Neurons in the caudal ventrolateral medulla mediate the somato-sympathetic inhibitory reflex response via GABA receptors in the rostral ventrolateral medulla.

In urethane-anesthetized rabbits, stimulation of the sural nerve, consisting of cutaneous afferents (A-fibers), evoked reflex responses consisting of an early small excitatory component followed by a prolonged inhibitory component in renal sympathetic nerve activity. Bilateral injections of GABA antagonist, bicuculline (4 nmol/site), into the rostral ventrolateral medulla (RVLM), where sympatho-excitatory reticulospinal neurons are located, attenuated the inhibitory component in a dose-dependent manner as well as the inhibition evoked by stimulation of the aortic nerve A-fibers (baroreceptor afferents). Bilateral injections of a neurotoxic agent, kainic acid (4 nmol/site, 3 sites/side), into the caudal ventrolateral medulla (CVLM), where sympatho-inhibitory neurons with axonal projection to the RVLM are located, diminished these sympatho-inhibitory responses. Therefore it is concluded that the sympatho-inhibition evoked by activation of somatic afferents was mediated by neurons in the CVLM and by GABA receptors in the RVLM, as was the sympatho-inhibition associated with the arterial baroreceptor reflex. Bilateral injections of kynurenic acid (4 nmol/site, 3 sites/side) into the CVLM did not affect the somato-sympathetic reflex response, but diminished the sympatho-inhibition produced by activation of the baroreceptor afferents. Sympatho-inhibitory neurons in the CVLM were activated by glutamate when baroreceptor afferents were activated, but another excitatory transmitter may participate in the somato-sympathetic reflex in the CVLM.     

GABAergic modulation of neurons in the nucleus of the solitary tract with ascending projections to the subfornical organ in the rat

Twenty-five neurons in the region of the nucleus of the solitary tract (NTS) were antidromically activated by electrical stimulation of the subfornical organ (SFO) in male rats under urethane anesthesia. Microiontophoretically applied bicuculline, a gamma-aminobutyric acid (GABA)(A) antagonist, but not phaclofen, a GABA(B) antagonist, attenuated the post-antidromic inhibitory response evoked by SFO stimulation of approximately two-third (n=17) of identified neurons, indicating the existence of recurrent inhibitory systems through GABA(A) receptors. Iontophoretically applied GABA decreased the spontaneous activity of all identified neurons, and the GABA-induced inhibition was prevented by simultaneously applied bicuculline, but not by phaclofen. Activation of peripheral baroreceptors, achieved by rising arterial blood pressure with an intravenous infusions of phenylepherine, suppressed the activity of the majority (n=20) of identified neurons. The inhibitory response of identified neurons (n=7) to baroreceptor activation was partially antagonized by iontophoretically applied bicuculline, but not by phaclofen. These results imply that GABAergic mechanisms may modulate the baroreceptor reflex acting on GABA(A) receptors of NTS neurons with ascending projections to the SFO in the region of the NTS.     

Effects of activation and blockade of P2x receptors in the ventrolateral medulla on arterial pressure and sympathetic activity.

Sympathoexcitatory and sympathoinhibitory neurons in the rostral and caudal ventrolateral medulla (VLM) play a crucial role in the tonic and reflex control of sympathetic vasomotor activity. Recent evidence also indicates that the VLM contains a high density of P2x purinoceptors. In this study, we investigated the cardiovascular effects of selective activation of P2x purinoceptors in the rostral and caudal VLM, and the effects of blockade of P2x purinoceptors in the rostral VLM on the tonic and reflex control of sympathetic vasomotor activity. In anesthetized barodenervated rabbits, microinjection into the rostral and caudal VLM of the P2x purinoceptor agonist, alpha,beta-methylene adenosine triphosphate (alpha,beta-meATP) (4-400 pmol) elicited dose-dependent increases and decreases, respectively, in arterial pressure (AP), heart rate (HR) and renal sympathetic nerve activity (RSNA). The response evoked by alpha,beta-meATP in the rostral VLM was blocked by prior injection into the same site of the P2 purinoceptor antagonist suramin but not by the ionotropic glutamate receptor antagonist kynurenic acid. Bilateral injections of suramin into the rostral VLM sympathoexcitatory region had no significant effect on resting cardiovascular variables, nor on the reflex increase in RSNA evoked by sciatic nerve stimulation (which is known to be mediated by the rostral VLM sympathoexcitatory neurons). The results demonstrate that: (1) activation of P2x purinoceptors in the VLM are capable of producing marked excitation of both sympathoexcitatory and sympathoinhibitory neurons; (2) these effects are not due to modulation of glutamatergic inputs to these neurons; and (3) P2x purinoceptors do not play a significant role in maintaining the tonic activity of rostral VLM sympathoexcitatory neurons or in modulating their responses to excitatory synaptic inputs evoked by stimulation of sciatic nerve afferents.     

