GABA-induced responses in electrophysiologically characterized neurons within the rat rostro-ventrolateral medulla in vitro

Glycinergic neurotransmission in the rostral ventrolateral medulla controls the time course of baroreflex-mediated sympathoinhibition

KEY POINTS

To maintain appropriate blood flow to various tissues of the body under a variety of physiological states, autonomic nervous system reflexes regulate regional sympathetic nerve activity and arterial blood pressure. Our data obtained in anaesthetized rats revealed that glycine released in the rostral ventrolateral medulla (RVLM) plays a critical role in maintaining arterial baroreflex sympathoinhibition. Manipulation of brainstem nuclei with known inputs to the RVLM (nucleus tractus solitarius and caudal VLM) unmasked tonic glycinergic inhibition in the RVLM. Whole-cell, patch clamp recordings demonstrate that both GABA and glycine inhibit RVLM neurons. Potentiation of neurotransmitter release from the active synaptic inputs in the RVLM produced saturation of GABAergic inhibition and emergence of glycinergic inhibition. Our data suggest that GABA controls threshold excitability, wherreas glycine increases the strength of inhibition under conditions of increased synaptic activity within the RVLM.

ABSTRACT

The arterial baroreflex is a rapid negative-feedback system that compensates changes in blood pressure by adjusting the output of presympathetic neurons in the rostral ventrolateral medulla (RVLM). GABAergic projections from the caudal VLM (CVLM) provide a primary inhibitory input to presympathetic RVLM neurons. Although glycine-dependent regulation of RVLM neurons has been proposed, its role in determining RVLM excitability is ill-defined. The present study aimed to determine the physiological role of glycinergic neurotransmission in baroreflex function, identify the mechanisms for glycine release, and evaluate co-inhibition of RVLM neurons by GABA and glycine. Microinjection of the glycine receptor antagonist strychnine (4 mm, 100 nL) into the RVLM decreased the duration of baroreflex-mediated inhibition of renal sympathetic nerve activity (control = 12 ± 1 min; RVLM-strychnine = 5.1 ± 1 min), suggesting that RVLM glycine plays a critical role in regulating the time course of sympathoinhibition. Blockade of output from the nucleus tractus solitarius and/or disinhibition of the CVLM unmasked tonic glycinergic inhibition of the RVLM. To evaluate cellular mechanisms, RVLM neurons were retrogradely labelled (prior injection of pseudorabies virus PRV-152) and whole-cell, patch clamp recordings were obtained in brainstem slices. Under steady-state conditions GABAergic inhibition of RVLM neurons predominated and glycine contributed less than 25% of the overall inhibition. By contrast, stimulation of synaptic inputs in the RVLM decreased GABAergic inhibition to 53%; and increased glycinergic inhibition to 47%. Thus, under conditions of increased synaptic activity in the RVLM, glycinergic inhibition is recruited to strengthen sympathoinhibition.

Effects of centrally administered glucagon-like peptide-2 on blood pressure and barosensitive neurons in spontaneously hypertensive rats

The central administration of glucagon-like peptide-2 (GLP-2) decreases blood pressure in rats. In the present study, we investigated the hypotensive effects of GLP-2 using spontaneously hypertensive rats (SHRs), an animal model of hypertension. The central administration of GLP-2 (0.6 μg) decreased mean arterial pressure (MAP) in SHRs (-24.1 ± 4.5%; P < 0.05), but not in normotensive Wistar-Kyoto (WKY) rats (-10.6 ± 7.4%; P > 0.05), whereas GLP-2 (6 μg) decreased MAP in WKY rats (-23.5 ± 4.2%; P < 0.05) and SHRs (-46.7 ± 11.6%; P < 0.01) under anesthesia with urethane and α-chloralose. Histological analyses revealed that the central administration of GLP-2 (6 μg) induced Fos immunoreactivity (Fos-IR) in the hypothalamic and medullary areas in WKY rats and SHRs. However, the distribution of Fos-IR in GABAergic neurons in the rostral ventrolateral medulla (RVLM) differed between WKY rats and SHRs. GLP-2 directly modulated the excitability of RVLM neurons in brainstem slices from SHRs, but not WKY rats. These results suggest that neuronal activity through the activation of GLP-2 receptors in the RVLM contributes to lowering blood pressure in SHRs.

