Newly identified GABAergic neurons in regions of the ventrolateral medulla which regulate blood pressure

Quantitative and immunohistochemical analysis of neuronal types in the mouse caudal nucleus tractus solitarius: focus on GABAergic neurons

gamma-Aminobutyric acid-ergic (GABAergic) neurons are major inhibitory interneurons that are widely distributed in the central nervous system. The caudal nucleus tractus solitarius (cNTS), which plays a key role in respiratory, cardiovascular, and gastrointestinal function, contains GABAergic neurons for regulation of neuronal firing. In the present study, GABAergic neuronal organization was analyzed in relation to the location of subnuclei in the mouse cNTS. According to the differential expression of glutamate decarboxylase 67 (GAD67), vesicular glutamate transporter 2 (VGLUT2), calbindin, and tyrosine hydroxylase (TH) mRNAs, the cNTS was divided into four subnuclei: the subpostrema, dorsomedial, commissural, and medial subnuclei. The numerical density and size of soma in the four subnuclei were then quantified by an unbiased dissector analysis. Calbindin-positive cells constituted subpopulations of small non-GABAergic neurons preferentially localized in the subpostrema subnucleus. TH-positive cells constituted large neurons preferentially localized in the medial subnucleus. GABAergic neurons constituted a subpopulation of small neurons, preferentially localized in the commissural and medial subnuclei, which represented > or =50% of small cells in these subnuclei. Thus, the GABAergic small neurons were located around TH-positive large cells in the ventrolateral portion of the cNTS. This finding, in combination with results of previous studies in the rat cNTS showing that large cells originate efferents from the cNTS, suggests that GABAergic small neurons in the commissural and medial subnuclei might regulate output from the cNTS.

Cellular and subcellular localization of androgen receptor immunoreactivity relative to C1 adrenergic neurons in the rostral ventrolateral medulla of male and female rats

In male and female rats, high androgen levels can increase blood pressure. The C1 area of the rostral ventrolateral medulla (RVLM), which is crucial for blood pressure regulation, contains estrogen receptors (ERs) in pre- and postsynaptic neuronal compartments and is modulated by estrogens (Wang et al. [2006] Brain Res 1094:163-178). In this study, the cellular and subcellular localization of androgen receptors (ARs) in the C1 area was examined in sections from male, proestrus (high estrogen) and diestrus (low estrogen) female rat brains that were immunocytochemically labeled for AR and tyrosine hydroxylase (TH). By light and electron microscopy, AR-labeled nuclei were scattered among TH-labeled somata in the RVLM; significantly more AR-labeled nuclei were seen males compared to females. Electron microscopy revealed that extranuclear AR-immunoreactivity (ir) was in similar profile types in male and female rats. AR-ir was almost exclusively in myelinated and unmyelinated axons and in glia. Rarely, AR-ir was in axon terminals that contacted TH-containing dendrites. AR-labeled axon terminals had large diameters and contained numerous dense-core vesicles, resembling peptide-containing hypothalamic or solitary tract inputs. No nuclear or extranuclear AR-ir was found in TH-labeled perikarya and dendrites although a few non-TH- labeled dendrites contained AR-ir. Qualitatively, more axonal profiles appeared to be present in males compared to females. These studies suggest that, unlike ERs, ARs in male and female rats are almost exclusively positioned on afferents and glia, suggesting that androgens modulate RVLM C1 neurons, and thus blood pressure, through presynaptic and glial signaling.

Acupuncture stimulation to the sacral segment affects state of vigilance in rats

The effects of acupuncture stimulation to the sacral segment on electroencephalograms (EEGs) and activity of locus coeruleus (LC) neurons were examined in urethane-anesthetized rats. In 71 of 112 trials, when EEGs displayed small amplitude and high frequency, stimulation to the sacral segment-induced large amplitude and slow EEGs with a latency of <450s and duration ranged from 32s to >42 min. Stimulus-induced EEGs comprised significant increases in delta power and significant decreases in theta and beta powers. After intraperitoneal administration of bicuculline, stimulation to the sacral segment failed to induce changes in EEG pattern. Firing rate of noradrenergic LC neurons decreased significantly from 2.9+/-1.5 to 1.1+/-0.8 Hz (n=11, p<0.001). Decreased neuronal activity exhibited close relationships with increased EEG amplitude. These results suggest that acupuncture stimulation to the sacral segment changes the state of animals from light anesthesia to deep anesthesia, and that this change is mediated by GABAergic systems suppressing the activity of noradrenergic LC neurons.

Cardiovascular effects and c-Fos expression in the rat hindbrain in response to innocuous stomach distension

The present work was planned to study the effects of non-noxious gastric distension on hemodynamic variables and on cardiovascular hindbrain areas detected by means of c-Fos immunoreactivity, to determine the afferent and central mechanisms involved. In anesthetized rats, innocuous stomach distension increased arterial blood pressure and heart rate and induced c-Fos immunoreactivity within nucleus tractus solitarii, nucleus ambiguus, ventrolateral medulla and lateral reticular nucleus. Bilateral vagotomy abolished the pressor response and c-Fos immunoreactivity in nucleus ambiguus and ventrolateral medulla. Also, c-Fos immunoreactivity was significantly decreased in nucleus tractus solitarii and lateral reticular nucleus. After bilateral splanchnicotomy the pressor and tachycardic responses caused by gastric distension were reduced. c-Fos immunoreactivity in nucleus tractus solitarii, lateral reticular nucleus and nucleus ambiguus was reduced in comparison to the intact rats. In ventrolateral medulla a preferential localization of c-Fos immunoreactivity was found within its caudal portion. It was shown that such gastric distension, known to activate low threshold mechanoreceptors, induced cardiovascular effects via both vagal and splanchnic afferents and involving their central convergence and interaction in modulating the baroreceptor buffer system.

Immunohistochemical localization of GAD67-expressing neurons and processes in the rat brainstem: subregional distribution in the nucleus tractus solitarius

The role of gamma-aminobutyric acid (GABA) in homeostatic control in the brainstem, in particular, in the nucleus tractus solitarius (NTS), is well established. However, to date, there is no detailed description of the distribution of GABAergic neurons within the NTS. The goal of the current study was to reexamine the efficacy of immunohistochemical localization of glutamic acid decarboxylase (GAD) protein, specifically the 67-kDa isoform (GAD67), as a marker for GABAergic neurons in the medulla and to provide a detailed map of GAD67-immunoreactive (-ir) cells within rat NTS by using a recently developed mouse monoclonal antibody. We describe a distribution of GAD67-ir cells in the medulla similar to that reported previously from in situ hybridization study. GAD67-ir cells were localized in regions known to contain high GABA content, including the ventrolateral medulla, raphe nuclei, and area postrema, but were absent from all motor nuclei, although dense terminal labeling was discerned in these regions. In the NTS, GAD67-ir was localized in all subregions. Semiquantitative analysis of the GAD67-ir distribution in the NTS revealed greater numbers of GAD67-ir cells medial to the solitary tract. Finally, dense GAD67 terminal labeling was found in the medial, central, intermediate, commissural, and subpostremal subregions, whereas sparse labeling was observed in the ventral subregion. Our findings support the use of immunohistochemistry for GAD67 as a marker for the localization of GABAergic cells and terminal processes in the rat brainstem. Furthermore, the reported heterogeneous distribution of GAD67-ir in the NTS suggests differential inhibitory modulation of sensory processing.

Modulatory inputs on sympathetic neurons in the rostral ventrolateral medulla in the rat

1. The first part of this study looks at spontaneously active neurons located in the rostral ventrolateral medulla (RVLM) with projections to the thoracic spinal cord. Sixteen neurons were intracellularly recorded in vivo. Four out of 16 neurons were antidromically activated from the thoracic spinal cord (axonal conduction velocities varied from 1.8 m/s to 9.5 m/s). 2. The simultaneous averages of the neuronal membrane potential and arterial blood pressure triggered by the pulsatile arterial wave or the EKG-R wave demonstrated changes in membrane potential (hyperpolarization or depolarization) locked to the cardiac cycle in four neurons in this group. These neurons (three of them bulbospinal) were further tested for barosensitivity by characterizing the responses to electrical stimulation of the aortic depressor nerve. Four neurons responded with inhibitory hyperpolarizing responses characterized as inhibitory postsynaptic potentials (IPSP) to aortic nerve stimulation (onset latency: 32.3 +/- 5.0 ms; mean +/- SEM). 3. In two neurons in the RVLM, one of them characterized as barosensitive, electrical stimulation of the opposite RVLM (0.5 Hz, 1.0 ms pulse duration, 25-100 microA) elicited excitatory postsynaptic potentials (EPSPs) with latencies of 9.07 and 10.5 ms. At resting membrane potential, the onset latency of the evoked EPSPs did not change with increasing stimulus intensities. Some of the recorded neurons were intracellularly labelled with biocytin for visualization. They were found in the RVLM. 4. These experiments in vivo would support the idea of a functional commissural pathway between the RVLM of both sides. 5. Anatomical data have shown that some of those commissural bundle fibers originate in the C1 adrenergic neuronal group in the RVLM. In the second part of this study, we used an intracellular recording technique in vitro to investigate the effects of the indirect adrenergic agonist tyramine on neurons in the RVLM with electrophysiological properties similar to premotor sympathetic neurons in vivo. 6. Tyramine (0.5-1 mM) produced a pronounced inhibitory effect with hyperpolarization and increase in membrane input resistance on two neurons characterized as regularly firing (R), and on one neuron characterized as irregularly firing (1). This effect was preceded by a transient depolarization with increases in firing rate. 7. These results would indicate that neurons in the RVLM recorded in vitro and with properties similar to premotor sympathetic neurons can be modulated by catecholamines released from terminals probably making synaptic contacts.

