central effect of angiotensin III on caudal hypoglossal neurons in rats

We communicated the central effect of angiotensin III (ANG III), a potent signal for extracellular dehydration, on single neurons in the caudal hypoglossal nucleus of Sprague-Dawley rats anesthetized with pentobarbital sodium. A significant number (121 of 168) of caudal hypoglossal neurons responded to intracerebroventricular application of ANG III (80 or 160 pmol), with either an increase (n = 83) or decrease (n = 38) in their spontaneous discharge. These effects of ANG III were significantly reversed by intracerebroventricular application of the nonpeptide angiotensin AT1 receptor antagonist losartan (40 nmol), but not by the AT2 antagonist, PD-123319. The hypoglossal neuronal responses to repeated administration of ANG III (80 pmol), delivered at an interval < or = 18 min, exhibited acute tachyphylaxis. Intracerebroventricular administration of the cholinergic dipsogen, carbachol (50 ng), or the osmotic stimulant, hypertonic saline (0.5 M), also elicited responses in ANG III-responsive hypoglossal neurons. These results suggest that neurons in the caudal hypoglossal nucleus may serve as the final common pathway for extracellular and, possibly, intracellular thirst in the rat. Furthermore, it is likely that the action of ANG III is mediated by the AT1 subtype of angiotensin receptors.

Role of fast inhibitory synaptic mechanisms in respiratory rhythm generation in the maturing mouse

1. The importance of glycinergic and GABAAergic synaptic mechanisms for respiratory rhythm generation in the maturing mouse were investigated in vivo and in an in vitro slice preparation generating respiratory rhythmic activity spontaneously at all postnatal ages. 2. The effect on respiration of topical application of strychnine or bicuculline to the surface of the ventrolateral medulla was assessed in spontaneously breathing anaesthetized mice of different ages (postnatal (P) days 0 to > 56). Glycine receptor antagonization with concentrations of strychnine up to 25 microM was ineffective in altering the breathing pattern in neonates (P1-P8). However, in mature mice (P > 15), low doses of strychnine (0.2-2 microM) abolished regular rhythmic discharge in the phrenic nerve. Bicuculline (0.5-50 microM) produced dose-dependent increases in inspiratory time, amplitude and cycle length of phrenic nerve discharge in anaesthetized neonatal mice whereas both cycle length and duration of inspiratory activity were reduced in mature animals. In addition, in both neonates and mature mice low concentrations of bicuculline (0.5-5 microM) abolished phrenic nerve discharge intermittently. 3. The response of respiratory-modulated hypoglossal (XII) neurones recorded in tilted sagittal slices from newborn and mature mice during blockade of glycine and GABAA receptors was similar to the phrenic nerve changes observed in vivo: in slices from neonates, the rhythmic activity of XII neurones was resistant to concentrations of strychnine up to 50 microM whereas low doses of strychnine (0.2-2 microM) abolished rhythmic activity in preparations from mature mice. Bicuculline (1-50 microM) produced a dose-dependent prolongation of burst duration and a slowing of rhythmic discharge in slices from neonatal mice whereas in mature mice rhythmic XII bursts were shortened and their frequency increased. At all maturational stages, bicuculline (1-50 microM) induced severe disruption of the regular rhythm of XII neurone activity causing maintained depolarizations and oscillations in membrane potential. 4. On-going inhibitory postsynaptic potentials of neurones located in the ventral respiratory group region of tilted sagittal slices from both immature and mature mice were sensitive to low concentrations of either bicuculline or strychnine (1-5 microM) indicating an absence of a maturational change in the sensitivity of GABAA and glycine receptors to their respective antagonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in phrenic, hypoglossal and recurrent laryngeal nerve activities after intravenous infusions of aminophylline in cats

Aminophylline is known to have respiratory stimulant properties, and it has been suggested that it may also be effective in sleep apnoea. However, its role in this disorder remains uncertain. Theoretically, increasing upper airway motoneural activity in order to maintain airway patency might alleviate obstructive sleep apnoea. On the other hand, increasing the respiratory drive may also prove beneficial in treating central sleep apnoea. In these studies, we attempted to determine the effect of aminophylline on neural activities of the upper airway and diaphragm. We administered intravenously either a low dose (4 mg.kg-1) or a high dose (16 mg.kg-1) of aminophylline to decerebrated, vagotomized and paralysed cats, and continuously recorded the phrenic hypoglossal and recurrent laryngeal nerve activities for 3 h. Results showed that a high dose of aminophylline induced a marked increase in phrenic nerve activity, but not hypoglossal or recurrent laryngeal nerve activity. In a group treated with a low dosage of aminophylline, a significant increase of activity was found in all three nerves. Furthermore, phrenic nerve activity increased more with a high dose than with a low dose. We confirmed that aminophylline has dose-dependent and selective effects on respiratory neural activity. A low dose acts on the upper airway and diaphragm, but a high dose induces a marked increase in central respiratory drive. According to our results, low dose aminophylline might be beneficial in obstructive sleep apnoea, whereas, a high or low dose might improve some cases of central sleep apnoea.

