5-HT at hypoglossal motor nucleus and respiratory control of genioglossus muscle in anesthetized rats

Serotonin (5-HT) from medullary raphe neurons excites hypoglossal motoneurons innervating genioglossus (GG) muscle. Since some raphe neurons also show increased activity in hypercapnia, we tested the hypothesis that serotonergic mechanisms at the hypoglossal motor nucleus (HMN) modulate GG activity and responses to CO2. Seventeen urethane-anesthetized, tracheotomized and vagotomized rats were studied. Microdialysis probes were used to deliver mianserin (5-HT receptor antagonist, 0 and 0.1 mM) or 5-HT (eight doses, 0-50 mM) to the HMN during room air or CO2-stimulated breathing. Mianserin decreased respiratory-related GG activity during room air and CO2-stimulated breathing (P<0.001), and also suppressed GG responses to CO2 (P=0.05). In contrast, GG activity was increased by 5-HT at the HMN, and was further increased in hypercapnia (P<0.02). However, 5-HT increased respiratory-related GG activity at levels lower (1 mM) than those eliciting tonic GG activity (10-30 mM 5-HT). The results show that 5-HT at the HMN contributes to the respiratory control of GG muscle.     

Modulation of Genioglossus Muscle Activity Across Sleep-Wake States by Histamine at the Hypoglossal Motor Pool

Study Objectives:

Histamine neurons comprise a major component of the aminergic arousal system and significantly influence sleep-wake states, with antihistamines widely used as sedative hypnotics. Unlike the serotonergic and noradrenergic components of this arousal system, however, the role of histamine in the central control of respiratory motor activity has not been determined. The aims of this study were to characterize the effects of histamine receptor agonists and antagonists at the hypoglossal motor pool on genioglossus muscle activity across sleep and awake states, and also determine if histamine contributes an endogenous excitatory drive to modulate hypoglossal motor outflow to genioglossus muscle.

Design, Participants, and Interventions:

Thirty-three rats were implanted with electroencephalogram and neck electrodes to record sleep-wake states, and genioglossus and diaphragm electrodes for respiratory muscle recordings. Microdialysis probes were inserted into the hypoglossal motor nucleus.

Measurements and Results:

Histamine at the hypoglossal motor nucleus significantly increased tonic genioglossus muscle activity in wakefulness, non-REM sleep and REM sleep. The activating effects of histamine on genioglossus muscle activity also occurred with a histamine type-1 (H₁) but not H₂ receptor agonist. However, H₁ receptor antagonism at the hypoglossal motor nucleus did not decrease genioglossus muscle activity in wakefulness or sleep.

Conclusions:

The results suggest that histamine at the hypoglossal motor pool increases genioglossus muscle activity in freely behaving rats in wakefulness, non-REM, and REM sleep via an H₁ receptor mechanism.

Citation:

Bastedo T; Chan E; Park E; Liu H; Horner RL. Modulation of genioglossus muscle activity across sleep-wake states by histamine at the hypoglossal motor pool. SLEEP 2009;32(10):1313-1324.     

Systemic vs. central administration of common hypnotics reveals opposing effects on genioglossus muscle activity in rats