Study of possible transmitters in the solitary tract nucleus of the cat involved in the carotid sinus baro- and chemoreceptor reflex

By using a multibarrelled microelectrode, the possibility that putative transmitters may influence on the field potential evoked in the solitary tract nucleus by electrical stimulation of the carotid sinus nerve (the NTS response) was examined electrophysiologically in the cat. After iontophoretic application of a selective antagonist to the putative transmitter, it was determined whether or not the NTS response was influenced. Both substance P antagonist and naloxone altered the NTS response recorded in the depressor and apneic (or hypopneic) response zone as well as in the pressor and apneustic (or inspiratory) response zone throughout the rostral, intermediate and commissure regions, suggesting that substance P and opioid peptide may play the role of excitatory transmitters. Under the polarizing cathodal current which may activate inhibitory inputs to the site of the NTS response, bicuculline and prazosin strongly enhanced the NTS response recorded in the pressor and apneustic zone, while naloxone weakly enhanced the NTS response recorded in the depressor and apneic zone. These results suggest that gamma-aminobutyric acid, alpha-adrenergic agonist and opioid peptide may have an inhibitory influence on the NTS response.     

Extended secondhand tobacco smoke exposure induces plasticity in nucleus tractus solitarius second-order lung afferent neurons in young guinea pigs

Infants and young children experiencing extended exposure to secondhand smoke (SHS) have an increased occurrence of asthma, as well as increased cough, wheeze, mucus production and airway hyper-reactivity. Plasticity in lung reflex pathways has been implicated in causing these symptoms, as have changes in substance P-related mechanisms. Using whole-cell voltage-clamp recordings and immunohistochemistry in brainstem slices containing anatomically identified second-order lung afferent nucleus tractus solitarius (NTS) neurons, we determined whether extended SHS exposure during the equivalent period of human childhood modified evoked or spontaneous excitatory synaptic transmission, and whether those modifications were altered by endogenous substance P. SHS exposure enhanced evoked synaptic transmission between sensory afferents and the NTS second-order neurons by eliminating synaptic depression of evoked excitatory postsynaptic currents (eEPSCs), an effect reversed by the neurokinin-1-receptor antagonist (SR140333). The recruitment of substance P in enhancing evoked synaptic transmission was further supported by an increased number of substance P-expressing lung afferent central terminals synapsing onto the second-order lung afferent neurons. SHS exposure did not change background spontaneous EPSCs. The data suggest that substance P in the NTS augments evoked synaptic transmission of lung sensory input following extended exposure to a pollutant. The mechanism may help to explain some of the exaggerated respiratory responses of children exposed to SHS.     

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.     

Importance of neurokinin‐1 receptors in the nucleus tractus solitarii of mice for the integration of cardiac vagal inputs