Evidence for a tonic GABA-ergic inhibition of excitatory respiratory-related afferents to presympathetic neurons in the rostral ventrolateral medulla

The effect of blockade of ionotropic GABA and glutamate receptors in the rostral ventrolateral medulla (RVLM) on the relationship between phrenic nerve, splanchnic sympathetic nerve and lumbar sympathetic nerve activities was examined in urethane anesthetized, paralyzed and vagotomized Sprague-Dawley rats. Bilateral microinjection of the GABA-A receptor antagonist, bicuculline (4 mM, 100 nl), into the RVLM dramatically, and almost exclusively, increased the post-inspiratory related discharge in both splanchnic sympathetic nerve and lumbar sympathetic nerve activities and elicited hypertension with fluctuations of arterial pressure phase locked to the discharge of the phrenic nerve. Subsequent bilateral microinjection of kynurenate, a non-selective ionotropic excitatory amino acid receptor antagonist (50 mM, 100 nl), into the RVLM significantly attenuated the sympathoexcitation and hypertension evoked by injection of bicuculline. This was accompanied by an abolition of the post-inspiratory related burst discharge of splanchnic sympathetic nerve and lumbar sympathetic nerve activities. These data suggest that the GABAergic inputs to RVLM tonically inhibit glutamatergic inputs from central respiratory neurons that normally act to increase the firing of presympathetic neurons in the RVLM. Inputs from post-inspiratory neurons appear to be an especially potent excitatory synaptic drive to the presympathetic neurons in the absence of the GABAergic inhibition.

Cellular relations between mu-opioid receptive, GABAergic and reticulospinal neurons in the rostral ventrolateral medulla

Physiological studies have suggested that mu-opioid receptor (MOR) activation can both excite and inhibit reticulospinal neurons in the rostral ventrolateral medulla (RVL), possibly via influences on GABAergic neurons. Thus, to determine the cellular relationships of MORs to GABAergic neurons in the RVL, two experimental approaches were used. First, single sections through the RVL were labeled for MOR using immunoperoxidase detection and for GABA using immunogold detection and examined by electron microscopy. These studies revealed that MOR-immunoreactive (IR) terminals were smaller on average than GABA-IR terminals and formed both asymmetric and symmetric synapses, whereas GABA-IR terminals formed exclusively symmetric synapses. MOR and GABA immunoreactivities rarely co-localized. Interactions between axons and terminals containing MOR or GABA immunoreactivity were primarily: (1) direct appositions with each other; or (2) convergence onto a common dendritic target that sometimes contained either MOR or GABA immunoreactivity. Since the identity of these target dendrites mostly was unknown, a second study was designed to determine if they might be reticulospinal neurons. For this study, reticulospinal neurons were identified with a retrograde tracer and both MOR and GABA were localized in the same sections of the RVL. These studies revealed that numerous GABA-IR terminals formed symmetric synapses on the perikarya and proximal dendrites of reticulospinal neurons. In contrast, few MOR-IR terminals contacted reticulospinal perikarya and large dendrites although they were often found nearby. These results provide anatomical evidence that MOR activation by endogenous or exogenous agonists may indirectly alter GABAergic neurotransmission in the RVL either through presynaptic interactions between cells or through competing influences on postsynaptic targets.

Effects of six antihypertensive drugs on blood pressure and hypothalamic GABA content in spontaneously hypertensive rats

In order to investigate the effects of antihypertensive drugs on blood pressure and gamma-amino butyric acid (GABA) content in the hypothalamus and the possible relationship between blood pressure decrease and GABA content changes, blood pressure and GABA contents after chronic (20 weeks) treatments of nitrendipine, atenolol, captopril, hydrochlorothiazide, dihydralazine and prazosin were studied in spontaneously-hypertensive rats (SHR). The acute and subacute (1 week) effects of nitrendipine on GABA contents was also observed in SHR. It was found that 20 week treatments with six different antihypertensive agents produced a decrease in systolic blood pressure and an increase in GABA content. The blood pressure level was significantly correlated with GABA content in the hypothalamus, but not with that in the cortex. Acute treatment with a single dose of nitrendipine, did not alter GABA content. Bicuculline, a GABA receptor antagonist, did not attenuate the hypotensive effect of nitrendipine. In conclusion, chronic treatments by different antihypertensive agents produced an increase of hypothalamic GABA content and a decrease of blood pressure. The increase of GABA content induced by nitrendipine seems likely to be secondary to blood pressure decrease.

Evidence that dopamine-beta-hydroxylase immunoreactive neurons in the lateral reticular nucleus project to the spinal cord in the rat

The existence of noradrenergic projections from the lateral reticular nucleus (LRt) to the dorsal quadrant of cervical, thoracic, or lumbar spinal cord was investigated using a combined method of WGA-apo-HRP-gold retrograde tracing and dopamine-beta-hydroxylase (DBH) immunocytochemistry. Preliminary retrograde tracing studies indicated that LRt neurons projecting to cervical, thoracic, or lumbar spinal cord were characteristically located near the perimeter of the LRt. Double-labeling experiments demonstrated that a portion of these peripherally-located, spinal-projecting neurons were DBH-immunoreactive. Double-labeled neurons were also located at the parvocellular division of the contralateral LRt in the thoracic injection cases. Double-labeled neurons were not observed at the subtrigeminal division in cervical, thoracic, or lumbar injection case. The results suggest the possibility that the noradrenergic LRt-spinal pathway might be involved in a variety of pain processing and cardiovascular regulatory functions in the rat.

Catecholamine monoamine oxidase a metabolite in adrenergic neurons is cytotoxic in vivo

3,4-Dihydroxyphenylglycolaldehyde is the monoamine oxidase-A metabolite of two catecholamine neurotransmitters, epinephrine and norepinephrine. This aldehyde metabolite and its synthesizing enzymes increase in cell bodies of catecholamine neurons in Alzheimer's disease. To test the hypothesis that 3,4-dihydroxyphenylglycolaldehyde, but not epinephrine or its major metabolite 4-hydroxy-3-methoxyphenylglycol, is a neurotoxin, we injected 3,4-dihydroxyphenylglycolaldehyde onto adrenergic neurons in the rostral ventrolateral medulla. Injections of epinephrine or 4-hydroxy-3-methoxyphenylglycol were made into the same area of controls. A dose response and time study were performed. Adrenergic neurons were identified by their content of the epinephrine synthesizing enzyme, phenylethanolamine N-methyltransferase, immunohistochemically. Apoptosis was evaluated by in situ terminal deoxynucleotidyl-transferase mediated dUTP nick end label staining. Injection of 3,4-dihydroxyphenylglycolaldehyde in amounts as low as 50 ng results in loss of adrenergic neurons and apoptosis after 18 h. The degree of neurotoxicity is dose and time dependent. Doses of 3,4-dihydroxyphenylglycolaldehyde 10-fold higher produce necrosis. Neither epinephrine nor 4-hydroxy-3-methoxyphenylglycol are toxic. A 2.5 microg injection of 3,4-dihydroxyphenylglycolaldehyde is toxic to cortical neurons but not glia. Active uptake of the catecholamine-derived aldehyde into differentiated PC-12 cells is demonstrated. Implications of these findings for catecholamine neuron death in neurodegenerative diseases are discussed.