STUDY OBJECTIVES: To determine if systemic administration of selected sedative-hypnotics that modulate the function of the y-amino-butyric acid-A (GABAA) receptor can: (i) delay arousal thereby allowing genioglossus (GG) activity to increase more in response to respiratory stimulation during sleep, (ii) also cause the robust increase in GG activity during undisturbed sleep recently observed with barbiturates. We also determined effects on GG activity with local application to the hypoglossal motor nucleus (HMN). DESIGN, PARTICIPANTS, AND INTERVENTIONS: Sleep-wake states, GG and diaphragm activities were recorded in freely-behaving rats after systemic administration of lorazepam (0.5 mg/kg and 1 mg/kg, n = 9 and 5 mg/kg, n = 7), zolpidem (5 mg/kg and 10 mg/kg, n = 6) and the antihistamine diphenhydramine (20 mg/kg, n = 9). Rats were also exposed to ramp increases in inspired CO2 in NREM sleep. The effects of lorazepam and zolpidem applied directly to the HMN were also determined in 37 anesthetized rats. MEASUREMENTS AND RESULTS: Lorazepam, zolpidem and diphenhydramine all increased arousal threshold, consistent with their sedative action. GG activity before arousal in response to hypercapnia was increased with lorazepam and zolpidem only, an effect mainly due to increased baseline activity before CO2 stimulation. Lorazepam and zolpidem applied directly to the HMN, however, decreased GG activity. CONCLUSIONS: Lorazepam and zolpidem have an inhibitory effect on GG activity via local effects at the HMN. Following systemic administration, however, this inhibitory effect can be outweighed both by a delay in arousal (allowing greater CO2-mediated respiratory stimulation in sleep) and excitatory influences on baseline GG activity via mechanisms operating outside the HMN.     

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.     

Activity of neurons located in the region of the hypoglossal motor nucleus during swallowing in sheep

Summary Extracellular activity of swallowing neurons (SN) in the region of the hypoglossal (XIIth) motor nucleus was studied in sheep anesthetized with halothane. Eighty six SN exhibited a discharge closely linked to swallowing electromyographic (EMG) activity of the geniohyoïd (GH) muscle induced by stimulation of the superior laryngeal nerve. Swallowing activation persisted after motor paralysis, indicating that this activity did not result from sensory feedback. SN were classified into two groups. Group I SN (N = 66) discharged a burst of up to 12 spikes for 50–300 ms during the response of GH. Mean frequencies ranged from 10 to 60 Hz, peak instantaneous frequencies from 10 to 100 Hz. Thirty two SN were antidromically activated by stimulating the XIIth nerve. Mean latencies of antidromic spikes were 2.6 ms and 2.4 ms for SN sending their axons in the medial and lateral branches respectively of XIIth nerve, corresponding to conduction velocities of 50.4 m/s and 53.7 m/s. The other 34 group I SN were located at sites with large antidromic field potentials obscuring the identification of unitary antidromic spikes. Thirty group I SN, 16 antidromically activated and 14 in areas with large field potentials, were histologically located in the XIIth motor nucleus between the transverse planes 1 mm caudal to 2 mm rostral to the obex. Group I SN are likely motoneurons supplying GH and tongue musculature. Group II SN (N = 20) were never antidromically activated by XIIth nerve stimulation, and were all located in the reticular formation adjacent to the lateral edge of the XIIth motor nucleus, particularly in transverse planes within 1 mm of the obex. During swallowing, group II SN exhibited an activation (10–50 spikes) lasting from 100 to 370 ms and generally starting before the onset of GH activity. Mean frequencies ranged from 60 to 200 Hz, and peak instantaneous frequencies from 120 to 400 Hz. Central microstimulation of group II SN evoked in the ipsilateral XIIth nerve a synaptic potential with a latency 0.8 to 1.3 ms longer than that induced by directly stimulating the XIIth motor nucleus. In addition, stimulation of group II SN was also effective in eliciting EMG activity of the contralateral GH. These results suggest that group II SN are interneurons involved in the bilateral activation of GH and tongue muscles.     

The motor innervation of the single-bellied digastric muscle in the rabbit: A retrograde horseradish peroxidase study

The digastric muscle of the rabbit consists of a single anterior belly which inserts onto the lower jaw. Horseradish peroxidase was injected into the muscle and into subcutaneous regions overlying the lower jaw to determine the sites of origin of the motor innervation to both the digastric muscle and the platysma muscles. After digastric muscle injection, labelled cells were found in the ipsilateral retrotrigeminal nucleus as well as in the intermediate subnucleus of the main facial nucleus on both sides. Subcutaneous injections produced labelling which was found bilaterally in the intermediate subnucleus and in the ventromedial portion of the medial subnucleus. These results are interpreted in relation to the common embryological origin of these two muscles and their innervation.     

