Distribution of glutamate- and GABA-immunoreactive neurons projecting to the cardioacceleratory center of the intermediolateral nucleus of the thoracic cord of SHR and WKY rats: a double-labeling study

Modulation of sympathetic preganglionic neuron activity via adrenergic receptors

The sympathetic preganglionic neurons (SPNs) play a key role in the sympathetic nervous system. Previous reports have suggested that norepinephrine (NE) directly affects SPNs via both inhibitory hyperpolarization interactions mediated by α2 receptors and excitatory depolarization interactions mediated by α1 receptors. It remains poorly understood, however, whether the excitability of SPNs can be inhibited indirectly (presynaptically) as well as directly (postsynaptically). We intracellularly recorded 41 SPNs using the whole-cell patch-clamp technique in spinal cord slice preparations of neonatal rats. We examined the effects of NE or dexmedetomidine hydrochloride (Dxm) (α2-adrenergic receptor agonist) on SPNs by analyzing the excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs). EPSPs were dominant in 15 SPNs (EPSP-SPNs) and IPSPs were dominant in 7 SPNs (IPSP-SPNs) at baseline. We were unable to analyze the postsynaptic potentials in the other 19 SPNs, due to high frequency of action potential firings (firing-SPNs). At baseline, the membrane potentials and resistances of each type of SPN were similar. NE (1  μM) gradually depolarized the EPSP-SPNs and IPSP-SPNs (P   < 0.001) and NE significantly increased the EPSP frequency of the EPSP-SPNs (P   < 0.05). Dxm (10  nM) after application of NE decreased the EPSP frequency of the EPSP-SPNs (P  <  0.001) and the EPSP voltage and IPSP voltage of the IPSP-SPNs (P  <  0.05). In 5 of the 19 firing-SPNs, NE induced membrane hyperpolarization (P   < 0.05) and completely inhibited firings. Dxm had no effect in these neurons. The SPNs received inhibitory modulation through α2-adrenergic receptors. Some SPNs can be directly inhibited via effects independent of the α2 receptors.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

How sympathetic are your spinal cord circuits?

NEW FINDINGS

What is the topic of this review? This review focuses on the role of gap junctions and interneurones in sympathetic control at the spinal cord level. What advances does it highlight? The review considers the importance of these local spinal circuits in contributing to rhythmic autonomic activity and enabling appropriate responses to homeostatic perturbations. Sympathetic control of end organs relies on the activity of sympathetic preganglionic neurones (SPNs) within the spinal cord. These SPNs exhibit heterogeneity with respect to function, neurochemistry, location, descending inputs and patterns of activity. Part of this heterogeneity is bestowed by local spinal circuitry. Our understanding of the role of these local circuits, including the significance of connections between the SPNs themselves through specialized gap junctions, is patchy. This report focuses on interneurones and gap junctions within these circuits. Gap junctions play a role in sympathetic control; they are located on SPNs in the intermediolateral cell column. Mefloquine, a chemical that blocks these gap junctions, reduces local rhythmic activity in the spinal cord slice and disrupts autonomic control in the working heart-brainstem preparation. The role that these gap junctions may play in health and disease in adult animals remains to be elucidated fully. Presympathetic interneurones are located in laminae V, VII and X and the intermediolateral cell column; those in lamina X are GABAergic and directly inhibit SPNs. The GABAergic inputs onto SPNs exert their effects through activation of synaptic and extrasynaptic receptors, which stabilize the membrane at negative potentials. The GABAergic interneurones contribute to rhythmic patterns of activity that can be generated in the spinal cord, because bicuculline reduces network oscillatory activity. These studies indicate that local spinal cord circuitry is critical in enabling appropriate levels and patterning of activity in sympathetic outflow. We need to understand how these circuits may be harnessed in the situation of spinal cord injury.

GABA(B) Mediated Regulation of Sympathetic Preganglionic Neurons: Pre- and Postsynaptic Sites of Action

Modulatory influences on sympathetic nervous system activity are diverse and far reaching, acting at select points in the complex pathways controlling sympathetic outflow to enable subtle changes or more global effects. Changes in the degree of sympathetic neuromodulation can have serious consequences on homeostatic variables such as heart rate, blood pressure and gut motility. At the level of the spinal cord, the sympathetic preganglionic neurons (SPNs) can be modulated by activation of presynaptic GABA(B) heteroreceptors on glutamatergic terminals and by postsynaptic GABA(B) receptors. Here we show that a low concentration of the GABA(B) agonist baclofen (1 μM) attenuated GABAergic inhibitory postsynaptic potentials in SPNs elicited from stimulation of either the central autonomic area or descending fibers in the lateral funiculus. This low baclofen concentration also elicited three categories of postsynaptic response: a large hyperpolarization with a decrease in input resistance, a moderate hyperpolarization with no change in input resistance and no response. Using cesium-loaded, tetraethylammonium chloride containing electrodes (to block potassium conductance), baclofen elicited moderate hyperpolarizations with no change in input resistance in 50% of SPNs; the remainder were unaffected. These modest hyperpolarizations were reduced in Ca(2+) free solution or cadmium. Hyperpolarizing responses were also observed in interneurons in the vicinity of SPNs. These studies provide the first evidence for GABA(B) autoreceptors involved in inhibitory GABAergic transmission onto SPNs and for postsynaptic GABA(B) receptors on interneurons. The data also indicate that there is heterogeneity in the postsynaptic responses of SPNs.

Cyclooxygenase and nitric oxide synthase in the presympathetic neurons in the paraventricular hypothalamic nucleus are involved in restraint stress-induced sympathetic activation in rats

Stress is one of the important factors to activate the sympathetic nervous system. We recently reported that central administration of corticotropin-releasing factor (CRF), known as a stress-related neuropeptide, increases the expression of both cyclooxygenase (COX) and nitric oxide synthase (NOS) in presympathetic neurons in the paraventricular hypothalamic nucleus (PVN). In the present study, therefore, we investigated whether brain COX and NOS can also mediate restraint stress (RS)-induced sympathetic activation by assessing the plasma catecholamine levels and neuronal activation of presympathetic neurons in the PVN. In addition, we examined effects of RS on the expression of both COX and NOS isozymes in the presympathetic PVN neurons. Intraperitoneal administration of an inhibitor for COX-1, COX-2 or inducible NOS (iNOS), but not for neuronal NOS (nNOS), reduced RS-induced elevation of plasma catecholamine levels and Fos expression in the presympathetic PVN neurons. Moreover, RS increased the expression of COX-1, COX-2 and iNOS in the presympathetic PVN neurons, whereas nNOS expression did not change. These results suggest that COX-1, COX-2 and iNOS in the presympathetic PVN neurons mediate acute RS-induced sympathetic activation.

Neurochemistry of bulbospinal presympathetic neurons of the medulla oblongata

This review focuses on presympathetic neurons in the medulla oblongata including the adrenergic cell groups C1-C3 in the rostral ventrolateral medulla and the serotonergic, GABAergic and glycinergic neurons in the ventromedial medulla. The phenotypes of these neurons including colocalized neuropeptides (e.g., neuropeptide Y, enkephalin, thyrotropin-releasing hormone, substance P) as well as their relative anatomical location are considered in relation to predicting their function in control of sympathetic outflow, in particular the sympathetic outflows controlling blood pressure and thermoregulation. Several explanations are considered for how the neuroeffectors coexisting in these neurons might be functioning, although their exact purpose remains unknown. Although there is abundant data on potential neurotransmitters and neuropeptides contained in the presympathetic neurons, we are still unable to predict function and physiology based solely on the phenotype of these neurons.