Unmyelinated vagal afferents from the heart terminate within the nucleus tractus solitarii (NTS) located in the dorsomedial medulla. The neurotransmitter and postsynaptic receptors mediating information from cardiac vagal receptors to the NTS are unknown. This study determined the effects of neurokinin-1 (NK₁) receptor blockade on: (i) the reflex response evoked following aortic root injection of either veratridine (1–3 μg/kg) or bradykinin (80–300 ng/kg) to stimulate cardiac receptors in in vivo anaesthetized mice; and (ii) the evoked synaptic response of cardioreceptive NTS neurons following both intraleft-ventricular injection of veratridine or bradykinin, and electrical stimulation of the ipsilateral vagus nerve in an arterially perfused working heart-brainstem preparation of mouse. Administration of CP-99,994 (0.75–1.5 mg/kg i.v.), a specific NK₁ antagonist, attenuated significantly the evoked reflex bradycardia and depressor response following cardiac receptor (n = 6), but not pulmonary chemoreflex stimulation in vivo. From extracellular recordings of cardioreceptive NTS neurons, CP-99,994 reduced reversibly the total number of evoked spikes, peak firing frequency and response duration evoked by intraventricular injections of veratridine (n = 5) or bradykinin (n = 5). The number of evoked action potentials following electrical stimulation of the vagus nerve was also reduced. In five whole cell recordings of NTS neurons, both the evoked depolarization following cardiac receptor stimulation, and the peak amplitude and duration of vagus nerve-evoked EPSPs were reduced by CP-99 994; synaptic inputs from both peripheral chemoreceptors or pulmonary C-fibres were unaffected. These data support a selective involvement of NK₁ receptors in the transmission of cardiac vagal afferent inputs to NTS neurons integrating cardiorespiratory information.     

Anatomical substrates for baroreflex sympathoinhibition in the rat

The fundamental neuronal substrates of the arterial baroreceptor reflex have been elucidated by combining anatomical, neurophysiological, and pharmacological approaches. A serial pathway between neurons located in the nuclei of the solitary tract (NTS), the caudal ventrolateral medulla (CVL), and the rostral ventrolateral medulla (RVL) plays a critical role in inhibition of sympathetic outflow following stimulation of baroreceptor afferents. In this paper, we summarize our studies using tract-tracing and electron microscopic immunocytochemistry to define the potential functional sites for synaptic transmission within this circuitry. The results are discussed as they relate to the literature showing: (1) baroreceptor afferents excite second-order neurons in NTS through the release of glutamate; (2) these NTS neurons in turn send excitatory projections to neurons in the CVL; (3) GABAergic CVL neurons directly inhibit RVL sympathoexcitatory neurons; and (4) activation of this NTS-->CVL-->RVL pathway leads to disfacilitation of sympathetic preganglionic neurons by promoting withdrawal of their tonic excitatory drive, which largely arises from neurons in the RVL. Baroreceptor control may also be regulated over direct reticulospinal pathways exemplified by a newly recognized sympathoinhibitory region of the medulla, the gigantocellular depressor area. This important autonomic reflex may also be influenced by parallel, multiple, and redundant networks.     

Membrane and synaptic properties of nucleus tractus solitarius neurons projecting to the caudal ventrolateral medulla

Projections from the nucleus of tractus solitarius (NTS) to the caudal ventrolateral medulla (CVLM) are important in mediating autonomic reflexes. However, little is known about the cellular properties of the CVLM-projecting NTS neurons. In this study, the CVLM-projecting NTS neurons were retrogradely labeled by fluorescent microspheres injected into the CVLM. Whole cell voltage- and current-clamp recordings were performed on labeled NTS neurons in coronal brainstem slices. Compared with unlabeled neurons, the labeled NTS neurons had more depolarized resting membrane potentials, larger input resistance, and higher firing activity in response to depolarizing currents. Bath application of an ionotropic glutamate receptor antagonist kynurenic acid and a non-NMDA receptor antagonist CNQX significantly decreased the firing activity in the majority of labeled NTS neurons. In contrast, an NMDA receptor antagonist AP5 failed to alter the firing activity in labeled neurons tested. While the glycine receptor antagonist strychnine had no effect on the firing activity, blockade of GABA(A)receptors with bicuculline significantly increased the firing rate in the majority of labeled NTS neurons. Furthermore, CNQX blocked the majority of spontaneous excitatory postsynaptic currents (EPSCs) and evoked EPSCs elicited by stimulation of the tractus solitarius. The residual spontaneous and evoked EPSCs were abolished by the nicotinic receptor antagonist mecamylamine and the purinergic P2X receptor antagonist iso-PPADS. Finally, while bicuculline completely blocked the miniature inhibitory postsynaptic currents (IPSCs), the spontaneous and evoked IPSCs were abolished by a combination of bicuculline and strychnine in labeled NTS neurons. Collectively, these data suggest that the CVLM-projecting neurons are a population of neurons with distinctive membrane properties.     