Protein kinase A activators produce a short-term,but not long-term,increase in respiratory-drive transmission at the hypoglossal motor nucleus in vivo

Synaptic plasticity is an intrinsic and conserved feature of neuronal activity that has been most extensively studied in the context of learning and memory in Aplysia and the mammalian hippocampus. However, the intracellular mechanisms underlying plasticity at motor nuclei, influencing motor behaviour, are less well studied. In vitro experiments in neonatal rodents indicate that protein kinase A (PKA) modulates respiratory-drive transmission at the hypoglossal motor nucleus (HMN), which innervates the genioglossus muscle of the tongue. We hypothesised that PKA activators at the HMN would increase genioglossus activity in vivo, whereas a PKA inhibitor would suppress activity indicative of constitutive PKA activation. Since PKA activators are importantly involved in models of long-term augmentation of neuronal activity following massed stimulation [16], we also hypothesised that application of PKA activators to the HMN would produce long-term facilitation of genioglossus activity. Experiments were performed in 25 isoflurane-anaesthetised, tracheotomised, spontaneously breathing adult rats. Microdialysis perfusion of 8-Br-cAMP (direct PKA activator) into the HMN increased genioglossus activity compared to baseline levels with artificial cerebrospinal fluid (P < 0.001). Application of forskolin (indirect PKA activator) had a similar effect (P < 0.002). Genioglossus activity progressively decreased back to baseline during a 90-min washout with artificial cerebrospinal fluid, demonstrating a lack of long-term facilitation of genioglossus activity. Similar to massed application of 8-Br-cAMP to the HMN, intermittent application produced a short-term (P < 0.001), but not long-term, increase in genioglossus activity in vivo. Application of Rp-8-Cl-cAMPS (PKA inhibitor) did not decrease genioglossus activity, indicating a lack of constitutive PKA activation.     

Respiratory reactions to microinjection of bombesin into the solitary tract nucleus and their mechanisms

Acute experiments were performed on urethane-anesthetized adult laboratory rats to investigate the effects of microinjections of 10⁻¹³−10⁻⁴ M bombesin into the solitary tract nucleus on measures of respiration. Bombesin microinjections were found to stimulate respiration, inducing significant increases in the level of pulmonary ventilation, increases in respiratory volume, and increases in the bioelectrical activity of the inspiratory muscles. The most marked respiratory reactions were seen after intermediate peptide doses (10⁻¹⁰−10⁻⁷ M). These respiratory effects of bombesin were found to result from its ability to suppress the inspiration-inhibiting Hering-Breuer reflex at the level of the solitary tract nucleus. The fact that ultralow doses of bombesin were active, along with the distribution of endogenous bombesin and its specific receptors in the solitary tract nucleus, and the ability of this peptide to modulate the Hering-Breuer reflex all provide evidence that bombesin is involved in controlling respiration at the level of the dorsal structures of the respiratory center. __________ Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 91, No. 5, pp. 521–529, May, 2005.     

The scaling of motor noise with muscle strength and motor unit number in humans

Understanding the origin of noise, or variability, in the motor system is an important step towards understanding how accurate movements are performed. Variability of joint torque during voluntary activation is affected by many factors such as the precision of the descending motor commands, the number of muscles that cross the joint, their size and the number of motor units in each. To investigate the relationship between the peripheral factors and motor noise, the maximum voluntary torque produced at a joint and the coefficient of variation of joint torque were recorded from six adult human subjects for four muscle/joint groups in the arm. It was found that the coefficient of variation of torque decreases systematically as the maximum voluntary torque increases. This decreasing coefficient of variation means that a given torque or force can be more accurately generated by a stronger muscle than a weaker muscle. Simulations demonstrated that muscles with different strengths and different numbers of motor units could account for the experimental data. In the simulations, the magnitude of the coefficient of variation of muscle force depended primarily on the number of motor units innervating the muscle, which relates positively to muscle strength. This result can be generalised to the situation where more than one muscle is available to perform a task, and a muscle activation pattern must be selected. The optimal muscle activation pattern required to generate a target torque using a group of muscles, while minimizing the consequences of signal dependent noise, is derived.     