Maps of cardiovascular and respiratory regions of rat ventral medulla: focus on the caudal medulla

The ventral medulla oblongata is critical for cardiorespiratory regulation. Here we review previous literature relating to sites within the ventral medulla that have been identified as having a 'cardiovascular' or 'respiratory' function. Together with the maps generated here, of sites from which cardiovascular and respiratory responses were evoked by glutamate microinjection, specific 'cardiovascular' regions have been defined and delineated. Commonly investigated regions, including the vasopressor rostral ventrolateral medulla (RVLM) and vasodepressor caudal ventrolateral medulla (CVLM), or areas only described by others, such as the medullary cerebral vasodilator area, are included for completeness. Emphasis is given to the caudal medulla, where three pressor regions, the caudal pressor area (CPA), the intermediate pressor area (IPA) and the medullo-cervical pressor area (MCPA), caudal to the vasodepressor CVLM were defined in the original data provided. The IPA is most responsive under pentobarbitone rather than urethane anaesthesia clearly delineating it from both the rostrally located CPA and the caudally located MCPA. The description of these multiple pressor areas appears to clarify the confusion that surrounds the identification of the 'CPA'. Also noted is a vasopressor region adjacent to the vasodepressor CVLM. Apart from the well described ventral respiratory column, a region medial to the pre-Bötzinger is described, from which increases in both phrenic nerve frequency and amplitude were evoked. Limitations associated with the technique of glutamate microinjection to define functionally specific regions are discussed. Particular effort has been made to define and delineate the regions with respect to ventrally located anatomical landmarks rather than the commonly used ventral surface or dorsal landmarks such as the obex or calamus scriptorius that may vary with the brain orientation or histological processing. This should ensure that a region can easily be defined by all investigators. Study of defined regions will help expedite the identification of the role of the multiple cell groups with diverse neurotransmitter complements that exist even within each of the regions described, in coordinating the delivery of oxygenated blood to the tissues.

Modulation of GABAergic synaptic transmission by terminal nicotinic acetylcholine receptors in the central autonomic nucleus of the neonatal rat spinal cord

Using patch clamp recordings from an in vitro spinal cord slice preparation of neonatal rats (9-15days old), we characterized the GABAergic synaptic transmission in sympathetic preganglionic neurones (SPN) of the central autonomic nucleus (CA) of lamina X. Local applications of isoguvacine (100microM), a selective agonist at GABA(A) receptors, induced in all cells tested a chloride current which was abolished by bicuculline, a competitive antagonist at GABA(A) receptors. In addition, 25% of the recorded cells displayed spontaneous tetrodotoxin-insensitive and bicuculline-sensitive chloride miniature inhibitory postsynaptic currents (mIPSCs). Acetylcholine (100microM) increased the frequency of GABAergic mIPSCs without affecting their amplitudes or their kinetic properties indicating a presynaptic site of action. The presynaptic effect of ACh was restricted to GABAergic neurones synapsing onto sympathetic preganglionic neurones. The facilitatory effect of ACh was abolished in the absence of external calcium or in the presence of 100microM cadmium added to the bath solution. Choline 10mM, an agonist at alpha7 nicotinic acetylcholine receptors (nAChRs) or muscarine (10microM), a muscarinic receptor agonist, did not reproduce the presynaptic effect of ACh. The presynaptic effect of ACh was blocked by 1microM of dihydro-beta-erythroidine (DHbetaE), an antagonist of non-alpha7 nAChRs but was insensitive to alpha7 nAChRs antagonists (strychnine, alpha-bungarotoxin and methyllycaconitine) or to the muscarinic receptor antagonist atropine (10microM). It was concluded that SPNs of the central autonomic nucleus displayed a functional GABAergic transmission which is facilitated by terminal non alpha7 nAChRs.

Upregulation of tachykinin NK-1 and NK-3 receptor binding sites in the spinal cord of spontaneously hypertensive rat: impact on the autonomic control of blood pressure

1 Effects of intrathecally (i.t.) injected tachykinin NK-1 and -3 receptor agonists and antagonists were measured on mean arterial blood pressure (MAP) and heart rate (HR) in awake unrestrained spontaneously hypertensive rats (SHR,15-week-old) and age-matched Wistar Kyoto rats (WKY). Quantitative in vitro autoradiography was also performed on the lower thoracic spinal cord of both strains and Wistar rats using specific radioligands for NK-1 receptor ([(125)I]HPP[Arg(3),Sar(9),Met(O(2))(11)]SP (3-11)) and NK-3 receptor ([(125)I]HPP-Asp-Asp-Phe-N-MePhe-Gly-Leu-Met-NH(2)). 2 The NK-1 agonist [Sar(9),Met(O(2))(11)]SP (650 and 6500 pmol) decreased MAP and increased HR in WKY. The fall in MAP was blunted in SHR and substituted by increases in MAP (65-6500 pmol) and more sustained tachycardia. The NK-3 agonist senktide (6.5-65 pmol) evoked marked increases in MAP and HR (SHR>>>WKY), yet this response was rapidly desensitized. Cardiovascular effects of [Sar(9),Met(O(2))(11)]SP (650 pmol) and senktide (6.5 pmol) were selectively blocked by the prior i.t. injection of LY303870 (NK-1 antagonist, 65 nmol) and SB235375 (NK-3 antagonist, 6.5 nmol), respectively. Antagonists had no direct effect on MAP and HR in both strains. 3 Densities of NK-1 and -3 receptor binding sites were significantly increased in all laminae of the spinal cord in SHR when compared to control WKY and Wistar rats. The dissociation constant was however not affected in SHR for both NK-1 (K(d)=2.5 nM) and NK-3 (K(d)=5 nM) receptors. 4 Data highlight an upregulation of NK-1 and -3 receptor binding sites in the thoracic spinal cord of SHR that may contribute to the hypersensitivity of the pressor response to agonists and to the greater sympathetic activity seen in this model of arterial hypertension.

Neuronal expression of nuclear transcription factor MafG in the rat medulla oblongata after baroreceptor stimulation

The medulla oblongata is the site of central baroreceptive neurons in mammals. These neurons express specific basic-leucine zipper transcription factors (bZIP) after baroreceptor stimulation. Previously we showed that activation of baroreceptors induced expression of nuclear transcription factors c-Fos and FosB in central baroreceptive neurons. Here we studied the effects of baroreceptor stimulation on induction of MafG, a member of small Maf protein family that functions as dimeric partners for various bZIP transcription factors by forming transcription-regulating complexes, in the rat medulla oblongata. To determine whether gene expression of MafG is induced by stimulation of arterial baroreceptors, we examined the expression of its mRNA by semi-quantitative reverse transcription-PCR method and its gene product by immunohistochemistry. We found that the number of MafG transcripts increased significantly in the medulla oblongata after baroreceptor stimulation. MafG-immunoreactive neurons were distributed in the nucleus tractus solitarii, the dorsal motor nucleus of the vagus nerve, the ambiguous nucleus and the ventrolateral medulla. The numbers of MafG-immunoreactive neurons in these nuclei were significantly greater in test rats than in saline-injected control rats. We also found approximately 20% of MafG-immunoreactive neurons coexpress FosB after baroreceptor stimulation. Our results suggest that MafG cooperates with FosB to play critical roles as an immediate early gene in the signal transduction of cardiovascular regulation mediated by baroreceptive signals in the medulla oblongata.