GABAergic neurotransmission at the nucleus tractus solitarii in the suppression of reflex bradycardia by parabrachial nucleus.

We investigated the role of GABAergic neurotransmission at the nucleus tractus solitarii (NTS) in the suppression of cardiac baroreceptor reflex (BRR) response induced by parabrachial nucleus (PBN) complex in adult Sprague-Dawley rats maintained under pentobarbital anesthesia. Based on in vivo microdialysis coupled with high-performance liquid chromatography-fluorescence detection for gamma-aminobutyric acid (GABA), we found that electrical stimulation of the ventrolateral regions and Koelliker-Fuse (KF) subnucleus of PBN complex resulted in a site-specific increase in GABA concentration in the dialysate collected from the NTS. The temporal increase in extracellular GABA concentration in the NTS coincided with the time course of PBN-induced cardiac BRR inhibition. In addition, the PBN-induced cardiac BRR suppression was reversed by microinjection bilaterally into the NTS of a GABA(A) receptor antagonist, bicuculline methiodide (5 pmol), or a GABA(B) receptor antagonist, 2-OH saclofen (500 pmol). Blockade of neuronal activity in the ventrolateral regions and KF subnucleus of PBN complex with lidocaine (5%) elicited an enhancement of the same reflex response. The time course of this facilitatory effect of lidocaine correlated positively with the temporal decrease in extracellular GABA concentration in the NTS. Anatomically, Fast Blue-labeled neurons were identified in the same subnuclei of the PBN complex after microinjection of the retrograde transport tracer into the NTS. Some of these Fast Blue-labeled neurons were also immunoreactive to glutamic acid decarboxylase. These results suggest that a direct GABAergic descending projection from the KF subnucleus and surrounding areas of the PBN complex to the NTS may inhibit cardiac BRR response by activating GABA(A) and GABA(B) receptors at the NTS.     

Ablation of NK1 receptor bearing neurons in the nucleus of the solitary tract blunts cardiovascular reflexes in awake rats

The nucleus of the solitary tract (NTS) receives primary afferents involved in cardiovascular regulation. We investigated the role of NK(1)-receptor bearing neurons in the NTS on cardiovascular reflexes in awake rats fitted with chronic venous and arterial cannulae. These neurons were lesioned selectively with saporin conjugated with substance P (SP-SAP, 2 microM, bilateral injections of 20 nL in the subpostremal NTS, or 200 nL in both the subpostremal and the commissural NTS). Before, and 7 and 14 days after injection of SP-SAP, we measured changes in blood pressure and heart rate induced by i.v. injection of phenylephrine and nitroprusside (baroreceptor reflex), cyanide (arterial chemoreceptor reflex), and phenylbiguanide (Bezold-Jarisch reflex). The smaller injections with SP-SAP completely abolished NK1 receptor staining in the subpostremal NTS. The larger injections abolished NK1 receptor immunoreactivity in an area that extended from the commissural NTS to the rostral end of the subpostremal NTS. The lesions seemed to affect only a limited number of neurons, since neutral red stained sections did not show any obvious reduction in cell number. The smaller lesions reduced the gain of baroreflex bradycardia and the hypotension induced by phenylbiguanide. The larger lesions completely abolished the response to phenylbiguanide, blocked the baroreflex bradycardia induced by phenylephrine, severely blunted the baroreflex tachycardia, and blocked the bradycardia and reduced the hypertension induced by cyanide. Thus, these responses depend critically on NK(1)-receptor bearing neurons in the NTS.     