The brainstem reticular formation is a small-world, not scale-free, network

Recently, it has been demonstrated that several complex systems may have simple graph-theoretic characterizations as so-called 'small-world' and 'scale-free' networks. These networks have also been applied to the gross neural connectivity between primate cortical areas and the nervous system of Caenorhabditis elegans. Here, we extend this work to a specific neural circuit of the vertebrate brain--the medial reticular formation (RF) of the brainstem--and, in doing so, we have made three key contributions. First, this work constitutes the first model (and quantitative review) of this important brain structure for over three decades. Second, we have developed the first graph-theoretic analysis of vertebrate brain connectivity at the neural network level. Third, we propose simple metrics to quantitatively assess the extent to which the networks studied are small-world or scale-free. We conclude that the medial RF is configured to create small-world (implying coherent rapid-processing capabilities), but not scale-free, type networks under assumptions which are amenable to quantitative measurement.

Genetics of human brain oscillations

In the last three decades, much emphasis has been placed on neural oscillations in vitro, in vivo, as well as in the human brain. These brain oscillations have been studied extensively in the resting electroencephalogram (EEG), as well as in the underlying evoked oscillations that make up the event-related potentials (ERPs). There are several approaches to elucidate the possible mechanisms of these brain oscillations. One approach is to look at the neurophysiology and neurochemistry involved in generating and modulating these oscillations. Another more recent approach is to examine the genetic underpinnings of these neural oscillations. It is proposed that the genetic underpinnings of these oscillations are likely to stem from regulatory genes which control the neurochemical processes of the brain, and therefore influence neural function. Genetic analyses of human brain oscillations may identify genetic loci underlying the functional organization of human neuroelectric activity. Brain oscillations represent important correlates of human information processing and cognition. They represent highly heritable traits that are less complex and more proximal to gene function than either diagnostic labels or traditional cognitive measures. Therefore these oscillations may be utilized as phenotypes of cognition and as valuable tools for the understanding of some complex genetic disorders. Genetic loci that have been recently identified regarding both resting and evoked brain oscillations involving the GABAergic and cholinergic neurotransmitter systems of the brain are discussed. It is concluded that the advent of genomics and proteomics and a fuller understanding of gene regulation will open new horizons on the critical electrical events so essential for human brain function.

The utility of neurophysiological markers in the study of alcoholism

OBJECTIVE

This review attempts to differentiate neuroelectric measures (electroencephalogram (EEG), event-related potentials (ERPs) and event-related oscillations (EROs)) related to acute and chronic effects of alcohol on the brain from those that reflect underlying deficits related to the predisposition to develop alcoholism and related disorders. The utility of these neuroelectric measures as endophenotypes for psychiatric genetics is evaluated.

METHODS

This article reviews the main findings of EEG and ERP abnormalities in alcoholics, offspring of alcoholics at high risk to develop alcoholism and the electrophysiological effects of alcohol on high risk compared to low-risk offspring. It highlights findings using EROs, a fast developing tool in examining brain function and cognition. It also reviews evidence of genetic findings related to these electrophysiological measures and their relationship to clinical diagnosis.

RESULTS

Many of these abnormal neuroelectric measures are under genetic control, may precede the development of alcoholism, and may be markers of a predisposition toward the development of a spectrum of disinhibitory conditions including alcoholism. Genetic loci underlying some neuroelectic measures that involve neurotransmitter systems of the brain have been identified.

CONCLUSIONS

Quantitative neuroelectric measures (EEG, ERPs, EROs) provide valuable endophenotypes in the study of genetic risk to develop alcoholism and related disorders.

SIGNIFICANCE

Genetic studies of neuroelectric endophenotypes offer a powerful strategy for identifying susceptibility genes for developing psychiatric disorders, and provide novel insights into etiological factors.