NMDA-induced rhythmical activity in XII nerve of isolated CNS from newborn rats

We tried to induce rhythmical oro-facial motor activities in an isolated brain stem-spinal cord preparation from newborn rats. Neural activities were monitored from the hypoglossal nerve (XII N) and the ventral roots of the cervical cord. Bath application of N-methyl-D-aspartate (NMDA) as well as glutamate induced rhythmical burst activity in XII N distinct from and much faster than respiratory rhythm. This NMDA-induced rhythmical activity was blocked by simultaneous application of 2-amino-5-phosphonovalerate (AP5). The results demonstrate that NMDA receptor activation can induce rhythmical XII N activity different from respiration in an isolated mammalian CNS. This preparation will be useful for the investigation of neural mechanisms underlying the central generation of food ingestive movements.

Fate of lysosomes transported to the dendrites by a colchicine-induced mechanism

Lysosomes in hypoglossal motoneurons were retrogradely labeled with fluorescent latex microspheres and their distribution as well as that of acid phosphatase was examined after a single 50 microgram intracerebroventricular (i.c.v.) injection of colchicine. In saline injected controls the fluorescent label was distributed mainly in cell bodies. Twenty-four hours after the colchicine injection we observed a re-distribution of fluorescent label from the cell body of neurons to the dendrites. Seventy-two hours after the colchicine injection the fluorescent label had returned from the dendrites to the cell body. A similar pattern was obtained by following the effect of colchicine on the distribution of acid phosphatase reaction product. We conclude that the reappearance of fluorescent label in cell bodies following colchicine treatment is the result of the retrograde transport of dendritic lysosomes.

Role of GABAB receptors in the control of hypoglossal motoneurons in vivo

There is little information on GABAB receptor-mediated effects on orofacial motoneurons. We recorded the inspiratory activity from both hypoglossal (XII) nerves in urethane-anesthetized, paralyzed, vagotomized and artificially ventilated rats. A GABAB receptor agonist, baclofen, or antagonist, CGP-35348, was microinjected into one XII nucleus. Baclofen rapidly reduced the XII nerve activity in a dose-dependent manner by over 50%. The antagonist caused a delayed suppression of activity by 40%. We conclude that: (1) GABAB receptors within the XII nucleus may suppress the activity of inspiratory XII motoneurons, but they are not tonically active under the conditions of our experiment; (2) there is a net endogenous excitatory effect in XII motoneurons that is mediated by GABAB receptors located in the reticular formation surrounding the XII nucleus.

Effect of muscimol microinjections into the prepositus hypoglossi and the medial vestibular nuclei on cat eye movements

1. For horizontal eye movements, previous observations led to the hypothesis that the legendary neural integrator necessary for correct gaze holding, adequate vestibuloocular reflex (VOR), and optokinetic nystagmus, was located in the region of the complex formed by the nucleus prepositus hypoglossi (NPH) and the medial vestibular nucleus (MVN). 2. The aim of the present study was to test the respective contributions of the NPH, of the rostral part of the MVN, which contains most second-order vestibular neurons, and of the central part of the MVN to the horizontal integrator. 3. An injection of muscimol was used to inactivate each of these three zones in the cat's brain. Muscimol is a gamma-aminobutyric acid (GABA) agonist. By binding to GABAA receptors, it induces a hyperpolarization of the neurons that nullifies their activity. Muscimol was injected into the brain stem of the alert cat through a micropipette by an air pressure system. 4. The search coil technique was used to record spontaneous eye movements and the VOR induced by rotating a turntable at a constant velocity. VOR was analyzed by a new method: transient analysis of vestibular nystagmus. 5. A unilateral injection of muscimol into the NPH induced a bilateral gaze-holding failure: saccades were followed by a centripetal postsaccadic drift. A vestibular imbalance was also present but it was moderate and variable. The VOR responses were distorted drastically. Through transient analysis of vestibular nystagmus, that distortion was revealed to be due more to a failure of the neural integrator than to an alteration of the vestibular input to the neural integrator. The responses to a rotation either toward the injected side or in the opposite direction were asymmetrical. The direction of that asymmetry was variable. 6. A unilateral injection of muscimol into the rostral part of the MVN caused a vestibular imbalance: in complete darkness, a nystagmus appeared, whose linear slow phases were directed toward the side of injection. 7. A unilateral injection of muscimol into the central part of the MVN induced a syndrome where a severe bilateral gaze-holding failure was combined with a vestibular imbalance. In the light, saccades were followed by a bilateral centripetal postsaccadic drift. In complete darkness, a nystagmus was observed, whose curved slow phases were directed towards the side of injection. The VOR responses were distorted drastically. Here again, that distortion was revealed by our analysis to be due more to a failure of the neural integrator than to an alteration of the vestibular input to the neural integrator.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of bicuculline on inspiratory activities in piglets