Modulation of the carotid baroreceptor reflex by substance P in the nucleus tractus solitarius

Previous studies have shown that administration of substance P (SP) into the nucleus tractus solitarius (NTS) can evoke a depressor response similar to that produced by activation of the arterial baroreceptors. In addition, some studies have suggested that SP increases the reflex responses to activation of baroreceptor input. The present study was performed to determine the effects of SP on the carotid sinus baroreceptor reflex at the level of the NTS by examining the effects of both exogenous SP microinjected into different rostrocaudal locations in the NTS and blockade of the effects of endogenous SP, through the microinjection of a substance P antagonist (SPa; [D-Pro, D-Trp]-substance P). Changes in pressure in an isolated carotid sinus in anesthetized dogs were used to evoke baroreflex changes in arterial blood pressure (BP) before and after microinjection of SP (0.5 microM) or SPa (10 microM) into barosensitive regions of the NTS. Microinjection of SP or its antagonist did not alter baseline, resting BP but did produce significant changes in baroreflex sensitivity. Microinjection of SP into different rostrocaudal regions of the NTS produced different responses, with rostral and caudal NTS microinjections producing significant increases in sensitivity. No effects on baroreflex sensitivity were obtained in response to SP microinjections into the intermediate NTS. Unlike SP, microinjection of the SPa significantly decreased baroreflex sensitivity at all rostrocaudal levels of the NTS. These data demonstrated that SP has the capability to modulate the carotid baroreflex at the level of the NTS and support a physiological role for endogenously released SP.     

Glycine, like glutamate, microinjected into the nucleus tractus solitarii of rat decreases arterial pressure and heart rate

Glycinergic mechanisms have been implicated in central cardiovascular regulation. However, the inhibitory amino acid's role in the nucleus tractus solitarii (NTS), the site of termination of cardiovascular afferents, has not been clarified. Thus, we sought to determine if the microinjection of glycine into the NTS alters arterial pressure and heart rate. Microinjections of glycine, like glutamate, confined to the NTS decreased arterial pressure and heart rate in a neurally mediated, dose-dependent manner. The glycine antagonist strychnine completely blocked these effects of glycine but did not itself alter arterial pressure or heart rate, or interfere with the baroreceptor reflex. The acute hypotensive, bradycardic response to glycine was followed by a period during which glycine essentially eliminated the cardiovascular responses to the microinjection into the NTS of glutamate, an amino acid reputed to be a transmitter in the baroreceptor reflex arc. These data suggest that glycine is involved in cardiovascular regulation by the NTS but do not support its being an integral transmitter in the baroreceptor reflex.     

Inhibition of baroreflex following microinjection of GABA or morphine into the nucleus tractus solitarii in rabbits

The possibility that putative transmitters may influence the aortic nerve stimulation-produced bradycardia and depressor responses was examined in urethane- and chloralose-anesthetized, paralyzed and artificially ventilated rabbits. The ipsilateral microinjection of gamma-aminobutyric acid (GABA, 2-4 micrograms) or morphine hydrochloride (2-4 micrograms) into the nucleus tractus solitarii (NTS) area could partially block the evoked bradycardia and depressor responses produced by stimulation of the aortic nerve without influencing the basal blood pressure and heart rate. This blocking effect of either GABA or morphine was dose-related. Pretreatment with GABA receptor antagonist bicuculline methiodide (0.15-0.20 micrograms) and opiate receptor antagonist naloxone hydrochloride (1-2 micrograms) into the same medullary area completely abolished the effect of GABA and morphine, respectively. Application of bicuculline also greatly antagonized the effect of morphine, but the blocking effect of GABA on the evoked bradycardia and depressor responses still existed following the pretreatment of naloxone. These results indicate that GABAergic and opiate systems present at the NTS exert an inhibitory influence on the evoked baroreflexes and inhibitory effect of the latter may be related to the activation of GABAergic receptor in this nucleus.     

Inhibition of the bradycardic component of the von Bezold–Jarisch reflex and carotid chemoreceptor reflex by periaqueductal gray stimulation: involvement of medullary receptors