Barosensitive neurons in the rat tractus solitarius and paratrigeminal nucleus: a new model for medullary, cardiovascular reflex regulation

The nucleus of the solitary tract (NTS), a termination site for primary afferent fibers from baroreceptors and other peripheral cardiovascular receptors, contains blood pressure-sensitive neurons, some of which have rhythmic activity locked to the cardiac cycle, making them key components of the central pathway for cardiovascular regulation. The paratrigeminal nucleus (Pa5), a small collection of medullary neurons in the dorsal lateral spinal trigeminal tract, like the NTS, receives primary somatosensory inputs of glossopharyngeal, vagal, and other nerves. Recent studies show that the Pa5 has efferent connections to the rostroventrolateral reticular nucleus (RVL), NTS, and ambiguus nucleus, suggesting that its structure may play a role in the baroreceptor reflex modulation. In the present study, simultaneous recording from multiple single neurons in freely behaving rats challenged with i.v. phenylephrine administration, showed that 83% of NTS units and 72% of Pa5 units were baroreceptor sensitive. Whereas most of the baroreceptor-sensitive NTS and Pa5 neurons (86 and 61%, respectively) increased firing rate during the ascending phase of the pressor response, about 16% of Pa5 and NTS baroreceptor-sensitive neurons had a decreased firing rate. On one hand, the decrease in firing rate occurred during the ascending phase of the pressor response, indicating sensitivity to rapid changes in arterial pressure. On the other hand, the increases in neuron activity in the Pa5 or NTS occurred during the entire pressor response to phenylephrine. Cross-correlational analysis showed that 71% of Pa5 and 93% of NTS baroreceptor-activated neurons possessed phasic discharge patterns locked to the cardiac cycle. These findings suggest that the Pa5, like the NTS, acts as a terminal for primary afferents in the medullary-baroreflex or cardiorespiratory-reflex pathways.Key words: cardiovascular reflexes, baroreflex response, arterial blood pressure, multiple single unit recording.

Levetiracetam opposes the action of GABAA antagonists in hypothalamic neurones

Whole-cell current-clamp recordings in guinea-pig brain slices were used to assess the effect of the novel antiepileptic drug levetiracetam (LEV; Keppra) on the gamma-aminobutyric acid-A (GABA(A)) responses evoked by exogenous applications of the agonists GABA and muscimol on hypothalamic neurones. LEV (40 microM) had no direct effect on GABA(A) responses but it occluded the GABA(A)-receptor blocking action of bicuculline-methiodide (100 microM) and, to a lesser extent, the GABA(A)-receptor blocking action of gabazine (50 microM). While previous reports have indicated that the inhibition by LEV of the epileptiform hyperexcitability induced by bicuculline in rat hippocampus might occur via non-GABAergic mechanisms, the present data suggest a possible indirect modulation by LEV of GABA-gated currents in guinea-pig hypothalamic neurones.

Detection of mRNA species in bulbospinal neurons isolated from the rostral ventrolateral medulla using single-cell RT-PCR