GABAergic and glycinergic presympathetic neurons of rat medulla oblongata identified by retrograde transport of pseudorabies virus and in situ hybridization

Electron microscopy suggests that up to half the synaptic input to sympathetic preganglionic neurons (SPGNs) is GABAergic or glycinergic. A proportion of this input is suspected to originate from neurons located within the medulla oblongata. The present study provides definitive evidence for the existence of these supraspinal presympathetic (PS) neurons with inhibitory phenotypes. PS neurons were identified by retrograde trans-synaptic migration of pseudorabies virus (PRV) injected into the adrenal gland. GABAergic or glycinergic cell bodies were identified by the presence of glutamate decarboxylase (GAD)-67 mRNA or glycine transporter (GlyT)-2 mRNA detected with in situ hybridization (ISH). Neither GABAergic nor glycinergic PS neurons were tyrosine hydroxylase (TH)-immunoreactive (ir). GABAergic PS neurons were located within the ventral gigantocellular nucleus, gigantocellular nucleus alpha, and medial reticular formation, mostly medial to the TH-ir PS neurons. About 30% of GABAergic PS neurons were serotonergic cells located in the raphe pallidus (RPa) and parapyramidal region (PPyr). Glycinergic PS neurons had the same general distribution as the GABAergic cells, except that no glycinergic neurons were located in the RPa or PPyr and none were serotonergic. PRV immunohistochemistry combined with ISH for both GlyT2 and GAD-67 mRNAs showed that at least 63% of midline medulla GABAergic PS neurons were also glycinergic and 76% of glycinergic PS neurons were GABAergic. In conclusion, the rostral ventromedial medulla contains large numbers of GABAergic and glycinergic neurons that innervate adrenal gland SPGNs. Over half of these PS neurons may release both transmitters. The physiological role of this medullary inhibitory input remains to be explored.

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.

Activation and integration of bilateral GABA-mediated synaptic inputs in neonatal rat sympathetic preganglionic neurones in vitro

The role of GABA receptors in synaptic transmission to neonatal rat sympathetic preganglionic neurones (SPNs) was investigated utilizing whole-cell patch clamp recording techniques in longitudinal and transverse spinal cord slice preparations. In the presence of glutamate receptor antagonists (NBQX, 5 microm and D-APV, 10 microm), electrical stimulation of the ipsilateral or contralateral lateral funiculi (iLF and cLF, respectively) revealed monosynaptic inhibitory postsynaptic potentials (IPSPs) in 75% and 65% of SPNs, respectively. IPSPs were sensitive to bicuculline (10 microM) in all neurones tested and reversed polarity around -55 mV, the latter indicating mediation via chloride conductances. In three neurones IPSPs evoked by stimulation of the iLF (n = 1) or cLF (n = 2) were partly sensitive to strychnine (2 microM). The expression of postsynaptic GABA(A) and GABA(B) receptors were confirmed by the sensitivity of SPNs to agonists, GABA (2 mm), muscimol (10-100 microM) or baclofen (10-100 microM), in the presence of TTX, each of which produced membrane hyperpolarization in all SPNs tested. Muscimol-induced responses were sensitive to bicuculline (1-10 microM) and SR95531 (10 microM) and baclofen-induced responses were sensitive to 2-hydroxy-saclofen (100-200 microM) and CGP55845 (200 nM). The GABA(C) receptor agonist CACA (200 microM) was without significant effect on SPNs. These results suggest that SPNs possess postsynaptic GABA(A) and GABA(B) receptors and that subsets of SPNs receive bilateral GABAergic inputs which activate GABA(A) receptors, coupled to a chloride conductance. At resting or holding potentials close to threshold either single or bursts (10-100 Hz) of IPSPs gave rise to a rebound excitation and action potential firing at the termination of the burst. This effect was mimicked by injection of small (10-20 pA) rectangular-wave current pulses, which revealed a time-dependent, Cs(+)-sensitive inward rectification and rebound excitation at the termination of the response to current injection. Synaptic activation of a rebound excitation mediated by a time-dependent inward rectification expressed intrinsically by SPNs may provide a novel mechanism enabling SPNs to be entrained to rhythms driven from the brainstem or higher centres.

Motor cortical control of cardiovascular bulbar neurones projecting to spinal autonomic areas

There is evidence that the motor cortex is involved in cardiovascular adjustments associated with somatic motor activity, as it has functional connections with the ventrolateral medulla, a brainstem region critically involved in the control of blood pressure and the regulation of plasma catecholamine levels. The ventrolateral medulla sends projections to the spinal intermediolateral nucleus, where preganglionic neurones controlling heart and blood vessels (T2 segment) and adrenal medulla (T8 segment) are found. The aim of the present study was to determine whether electrical stimulation of the rat motor cortex induces cardiovascular responses and Fos expression in ventrolateral medulla neurones projecting to the T2 and T8 segments. After a set of experiments designed to record cardiovascular parameters (blood pressure and plasma catecholamine levels), injections of retrograde tracer (Fluorogold) were performed in the intermediolateral nucleus of two groups of rats, at the T2 or at the T8 segmental levels. Five days later, the motor cortex was stimulated in order to induce Fos expression in the ventrolateral medulla. Stimulation of the motor cortex induced: (1). hypotension and a significant decrease in plasma noradrenaline levels, and (2). a significant increase in the number of the double-labelled neurones in the rostral ventrolateral medulla projecting to T2. These data demonstrate that cardiovascular adjustments, preparatory to, or concomitant with, motor activity may be initiated in the motor cortex and transmitted to cardiac and vasomotor spinal preganglionic neurones, via the ventrolateral medulla.

Evidence for a glutamatergic input to pontine preganglionic neurons of the superior salivatory nucleus in rat

Parasympathetic preganglionic neurons of the superior salivatory nucleus (SSN), which projects to the pterygopalatine ganglion (PPG), modulate salivation, lacrimation, and cerebrovascular tone. Our previous studies suggest that excitatory projections from the nucleus tractus solitarii modulate cerebrovascular tone by actions on SSN neurons. In this study we sought to test the hypothesis that N-methyl-D-aspartate (NMDA) type glutamate receptors and vesicular glutamate transporters (VGLUT) are present in the SSN and that SSN neurons receive glutamatergic input. In six rats we injected tetramethylrhodamine dextran (TRD), a fluorescent tracer, unilaterally into the PPG to label SSN neurons. Four days later, rats were perfused and brain stem sections containing the SSN were processed for fluorescent immunohistochemistry for N-methyl-D-aspartate receptor subunit 1 (NMDAR1) and vesicular glutamate transporters (VGLUT1 and VGLUT2). Confocal laser scanning microscopy showed that 88+/-3% of TRD-labeled SSN neurons contained NMDAR1-immunoreactivity (IR). The surrounding neuropil contained numerous fibers labeled for VGLUT2-IR, but not VGLUT1-IR. Double fluorescent immunohistochemistry for NMDAR1 and VGLUT2 revealed that fibers containing VGLUT2-IR were often in close proximity to cell bodies or proximal dendrites of TRD-labeled SSN neurons that were positive for NMDAR1-IR. These studies support our hypothesis that NMDA receptors and VGLUT are present in the SSN. They further provide support for the suggestion that there are glutamatergic inputs to SSN neurons and would be consistent with an excitatory input that could regulate cerebrovascular tone.