Functional roles of 5-hydroxytryptamine 3/4 receptors in neurons of rat dorsal motor nucleus of the vagus

In neurons of dorsal motor nucleus of the vagus that is involved in the gastric motility and possibly emesis, application of 5-hydroxytryptamine produces membrane depolarization, and suppresses spike-repolarization and spike-afterhyperpolarization, suggesting divergent effects of 5-hydroxytryptamine through activating multiple subtypes of 5-hydroxytryptamine receptors. However, only the role of 5-hydroxytryptamine 2A receptors has been established to be responsible for the depolarization, and the mechanisms underlying the modulation of spikes remain unknown although a role of 5-hydroxytryptamine 4 receptors was implicated in modulations of spikes. There is now increasing evidence for the role of 5-hydroxytryptamine receptors in neurons involved in generating emesis following administration of anticancer drug. Since antagonists of 5-hydroxytryptamine 3/4 receptors are widely used as anti-emetic drugs, we have reevaluated the functional roles of 5-hydroxytryptamine 3/4 receptors of dorsal motor nucleus of the vagus neurons, especially in modulating transient outward currents that are presumed to be involved in spike-repolarization and spike-afterhyperpolarization. Whole-cell patch-clamp recordings were made from the dorsal motor nucleus of the vagus neurons, which were identified by a retrograde tracing method with dextran-tetramethylrhodamine-lysine injected into a bundle of abdominal vagus nerves. Under a voltage-clamp condition, dorsal motor nucleus of the vagus neurons expressed a prominent A-like current. The activation of 5-hydroxytryptamine 3 receptors reversibly increased the resting membrane conductance while the activation of 5-hydroxytryptamine 4 receptors led to an almost irreversible decrease in the A-like current. A long-lasting suppression of A-like current by transient activation of 5-hydroxytryptamine 4 receptors would result in a long-lasting increase in the excitability of dorsal motor nucleus of the vagus neurons, which might be involved in generation of the long-lasting facilitation of gastric motility or in generation of the long-lasting gastric relaxation through the activation of enteric non-adrenergic non-cholinergic neurons as implicated in the delayed emesis induced by anticancer drugs.     

Vestibulospinal and reticulospinal interactions in the activation of back muscle EMG in the rat

Summary The effects of electrical stimulation of the lateral vestibular nucleus (LVN) and medullary reticular formation (RF) on electromyographic activity in axial muscles medial longissimus (ML) and lateral longissimus (LL) in the rat were studied. Long trains (150–500 ms) at 200–330 Hz and 20–100 A were sufficient to activate ML and LL at latencies of 20–100 ms from the beginning of the train. Results of stimulation at 200–330 Hz to RF or LVN showed that muscle units were activated at a fixed latency from any effective pulse in the stimulus train. Using high frequency (1 kHz) trains of 3–6 pulses to LVN, EMG activity was detected at minimum latencies of 3.5–6 ms. When conduction times from the medulla to the spinal cord, and the spinal cord to the muscle are subtracted, this latency range is consistent with monosynaptic activation. In many cases, muscle units were recruited in order of size, with both RF and LVN stimulation. Combined stimulation of LVN and RF sites in n. gigantocellularis led to EMG activity in ML and LL at currents which were insufficient to evoke activity when presented singly. When stimulation of one site (300–400 ms train) was just sufficient to evoke a response, a shorter, overlapping train (100–150 ms) to the other site led to a higher rate of muscle activity that continued through the end of the long train, even after the short train had ended. In all cases, the effect of RF facilitating LVN was similar to the effect of LVN facilitating RF. The evidence for convergence between these two systems in the medulla and the spinal cord is discussed.     