The rostral ventrolateral medulla (RVL) contains neurons which are critically involved in the tonic and reflex control of blood pressure. Some of these neurons project to the intermediolateral cell column of the thoracolumbar spinal cord and excite preganglionic sympathetic neurons. In order to gain a better understanding of the properties of the RVL neurons at the cellular and molecular level, a protocol was developed utilizing acute dissociation and the reverse transcription-polymerase chain reaction (RT-PCR) to study the expression of several genes in single RVL neurons. Neurons were dissociated from the RVL region of young rats, and classified as spinally projecting or non-spinal by the presence or absence of retrogradely transported fluorescent beads injected into the upper thoracic segments of the spinal cord. Individual neurons were collected by aspiration into a glass micropipette and analysed by RT-PCR. The presence of either glyceraldehyde 3-phosphate dehydrogenase (GAPDH) or neuron-specific enolase (NSE) mRNA was used as the criterion for selecting cells for further analysis. A subpopulation (50%) of spinally projecting, GAPDH- or NSE-positive neurons expressed mRNA for tyrosine hydroxylase (TH) or phenylethanolamine N-methyltransferase (PNMT), indicative of catecholaminergic or C1 adrenergic neurons, respectively. Some bulbospinal RVL neurons, including those that were TH- or PNMT-positive, were also found to express mRNA for the mineralocorticoid receptor (MR), the glucocorticoid receptor (GR), noradrenaline transporter (NET), and neuronal glutamate transporter (EAAC1). The glial glutamate transporter (GLT), glycine transporter (GLYT2), glutamic acid decarboxylase (GAD67) and gamma-amino butyric acid (GABA) transporter (GAT-1) were not expressed. The single-cell RT-PCR protocol is a powerful, yet simple and relatively rapid method for analysis of mRNA expression in a defined neuronal population. It can be combined with whole-cell patch-clamp recording prior to RT-PCR analysis, allowing linkage of the molecular analysis of mRNA expression to the electrophysiological and pharmacological properties of single neurons. The method is very sensitive, enabling mRNA transcripts in low abundance to be detected, and its application in our recent studies provided novel information about neurons involved in blood-pressure regulation at the molecular and cellular level.

Adrenergic responses in silent and putative inhibitory pacemaker-like neurons of the rat rostral ventrolateral medulla in vitro

Noradrenaline and adrenergic agonists were tested on pacemaker-like and silent neurons of the rat rostral ventrolateral medulla using intracellular recording in coronal brainstem slices as well as in punches containing only the rostral ventrolateral medullary region. Noradrenaline (1-100 microM) depolarized or increased the frequency of discharge of all cells tested in a dose-dependent manner. The noradrenaline-induced depolarization was associated with an apparent increase in cell input resistance at low concentrations and a decrease or no significant change at higher concentrations. Moreover, it was voltage dependent and its amplitude decreased with membrane potential hyperpolarization. Noradrenaline caused a dose-related increase in the frequency and amplitude of spontaneous inhibitory postsynaptic potentials. The alpha 1-adrenoceptor antagonist prazosin (0.5 microM) abolished the noradrenaline depolarizing response as well as-the noradrenaline-evoked increase in synaptic activity and unmasked an underlying noradrenaline dose-dependent hyperpolarizing response associated with a decrease in cell input resistance and sensitive to the alpha 2-adrenoceptor/antagonist yohimbine (0.5 microM). The alpha 1-adrenoceptor agonist phenylephrine (10 microM) mimicked the noradrenaline depolarizing response associated with an increase in membrane resistance as well as the noradrenaline-induced increase in synaptic activity. The alpha 2-adrenoceptor agonists UK-14,304 (1-3 microM) and clonidine (10-30 microM) produced only a small hyperpolarizing response, whereas the beta-adrenoceptor agonist isoproterenol (10-30 microM) had no effect. Baseline spontaneous postsynaptic potentials were abolished by strychnine (1 microM), bicuculline (30 microM) or both. However, only the strychnine-sensitive postsynaptic potentials had their frequency increased by noradrenaline or phenylephrine and they usually occurred with a regular pattern. Tetrodotoxin (1 microM) eliminated 80-95% of baseline spontaneous postsynaptic potentials and prevented the increase in synaptic activity evoked by noradrenaline and phenylephrine. Similar results were obtained in rostral ventrolateral medulla neurons impaled in both coronal slices and punches of the rostral ventrolateral medulla. It is concluded that noradrenaline could play an important inhibitory role in the rostral ventrolateral medulla via at least two mechanisms: an alpha 2-adrenoceptor-mediated hyperpolarization and an enhancement of inhibitory synaptic transmission through activation of alpha 1-adrenoceptors located on the somatic membrane of glycinergic interneurons. Some of these interneurons exhibit a regular discharge similar to the pacemaker-like neurons and might, at least in part, constitute a central inhibitory link in the baroreceptor-vasomotor reflex pathway.