Stimulation of the dorsolateral periaqueductal gray matter (dlPAG) and the B3 cell group inhibits the cardiovagal component of the baroreflex in rats. Our aim was to determine whether the defence reaction induces similar modulatory effects on the cardiac response of the von Bezold–Jarisch reflex and the carotid chemoreceptor reflex. We examined the effects of dlPAG stimulation on the reflex bradycardia triggered by systemic administration of phenylbiguanide or potassium cyanide. Electrical and chemical stimulation of the dlPAG produced marked inhibition of the cardiovagal components of the von Bezold–Jarisch and the carotid chemoreceptor reflexes. In addition, as 5-HT₃, NK₁ and GABA_A receptor activation blocks cardiac reflex responses, we studied whether these receptors were involved in the dlPAG-induced inhibitory effects. We found that, after microinjection of granisetron (a 5-HT₃ receptor antagonist), bicuculline (a GABA_A receptor antagonist) and GR-205171 (an NK₁ receptor antagonist) into the nucleus of the solitary tract (NTS), reflex bradycardic responses were preserved during dlPAG stimulation. Finally, activation of the B3 region also inhibited both reflex bradycardic responses, and these effects were prevented by prior blockade of 5-HT₃ receptors in the NTS. The inhibitory effect of dlPAG stimulation on the cardiac reflex responses was prevented by inhibition of neurons in the medullary B3 region. In conclusion, 5-HT₃, GABA_A and NK₁ receptors in the NTS appear to be involved in the inhibition of the von Bezold–Jarisch reflex and the carotid chemoreceptor reflex bradycardia evoked by activation of neurons in the dlPAG and the raphé magnus.     

Rostral ventrolateral medulla suppresses reflex bradycardia by the release of gamma-aminobutyric acid in nucleus tractus solitarii of the rat

We investigated the role of gamma-aminobutyric acid (GABA) in the nucleus tractus solitarii (NTS), the principal recipient of baroreceptor afferent fibers in the medulla oblongata, in the suppression of cardiac baroreceptor reflex (BRR) response by the rostral ventrolateral medulla (RVLM). Direct microinfusion via reverse microdialysis of L-glutamate (50 microM) into the RVLM promoted an inhibition of the BRR response, alongside an increase in the concentration of GABA in the dialysate collected from the ipsilateral NTS. Such an increase in GABA concentration in the NTS to RVLM activation was site-specific, as microinfusion of L-glutamate into areas outside the confines of RVLM resulted in no discernible change in GABA concentration in the dialysate of the NTS and minimal effect on the cardiac BRR response. The RVLM-induced BRR suppression of cardiac BRR response to microinjection into the bilateral RVLM of L-glutamate (1 nmol) was antagonized by administration into the bilateral NTS of the GABA(A) receptor antagonist, bicuculline methiodide (1 or 5 pmol), or the GABA(B) receptor antagonist, 2-hydroxy-saclofen (100 or 500 pmol). These results suggest that GABA released in the NTS may participate in cardiac BRR suppression induced by glutamatergic activation of the RVLM, via an action on both GABA(A) and GABA(B) receptor subtypes.     

Modulation of cardiovascular and electrocortical activity through serotonergic mechanisms in the nucleus tractus solitarius of the rat

The nucleus tractus solitarius (NTS) is an integral part of the baroreceptor reflex arc. Thus, stimulation of the NTS elicits changes in arterial pressure and heart rate as well as in numerous other physiologic parameters including electrocortical activity. Serotonin (5-HT), which has been implicated in cardiovascular and electrocortical control, is present in nerve terminals within the NTS. Therefore, this study was designed to determine whether 5-HT may effect that control within the NTS. Serotonin injected into the NTS of anesthetized rats produced marked changes in the EEG, arterial pressure, and heart rate. EEG activity changed from irregular 1-5 Hz, 350-500 microV waves with an overlying 13-15 Hz, low voltage rhythm to a regular, 5 Hz, 250-300 microV rhythm. The dose-dependent cardiovascular changes were maximal at a dose of 400 pmol which produced a fall of mean arterial pressure of 48 +/- 2 mm Hg from a baseline of 96 +/- 4 mm Hg and of heart rate of 90 +/- 9 bpm from a baseline of 400 +/- 18 bpm (n = 6; P less than 0.001). Both the cardiovascular and EEG effects of 5-HT injected into the NTS were blocked by the prior injection of the 5-HT antagonist metergoline at the same site. However, the bilateral microinjection of metergoline into the NTS did not affect the baroreceptor reflex. Thus, although serotonergic mechanisms in the NTS may be involved in the modulation of electrocortical and cardiovascular activity, they are not integral to the baroreceptor reflex arc.     