Glutamatergic and GABAergic inputs to the RVL mediate cardiovascular adjustments to noxious stimulation

Stimulation of cutaneous and muscle afferents induces several cardiovascular adjustments such as hypertension, tachycardia, and muscle vasodilation. Although previous studies have demonstrated that the rostral ventrolateral medulla (RVL) mediates sympathoexcitation and pressor responses to sciatic nerve stimulation (SNS), whether it also mediates blood flow adjustments remains unclear. Therefore, in the present study, we examined the role of the RVL in the vasodilation induced by SNS and the possible neurotransmitters involved. In Urethane-anesthetized, paralyzed, and artificially ventilated rats, SNS (square pulses, 1 ms, 20 Hz, 800--1200 microA, 10 s) produced increases in blood pressure, heart rate, blood flow, and vascular conductance of the stimulated limb. Unilateral microinjection of kainic acid (2 nmol/100 nl) into the RVL contralateral to the stimulated limb abolished cardiovascular adjustments to SNS. Unilateral microinjections of kynurenic acid (2 nmol/100 nl) selectively abolished the pressor response to SNS, whereas bicuculline (400 pmol/100 nl) abolished the increases in blood flow without changing the pressor response. These results suggest that glutamatergic synapses within the RVL mediate pressor responses, whereas GABAergic synapses may mediate the vasodilation to SNS.

Sympathoexcitatory CVLM neurons mediate responses to caudal pressor area stimulation

Neurons in the caudal pressor area (CPA) are a source of tonic sympathoexcitation that is dependent on activation of cardiovascular sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM). In the present study, we sought to clarify the mechanism through which CPA neurons elicit increases in RVLM neuronal discharge, vasoconstrictor sympathetic tone, and arterial pressure. In urethan-chloralose-anesthetized, paralyzed, and artificially ventilated rats, bilateral disinhibition of CPA with bicuculline (Bic) after bilateral disinhibition of caudal ventrolateral medulla (CVLM) caused increases in splanchnic sympathetic nerve activity (+277% control) and arterial pressure (+54 mmHg). Inhibition of CVLM neurons with muscimol abolished the pressor response to activation of CPA neurons, suggesting that neurons within CVLM mediate the excitatory responses from CPA. Disinhibition of CVLM and CPA with Bic enhanced the sympathoexcitatory responses to stimulation of CPA with DL-homocysteic acid, which were blocked by microinjections of kynurenic acid into CVLM. We conclude that the pathway from CPA to RVLM involves an obligatory glutamatergic activation of sympathoexcitatory neurons in the vicinity of CVLM.

Fluorescent double-label study of lateral reticular nucleus projections to the spinal cord and periaqueductal gray in the rat

Following injections of WGA-HRP into either the spinal cord or periaqueductal gray, labeled neurons were observed bilaterally along the periphery of the lateral reticular nucleus (LRN) magnocellular division. The possibility that some of these neurons in the LRN provide collateral axonal branches to both the periaqueductal gray and the spinal cord was investigated in rats using a retrograde double-labeling method employing two different fluorescent tracers, True Blue and Nuclear Yellow. Following sequential injection of the two fluorescent axonal tracers into the spinal cord and periaqueductal gray in the same animal, a modest number of double-labeled neurons were observed bilaterally near the medial and dorsal perimeter of the magnocellular division of the LRN. The labeled neurons were distinctly multipolar in shape and measured approximately 15-18 mu in their greatest transverse diameter. No double-labeled neurons were observed in the parvocellular or subtrigeminal divisions of the LRN. Based upon these observations, it is suggested that collaterals of the LRN-spinal pathway provide feedback information to the periaqueductal gray that might then be used to modulate the participation of the latter cell group in a variety of pain processing and cardiovascular regulatory functions.

Evidence for glycinergic respiratory neurons: Bötzinger neurons express mRNA for glycinergic transporter 2

Bötzinger (BOTZ) neurons in the rostral ventrolateral medulla fire during the late expiratory phase of the respiratory cycle. These cells inhibit phrenic motor neurons and several types of respiratory neurons in the medulla oblongata. BOTZ cells produce a fast, chloride-mediated inhibition of their target neurons, but the neurotransmitter used by these cells has not been determined. In the present study, we examine whether gamma-aminobutyric acid (GABA) or glycine could be the inhibitory neurotransmitter of BOTZ cells. In chloralose-anesthetized rats, we individually filled 20 physiologically characterized BOTZ neurons with biotinamide by using a juxtacellular labeling method. Medullary sections containing the labeled BOTZ neurons were processed for in situ hybridization by using digoxigenin-labeled riboprobes for glutamic acid decarboxylase isoform 67 (GAD67), a marker for GABAergic neurons, or for glycine transporter 2 (GLYT2), a marker for glycinergic neurons. All BOTZ cells examined contained GLYT2 mRNA (n = 10), whereas none had detectable levels of GAD67 mRNA (n = 10). For a positive control, 12 GABAergic neurons in the substantia nigra pars reticulata also were recorded and filled with biotinamide in vivo. Most of these cells, as expected, had detectable levels of GAD67 mRNA (11 out of 12). These results demonstrate that the juxtacellular labeling method can be combined with in situ hybridization to identify physiologically characterized cells with probable GABAergic or glycinergic phenotypes. Furthermore, these data suggest that BOTZ neurons use the neurotransmitter glycine and not GABA to provide widespread inhibition of respiratory-related neurons.

Adrenal epinephrine secretion is not regulated by sympathoinhibitory neurons in the caudal ventrolateral medulla

By providing the principal inhibitory regulation of the discharge of sympathetic premotor neurons in the rostral ventrolateral medulla (RVLM), neurons in the caudal ventrolateral medulla (CVLM) play a major role in regulating the level of sympathetic nerve activity (SNA) to cardiovascular targets. To determine whether adrenal medullary secretion of epinephrine (EPI) is also regulated by sympathoinhibitory inputs from the CVLM to the RVLM, we compared levels of plasma EPI obtained after disinhibition of RVLM neurons with levels obtained after inhibition of CVLM neurons, both of which result in sustained elevations in arterial blood pressure (AP), SNA, and heart rate (HR). Plasma norepinephrine (NE) concentrations were significantly elevated following bilateral microinjection either of bicuculline (BIC) into the RVLM or of muscimol into the CVLM of urethane/chloralose-anesthetized, artificially-ventilated rats. In sharp contrast, although plasma EPI concentrations were significantly elevated following disinhibition of neurons in the RVLM, they were unchanged by inhibition of neurons in the CVLM. These results demonstrate that the discharge of sympathetic premotor neurons in the RVLM regulating adrenal secretion of EPI is modulated by a tonic, GABA-ergic inhibition that arises from a source that is different from the sympathoinhibitory neurons in the CVLM that project to RVLM sympathetic premotor neurons controlling vasoconstrictor and cardiac targets.

Ventrolateral medullary control of cardiovascular activity during muscle contraction

An overview of the role of ventrolateral medulla (VLM) in regulation of cardiovascular activity is presented. A summary of VLM anatomy and its functional relation to other areas in the central nervous system is described. Over the past few years, various studies have investigated the VLM and its involvement in cardiovascular regulation during static muscle contraction, a type of static exercise as seen, for example, during knee extension or hand-grip exercise. Understanding the neural mechanisms that are responsible for regulation of cardiovascular activity during static muscle contraction is of particular interest since it helps understand circulatory adjustments in response to an increase in physical activity. This review surveys the role of several receptors and neurotransmitters in the VLM that are associated with changes in mean arterial pressure and heart rate during static muscle contraction in anesthetized animals. Possible mechanisms in the VLM that modulate cardiovascular changes during static muscle contraction are summarized and discussed. Localized administration of an excitatory amino-acid antagonist into the rostral portion of the VLM (RVLM) attenuates increases in blood pressure and heart rate during static muscle contraction, whereas its administration into the caudal part of the VLM (CVLM) augments these responses. Opioid or 5-HT1A receptor stimulation in the RVLM, but not in the CVLM, attenuates cardiovascular responses to muscle contraction. Furthermore, intravenous, intracerebroventricular or intracisternal injection of an alpha 2-adrenoceptor agonist or a cholinesterase inhibitor attenuates increases in blood pressure and heart rate during static muscle contraction. Finally, the possible involvement of endogenous neurotransmitters in the RVLM and the CVLM associated with cardiovascular responses during static muscle contraction is discussed. An overview of the role of the VLM in the overall cardiovascular control network in the brain is presented and critically reviewed.

Evidence of disynaptic projections from the rostral ventrolateral medulla to the thoracic spinal cord

Sympathetic outflow is regulated by a direct pathway of the rostral ventrolateral reticular formation (rvlm) to the thoracic spinal cord. For the first time, a dual retrograde/anterograde transport technique was used to demonstrate by light microscopy, potential disynaptic pathways from the rvlm to the thoracic spinal cord in the rat. An anterograde tracer, biotinylated dextran amine (BDA) was injected into the rvlm and a retrograde tracer, FluoroGold (FG) deposited into the upper thoracic spinal cord in the same animal. Rostral ventrolateral medullary efferents labeled with BDA were apposed to thoracic reticulospinal neurons labeled with FG in the ventrolateral tegmentum, ipsilateral and contralateral to the injection site in the rvlm. Suggestive evidence was obtained of synaptic interactions with neuronal somata and proximal dendrites. The results support the idea that the rvlm projects to the thoracic cord via disynaptic, intrareticular pathways paralleling the well established monosynaptic projection.