Evidence for a GABA(B) receptor component in the spinal action of Substance P (SP) on arterial blood pressure in the awake rat

1 The activation of tachykinin NK(1) receptors in the rat spinal cord produced a transient drop in arterial blood pressure followed by a more prolonged pressor effect which is mediated by the stimulation of the sympatho-adrenal system. This study aims at characterizing the spinal mechanism of that initial hypotension occurring in awake unrestrained rats. 2 The initial hypotension (-18+/-2.0 mmHg at 1 min) and the tachycardia (110+/-10 b.p.m.) produced by the intrathecal (i.t.) injection of the stable NK(1) receptor agonist [Sar(9), Met(O(2))(11)]-SP (Sar9, 0.65 nmol) at T-9 spinal cord level was inhibited by the prior injection of 65 nmol LY306740 or LY303870 (NK(1) receptor antagonists). No inhibition was seen when a similar dose of antagonists was given intravenously. 3 The prior i.t. injection of the GABA(B) receptor antagonist CGP52432 (100 nmol) reduced the hypotension evoked by Sar9 (0.65 nmol) and by the GABA(B) receptor agonist baclofen (100 nmol). The GABA(A) receptor antagonist bicuculline (25 nmol, i.t.) was without effect against Sar9, and the GABA(A) agonist muscimol (100 nmol, i.t.) had no cardiovascular effect. 4 The putative involvement of other mediators (dopamine, serotonine, glycine and glutamate) in Sar9-induced hypotension was made unlikely on the basis of various pharmacological treatments. Thus data, suggest that the transient hypotension which occurs upon the activation of NK(1) receptors in the spinal cord is due to the release of GABA which in turn activates GABA(B) receptors to inhibit sympathetic pre-ganglionic fibres. This mechanism may have a physiological significance in the spinal reflex autonomic control of arterial blood pressure.

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.

Activation of ventrolateral medullary neurons projecting to spinal autonomic areas after chemical stimulation of the central nucleus of amygdala: a neuroanatomical study in the rat

Several studies have shown that the central nucleus of amygdala is involved in cardiovascular regulation. The control of this function may be mediated by activation of the ventrolateral medulla neurons that project to preganglionic neurons located in the intermediolateral nucleus of the spinal cord. The aim of the present study was to examine whether stimulation of the central nucleus of amygdala activated ventrolateral medulla neurons projecting to the intermediolateral nucleus. For this purpose, the injection of a retrograde tracer, the cholera toxin b subunit (CTb), into the intermediolateral nucleus of the T2 segment was combined with immunohistochemical detection of Fos protein following chemical stimulation of the central nucleus of amygdala. Results showed that retrogradely labeled neurons were found throughout the ventrolateral medulla. Moreover, chemical stimulation of the central nucleus of amygdala induced: (1) a decrease of arterial blood pressure; (2) an expression of Fos protein mainly in sub-populations of neurons located in the intermediate and caudal parts of the ventrolateral medulla; (3) a significantly higher number of double labeled neurons (CTb-immunoreactive/Fos-immunoreactive) in the rostral part of the ventrolateral medulla than in the other parts of this region. These results show that the central nucleus of amygdala influences the activity of brainstem neurons projecting to the intermediolateral nucleus. Data were discussed in terms of descending amygdalofugal pathways involved in the hypotension.

Melatonin sees the light: blocking GABA-ergic transmission in the paraventricular nucleus induces daytime secretion of melatonin

Despite a pronounced inhibitory effect of light on pineal melatonin synthesis, usually the daily melatonin rhythm is not a passive response to the surrounding world. In mammals, and almost every other vertebrate species studied so far, the melatonin rhythm is coupled to an endogenous pacemaker, i.e. a circadian clock. In mammals the principal circadian pacemaker is located in the suprachiasmatic nuclei (SCN), a bilateral cluster of neurons in the anterior hypothalamus. In the present paper we show in the rat that bilateral abolition of gamma-aminobutyric acid (GABA), but not vasopressin, neurotransmission in an SCN target area, i.e. the paraventricular nucleus of the hypothalamus, during (subjective) daytime results in increased pineal melatonin levels. The fact that complete removal of the SCN results in a pronounced increase of daytime pineal mRNA levels for arylalkylamine N-acetyltransferase (AA-NAT), i.e. the rate-limiting enzyme of melatonin synthesis, further substantiates the existence of a major inhibitory SCN output controlling the circadian melatonin rhythm.

Evidence for a GABAergic projection from the central nucleus of the amygdala to the brainstem of the macaque monkey: a combined retrograde tracing and in situ hybridization study

The central nucleus of the amygdala is interconnected with a variety of visceral and autonomic nuclei of the brainstem. These include the parabrachial nucleus, the nucleus of the solitary tract, the nucleus ambiguus and the dorsal motor nucleus of the vagus. Despite repeated attempts, neurochemical characterization of the major subcortical connections of the central nucleus has not yet been accomplished. Based on earlier immunohistochemical and in situ hybridization evidence indicating the presence of numerous GABAergic neurons in the macaque monkey central nucleus, we predicted that a sizeable portion of the descending projections may be GABAergic. We tested this hypothesis using a novel double labelling method with gold conjugated WGA-apoHRP as a retrograde tracer and in situ hybridization for detecting the mRNA that encodes the enzyme glutamic acid decarboxylase (GAD67) as a marker for GABAergic cells. Following WGA-apoHRP-gold injections into the brainstem, a large number of retrogradely labelled cells was observed in the medial and lateral divisions of the central nucleus. Of the retrogradely labelled cells observed in the medial division of the central nucleus, approximately half were double-labelled for GAD67 mRNA; about 30% double labelling was observed in the lateral division. These data support the view that a sizeable component of the central nucleus projection to the brainstem is GABAergic.

GABA- and glutamate-immunoreactive synapses on sympathetic preganglionic neurons projecting to the superior cervical ganglion

Our previous work suggests that virtually all of the synapses on sympathetic preganglionic neurons projecting to the rat adrenal medulla are immunoreactive for either the inhibitory amino acid, gamma-aminobutyric acid (GABA) or the excitatory amino acid, L-glutamate. To investigate whether or not this is true for other groups of sympathetic preganglionic neurons, and to determine whether or not the proportion of inputs containing each type of amino acid neurotransmitter is the same for different groups of sympathetic preganglionic neurons, we retrogradely labelled rat and rabbit sympathetic preganglionic neurons projecting to the superior cervical ganglion and used post-embedding immunogold on ultrathin sections to localise GABA- and glutamate-immunoreactivity. The cell bodies and dendrites of both rat and rabbit sympathetic preganglionic neurons projecting to the superior cervical ganglion received synapses and direct contacts from nerve fibres immunoreactive for GABA and from nerve fibres immunoreactive for glutamate. In the rat, GABA was present in 48.9% of the inputs to sympathetic preganglionic neurons projecting to the superior cervical ganglion, and glutamate was present in 51.7% of inputs. Double immunogold labelling for glutamate and GABA on the same section, as well as labelling of consecutive serial sections for the two antigens, indicated that GABA and glutamate occur in separate populations of nerve fibres that provide input to rat sympathetic preganglionic neurons projecting to the superior cervical ganglion. We now have shown that GABA or glutamate is present in virtually all of the inputs to sympathetic preganglionic neurons projecting to the superior cervical ganglion and in essentially all of the inputs to sympathetic preganglionic neurons supplying the adrenal medulla. These findings are consistent with the hypothesis that all fast synaptic transmission in central autonomic pathways may be mediated by either excitatory or inhibitory amino acids. Furthermore, we showed a statistically significant difference in the proportion of glutamate-immunoreactive inputs between sympathetic preganglionic neurons projecting to the superior cervical ganglion and sympathoadrenal neurons (data from Llewellyn-Smith et al. [Llewellyn-Smith, I.J., Phend, K.D., Minson, J.B., Pilowsky, P.M., Chalmers, J.P., 1992. Glutamate immunoreactive synapses on retrogradely labelled sympathetic neurons in rat thoracic spinal cord. Brain Res. 581, 67-80]), with preganglionics supplying the adrenal medulla receiving more excitatory inputs than those supplying the superior cervical ganglion. This increased excitatory input to sympathoadrenal neurons may explain the predominant activation of these neurons following baroreceptor unloading.