Medullary substrates mediating antinociception produced by electrical stimulation of the vagus

Electrical stimulation of afferents of the right cervical vagus inhibited the tail-flick reflex elicited by noxious heat in barbiturate-anesthetized rats. This inhibitory effect was eliminated in rats receiving local anesthetic blockade of either the nucleus tractus solitarii (NTS), the lateral reticular nuclei, the nucleus raphe magnus-medullary reticular formation, or nucleus raphe obscurus regions of the medulla. Similarly, the vasodepressor and bradycardic effects of vagal stimulation were either attenuated or eliminated by local anesthetic blockade of these regions. Microinjection of the non-specific glutamate antagonist gamma-D-glutamylglycine (DGG) into the NTS region also eliminated vagally evoked inhibition of the tail-flick reflex, hypotension, and bradycardia. Conversely, microinjection of glutamate into the NTS region resulted in inhibition of the tail-flick reflex, hypotension, and bradycardia. These findings with DGG and glutamate are consistent with the view that glutamate serves as a neurotransmitter of the primary vagal afferents mediating these antinociceptive and cardiovascular responses. These results are discussed in terms of vagal afferent influences on somatosensory, somatomotor, and cardiovascular function.     

Origin of tonic GABAergic inputs to vasopressor neurons in the subretrofacial nucleus of the rabbit

The aims of this study were to determine (1) whether the vasomotor effects reflexly elicited by baroreceptor stimulation are dependent upon gamma-aminobutyric acid (GABA) receptors in the subretrofacial (SRF) nucleus in the rostral ventrolateral medulla; (2) the extent to which inputs other than those arising from peripheral baroreceptors, or transmitted via the nucleus tractus solitarius (NTS), contribute to the tonic GABAergic inhibition of SRF vasopressor cells. Following bilateral injection of a mixture of the GABA antagonist bicuculline methiodide (500 pmol) and GABA agonist muscimol (500 pmol) into the SRF nucleus, the sympathoinhibitory response normally evoked by a rise in arterial pressure (induced by inflating an aortic cuff) was abolished in 4 out of 8 rabbits and reduced in the remainder. For the whole group, the mean reduction in this response was 71%. In other experiments, the pressor response produced by injection of bicuculline methiodide into the SRF nucleus was still present after (1) destruction of the intermediate portion of the NTS, and (2) complete removal of the brain rostral to the pons. We conclude that (1) an inhibitory GABAergic input into the SRF nucleus is an important component of the central pathways mediating baroreceptor inhibition of sympathetic vasomotor tone; (2) the SRF nucleus also receives tonic GABAergic inputs that are intrinsic to the lower brainstem and are independent of baroreceptor or other cardiovascular inputs relayed by the NTS.     

Excitatory and inhibitory local circuit input to the rat dorsal motor nucleus of the vagus originating from the nucleus tractus solitarius

The nucleus tractus solitarius (NTS) and dorsal motor nucleus of the vagus nerve (DMV) constitute sensory and motor nuclei of the dorsal vagal complex, respectively. We used whole-cell patch-clamp recordings from DMV neurons in rat brain slices and three methods of stimulation (electrical, glutamate microdrop, glutamate photostimulation) to test the hypothesis that convergent excitatory and inhibitory inputs to DMV neurons originate from intact neurons in multiple NTS areas. Electrical stimulation of the NTS resulted in evoked excitatory and inhibitory postsynaptic currents (eEPSCs and eIPSCs) in DMV neurons. Stimulation of the dorsal NTS with glutamate microdrops, which selectively stimulates the soma and dendrites of intact neurons, resulted in 31% of DMV neurons receiving eEPSCs, 44% receiving eIPSCs, and 6% receiving convergent excitatory and inhibitory inputs. Glutamate photostimulation allowed selective activation of intact neurons in multiple, discrete areas of the NTS and resulted in 36% of DMV neurons receiving eEPSCs, 65% receiving eIPSCs and 20% receiving both inputs. Data obtained by stimulation of multiple NTS areas support the hypothesis that there are anatomically convergent inputs to DMV neurons originating from intact neurons within the NTS. These data support the hypothesis that there is transfer of convergent information from the NTS to the DMV, implying that significant sensory-motor processing occurs within the brainstem.