Organization and transmitter specificity of medullary neurons activated by sustained hypertension: implications for understanding baroreceptor reflex circuitry

In situ expression of c-fos observed in response to phenylephrine (PE)-induced hypertension provided a basis for characterizing the organization and neurotransmitter specificity of neurons at nodal points of medullary baroreflex circuitry. Sustained hypertension induced by a moderate dose of PE provoked patterns of c-fos mRNA and protein expression that conformed in the nucleus of the solitary tract (NTS) to the termination patterns of primary baroreceptor afferents and in the caudal ventrolateral medulla (CVLM) to a physiologically defined depressor region. A majority of barosensitive CVLM neurons concurrently displayed markers for the GABAergic phenotype; few were glycinergic. Phenylephrine-sensitive GABAergic neurons that were retrogradely labeled after tracer deposits in pressor sites of the rostral ventrolateral medulla (RVLM) occupied a zone extending approximately 1.4 mm rostrally from the level of the calamus scriptorius, intermingled partly with catecholaminergic neurons of the A1 and C1 cell groups. By contrast, barosensitive neurons of the NTS were found to be phenotypically complex, with very few projecting directly to the RVLM. Extensive colocalization of PE-induced Fos-IR and markers for the nitric oxide phenotype were seen in a circumscribed, rostral, portion of the baroreceptor afferent zone of the NTS, whereas only a small proportion of PE-sensitive neurons in the NTS were found to be GABAergic. PE treatment parameters have been identified that provide a basis for defining and characterizing populations of neurons at the first station in the central processing of primary baroreceptor input and at a key inhibitory relay in the CVLM.

Co-localization of c-Fos and neurotransmitter immunoreactivities in the cat brain stem after carotid sinus nerve stimulation

To reveal neurones in the cat medulla oblongata involved in carotid baroreceptor/chemoreceptor reflexes, the distribution of c-Fos oncoprotein immunoreactivity was studied following electrical stimulation of the right carotid sinus nerve. The neurochemistry of the activated neurones was investigated using antisera to tyrosine hydroxylase, neuropeptide Y, somatostatin, and glutamate. Nitric oxide containing neurones were identified using antiserum to nitric oxide synthase (NOS) and by the histochemical localization of nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase. Following sinus nerve stimulation numerous c-Fos-IR cells were detected both ipsilaterally and contralaterally in the nucleus tractus solitarii, the area postrema and throughout the ventrolateral medulla. Dual labelling studies revealed that 3.3% of c-Fos-immunoreactive cells in the nucleus tractus solitarii were also immunoreactive for tyrosine hydroxylase. The double labelled cells were scattered within the medial and ventrolateral subnuclei, predominantly rostral to obex. A higher proportion (10.3%) of c-Fos-IR cells in the ventrolateral medulla also showed tyrosine hydroxylase immunoreactivity. Caudal to obex, these were scattered in the reticular formation between the spinal trigeminal nucleus and the lateral reticular nucleus, while more rostrally they were found within the lateral reticular nucleus, the nucleus ambiguus and the lateral tegmental field. Cells expressing c-fos and reactive for glutamate, neuropeptide Y or NADPH-diaphorase (or NOS) were only rarely seen, and co-localization of c-Fos and somatostatin immunoreactivities was not seen. These results suggest that of the neurones forming pathways within the medulla activated on carotid sinus nerve stimulation, presumably mediating baro- and chemoreceptor reflexes, relatively few utilize catecholamines, glutamate, neuropeptide Y or nitric oxide as their transmitter substance.

Evidence of gamma-aminobutyric acidergic control over the catecholaminergic projection from the medulla oblongata to the central nucleus of the amygdala

It is known that the nucleus of the solitary tract (NTS) and the ventrolateral medulla (VLM) project to the central nucleus of the amygdala (Ce), conveying visceral information. Conversely, the Ce sends projections to the NTS and the VLM. To understand better the role of catecholamine and gamma-aminobutyric acid (GABA) in these reciprocal connections, experiments were performed by combining lectin-conjugated horseradish peroxidase (WGA-HRP) anterograde and retrograde transport with preembedding immunocytochemistry to detect tyrosine hydroxylase (TH), and postembedding immunocytochemistry to detect GABA. The light microscopic study suggested that the majority of neurons in the NTS and the VLM projecting to the Ce were TH immunoreactive (TH-IR). Most of them were located at the level of the obex. Under the electron microscope, the GABAergic and non-GABAergic terminals were found to form synaptic contacts with the TH-(IR) or Ce-projecting or TH-IR/Ce-projecting double-labelled neurons of the NTS and VLM. The GABAergic terminals mostly formed symmetrical synaptic contacts with the postsynaptic structure in which perikarya (14-19%), dendrites (79-84%), and spines (2%) were observed. Approximately 94% of the axon terminals in the NTS and 90% of those in the VLM arising from the Ce were GABAergic and appeared not to form synaptic contacts with the TH-IR or Ce-projecting neurons in these regions. The present results demonstrated that the catecholaminergic neurons of the NTS and VLM projecting to the Ce receive an extensive GABAergic innervation and that the amygdala projection to the medulla is mostly GABAergic.

Neurons in rostral ventrolateral medulla mediate vestibular inhibition of locus coeruleus in rats

The effects of caloric vestibular stimulation on the central noradrenergic neurons system were examined in the rat. In urethane-anesthetized rats, caloric stimulation inhibited the spontaneous activity of noradrenergic locus coeruleus neurons and increased systemic blood pressure. Electrical and chemical lesions in the ventrolateral medulla attenuated both the locus coeruleus inhibition and the blood pressure increase in response to caloric stimulation. Neither the neuronal inhibition nor the pressor effect was attenuated by any deafferentation of the forebrain or baroreceptors, or lesioning of the nucleus tractus solitarius. These findings indicate that the caloric stimulation-induced locus coeruleus inhibition is mediated by neurons in the ventrolateral medulla, and that these neurons also mediate the vestibulo-pressor responses. The locus coeruleus inhibition via the ventrolateral medulla is, however, considered to be independent of ventrolateral medulla-mediated systemic pressor effect. Collectively these findings suggest that the ventrolateral medulla is the major origin of inhibitory vestibular input to the noradrenergic neurons of the locus coeruleus, and that the ventrolateral medulla plays an important role in the vestibulo-autonomic response.

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.

Barosensitive and chemosensitive neurons in the rat medulla: a double labeling study with c-Fos/glutamate, GAD, PNMT and calbindin

The purpose of this study was to survey distribution and density of the barosensitive and chemosensitive neurons in the medulla of rats anesthetized with fentanyl/midazolam, using immunohistochemical methods. After stimulation of the arterial baroreceptor or the chemoreceptor, we identified c-Fos-labeled neurons with immunoreactions to antisera of glutamate. PNMT, GAD and calbindin in the nucleus tractus solitarii (NTS) and the ventrolateral medulla (VLM). The double labeled neurons were located in the medical part of the NTS, and in the lateral part of the paragigantocellular reticular nucleus and the ventral division of the ambiguus nucleus. Main findings were as follows: (1) No significant difference was found in distribution and density of glutamatergic, adrenergic and calbindin-containing neurons between the barosensitive and chemosensitivie types; (2) a few GABAergic neurons were distributed almost evenly in the NTS and VLM, and in these neurons the barosensitive type outnumbered the chemosensitive one; (3) glutamatergic and calbindin-containing neurons were dominant in the NTS; adrenergic neurons in the VLM. (4) as for the adrenergic neurons in the NTS, the chemosensitive type significantly outnumbered the barosensitive one. This study showed that distribution and density of the barosensitive neurons, either glutamatergic, adrenergic, or calbindin-containing neurons, overlapped with those of the chemosensitive corresponding neurons, suggesting presence of the neural matrix of the cardiopulmonary interaction. Exceptionally, the number of the barosensitive GABAergic neurons was significantly larger than that of the chemosensitive GABAergic ones.