Antagonism of the alpha receptor sub-type at gamma-aminobutyric acid (GABA) recognition sites in developing pigs was evaluated using the GABAA receptor antagonist bicuculline. The effect of bicuculline infusions was to produce an increase of phrenic and hypoglossal discharge amplitudes. This bicuculline-induced effect on discharge amplitude was manifested in autopower spectra as an increase in the power of peaks located in the medium-frequency (10-50 Hz) band. More importantly, coherence estimates were increased by bicuculline administration demonstrating GABA-mediated influences on a central pattern generator with output in the 10-50 Hz band.

Distinct neurotrophic responses of axotomized motor neurons to BDNF and CNTF in adult rats

In adult mammals, transection of the hypoglossal nerve results in a dramatic loss of choline acetyltransferase (ChAT) in the hypoglossal motor neurons without affecting their cell number. This injury-induced reduction in ChAT is prevented when brain-derived neurotrophic factor (BDNF) is applied to the proximal end of the transected nerve. In contrast, application of ciliary neurotrophic factor (CNTF) has no such effect, even though both factors are known to rescue developing motor neurons from cell death. These results suggest that BDNF may regulate the phenotypic expression of ChAT in mature motor neurons, and indicate that the sensitivity and response of motor neurons to such neurotrophic agents change with development.

Possible involvement of serotonin in obstructive apnea of the newborn

Experiments were conducted on newborn rats in order to study the serotonergic modulation of the central respiratory activity during the neonatal period. In vitro experiments performed on isolated brainstem spinal cord preparations demonstrated that serotonin exerts a permanent excitatory modulation on the central respiratory rhythm generator but depresses the inspiratory motor output to the genioglossus muscle. In vivo experiments confirmed that increasing endogenous serotonin levels decreases the inspiratory activity of the genioglossus and elicits drastic obstructive apneas.

GAP-43 (B50/F1) gene regulation by axonal injury of the hypoglossal nerve in the adult rat

The expression of mRNA encoding the growth associated protein, GAP-43, was investigated in rat hypoglossal motor neurons when the hypoglossal nerve was either resected or crushed unilaterally. For the detection of GAP-43 mRNA, a histochemical in situ hybridization method. using an alkaline phosphatase labeled probe, was used. The temporal profiles of GAP-43 mRNA expression were not identical following the two types of injuries. Increased expression in the hypoglossal nucleus contralateral to the injured nerve was observed from 1 day to 4-6 weeks after nerve crush, but lasted up to 7-8 weeks after resection. The magnitude and duration of increased GAP-43 mRNA expression were significantly greater following resection than crush injury. Local treatment with vinblastine, which is known to disturb the fast axonal flow by depolymerizing tubulin, also induced GAP-43 mRNA expression. The patterns of gene regulation following these nerve injuries may be due to the extent of nerve damage, to tubulin disturbance, or to some other factors derived from outside the nerve.

Inhibition of salt-induced gustatory responses in the frog (Rana catesbeiana) by 5'-GMP

Millimolar concentration of sodium 5'-guanylate (5'-GMP), a 'umami' substance, inhibited salt-induced gustatory neural responses, particularly tonic components, of the bullfrog when the tongue was adapted to a low salt (5 mM NaCl) solution but not when adapted to normal saline that contained 115 mM NaCl. The result suggests that 5'-GMP is a modulator of adaptation process in salt response in the bullfrog taste system.

Effects of GABAA receptor antagonism on inspiratory activities in kittens

In anesthetized kittens (< 1 to 24 days old), the effects of GABAA receptor antagonism on phrenic, hypoglossal and cervical sympathetic discharges were examined by i.v. bicuculline infusions during hyperoxia and hypoxia. Administration of bicuculline during hyperoxia produced marked increases in the amplitudes of inspiratory nerve discharges. During hypoxic stimulation (10% O2), the amplitudes of inspiratory activities decreased towards or below those observed during hyperoxia; bicuculline reversed this depression and restored inspiratory discharges. Our results indicated that GABAA receptors were functional shortly after birth, acting to mediate influences shaping inspiratory activity during hyperoxic breathing and during conditions of increased chemical drive.