Difference in topology and numbers of barosensitive catecholaminergic and cholinergic neurons in the medulla between SHR and WKY rats

We hypothesized that there may be a significant difference in the neuronal composition of the baroreceptor reflex pathway between normotensive Wistar Kyoto (WKY) and spontaneously hypertensive SHR rats. Using the double-immunoreactive (IR) method, the topology and numbers of barosensitive neurons that contain glutamate (Glu), glutamic acid decarboxylase (GAD), tyrosine hydroxylase (TH), phenylethanolamine N-methyltransferase (PNMT) and choline acetyltransferase (ChAT) were compared between the two strains. The control rats were sham-operated only for cannulation of the trachea and femoral artery/vein. The test rats were injected with the pressor agent phenylephrine to raise blood pressure and stimulate arterial baroreceptors. In both the control and test experiments, the c-Fos/Glu-, GAD-, TH- and PNMT-IR neurons were found in the nucleus tractus solitarii (NTS) and ventrolateral medulla (VLM), while the FosB/ChAT-IR neurons were found in the NTS, dorsal motor nucleus of the vagus (DMX) and nucleus ambiguus (AMB). In the control experiment, no significant difference in numbers was recognized in any of the double-IR neurons between the two strains. In the test experiment, the numbers of FosB/ChAT-IR neurons in the NTS, DMX and AMB were significantly smaller in SHR than in WKY. The numbers of c-Fos/TH-IR neurons in the caudal VLM were significantly larger in SHR than in WKY. These results suggest that a smaller number of barosensitive cholinergic neurons in the DMX and AMB in SHR causes the weaker baroreceptor-cardiac vagal reflex in SHR, and that a larger number of barosensitive catecholaminergic neurons in the caudal VLM in SHR are involved in the stronger baroreceptor-vasopressin reflex in SHR.

GABA- and glycine-mediated inhibitory postsynaptic potentials in neonatal rat rostral ventrolateral medulla neurons in vitro

Whole-cell patch recordings were made from rostral ventrolateral medulla neurons of two in vitro preparations: (i) brainstem spinal cords of two- to five-day-old rats, and (ii) coronal brainstem slices of eight- to 12-day-old rats, and the inhibitory synaptic activities in these neurons have been studied. In brainstem spinal cord preparations, Lucifer Yellow was diffused into the recording neurons at the end of experiments. Medullary neurons were characterized as: (i) spinally projecting by the appearance of an antidromic spike following electrical stimulation of the spinal tract between T2 and T3 segments, and (ii) adrenergic by the detection of phenylethanolamine-N-methyltransferase immunoreactivity in Lucifer Yellow-filled neurons. Of the 13 spinally projecting and phenylethanolamine-N-methyltransferase-positive medullary neurons, focal stimulation elicited in the presence of glutamate receptor antagonists an inhibitory postsynaptic potential in nine neurons. Inhibitory synaptic potentials were reversibly eliminated by the GABA(A) receptor antagonist bicuculline (10-20 microM) in six of nine neurons, by the glycine receptor antagonist strychnine (0.1-1 microM) in two and by a combination of bicuculline and strychnine in one neuron. In brainstem slice preparations, focal stimulation elicited three types of synaptic potential: (i) an excitatory postsynaptic potential, (ii) an inhibitory postsynaptic potential and (iii) a biphasic synaptic potential consisting of an excitatory synaptic potential followed by an inhibitory synaptic potential. Inhibitory synaptic potentials had a reversal potential between -70 and -80 mV, reversed their polarity in a low (6.7 mM) Cl- Krebs' solution, and suppressed or blocked by either bicuculline or strychnine or both. Elimination of inhibitory synaptic potentials unmasked in some cells an excitatory synaptic potential or enhanced the excitatory synaptic potential component in medullary neurons with a biphasic response, indicating a marked convergence of excitatory and inhibitory inputs onto a single neuron. A population of medullary neurons appeared to be pacemaker neurons whereby they discharged spontaneously. When discharges were suppressed by membrane hyperpolarization, focal stimulation elicited inhibitory synaptic potentials in 8/23 neurons tested. Our results suggest that inhibitory synaptic potentials in medullary neurons are mediated by either GABA and/or glycine which open primarily Cl- channels. The prevalence of inhibitory synaptic potentials in medullary neurons indicates an essential role of inhibitory transmission in controlling the input and output ratio of these neurons.

Role of spinal GABA receptors in depressor responses to chemical stimulation of the A5 area in normal and hypertensive rats

Chemical stimulation of neurons in the pontine A5 area by microinjection of L-glutamate lowers arterial blood pressure. The mechanism of this 'A5 depressor response' is not well-established. Here, we examine the involvement of spinal cord gamma-aminobutyric acid (GABA) receptors in this depressor response in normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). Experiments were conducted in male WKY and age-matched SHR anaesthetised with sodium pentobarbitone and chloral hydrate. An intrathecal catheter was implanted with the tip located between T9 and L2. Three days later, rats were re-anaesthetised and 10 nl of 40 mM L-glutamate was injected into the A5 area before, during and after, blockade of spinal cord GABA-A receptors by intrathecal injection of bicuculline methiodide (1 mM in 10 microliters phosphate-buffered saline). Injection of L-glutamate (10, 20, 40, 80 mM in 10 nl) produced depressor responses that were similar in WKY (n = 6) and SHR (n = 6). Intrathecal injection of bicuculline elicited a pressor response that was greater in SHR (n = 7, 28.5 +/- 7.6% increase in mean arterial pressure) than WKY (n = 11, 11.6 +/- 3.6%, p < 0.05). After bicuculline, the depressor response to injection of L-glutamate into the A5 area was eliminated in both WKY (n = 7) and SHR (n = 6). Intrathecal injection of vehicle had no effect on either resting arterial blood pressure or the depressor response to A5 stimulation. Basal blood pressure and control responses to A5 stimulation were fully restored by around 90 min after bicuculline injection in each animal. In separate groups of rats, intrathecal injection of muscimol elicited depressor responses that were greater in SHR (n = 6, -32.0 +/- 6.2%) than WKY (n = 6, -17.3 +/- 1.5%, p < 0.05). Our results suggest that the A5 depressor response is due to a projection from the A5 area to the spinal cord. This projection acts directly, or through a spinal interneuron and uses GABA as a neurotransmitter. Furthermore, our results indicate a hyper-responsiveness to GABA-A receptor stimulation in SHR since intrathecal bicuculline elicited much greater increases and intrathecal muscimol elicited much greater decreases, in blood pressure in SHR than in WKY. Finally, it seems likely that the A5-spinal depressor pathway is less effective in SHR than WKY under physiological conditions since chemical stimulation of the A5 area with L-glutamate produced a comparable depressor response in both strains.