The ventrolateral medulla of the rat is connected with the spinal cord dorsal horn by an indirect descending pathway relayed in the A5 noradrenergic cell group

The pathway conveying the descending inhibitory noradrenergic input elicited from the caudal ventrolateral medulla (VLM) onto the spinal cord dorsal horn was studied in the rat. Retrograde labeling with cholera toxin subunit B (CTb) injected into the dorsal horn was combined with immunostaining for dopamine-beta-hydroxylase (DBH) in the VLM and other brainstem nuclei containing noradrenergic cells. CTb-labeled neurons occurred in the lateral part of the VLM (VLMlat), located ventrolaterally to the DBH-immunoreactive cells of the A1 noradrenergic cell group. Neuronal profiles stained for CTb and DBH (double labeled) occurred in the A5 (31%), A6 (57%), and A7 (12%) noradrenergic cell groups. To ascertain whether noradrenergic cells targeting the spinal cord in those groups received projections from the VLMlat, this area was injected with the anterograde tracer biotinylated dextran amine (BDA). Labeled terminal fibers with boutons en passant were apposed to numerous double-stained neurons in the A5 cell group. Similar appositions occurred in small amounts in the ventral subcoerulear component of the A6. Correlated light and electron microscopic analyses of the labeled appositions revealed that the BDA-labeled axonal boutons contained spherical vesicles and were presynaptic at asymmetrical contacts to somata and dendritic profiles of the double-stained A5 neurons. These data indicate the occurrence of an indirect dysynaptic pathway connecting the VLM to the spinal cord, with a relay in the A5 cells. This pathway may convey the antinociceptive effects mediated by alpha 2-adrenoreceptors, which have been previously observed in the spinal cord following VLM stimulation.

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

Rostro-ventrolateral medulla (RVL) neurons were recorded using conventional intracellular recording techniques in brain slices maintained in vitro at 32 degrees C and classified into 3 major groups. The first group included neurons having endogenous pacemaker-like (PL) activity with regular firing frequency (mean 8 Hz) and a linear current-voltage relationship (I-V). The second group of neurons were slowly and irregularly firing (IF) or quiescent, presenting membrane potential oscillations and their I-V usually displayed an inward rectification. These neurons had a relatively longer action potential duration. The third group included silent neurons (S) with no apparent membrane oscillations and they differed from the first two groups by having relatively shorter action potential duration and amplitude and lower cell input resistance. When recorded with KCl-filled electrodes, the majority of silent neurons displayed a time-dependent inward rectification. With KAc-filled electrodes, irregular slow hyperpolarizing and depolarizing spontaneous potentials could be recorded primarily on PL and IF neurons, respectively. Moreover, fast spontaneous inhibitory postsynaptic potentials (PSPs) were detected in about 15% of PL and S neurons. They generally exhibited a regular pattern and were depolarizing when KCl-filled electrodes were used for recording. The amplitude of these inhibitory PSPS was reversibly reduced by the GABA A antagonists bicuculline, SR 95531 and picrotoxin. With KAc-filled electrodes, pressure-applied GABA (20 mM) evoked complex responses. In PL neurons, it consisted of a fast hyperpolarization followed by a slower depolarization that were both sensitive to SR 95531 and picrotoxin. The response was terminated by a long-lasting hyperpolarization that was reduced, but not abolished, by the GABA B antagonist CGP 35348. In IF and S neurons, GABA application usually produced a fast followed by a slow monophasic hyperpolarization and depolarization, respectively. The fast component of these responses was sensitive to the GABA A antagonists. Pressure application of isoguvacine (10 mM) always induced monophasic responses in all types of neurons recorded. Baclofen (1-30 mu M) reduced the firing frequency and hyperpolarized PL and IF neurons, an effect that was antagonized by CGP 35348 (50-100 mu M); however, it had little effect on silent neurons. It is concluded that RVL neurons have heterogeneous electrophysiological characteristics. Their predominant synaptic input and GABA responsiveness might be additional criteria to identify the excitatory and inhibitory elements in the RVL circuitry. All neuronal types seem to have functional GABA A and GABA B receptors; however, only a subpopulation is under tonic inhibitory control in vitro, probably from local GABAergic pacemaker interneurons. Our results further emphasize the role of GABA as an important neurotransmitter in the RVL network.

Properties of presympathetic neurones in the rostral ventrolateral medulla in the rat: an intracellular study "in vivo'

1. Intracellular recordings were made in pentobarbitone-anaesthetized rats from sixty-eight neurones located in the rostral ventrolateral medulla (RVLM), which responded with inhibition (latency, 33.6 +/- 9.3 ms) after stimulation of the aortic depressor nerve with short bursts of pulses. This inhibition was due to chloride- and voltage-dependent IPSPs. 2. Seventeen neurones could be excited antidromically after stimulation in the T2 spinal segment (conduction velocity 1.9-8.5 m.s-1) and were classified as RVLM presympathetic vasomotor neurones. 3. "Spontaneously' active neurones (n = 29) displayed a largely irregular pattern of firing, with no clear relationship between the level of the membrane potential and cycles of phrenic nerve activity at end-tidal CO2 < 5.0%. Cardiac cycle-related shifts of the membrane potential were not considered indicative of baroreceptor input as they could be due to movement artifacts. 4. All neurones displayed large synaptic activity (EPSPs and IPSPs, peak-to-peak amplitude > 5.0 mV). The depolarizing IPSPs observed during injection of chloride and/or negative current consisted of a phasic and a tonic component. 5. The on-going activity of these neurones resulted from synaptic inputs, with individual action potentials usually preceded by identifiable fast EPSPs. 6. No evidence was found for the presence of gradual depolarizations (autodepolarizations) between individual action potentials, and therefore under these experimental conditions the activity of RVLM presympathetic neurones did not depend on intrinsic pacemaker properties. 7. These results are consistent with the "network' hypothesis for the generation of sympathetic vasomotor tone.

Hypotension-induced expression of the c-fos gene in the medulla oblongata of piglets

Neural networks that mediate the reflex response to baroreceptor withdrawal were explored in Sus scrofa. Induction of c-fos was used as a monitor of synaptic activity in response to hypotension sustained by systemic administration of a peripheral vasodilator, sodium nitroprusside. Patterns of c-fos gene expression were compared between Saffan-anesthetized experimental animals and age-matched normotensive controls administered vehicle. Effects of other variables were controlled including 1 h preoperative accommodation to the novel environment, anesthesia, blood gases and pH. Identical post-stimulus survival periods were allowed for accumulation of transcript. The c-fos protein, Fos, was identified immunocytochemically with two rabbit antisera raised against amino acids 1-131 of Fos or residues 4-17 of synthetic human transcript. Fos was identified in catecholaminergic neurons labeled with an antiserum to tyrosine hydroxylase (TH). Fos was induced in the nucleus tractus solitarii (NTS) of hypotensive piglets. Neurons encoding Fos matched projection patterns of first order visceral afferents. Induction was prominent in the dorsolateral nucleus coinciding with the baroreceptor field. Indices of increased neuronal activity were evident in other baroreceptor terminal sites, e.g., medial subnucleus, the medial commissural field, the intermediate subnucleus and a ventral A2 noradrenergic area. In reticular formation c-fos protein was induced in circumscribed columns in the lateral tegmental field (LTF) extending from facial nucleus to calamus scriptorius. Catecholaminergic (TH-positive) neurons expressed Fos in the porcine C1 and A1 areas of ventrolateral medulla. Fos was also induced in a dorsal intermediate reticular zone of LTF. Minor or inconsistent differences between experimental and control were observed in nucleus raphe pallidus, rostral paramedian reticular formation, upper thoracic intermediolateral cell column, and stellate ganglia. In conclusion, baroreceptor withdrawal in young animals induced patterns of neuronal response along established cardiovascular reflex pathways.

Medullary visceral reflex circuits: local afferents to nucleus tractus solitarii synthesize catecholamines and project to thoracic spinal cord

Visceral feedback circuits in lower brainstem were elucidated with retrograde tracers by mapping neurons that issue local projections to the general visceral afferent division of the nucleus tractus solitarii (NTS) and dorsomotor vagal nucleus (DMX) in adult male rats. In study 1, spinal and intramedullary afferents to the visceral-sensorimotor complex (NTS-X) were traced to contiguous populations of cell bodies arranged in cylindrical segmental organization. NTS-X afferents derive from curvilinear arrays of neurons that parallel the efferent radiations of the solitariotegmental tract. Newly discovered afferents arise from circumscribed cell groups in the dorsal reticular formation and periventricular zone. Another source was traced to a paraambigual cell column in the apex of the rostral ventrolateral reticular nucleus (n.RVL). In study 2, catecholaminergic afferents were initially defined with combined retrograde transport-immunocytochemical methods. Deposits of retrograde tracers into NTS-X transported to neurons containing tyrosine hydroxylase (TH) in the A1, C1, and C3 areas or phenylethanolamine N-methyltransferase (PNMT) in the C1 area of the n.RVL and C3 area. In study 3, it was revealed that NTS-X afferents arise, in part, as collaterals of thoracic reticulospinal neurons. Deposits of the retrograde fluorescent tracer Fluorogold into the upper thoracic cord and rhodamine-labeled microbeads into NTS-X transported to the same neurons within a subambigual locus in n.RVL and parts of nucleus raphe magnus. In study 4, dual retrograde tracer-immunocytochemical analysis demonstrated that catecholamines are synthesized by a subset of neurons in the n.RVL that issue collaterals to the NTS-X and thoracic cord. Double retrogradely labeled TH- or PNMT-immunoreactive cell bodies were restricted to the C1 area within a 450-microns column bordered rostrally by the facial nucleus and ventrally by the medullary subpial surface. We conclude that visceral reflex arcs are reciprocally organized. Targets of NTS projection are also sources of local NTS-X afferent innervation. Catecholaminergic and other local afferents from reticular formation, periventricular, and spinal gray may, via collaterals, simultaneously modulate visceral reflex excitability at the level of NTS and the outflow of autonomic and respiratory motoneurons.