Expression of glutamate receptor subunits 2/3 and 4 in the hypoglossal nucleus of the rat after neurectomy

The immunoreactivity of glutamate receptor subunits 2/3 (GluR2/3) and 4 (GluR4) was studied following neurectomy of the hypoglossal nucleus (NH). After a short period of survival (at 1, 2, and 7 days postoperation, dpo), GluR2/3 immunoreactivity was barely dectectable in the operated side of HN. During these periods, GluR4 immunoreactivity was present, but was greatly reduced when compared with the GluR4 immunoreactivity in the unoperated side. The data suggest that of the 4 subunits of the AMPA receptor, GluR2/3 is the most susceptible receptor to the early stage of hypoglossal neurectomy, and GluR4 tolerated the lesion more than the others. It is also suggested that both GluR2/3 and 4 may play a very important neuroprotective role in the early stage of neuronal degeneration after axotomy, especially the former. Following a midterm survival period (14, 21, and 35 dpo), GluR2/3 immunoreactivity gradually reappeared in some neurons on the operated side of HN, which may indicate functional recovery. However, the number of GluR4-immunopositive neurons on the operated side of HN was greatly reduced. The reason for such a reduction is not known, but, from the speculative point of view, it is possible that the disappearance of GluR4-positive neurons may be related to their excitotoxic property, especially at 35 dpo, when neuronal cell death had already occurred. Following a long-term period of survival (i.e., 56, 90, and 120 dpo), the numbers of surviving neurons remained fairly constant, suggesting the possible cessation of neuronal death. Received: 15 April 1997 / Accepted: 13 June 1997     

Cerebellar control of the red nucleus and adjacent structures in the young rabbit

Maturation of the cerebellar influence on rubral cells was studied in70 young rabbits aged from 3 to 17 days. Extracellular spike potentials of 187 neurons belonging to the magnocellular part of the red nucleus were recorded with glass microelectrodes. The differences observed between young and old animals are quantitative and particularly striking. Out of 107 cells in animals less than 8 days old, only 7 were facilitated or inhibited by cerebellar stimulation, whereas in older animals, out of 80 cells, 76 were facilitated or inhibited. Similar results were obtained for 273 perirubral cells and 72 oculomotor cells. The present results suggest that cerebello-interposital or interposito-rubral connections are not yet functional during the first postnatal week.     

Postnatal changes in the expressions of serotonin 1A, 1B,and 2A receptors in ten brain stem nuclei of the rat: implication for a sensitive period

A critical period in respiratory network development occurs in the rat around postnatal days (P) 12–13, when abrupt neurochemical, metabolic, and physiological changes were evident. As serotonin and its receptors are involved in respiratory modulation, and serotonergic abnormality is implicated in sudden infant death syndrome, we hypothesized that 5-HT receptors are significantly downregulated during the critical period. This was documented recently for 5-HT_2AR in several respiratory nuclei. The present study represents a comprehensive analysis of postnatal development of 5-HT_1AR and 5-HT_1BR in 10 brain stem nuclei and 5-HT_2AR in six nuclei not previously examined. Optical densitometric analysis of immunohistochemically-reacted neurons from P2 to P21 indicated four developmental patterns of expression: (1) Pattern I: a high level of expression at P2-P11, an abrupt and significant reduction at P12, followed by a plateau until P21 (5-HT_1AR and 5-HT_1BR in raphé magnus [RM], raphé obscurus [ROb], raphé pallidus [RP], pre-Bötzinger complex [PBC], nucleus ambiguus [Amb], and hypoglossal nucleus [XII; 5-HT_1AR only]). (2) Pattern II: a high level at P2-P9, a gradual decline from P9 to P12, followed by a plateau until P21 (5-HT_1AR and 5-HT_1BR in the retrotrapezoid nucleus (RTN)/parafacial respiratory group (pFRG)). (3) Pattern III: a high level at P2-P11, followed by a gradual decline until P21 (5-HT_1AR in the ventrolateral subnucleus of solitary tract nucleus [NTS_VL] and the non-respiratory cuneate nucleus [CN]). (4) Pattern IV: a relatively constant level maintained from P2 to P21 (5-HT_1AR in the commissural subnucleus of solitary tract nucleus (NTS_COM); 5-HT_1BR in XII, NTS_VL, NTS_COM, and CN; and 5-HT_2AR in RM, ROb, RP, RTN/pFRG, NTS_VL, and NTS_COM). Thus, a significant reduction in the expression of 5-HT_1AR, 5-HT_1BR, and 5-HT_2AR in multiple respiratory-related nuclei at P12 is consistent with reduced serotonergic transmission during the critical period, thereby rendering the animals less able to respond adequately to ventilatory distress.     

