A GABAergic mechanism in the inhibition of cardiac vagal reflexes

GABA in nucleus tractus solitarius participates in electroacupuncture modulation of cardiopulmonary bradycardia reflex

Phenylbiguanide (PBG) stimulates cardiopulmonary receptors and cardiovascular reflex responses, including decreases in blood pressure and heart rate mediated by the brain stem parasympathetic cardiac neurons in the nucleus ambiguus and nucleus tractus solitarius (NTS). Electroacupuncture (EA) at P5-6 stimulates sensory fibers in the median nerve and modulates these reflex responses. Stimulation of median nerves reverses bradycardia through action of γ-aminobutyric acid (GABA) in the nucleus ambiguus, important in the regulation of heart rate. We do not know whether the NTS or the neurotransmitter mechanisms in this nucleus participate in these modulatory actions by acupuncture. We hypothesized that somatic nerve stimulation during EA (P5-6) modulates cardiopulmonary inhibitory responses through a GABAergic mechanism in the NTS. Anesthetized and ventilated cats were examined during either PBG or direct vagal afferent stimulation while 30 min of EA was applied at P5-6. Reflex heart rate and blood pressure responses and NTS-evoked discharge were recorded. EA reduced the PBG-induced depressor and bradycardia reflexes by 67% and 60%, respectively. Blockade of GABAA receptors in the NTS reversed EA modulation of bradycardia but not the depressor response. During EA, gabazine reversed the vagally evoked discharge activity of cardiovascular NTS neurons. EA modulated the vagal-evoked cardiovascular NTS cellular activity for 60 min. Immunohistochemistry using triple labeling showed GABA immunoreactive fibers juxtaposed to glutamatergic nucleus ambiguus-projecting NTS neurons in rats. These glutamatergic neurons expressed GABAA receptors. These findings suggest that EA inhibits PBG-evoked bradycardia and vagally evoked NTS activity through a GABAergic mechanism, likely involving glutamatergic nucleus ambiguus-projecting NTS neurons.

Mechanism of the inhibitory effect of electroacupuncture on experimental arrhythmias

Clinical observations reported that acupuncture can alleviate several kinds of arrhythmias. To explore its efficacy and mechanism, we have studied the electroacupuncture (EA) inhibition on experimental arrhythmias in rabbits since 1980s and analyzed its mechanism. These studies were mostly conducted in the Department of Physiology, Shanghai Medical University; recently the mechanism of acupuncture's effect on arrhythmias was analyzed in the School of Medicine, University of California, Irvine, which involves the following: (1)the inhibitory effect of EA on ventricular extrasystoles can be induced by hypothalamic defense area stimulation: a low-current and low-frequency stimulation of the median nerve underneath acupoints P 5 or deep peroneal nerve underneath S 36 can activate arcuate nucleus-ventral periaqueductal gray -nuclei raphe pathway and release endorphin, enkephaline, gamma-aminobutyric acid (GABA), 5-hydroxytryptamine (5-HT), etc., thus inhibiting the rostral ventrolateral medulla (rVLM), decreasing sympathetic outflow, and alleviating ventricular extrasystoles; (2) the bradycardia induced by stimulation of aortic depressor nerve can be blocked by stimulation of superficial radial nerve (underneath LI 6-7) or superficial peroneal nerve (underneath G37-39): these stimulations activate the rVLM release opioids and GABA to inhibit nucleus tractus solitarius and vagal nuclei, and block vagal bradycardia. These experimental data explore the mechanism of acupuncture's effect on arrhythmias and are useful for clinical application.

Modulation of cardiopulmonary depressor reflex in nucleus ambiguus by electroacupuncture: roles of opioids and γ-aminobutyric acid

Stimulation of cardiopulmonary receptors with phenylbiguanide (PBG) elicits depressor cardiovascular reflex responses, including decreases in blood pressure and heart rate mediated in part by the brain stem parasympathetic cardiac neurons in the nucleus ambiguus (NAmb). The present study examined NAmb neurotransmitter mechanisms underlying the influence of electroacupuncture (EA) on the PBG-induced hypotension and bradycardia. We hypothesized that somatic stimulation during EA modulates PBG responses through opioid and γ-aminobutyric acid (GABA) modulation in the NAmb. Anesthetized and ventilated cats were studied during repeated stimulation with PBG or cardiac vagal afferents while low-frequency EA (2 Hz) was applied at P5-6 acupoints overlying the median nerve for 30 min and NAmb neuronal activity, heart rate, and blood pressure were recorded. Microinjection of kainic acid into the NAmb attenuated the PBG-induced bradycardia from -60 ± 11 to -36 ± 11 beats/min. Likewise, EA reduced the PBG-induced depressor and bradycardia reflex by 52 and 61%, respectively. Cardiac vagal afferent evoked preganglionic cellular activity in the NAmb was reduced by EA for about 60 min. Blockade of opioid or GABA(A) receptors using naloxone and gabazine reversed the EA-related modulation of the evoked cardiac vagal activity by 73 and 53%, respectively. Similarly, naloxone and gabazine reversed EA modulation of the negative chronotropic responses from -11 ± 5 to -23 ± 6 and -13 ± 4 to -24 ± 3 beats/min, respectively. Thus EA at P5-6 decreases PBG evoked hypotension and bradycardia as well as the NAmb PBG-sensitive preganglionic cardiac vagal outflow through opioid and GABA neurotransmitter systems.