Excitatory amino acid projections to the nucleus of the solitary tract in the rat: a retrograde transport study utilizing D-[3H]aspartate and [3H]GABA

Afferents to the nucleus tractus solitarius utilizing excitatory amino acid transmitters were described in rat brain by autoradiography following microinfusion and retrograde transport of D-[3H]aspartate. Parallel experiments with the injection of [3H]GABA were employed to establish the transmitter-selective nature of the retrograde labelling found with D-[3H]aspartate. Following infusion of D-[3H]aspartate, perikaryal labelling was heaviest in myencephalon, where at least 16 discrete nuclei were labelled. Heaviest labelling was localized bilaterally in the trigeminal nucleus with cells extending through its subdivisions and the entire rostrocaudal axis. Intense labelling was also obtained in the inferior olive, predominantly contralaterally, and non-perikaryal labelling noted. Vestibular, reticular and raphe nuclei contained heavily labelled perikarya. In cervical spinal cord, a moderate density of labelled cells was found in dorsal horn, adjoining the central canal (lamina X) and in the central cervical nucleus, along with appreciable labelling of processes and non-perikaryal labelling. The relative density of labelled perikarya in mesencephalic nuclei was much lower than found in myencephalon, although D-[3H]aspartate produced topographic and precise labelling of a small number of cells in the periaqueductal gray, medial parabrachial nucleus and Koelliker-Fuse nucleus. Only weak labelling was found in cortex and hypothalamus. Labelled cells were not consistently observed in other regions (stria terminalis, amygdala, fastigial nucleus, locus coeruleus and rostral ventrolateral medulla) known to innervate the nucleus tractus solitarius. Lower densities of labelled perikarya were found after the microinjection of [3H]GABA, and the only regions in which a small number of cells were labelled by both D-[3H]aspartate and [3H]GABA were trigeminal nucleus, reticular nuclei and raphe obscurus. An exception was the ventrolateral medulla, where [3H]GABA produced precise labelling in the nucleus ambiguus and facial nucleus consistent with previous evidence for a GABAergic pathway from this area to the nucleus tractus solitarius. Our findings confirm the selectivity of the retrograde transport of D-[3H]aspartate and [3H]GABA. Overall, the transport of D-[3H]aspartate revealed a complex topographic and convergent network of afferent pathways to the nucleus tractus solitarius likely to utilize an excitatory amino acid transmitter.

Distribution of cholinergic, GABAergic and serotonergic neurons in the medial medullary reticular formation and their projections studied by cytotoxic lesions in the cat

As part of a larger study concerning the role of neurons in the medial medullary reticular formation in sleep-wake states, the distribution and projections of cholinergic, GABAergic and serotonergic neurons were studied within the lower brainstem of the cat. Cells were plotted with the aid of an image analysis system through the medullary reticular formation and raphe in adjacent sections immunostained for choline acetyltransferase, glutamic acid decarboxylase and serotonin. Immunostained fibres and varicosities were examined and quantified by microdensitometry in regions of the medulla, pons and upper spinal cord in normal and quisqualate-injected animals to assess the loss of local and distant projections following cytotoxic destruction of neurons in the medial medullary reticular formation. Choline acetyltransferase-immunoreactive neurons are unevenly and sparsely distributed, though none the less in significant numbers (estimated at approximately 9080 in total), through the medial medullary reticular formation, and are present in all tegmental fields, including the gigantocellular (approximately 3700) and magnocellular (approximately 1760) rostrally and the ventral (approximately 3240) and paramedian (approximately 380) caudally, and are absent in the midline raphe. Glutamic acid decarboxylase-immunoreactive neurons are more evenly and densely distributed in large numbers (estimated at approximately 18,720) through the medial medullary reticular formation, being present in the gigantocellular (approximately 5960), magnocellular (approximately 8260), ventral (approximately 2280) and paramedian (approximately 2220) tegmental fields, and are also numerous within the raphe magnus and pallidus-obscurus nuclei (approximately 3880). Serotonin-immunoreactive cells are sparse in the medial medullary reticular formation (estimated to total approximately 1540), where they are mainly located in the magnocellular tegmental field (approximately 1340), and are concentrated in larger numbers within the raphe nuclei (approximately 8060). Cholinergic varicose fibres were moderately densely distributed through the medial medullary reticular formation, as well as through more distant lateral, rostral and caudal brainstem and upper spinal regions. After cytotoxic lesions focussed in the gigantocellular and magnocellular tegmental fields, a loss of approximately 55% of the cholinergic neurons in the medial medullary reticular formation was associated with a minor decrease (approximately 35% in optical density measures) of local cholinergic fibres. Small and variable reductions in varicose fibres (and their optical density measures) were detected in distant structures (including the pontine lateral, gigantocellular and subcoerular tegmental fields and the caudal spinal trigeminal nucleus), that were none the less correlated with the number of intact medial medullary cholinergic cells, suggesting that these cells may project to distant brainstem targets, in addition to providing a minor proportion of the local cholinergic innervation of the medial medullary reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Innervation of serotonergic medullary raphe neurons from cells of the rostral ventrolateral medulla in rats

The rostral ventral medulla has been shown to consist of three distinct subregions: the midline or raphé region, the lateral paragigantocellular-gigantocellular region and the rostro-ventrolateral reticular nucleus. All three regions have been shown to contribute to central vaso-regulation and to project towards sympathetic preganglionic neurons of the thoracic spinal cord. Therefore it is of particular interest to describe the interconnections between the three regions and to see if local afferents reach cells which have been implicated in the regulation of descending inputs. Following injections of the anterograde tract tracer Phaseolus vulgaris leucoagglutinin into the lateral paragigantocellular nucleus or the rostroventrolateral reticular nucleus, labelled axons were traced into the medullary raphé nuclei and the contralateral rostral ventrolateral medulla. Efferents originating from both regions innervated the raphé pallidus, raphé obscurus and raphé magnus. However the distribution of terminals originating from the two regions was different in the contralateral ventrolateral medulla oblongata. The data indicate that the connection between the ipsi- and contralateral equivalents of both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus are stronger than the cross-connection between the ipsi- and contralateral parts of the two different regions. In the second part of the study, the existence of direct projections from the rostroventrolateral reticular nucleus and the lateral paragigantocellular-gigantocellular region onto serotonin-immunogold-labelled cells of the ventromedial medulla were investigated. The correlated light and electron microscopic analysis revealed direct synaptic contacts between axons originating from both the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus, and serotonin-immunoreactive cells of the raphé obscurus and raphé pallidus. The results of the present light microscopic tract-tracing study revealed a different pattern of the intramedullary projection of the lateral paragigantocellular-gigantocellular region and the rostroventrolateral reticular nucleus. These data are in support of the proposed parcellation of the two cytoarchitectonically different areas of the rostral ventrolateral medulla into two functionally distinct subdivisions. Furthermore, the direct anatomical connection revealed in the present study between cells of the rostral ventrolateral and ventromedial medulla oblongata indicates the possibility that vasoregulatory effects of some cells of the rostral ventrolateral medulla oblongata might be executed via direct projections onto serotonin-immunoreactive cells of the medullary raphé nuclei.

Non-NMDA receptors in the rostral ventrolateral medulla mediate somatosympathetic pressor responses

The role of excitatory amino acid receptors in the rostral ventrolateral medulla (RVLM) in mediating a somatosympathetic pressor response (SPR) was studied. Rats were anesthetized with urethane, bilaterally vagotomized, paralyzed and respirated. Increases in mean arterial pressure were evoked by 10-s trains of electrical stimulation of sciatic nerve afferents before and after bilateral microinjections into the RVLM of the N-methyl-D-aspartic acid (NMDA) receptor antagonist D-2-amino-7-phosphono-heptanoic acid (D-AP7) or the non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX). DNQX reversed or markedly attenuated the SPR. In contrast, the SPR was not significantly altered by blockade of NMDA receptors in the RVLM with D-AP7. However, prior administration of D-AP7 prevented reversal of the SPR by DNQX, while administration of D-AP7 after DNQX partially restored the SPR. These results indicate that pressor responses evoked by electrical stimulation of sciatic nerve afferents require synaptic activation of non-NMDA receptors in the RVLM. A somatic depressor response, revealed after blockade of non-NMDA receptors within the RVLM, may be mediated by activation of NMDA receptors in this region of the brainstem.