Modulation of TASK-1/3 channels at the hypoglossal motoneuron pool and effects on tongue motor output and responses to excitatory inputs in vivo: implications for strategies for obstructive sleep apnea pharmacotherapy

Obstructive sleep apnea (OSA) occurs exclusively during sleep due to reduced tongue motor activity. Withdrawal of excitatory inputs to the hypoglossal motor nucleus (HMN) from wake to sleep contributes to this reduced activity. Several awake–active neurotransmitters with inputs to the HMN (e.g. serotonin [5-HT]) inhibit K⁺ leak mediated by TASK-1/3 channels on hypoglossal motoneurons, leading to increased neuronal activity in vitro. We hypothesize that TASK channel inhibition at the HMN will increase tongue muscle activity in vivo and modulate responses to 5-HT. We first microperfused the HMN of anesthetized rats with TASK channel inhibitors: doxapram (75 μM, n = 9), A1899 (25 μM, n = 9), ML365 (25 μM, n = 9), acidified artificial cerebrospinal fluid (ACSF, pH = 6.25, n = 9); and a TASK channel activator terbinafine (50 μM, n = 9); all with and without co-applied 5-HT (10 mM). 5-HT alone at the HMN increased tongue motor activity (202.8% ± 45.9%, p < 0.001). However, neither the TASK channel inhibitors, nor activator, at the HMN changed baseline tongue activity (p > 0.716) or responses to 5-HT (p > 0.127). Tonic tongue motor responses to 5-HT at the HMN were also not different (p > 0.05) between ChAT-Cre:TASK^f/f mice (n = 8) lacking TASK-1/3 channels on cholinergic neurons versus controls (n = 10). In freely behaving rats (n = 9), microperfusion of A1899 into the HMN increased within-breath phasic tongue motor activity in wakefulness only (p = 0.005) but not sleep, with no effects on tonic activity across all sleep–wake states. Together, the findings suggest robust maintenance of tongue motor activity despite various strategies for TASK channel manipulation targeting the HMN in vivo, and thus currently do not support this target and direction for potential OSA pharmacotherapy.     

N-甲基-D-天冬氨酸受体在大鼠丘脑前核和扣带后回的表达

目的探讨大鼠丘脑前核和扣带后回内N-甲基-D-天冬氨酸受体NMDAR2A,NMDAR2B及NMDAR1(NR2A.NR2B及NR1)mRNA的表达,从形态学角度为系统研究丘脑前核的功能提供了依据。方法应用原位杂交方法检测丘脑前核及扣带后回内NR2A,NR2B以及NR1 mRNA的表达。结果原位杂交阳性反应产物为棕黄色,主要分布在神经元核周围胞浆中,胞核基本不着色。在丘脑前核,阳性神经元分布较密集,细胞形态较一致。扣带后回皮质阳性神经元在在大脑皮质不同部位分布有差异,分子层染色最弱,外颗粒层密集分布且染色增强,余四层较外颗粒层松散,染色细胞减少。结论 NR2A,NR2B以及NR1广泛分布在丘脑前核和扣带后回。     

Intrinsic gamma-aminobutyric acid neurons in the nucleus of the solitary tract and the rostral ventrolateral medulla of the rat: an immunocytochemical and biochemical study