GABA(A) receptor activation at medullary sympathetic neurons contributes to postexercise hypotension

A single bout of exercise results in a postexercise hypotension (PEH) that is accompanied by a reduced baroreflex function. Based on the role of rostral ventrolateral medulla (RVLM) neurons in controlling sympathetic nerve activity (SNA) and blood pressure, the role of gamma-aminobutyric acid (GABA) in controlling RVLM neuronal activity, and the reduced baroreflex-SNA relationship during PEH, we determined whether: 1) RVLM neuronal activity is decreased during PEH, 2) GABA(A)-receptor mechanisms mediate the decrease, and 3) baroreflex control of RVLM activity is reduced. Spontaneously hypertensive rats (SHR) were subjected to 40 min of treadmill or sham exercise (Sham PEH). PEH lasted 10 h in conscious and anesthetized SHR, indicating that the anesthetics did not affect the expression of PEH. Extracellular RVLM neuronal activity having a cardiac and sympathetic rhythm, lumbar SNA, and blood pressure were recorded at rest and during baroreflex function curves. Resting RVLM neuronal activity was lower and was increased to a greater extent by GABA(A)-receptor antagonism in PEH versus Sham PEH (P < 0.05). Baroreflex control of RVLM neuronal activity operated with a reduced gain (P < 0.05). Thus increased GABA signaling at RVLM neurons may contribute to PEH.

Characteristics of spontaneous and evoked GABAergic synaptic currents in cardiac vagal neurons in rats

Despite the importance of GABAergic input to cardiac vagal neurons the electrophysiological properties and possible origins of this innervation have not yet been studied. Individual cardiac vagal neurons were identified by a retrograde fluorescent tracer and were studied in an in vitro slice preparation using patch-clamp electrophysiology. Cardiac vagal neurons received spontaneous GABAergic inhibitory post-synaptic currents (IPSCs) that were blocked by the GABA(A) receptor antagonist bicuculline. The spontaneous presynaptic GABAergic input to cardiac vagal neurons in the nucleus ambiguus occurred at a significantly lower frequency than that recorded in cardiac vagal neurons in the dorsal motor nucleus of the vagus. To identify a possible source of the GABAergic innervation to cardiac vagal neurons the nucleus tractus solitarius was electrically stimulated. GABAergic synaptic currents in cardiac vagal neurons, in both the dorsal motor nucleus of the vagus (DMNX) and the nucleus ambiguus (NA), were consistently evoked upon stimulation of the nucleus tractus solitarius and these responses were also blocked by bicuculline.

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.

Inhibition of baroreflex bradycardia by ethanol involves both GABAA and GABAB receptors in the brainstem of the rat

The effects of ethanol on baroreceptor reflex bradycardia and its interactions with cardiovascular effects mediated by GABA receptors in the dorsal vagal complex were studied in urethane-anaesthetised rats. Ethanol, 1 g kg-1 administered i.v. or 25-200 nmol microinjected bilaterally into the dorsal vagal complex, inhibited the reflex bradycardic response to bolus i.v. doses of phenylephrine both in spontaneously breathing and in paralysed, artificially ventilated animals, and this effect could be prevented by pretreatment with the GABA-depleting agent, 3-mercaptopropionate in both groups of rats. Ethanol, 1 g kg-1 i.v., did not influence the bradycardic response to electrical stimulation of the cervical vagus. Microinjection of muscimol into the dorsal vagal complex caused a dose-dependent pressor response and inhibited baroreflex bradycardia. The pressor response was potentiated and a tachycardic response to muscimol emerged following microinjection of ethanol into the dorsal vagal complex. Similar administration of baclofen caused dose-dependent increases in blood pressure and heart rate and inhibited baroreflex bradycardia. Injection of ethanol into the dorsal vagal complex potentiated the pressor response to a low dose of baclofen but did not affect the tachycardic response. Bicuculline, 10 pmol/side into the dorsal vagal complex, blocked the effects of muscimol but not those of baclofen and reduced the baroreflex inhibitory action of ethanol. 2-Hydroxysaclofen, microinjected at 400 pmol to 1 nmol/side, blocked the effects of baclofen but not those of muscimol. 2-Hydroxysaclofen or phaclofen, 2 mg kg-1 s.c., prevented the baroreflex inhibitory action of ethanol and also prevented ethanol potentiation of the pressor and, less effectively, the tachycardic effects of muscimol. It is concluded that ethanol inhibits baroreflex bradycardia through potentiation of the actions of endogenous GABA in the dorsal vagal complex. Both GABAA and GABAB receptors appear to be involved in this action of ethanol.

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.