Differences in the density of barosensitive neurons in the medulla of spontaneously hypertensive and Wistar-Kyoto rats

1. The density of barosensitive neurons in the medulla was examined in spontaneously hypertensive rats (SHR) and in normotensive Wistar-Kyoto (WKY) rats. In control experiments, rats were sham-operated, while in test experiments arterial baroreceptors were stimulated by pressor responses to i.v. administration of phenylephrine and the density of c-Fos-labelled neurons was immunocytologically examined. 2. In both control and test experiments, c-Fos-labelled neurons were distributed in cardiovascular control sites: the nucleus tractus solitarii (NTS) and the caudal and rostral ventrolateral medullas (CVLM/RVLM). 3. In both WKY rats and in SHR, the total density of labelled neurons in test experiments was significantly higher than in control experiments. 4. In control experiments, no significant difference was found in the distribution and density of labelled neurons in the NTS and in the CVLM/RVLM between rats and SHR. 5. In test experiments, no significant difference was found in the distribution and density of labelled neurons in the NTS between WKY rats and SHR. 6. In test experiments in SHR, the density of labelled neurons in the CVLM just caudal to the obex level was significantly higher than that in WKY rats, whereas the density of labelled neurons in WKY rats in the RVLM just rostral to the obex level was significantly higher than that in SHR. 7. These results indicate that stimulation of the arterial baroreceptor induces strain-specific differences in the density of barosensitive neurons in the CVLM/RVLM near the obex level.

Stimulation within the rostral ventrolateral medulla can evoke monosynaptic GABAergic IPSPs in sympathetic preganglionic neurons in vitro

The inhibitory responses of identified sympathetic preganglionic neurons (SPNs) to stimulation within the rostral ventrolateral medulla (RVLM) were studied to determine their nature and pharmacology. Whole cell patch-clamp recordings were made from 36 SPNs in the upper thoracic segments of the spinal cord in a neonatal rat brain stem-spinal cord preparation. Neurons were identified as SPNs on the basis of their antidromic activation after stimulation of the ipsilateral segmental ventral root and their morphology and location in the intermediolateral cell column and intercalated nucleus. In all SPNs, electrical stimulation of the RVLM evoked fast excitatory postsynaptic potentials (EPSPs) that were mediated by non-N-methyl-D-aspartate (NMDA) and NMDA receptors. These excitatory responses were the most prominent response in control artificial cerebrospinal fluid and have been studied previously. In 22 of the SPNs, RVLM stimulation also elicited fast inhibitory postsynaptic potentials (IPSPs), which increased in amplitude as the membrane was depolarized. Five of these neurons were not studied further as they responded occasionally with IPSPs that had highly variable onset latencies indicating the involvement of a polysynaptic pathway. In the remaining SPNs (n = 17), the evoked IPSPs persisted in the presence of the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3,-dione and D,L-2-amino-5-phosphonopentanoic acid. In eight of these SPNs, it was necessary to block the EPSPs to reveal the IPSPs. In the 7 SPNs tested, the onset latencies of the IPSPs were not significantly different from the onset latencies of the fast EPSPs. The low sweep-to-sweep fluctuations in onset latency of individual IPSPs (absolute average deviation: 0.4 ms) indicated that the IPSPs were elicited by activation of a monosynaptic pathway. The amplitudes of the IPSPs decreased in amplitude as the membrane was hyperpolarized and reversed in polarity at -70.3 +/- 1.7 mV (mean +/- SD), which was close to the equilibrium potential for chloride ions. In addition, in seven SPNs, bath applications of 5 microM bicuculline, a gamma-aminobuturic acid-A (GABAA) antagonist, abolished or reduced the evoked IPSPs. Five SPNs also were studied that displayed ongoing IPSPs. The amplitudes of these IPSPs increased with membrane depolarization and were blocked by bath applications of 5 microM bicuculline, suggesting that they also were mediated by activation of GABAA receptors. These results demonstrate the existence of a bulbospinal GABAergic pathway impinging directly onto SPNs. This pathway may be tonically active in the neonatal rat brain stem-spinal cord preparation.

Distribution and projection of the medullary cardiovascular control neurons containing glutamate, glutamic acid decarboxylase, tyrosine hydroxylase and phenylethanolamine N-methyltransferase in rats

This study was aimed at showing the distribution and projection of the medullary cardiovascular control neurons that contain a standard neurotransmitter or a related enzyme in the rat. A small amount of HRP was injected into either the depressor area of the caudal ventrolateral medulla (D-CVLM) or the pressor area of the rostral ventrolateral medulla (P-RVLM). Using an immunohistochemical method, we identified HRP-labelled neurons which were stained with antiserum to glutamate (Glu), glutamic acid decarboxylase (GAD), tyrosine hydroxylase (TH) or phenylethanolamine N-methyltransferase (PNMT). Our findings are summarized as follows. (1) The Glu-containing neurons in the nucleus tractus solitararii (NTS) project to the D-CVLM (n = 279, 100% assumed as a standard value) and P-RVLM (n = 225, 81% against the standard), indicating divergent excitatory projection. (2) The GAD-containing neurons in the NTS (n = 74, 27% against the standard) project to the P-RVLM, indicating the convergent inhibitory projection. (3) The projections of the TH-containing neurons from the NTS (n = 19, 7% against the standard) and CVLM (n = 4, 1% against the standard) to the P-RVLM are weaker than those of the GAD-containing neurons, suggesting that the catecholaminergic neurons play a minor role in inhibition of the sympathetic activity of the P-RVLM neurons. These results suggest that the glutamatergic NTS neurons excite both the P-RVLM and D-CVLM neurons, and the gamma-aminobutyric acid (GABA)ergic NTS and CVLM neurons inhibit the sympathetic activity of the P-RVLM neurons.

Nicotinic actions on neurones of the central autonomic area in rat spinal cord slices

1. Nicotinic responses and actions on excitatory synaptic activity were studied in eighty-four neurones in the region dorsal to the central canal (lamina X) in transverse thoracolumbar spinal cord slices of neonate (P2-P10) rats by using the whole-cell patch-clamp technique. 2. Neurones (n = 15) labelled with Lucifer Yellow, showed the typical morphology of sympathetic preganglionic neurones (SPNs) in the central autonomic area (CA). Unlabelled neurones of comparable morphology were visually identified and recorded. 3. All neurones recorded responded to the nicotinic acetylcholine receptor (nAChR) agonist, DMPP. Under current-clamp conditions, pressure ejections of DMPP depolarized cells and induced the discharge of action potentials. Tetrodotoxin suppressed action potentials but not DMPP-induced depolarization. 4. Under voltage-clamp conditions at a holding potential (Vh) of -50 mV, DMPP induced a transient inward current (which reversed around 0 mV) and an increase in membrane current noise in 50% of the recorded neurones. In the others, DMPP increased membrane current noise without measurable inward current. The current-voltage relationship showed strong inward rectification at holding potentials more positive than 0 mV. 5. In neurones displaying a detectable current response to DMPP, the following agonist rank order potency could be established: DMPP = nicotine > cytisine > ACh. The DMPP response could be blocked by mecamylamine but was insensitive to methyllycaconite. 6. Pressure application of glutamate induced inward currents in all cells tested at a Vh of -50 mV. This response reversed at 10 mV, displayed a region of negative slope conductance at Vh more negative than -30 mV and was partially blocked by CNQX. Pressure application of DMPP transiently increased the amplitude of the glutamate-induced current in six out of nine cells tested. This potentiation persisted in the presence of tetrodotoxin. 7. Forty per cent of the recorded neurones displayed spontaneous excitatory postsynaptic currents (sEPSCs). At a Vh of -50 mV the sEPSCs had a mean amplitude of -19.3 pA and occurred at a frequency below 0.5 Hz. sEPSCs were blocked by CNQX and inverted around 0 mV. Brief application of DMPP increased the discharge frequency of sEPSCs without affecting their kinetics. Additionally, in some cells DMPP increased mean sEPSC amplitude. 8. Focal electrically evoked EPSCs reversed close to 10 mV and were sensitive to CNQX. They occurred with a constant latency, rise time and a mono-exponential decay time. Application of DMPP decreased the percentage of stimulation failures and increased the amplitude of evoked EPSCs, in all cells tested. 9. It is concluded that neurones in the CA, presumed to be SPNs, have functional nAChRs with activation having two distinct effects: firstly, a direct depolarization of the postsynaptic membrane; and secondly, a facilitation of the excitatory transmission onto these cells. This second effect is achieved by an increase of the size of the glutamate-induced current at the postsynaptic level as well as by an enhancement of the presynaptic release of glutamate.