Sympathoexcitatory neurons in the C1 adrenergic area of the rostral ventrolateral medulla (RVL) are tonically inhibited by gamma-aminobutyric acid (GABA). To identify the source of this GABAergic input, the distribution of neurons containing glutamate decarboxylase (GAD) was determined immunocytochemically in rats treated with colchicine. Numerous GAD-stained neurons were located in the nucleus of the solitary tract (NTS) and in RVL. Unilateral lesions in NTS did not alter GABA content or GAD activity in RVL, indicating that the afferent projection from NTS to RVL is not GABAergic. Intrinsic GABAergic neurons in RVL may provide tonic inhibition of vasomotor neurons in the C1 area.     

Metabotropic glutamate receptors subtype 5 are necessary for the enhancement of auditory evoked potentials in the lateral nucleus of the amygdala by tetanic stimulation of the auditory thalamus

The lateral nucleus of the amygdala (LA) receives axonal projections from the auditory thalamus, the medial geniculate nucleus (MGN), and mediates auditory fear conditioning. Tetanic electrical stimulation of the MGN can induce long-term potentiation of acoustically-evoked responses (AEPs) recorded in the LA of anesthetized rats. The present study investigated the temporal development of tetanus-induced AEP potentiation recorded in the LA of anesthetized rats during the recording time up to 120 min after tetanization. In addition, the present study investigated whether the artificially-induced AEP potentiation is mediated by the metabotropic glutamate receptors subtype 5 (mGluR5). The results show that AEPs recorded in the LA to a broadband-noise burst were significantly enhanced immediately after tetanic but not low-frequency stimulation of the MGN. The AEP potentiation was well retained up to 120 min after tetanization. High-dose (1.5 microg/4 microl) microinjection of the selective antagonist of mGluR5, 2-methyl-6-(phenylethynyl)-pyridine (MPEP), into the ipsilateral lateral ventricle 30 min before tetanization completely blocked the AEP potentiation without affecting the baseline AEP. Low-dose (0.5 microg/4 microl) microinjection partially suppressed the AEP potentiation. When the high-dose MPEP was injected 40 min after tetanization, the AEP potentiation was not affected. These results indicate that in anesthetized rats mGluR5 receptors are necessary for the induction or early maintenance (40 min) of AEP potentiation in the LA by tetanic stimulation of the MGN.     

Subcellular and subsynaptic localization of group I metabotropic glutamate receptors in the nucleus accumbens of cocaine-treated rats

There is significant pharmacological and behavioral evidence that group I metabotropic glutamate receptors (mGluR1a and mGluR5) in the nucleus accumbens play an important role in the neurochemical and pathophysiological mechanisms that underlie addiction to psychostimulants. To further address this issue, we undertook a detailed ultrastructural analysis to characterize changes in the subcellular and subsynaptic localization of mGluR1a and mGluR5 in the core and shell of nucleus accumbens following acute or chronic cocaine administration in rats. After a single cocaine injection (30 mg/kg) and 45 min withdrawal, there was a significant decrease in the proportion of plasma membrane-bound mGluR1a in accumbens shell dendrites. Similarly, the proportion of plasma membrane-bound mGluR1a was decreased in large dendrites of accumbens core neurons following chronic cocaine exposure (i.e. 1-week treatment followed by 3-week withdrawal). However, neither acute nor chronic cocaine treatments induced significant change in the localization of mGluR5 in accumbens core and shell, which is in contrast with the significant reduction of plasma membrane-bound mGluR1a and mGluR5 induced by local intra-accumbens administration of the group I mGluR agonist, (RS)-3,5-dihydroxyphenylglycine (DHPG). In conclusion, these findings demonstrate that cocaine-induced glutamate imbalance has modest effects on the trafficking of group I mGluRs in the nucleus accumbens. These results provide valuable information on the neuroadaptive mechanisms of accumbens group I mGluRs in response to cocaine administration.