Effects of cAMP on respiratory rhythm generation in brainstem-spinal cord preparation from newborn rat

Involvement of cAMP in the generation of respiratory rhythm was studied in newborn rat brainstem-spinal cord preparations. The respiratory rhythm was monitored by C4 inspiratory activity and/or pre-inspiratory (Pre-I) activity of neurons in the rostral ventrolateral medulla; previously suggested to be primary rhythm generating neurons which have pacemaker properties. The effects of four cAMP-increasing agents (forskolin, IBMX, Db-cAMP, and 8-Br-cAMP) on this neuronal activity were examined. Perfusion with forskolin (3-10 microM) increased the burst rate of C4 inspiratory activity in 20 of 32 preparations, but in 8 of those the increase was preceded by transient depression. The facilitation of the respiratory rhythm was greater whenever the burst rate before forskolin treatment was lower. The Pre-I neuron burst rate, which was recorded together with C4 activity, predominantly increased with forskolin treatment. The effects of IBMX, Db-cAMP and 8-Br-cAMP were similar to those of forskolin, but they were slightly less potent. Long-lasting depression of the respiratory rhythm (C4 and Pre-I activity) by clonidine, which might decrease intracellular cAMP level via alpha 2-receptors, was reversed by forskolin. To investigate the direct effects of the cAMP-increasing agents on Pre-I neurons, Pre-I activity was isolated by blocking the chemical synaptic transmission by incubation in a low Ca solution (0.2 mM Ca2+, 5 mM Mg2+). Forskolin (5-10 microM), IBMX (5-10 microM), Db-cAMP (0.2-0.4 mM), and 8-Br-cAMP (0.4-0.75 mM) all enhanced the burst rate of isolated Pre-I neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA-like immunoreactivity in the gustatory zone of the nucleus of the solitary tract in the hamster: light and electron microscopic studies

The distribution of GABA-like immunoreactive (GABA-LI) somata was studied in the gustatory zone of the nucleus of the solitary tract (NST) in the hamster in order to identify putative inhibitory circuitry in gustatory processing. Immunoreactive somata were located throughout the gustatory NST, in accordance to the distribution of large and small types of neurons as determined in previous morphometric studies. Consequently, GABA-LI somata were mostly found in the dorsal two-thirds of the gustatory zone. Such somata were mostly ovoid in shape and possessed somal areas that averaged 85.5 +/- 2.8 microns 2 (12.7 x 8.4 microns). A narrow range of somal areas (50-125 microns 2) suggested a single functional group. At the electron microscopic level, 18% of the neurons encountered were immunoreactive and their nuclei always possessed deeply invaginated boundaries. This morphological feature indicated that GABA-LI neurons are smaller members of the most common class of neurons within the gustatory NST. Because GABA is often implicated as the neurotransmitter of small inhibitory local circuit neurons, these findings indicate a possible inhibitory aspect to the processing of taste information at the level of the first relay in the brainstem.

GABA-synthesizing neurons in the medulla: their relationship to serotonin-containing and spinally projecting neurons in the rat

GABA-synthesizing neurons were identified in the medulla of the rat by peroxidase-antiperoxidase (PAP) immunohistochemistry for glutamic acid decarboxylase (GAD). Using diaminobenzidine (DAB) either alone or intensified with silver, a relatively large number of GAD-immunoreactive neurons were evident within the reticular formation, raphe nuclei and vestibular nuclei. In all these areas, profuse GAD-immunoreactive varicosities appeared to contact the soma and dendrites of both non-GABA and GABA neurons. These observations suggest that GABA neurons may act as interneurons or local projection neurons within the medulla and accordingly exert a potent inhibitory and/or disinhibitory control on bulbar projection neurons. Within the ventral reticular formation (pars alpha and ventralis of the gigantocellular reticular field) and raphe magnus, large numbers of prominent GAD-immunoreactive neurons resembled in size and morphology and overlapped in distribution the serotonin-immunoreactive neurons of the same regions. However, by sequential double immunostaining utilizing DAB as a chromogen for serotonin (5-HT) and benzidine dihydrochloride (BDHC) for GAD, it was found that GAD-containing neurons were distinct from 5-HT-containing neurons. Following injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into the upper cervical spinal cord and combined processing for WGA-HRP (using tetramethylbenzidine [TMB] with cobalt) and immunohistochemistry (with DAB), a contingent of spinally projecting neurons were found to contain GAD. The GAD-immunoreactive reticulo- and raphe-spinal neurons were most frequent within the pars alpha and ventralis of the gigantocellular reticular fields and the raphe magnus, where they were approximately equal in number to the coexistent, but distinct 5-HT spinally projecting neurons. GABA neurons of the medulla may thus contribute directly to the bulbar inhibitory influence upon spinal sensory and motor systems.

Interaction between pressor and depressor areas in cat ventrolateral medulla

The purpose of the present study was to investigate whether stimulation of the caudal depressor area (CA) in the medulla lowered blood pressure (BP) by enhancing GABA release at the intermediate pressor area (IA) of the medulla. Application of the excitotoxin kainic acid (KA; 40 mM solution) to the caudal area lowered BP as has been previously described (Gatti, et al., Brain Research, 330 (1985) 21-29). Subsequent IA application of the GABA receptor antagonists picrotoxin (100 micrograms/side) or bicuculline (10 micrograms/side) consistently reversed this hypotensive effect. Picrotoxin raised BP by 92 +/- 10 mmHg (n = 5) after KA while by itself, picrotoxin only increased BP by 23 +/- 7.0 mmHg (n = 3) when applied to the IA. This effect was significantly greater following KA application to the caudal area (P less than 0.05). Likewise, bicuculline raised BP by 74 +/- 8.7 mmHg (n = 7) following KA while by itself, bicuculline only increased BP by 24 +/- 8.2 mmHg when applied to the IA (n = 4). This also was significantly different. This potentiation was not seen with the glycine receptor antagonist, strychnine. These data indicate that there is a GABAergic input from the caudal to the intermediate ventral surface areas of the cat which is involved in the central control of BP.

Neurons in the caudal ventrolateral medulla mediate the arterial baroreceptor reflex by inhibiting barosensitive reticulospinal neurons in the rostral ventrolateral medulla in rabbits

Participation of the caudal ventrolateral medulla in the arterial baroreceptor reflex was examined in urethane-anesthetized, vagotomized and immobilized rabbits whose aortic nerve was cut bilaterally. The extent of the caudal ventrolateral medulla was mapped by decreases in the renal sympathetic nerve activity and arterial pressure following a local microinjection of a neuroexcitatory amino acid, sodium glutamate (0.075-1.5 nmol). It extended between the levels 1.3 mm rostral and 3.0 mm caudal to the obex. An injection of sodium glutamate into the caudal ventrolateral medulla also diminished spontaneous activity of barosensitive reticulospinal neurons in the rostral ventrolateral medulla. In the 'split medulla preparation' in which the medulla was split along the midsagittal plane to disrupt fiber connections associating both sides, a neurotoxic agent, kainic acid, was injected unilaterally into the rostral ventrolateral medulla. This treatment markedly attenuated responses of renal sympathetic nerve activity and arterial pressure induced by a sodium glutamate injection into the ipsilateral caudal ventrolateral medulla, whereas responses to an injection into the contralateral caudal ventrolateral medulla were totally preserved. In four separate experiments, three to five injections of kainic acid were made unilaterally to cover the whole extent of the caudal ventrolateral medulla. The sympathoinhibitory and depressor responses to stimulation of the ipsilateral aortic nerve were then totally abolished. Simultaneously, the cardiac cycle-related rhythmic fluctuation of renal sympathetic nerve activity, which represented activity of the carotid sinus baroreceptor reflex, was attenuated to the noise level. These results, together with our previous electrophysiological demonstration of barosensitive caudal ventrolateral medulla neurons with axonal projections to the rostral ventrolateral medulla, strongly support the hypothesis that neurons in the caudal ventrolateral medulla mediate the arterial baroreceptor-vasomotor reflex through inhibition of barosensitive reticulospinal neurons in the rostral ventrolateral medulla.