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.

Vesicle shape and amino acids in synaptic inputs to phrenic motoneurons: do all inputs contain either glutamate or GABA?

Varicosities that made synapses or direct contacts with retrogradely labelled rat phrenic motoneurons were examined for their content of immunoreactivity for either glutamate or glutamate decarboxylase, the enzyme involved in synthesis of gamma-aminobutyric acid (GABA). Phrenic motoneurons were identified by retrograde tracing from the diaphragm with cholera toxin B subunit conjugated to horseradish peroxidase. Cell bodies and medium-sized to large dendrites were labelled. Preembedding immunocytochemistry identified glutamate decarboxylase-immunoreactive nerve fibres; glutamate-immunoreactive nerve terminals were identified using postembedding immunogold labelling of ultrathin sections. The presence of glutamate- or glutamate decarboxylase immunoreactivity in nerve terminals was correlated with the morphology of the synaptic vesicles. Two major classes of nerve terminals were identified. Nerve terminals with round (presumably spherical) synaptic vesicles (S terminals) comprised 55% of synapses and contacts on phrenic motoneuron somata and 58% of synapses and direct contacts with dendrites. Nerve terminals with flattened synaptic vesicles (F terminals) comprised 42% of synapses direct contacts with somata and 41% of synapses and direct contacts with dendrites. Analysis of immunogold-labelled sections showed that S terminals contained statistically higher levels of glutamate immunoreactivity than F terminals. At the light microscope level, many glutamate decarboxylase-immunoreactive nerve terminals surrounded retrogradely labelled motoneurons. Varicosities with glutamate decarboxylase immunoreactivity made 33% of all synapses and direct contacts on somata, and 33% of synapses and direct contacts with dendrites of the retrogradely labelled phrenic motoneurons. Flattened synaptic vesicles were present in those glutamate decarboxylase-immunoreactive nerve terminals in which synaptic vesicle morphology could be judged. An additional 10% of all nerve terminals were of the F type, but were not glutamate decarboxylase-immunoreactive. Three percent of terminals on somata and 1% of nerve terminals on dendrites could not be classified as S or F types. These findings suggest that more than 90% of all inputs to phrenic motoneuron cell bodies and proximal dendrites could contain either GABA or glutamate. Some of these glutamatergic and GABAergic nerve fibres undoubtedly represent the source of inspiratory drive to, or expiratory inhibition of, phrenic motoneurons.

Hypotensive and hypertensive effects of catecholamines intrathecally injected in anesthetized rats

The cardiovascular effects of catecholamines intrathecally (i.t.) injected at the T12-L1 level were analyzed in pentobarbital anesthetized rats. Volumes of injection were not greater than 3 microliters. Noradrenaline in doses ranging from 0.03 to 0.3 micrograms (i.t.) did not alter the mean blood pressure (MBP) while higher doses (1, 3 and 10 micrograms, i.t.) caused a dose-dependent increase in MBP. Adrenaline induced hypotensive effects at low doses (0.03-0.3 micrograms i.t.) and pressor effects at high doses (3 and 10 micrograms, i.t.). Neither adrenaline nor noradrenaline modified the heart rate. The pressor responses to both catecholamines were antagonized by the alpha 1-adrenoceptor blocker prazosin (0.05-1 microgram, i.t.) and by the selective alpha 1A-adrenoceptor antagonist 5-methyl urapidil (10 and 15 micrograms, i.t.). In contrast, these pressor effects were not modified by the alpha 1B-adrenoceptor antagonist chloroethylclonidine (90 micrograms i.t.). In animals pretreated with 1 microgram prazosin (i.t.), low doses of noradrenaline (0.03 and 0.1 microgram, i.t.) caused a hypotensive effect. Prazosin (1 microgram i.t.) failed to alter the hypotension caused by 0.1 microgram adrenaline. The hypotensive response induced by either 0.1 microgram noradrenaline (in the presence of prazosin) or 0.1 microgram adrenaline was blocked by the alpha 2-adrenoceptor antagonist yohimbine (1 mg/kg, i.v.), by the GABA-A antagonists bicuculline (3.2 micrograms, i.t.) and picrotoxin (2.7 micrograms, i.t.), and by the GABA-B antagonist 2-hydroxy saclofen (30 micrograms, i.t.). The glycine-receptor antagonist strychnine (25 micrograms, i.t.) did not modify the hypotension induced by either noradrenaline (in the presence of prazosin) or adrenaline. These findings suggest that in the low thoracolumbar spinal cord of pentobarbital-anesthetized rats, noradrenaline and adrenaline have excitatory as well as inhibitory effects on the control of the BP. The pressor responses of high doses of i.t. injected catecholamines could be mediated by the activation of spinal alpha 1A-adrenoceptors, although the participation of alpha 1B-adrenoceptors cannot be rule out entirely. The hypotensive responses induced by low doses of i.t. injected catecholamines seem to involve the activation of spinal alpha 2A-adrenoceptors and the stimulation of an inhibitory GABAergic neuron in the spinal cord.

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.

Monosynaptic projections from the medullary gigantocellular reticular formation to sympathetic preganglionic neurons in the thoracic spinal cord

Microinjection of L-glutamate into a restricted area of the medullary gigantocellular reticular formation, the gigantocellular depressor area (GiDA), lowers arterial pressure. Unlike the nuclei tractus solitarii and the caudal ventrolateral medulla, the two principle medullary vasodepressor areas, the GiDA projects directly to the spinal cord and not to the rostral ventrolateral medulla (Aicher et al. [1994] Neuroscience 60:761-779). We investigated whether neurons within GiDA directly innervate autonomic areas of the thoracic spinal cord. Fluoro-Gold injected into the thoracic spinal cord labeled neurons within functionally defined vasodepressor sites in the GiDA in the same animal. To examine the morphology of GiDA efferents to the spinal cord, the anterograde tracer Phaseolus vulgaris-leucoagglutinin was iontophoresed into the GiDA, and efferent processes in the intermediolateral cell column and nucleus intercalatus spinalis were examined by electron microscopy. Labeling was confined to axons and axon terminals (n = 144) that usually contained primarily small clear vesicles, contacted large and small dendrites, and formed symmetric (inhibitory) synapses. To determine whether some of the postsynaptic targets of GiDA efferent terminals in the thoracic spinal cord were sympathoadrenal preganglionic neurons, these neurons were retrogradely labeled from the adrenal gland with Fluoro-Gold in rats that had deposits of the anterograde tracer, biotinylated dextran amine (BDA), in the GiDA. Some BDA-containing terminals formed symmetric synapses with dendrites containing Fluoro-Gold. We conclude that a population of neurons in the GiDA monosynaptically innervates some sympathetic preganglionic neurons. The findings suggest the presence of a novel reticulospinal sympathoinhibitory projection originating in the GiDA.