A sympathoinhibitory area in cat rostral medulla: its role in cardiovascular tone regulation and baroreflex

We have found a medullary vasodepressor area in cat centered 3 mm rostral to the obex and just lateral to the compact division of the ambiguus nucleus. The area is compact, extending at most 1 mm in any direction. Microinjection of L-glutamate into this rostral depressor area (RDA) elicited acute hypotension and bradycardia. These responses were not reduced by either peripheral atropine blockade or bilateral vagotomy, but they were nearly abolished by peripheral phentolamine/propranolol blockade or high cervical cord transection. Bilateral reversible blockade of the RDA by local microinjection of the neuronal hyperpolarizing agent muscimol yielded chronic hypertension and tachycardia. Sympathetically mediated baroreflex, observed as a bradycardic response to a peripherally administered phenylephrine bolus in atropinized animals, was partially (50%) abolished during this same blockade. We conclude that the RDA contains sympathoinhibitory cells which are involved in the regulation of cardiovascular tone and in the expression of the sympathetic component of baroreflex.

GABA-mediated inhibition of medullary vasomotor neurones by area postrema stimulation in rats

1. The cardiovascular responses, together with the effects on medullary sympathoexcitatory (vasomotor) neurones of the rostral ventrolateral medulla, of area postrema stimulation have been studied in vivo. 2. Electrical (10 Hz) or chemical stimulation using microinjections of L-glutamate of the area postrema produced a vasodepressor response and an inhibition of the medullary sympathoexcitatory neurones in the nucleus reticularis rostroventrolateralis (RVL), while similar stimulation in the adjacent nucleus tractus solitarii (NTS) caused increases in arterial pressure. 3. Single-pulse stimulation of the area postrema revealed at least three influences on the activity of RVL vasomotor neurones, one being excitatory and two inhibitory. 4. The inhibitions evoked in the medullary vasomotor neurones on area postrema stimulation were blocked by ionophoretic application of bicuculline, a GABAA receptor antagonist, without altering the excitatory input to the same neurones. Bilateral microinjections of bicuculline into the RVL in regions where the vasomotor neurones had been identified totally eliminated the vasodepression due to area postrema stimulation. 5. These data support a role for the area postrema in cardiovascular control. It is concluded that the area postrema exerts its action on cardiovascular control in part via GABAergic inhibition of the 'vasomotor' neurones in the nucleus reticularis rostroventrolateralis.

GABAergic circuitry in the rostral ventral medulla of the rat and its relationship to descending antinociceptive controls

This study used postembedding immunocytochemistry to examine the organization of GABA-immunoreactive synapse in the rostral ventral medulla (RVM) of the rat. To determine whether the outflow neurons of the RVM are under GABAergic control, we examined the distribution of GABA-immunoreactive synapses upon bulbospinal projection neurons that were labelled by retrograde transport of wheatgerm agglutinin-HRP from the cervical spinal cord. To study the possible convergence of GABAergic and periaqueductal gray (PAG) synaptic inputs to RVM neurons, we also made lesions in the PAG and examined the relationship between degenerating PAG axons and GABA-immunoreactive terminals. Approximately 45% of all synapses in the RVM, which includes the midline nucleus raphe magnus and the nucleus reticularis paragigantocellularis lateralis, were GABA-immunoreactive. The vast majority of GABA-immunoreactive terminals contained round, clear, and pleomorphic vesicles and made symmetrical axodendritic synapses; axoaxonic synapses were not found. Almost 50% of the retrogradely labeled dendrites in the NRM were postsynaptic to GABA-immunoreactive terminals. Several examples of convergence of degenerating PAG terminals and GABAergic terminals onto the same unlabelled dendrite were also found. These data indicate that the projection neurons of the RVM are under profound GABAergic inhibitory control. The results are discussed with regard to the hypothesis that the analgesic action of narcotics and electrical stimulation of the midbrain PAG involves the regulation of tonic GABAergic inhibitory controls that are exerted upon spinally-projecting neurons of the nucleus raphe magnus.

Evidence for clonidine presynaptically modulating amino acid release in the rostral ventral medulla: role in hypertension

Reports suggested that the predominant site of action for the antihypertensive effects of clonidine is the rostral ventrolateral medulla (RVL), the presumed tonic vasomotor center. This study examined whether clonidine directly interacts with nerve terminal alpha 2-adrenergic receptors in the RVL to inhibit the release of sympathoexcitatory transmitters like glutamate (Glu) and aspartate (Asp), and/or facilitate the release of sympathoinhibitory transmitters like gamma-aminobutyric acid (GABA). Release of GABA and Glu was measured from synaptosomes prepared from the rostral ventral medulla of spontaneously hypertensive rats (SHR), a genetic model of hypertension, and normotensive Wistar-Kyoto rats (WKY). Quantification of neurotransmitter release was performed by high-performance liquid chromatography. Depolarization with 35 mM K+ significantly increased by 58-110% the release of GABA, Glu and Asp; however, no strain differences were observed. In contrast, spontaneous release of GABA and Asp was significantly lower in SHR than that of WKY (-36 and -41%, respectively); this effect was not observed for Glu. Clonidine (1 and 10 microM) enhanced the spontaneous release of GABA (+44%), Asp (+50%) and Glu (+70%) in SHR, but not WKY; this effect was prevented by yohimbine (1 microM). These data, together with previous findings, support the presence of facilitory alpha 2-adrenergic receptors on nerve terminals of GABAergic, glutamatergic and aspartatergic neurons in the rostral ventral medulla. These findings also suggest the existence of another inhibitory transmitter that may mediate the actions of clonidine to decrease sympathetic outflow from the RVL.

Localization of vasodepressor neurons in the caudal ventrolateral medulla in the rabbit

The precise distribution of vasodepressor neurons in the caudal ventrolateral medulla has been mapped in the rabbit using very small injections of L-glutamate (0.5 nmol in 25 nl or 0.1 nmol of 10 nl of Ringer solution). The vasodepressor neurons are discretely located in a circumscribed region of the lateral reticular formation, ventral and ventrolateral to the nucleus ambiguus, from 0.5 mm to 2.5 mm caudal to the obex. The results indicate that the vasodepressor neurons in the caudal ventrolateral medulla are confined to a relatively limited region, not corresponding to any defined nucleus.

Synthesis, release and receptor binding of acetylcholine in the C1 area of the rostral ventrolateral medulla: contributions in regulating arterial pressure

This study sought to determine whether release of acetylcholine (ACh) within the C1 area of nucleus reticularis rostroventrolateralis (RVL) contributes to the tonic maintenance of arterial pressure (AP) in the rat. The activity of choline acetyltransferase (ChAT), the biosynthetic enzyme for ACh, varied 5.5-fold in micropunches of the 6 medullary regions examined. ChAT activity in the C1 area (179 +/- 35 nmol [14C]ACh formed/mg protein/60 min; n = 4) was intermediate between that of the hypoglossal nucleus (249 +/- 38; highest) and the pyramids (45 +/- 11; lowest) and equivalent to that found in the parietal cortex (147 +/- 15). Release of [3H]ACh from C1 area micropunches was increased by raising extracellular K+ concentrations (5-55 mM) and was entirely Ca2(+)-dependent. Muscarinic receptor binding density was assessed using [3H]quinuclidinyl benzylate ([3H]QNB) as ligand and a recently developed 'electronic micropunch' technique which allows measurement of quench-corrected [3H]QNB binding within corresponding cylinders of tissue obtained by the mechanical micropunch cannula. [3H]QNB binding density varied 2.6-fold: lateral reticular nucleus pars lateralis greater than C1 area greater than nucleus ambiguus = hypoglossal nucleus = pyramid = oral spinal trigeminal nucleus. In urethane-anesthetized rats, inhibition of ACh synthesis by hemicholinium-3 (HC-3, 3 nmol/50 nl), or blockade of muscarinic receptors by scopolamine (SCOP, 3 nmol/50 nl), reduced resting mean AP by 18-24 mm Hg following bilateral microinjection into the C1 area. These concentrations of HC-3 and SCOP were sufficient to attenuate by 70-80% the increase in local cholinergic neurotransmission elicited by the cholinesterase inhibitor physostigmine given systemically. High concentrations of SCOP (30-150 nmol/50 nl) lowered AP by 46-60 mm Hg. Similarly, bilateral microinjections of GABA (10 nmol/50 nl) into the C1 area markedly reduced mean AP by 51 +/- 6 mm Hg to levels normally found after transection of the spinal cord. Thus, a substantial portion of tonic sympathetic activity may be driven by activation of muscarinic receptors in the C1 area. In the spontaneously hypertensive rat (SHR), a genetic model of hypertension, neither spontaneous nor K(+)-evoked release of [3H]ACh from the C1 area differed from that of normotensive Wistar-Kyoto rats (WKY).(ABSTRACT TRUNCATED AT 400 WORDS)