Fos expression in neurons projecting to the pressor region in the rostral ventrolateral medulla after sustained hypertension in conscious rabbits

Previous studies in anaesthetized animals have shown that the baroreflex control of sympathetic vasomotor activity is mediated to a large extent by inhibitory inputs to sympathoexcitatory pressor neurons in the rostral part of the ventrolateral medulla. The aim of this study was to determine, in conscious rabbits, the distribution of neurons within the brain that have two properties characteristic of interneurons conveying baroreceptor signals to the rostral ventrolateral medulla: (i) they are activated by an increase in arterial pressure; and (ii) they project specifically to the rostral ventrolateral medulla pressor region. In a preliminary operation, an injection of the retrogradely transported tracer, fluorescent-labelled microspheres, was made into the physiologically identified pressor region in the rostral ventrolateral medulla. After a waiting period of one to eight weeks, hypertension was produced in the conscious rabbit by continuous intravenous infusion of phenylephrine at a rate sufficient to increase arterial pressure by approximately 20 mmHg, maintained for a period of 60 min. A control group of animals was infused with the vehicle solution alone. In confirmation of our previous study, hypertension produced by phenylephrine resulted in the neuronal expression of Fos (a marker of neuronal activation) in the nucleus of the solitary tract, area postrema, the intermediate and caudal parts of the ventrolateral medulla parabrachial complex, and in the central nucleus of the amygdala. Approximately 50% of the Fos-immunoreactive neurons in both the caudal and intermediate parts of the ventrolateral medulla were also retrogradely labelled from the rostral ventrolateral medulla pressor region; such double-labelled neurons were confined to a discrete longitudinal column located just ventrolateral to the nucleus ambiguus. Significant numbers of double-labelled neurons were also found in the nucleus of the solitary tract and area postrema, although these represented a much lower proportion (13-16%) of the total number of Fos-immunoreactive neurons in these regions. In the parabrachial complex, Fos-immunoreactive and retrogradely labelled neurons were largely separate populations, while in the amygdala they were entirely separate populations. In the control group of rabbits, virtually no double-labelled neurons were found in any of these regions. The results indicate that putative baroreceptor interneurons that project to the pressor region of the rostral ventrolateral medulla are virtually confined to the lower brainstem. In particular, they support the results of previous studies in anaesthetized animals indicating that neurons in the intermediate and caudal ventrolateral medulla convey baroreceptor signals to the rostral ventrolateral medulla pressor region, and extend them by demonstrating the precise anatomical distribution of these neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Neuronal expression of Fos protein in the rat brain after baroreceptor stimulation

The purpose of this study was to identify the CNS neurons that express Fos protein after repeated activation of the baroreceptor reflex. This was done in Wistar rats anesthetized with urethane and alpha-chloralose with careful physiological controls. The intact control rat showed few Fos-immunoreactive (ir) neurons, whereas the anesthetized control rat showed many Fos-ir neurons in the CNS from the medulla oblongata to the forebrain. After repeated stimulation of baroreceptors by pressor responses to phenylephrine (dose), we counted the amounts of Fos-ir neurons (response). The correlation coefficient of the dose-response relationship was high, and significant only in the medial part of the nucleus tractus solitarii (NTS) in the medulla and periaqueductal gray (PAG) in the midbrain, whereas it was comparatively high but insignificant in the commissure and lateral parts of the NTS, caudal and rostral ventrolateral medulla, periambiguus nucleus, dorsal and ventral medullary reticular nuclei, lateral parabrachial nucleus, paraventricular nucleus thalamus, and dorsomedial nucleus hypothalamus. No significant correlation was found in the humoral control nuclei in the preoptico-hypothalamic structure. Fos expression was never detected in the sensory neurons in the ganglia petrosum and nodosum, and in the sympathetic preganglionic neurons in the intermediolateral nucleus of the thoracic spinal cord. This study shows that Fos expression in the CNS neurons is induced not only by baroreceptor stimulation but also by anesthesia and/or sham-operation, and that Fos expression in the NTSm and PAG neurons faithfully responds to baroreceptor stimulation.

Influence of humoral control peptides on medullary vasomotor control neurons: microstimulation and double-labeling studies using SHR and WKY rats

To study the influence of humoral control peptides on medullary vasomotor control neurons, angiotensin II (AII), arginine vasopressin (AVP) and atrial natriuretic peptide (ANP) were microinjected into three vasomotor control areas, i.e., the area postrema (AP), the nucleus tractus solitarii (NTS) and the rostral ventrolateral medulla (RVLM), of spontaneously hypertensive rats (SHR) and normotensive Wistar Kyoto rats (WKY), and the evoked cardiovascular response was observed. Unlike the injection areas, the threshold dose of one peptide for the cardiovascular response was similar, but the threshold dose differed from peptide to peptide. The threshold dose was lower for AII (0.15-0.29 pmol), in-between for ANP (0.9-1.5 pmol) and higher for AVP (14-30 pmol). No significant difference in the threshold dose was observed between SHR and WKY, suggesting that hypertension in SHR may not be due to the abnormal sensitivity to the three peptides of the vasomotor control neurons in the AP, NTS, and RVLM. The structural basis of the results of the microstimulation experiment was supported by the double-labeling study. The NTS neurons were innervated by (1) the AII-immunoreactive (ir) neurons in both sides of the lateral hypothalamic area (LH), the RVLM and the caudal ventrolateral medulla, and (2) the ANP-ir neurons in both sides of the paraventricular nucleus (Pa) and the LH. The RVLM neurons were innervated by (1) the AII-ir neurons in both sides of the LH and ipsilateral side of the lateral parabrachial nucleus (Pbl) and (2) the ANP-ir neurons in the ipsilateral Pbl. There was no evidence that the AVP-ir neurons in the Pa and the supraoptic nucleus innervate the NTS and the RVLM neurons, or that the AII, ANP or AVP-ir neurons innervate the AP neurons. This study suggests that in common with SHR and WKY rats AII and ANP may influence both the NTS and RVLM not by the humoral pathway but by the neural pathway, and AVP may not influence the three vasomotor control areas by the neural pathway.

Distribution of glutamate- and GABA-immunoreactive neurons projecting to the vasomotor center of the intermediolateral nucleus of the lower thoracic cord of Wistar rats: a double-labeling study

The purpose of this study was to survey distribution of glutamatergic and GABAergic neurons in the brainstem with projections to the vasomotor center in the intermediolateral nucleus of the T11 segment of Wistar rats. This was done with retrograde transport of WGA-HRP and immunoreaction to glutamate and GABA. HRP/glutamate-labeled neurons were distributed in the rostral ventrolateral medulla (RVLM, 29%), A5 noradrenaline cell area (16%), paraventricular nucleus of the hypothalamus (14%) and caudal ventrolateral medulla (CVLM, 11%), while HRP/GABA-labeled neurons in the RVLM (65%) and CVLM (24%). This indicates that the glutamatergic and GABAergic neurons projecting to the spinal vasomotor center originate mainly from the RVLM and CVLM.