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.

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 GABAA 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.

Afferent pathways in cardiovascular adjustments induced by volume expansion in anesthetized rats

The role of baroreceptors, cardiopulmonary receptors, and renal nerves in the cardiovascular adjustments to volume expansion (VE) with 4% Ficoll (Pharmacia; 1% body wt, 0.4 ml/min) were studied in urethan-anesthetized rats. In control animals, VE produced a transitory increase in mean arterial pressure (MAP), which peaked at 10 min (17 ± 4 mmHg) and increases in renal (128 ± 6 and 169 ± 19% of baseline at 10 and 40 min, respectively) and hindlimb vascular conductance (143 ± 6 and 150 ± 10%). These cardiovascular adjustments to VE were unaffected by bilateral vagotomy. After sinoaortic denervation, the increase in MAP induced by VE was greater than in control rats (30 ± 4 mmHg). However, renal vasodilation in response to VE was blocked, whereas hindlimb vasodilation was similar to that observed in control rats. After unilateral renal denervation (ipsilateral to flow recording), the initial renal vasodilation was blocked. However, 40 min after VE, a significant renal vasodilation (125 ± 4%) appeared. The hindlimb vasodilation and MAP responses were unaffected by renal denervation. These results demonstrate that the baroreceptor afferents are an essential component of cardiovascular adjustments to VE, especially in the control of renal vascular conductance. They also suggest that renal vasodilation induced by VE is mediated by neural and hormonal mechanisms.

Arterial pulse pressure and vasopressin release during graded water immersion in humans

Previous results indicate that arterial pulse pressure modulates release of arginine vasopressin (AVP) in humans. The hypothesis was therefore tested that an increase in arterial pulse pressure is the stimulus for suppression of AVP release during central blood volume expansion by water immersion. A two-step immersion model ( n = 8) to the xiphoid process and neck, respectively, was used to attain two different levels of augmented cardiac distension. Left atrial diameter (echocardiography) increased from 28 ± 1 to 34 ± 1 mm ( P < 0.05) during immersion to the xiphoid process and more so ( P < 0.05), to 36 ± 1 mm, during immersion to the neck. During immersion to the xiphoid process, arterial pulse pressure (invasively measured in a brachial artery) increased ( P < 0.05) from 44 ± 1 to 51 ± 2 mmHg and to the same extent from 42 ± 1 to 52 ± 2 mmHg during immersion to the neck. Mean arterial pressure was unchanged during immersion to the xiphoid process and increased during immersion to the neck by 7 ± 1 mmHg ( P < 0.05). Arterial plasma AVP decreased from 2.5 ± 0.7 to 1.8 ± 0.5 pg/ml ( P < 0.05) during immersion to the xiphoid process and significantly more so ( P < 0.05), to 1.4 ± 0.5 pg/ml, during immersion to the neck. In conclusion, other factors besides the increase in arterial pulse pressure must have participated in the graded suppression of AVP release, comparing immersion to the xiphoid process with immersion to the neck. We suggest that when arterial pulse pressure is increased, graded distension of cardiopulmonary receptors modulate AVP release.

Atrial natriuretic peptide and mechanisms of cardiovascular control. Role of serotonergic receptors

Atrial natriuretic peptide (ANP) inhibits renal sympathetic nerve activity (RSNA), provided the vagi are intact. Afferents from chemosensitive cardiopulmonary receptors are specifically required. Such receptors produce the Bezold-Jarisch reflex, are prominent on the ventricular epicardium, and are richly supplied with 5-hydroxytryptamine type 3 (5-HT3) receptors. We tested the hypothesis that epicardial 5-HT3-sensitive neurons mediate depressor effects of ANP. Through a special catheter, anesthetized, sinoaortically denervated rats received pericardial test injections of ANP (28-amino acid rat ANP; 100 and 1,000 ng) in the presence or absence of 5-HT3 antagonist (Ondansetron, 20 μg/kg; n = 9). In other groups we observed the effects of systemic ANP while blocking either epicardial or systemic 5-HT3 receptors. Arterial blood pressure (ABP), heart rate, and RSNA were recorded continuously. Intravenous ANP (100 or 200 ng) decreased ABP and RSNA significantly. In contrast, intrapericardial ANP (100 or 1,000 ng) caused no significant fall in ABP or RSNA. Both intravenous and pericardial Ondansetron reduced the effects of intravenous ANP significantly, but the intravenous antagonism was significantly greater. We conclude that epicardial chemosensitive afferents are not sensitive to ANP and that sympathoinhibitory effects of ANP arise from a 5-HT3 agonist that cannot be produced when ANP is confined to the pericardial space.

Vasopressin and V1-receptor antagonists modulate the activity of NTS neurons receiving baroreceptor input

Arginine vasopressin (AVP) may act as a neurotransmitter or neuromodulator in the solitary tract nucleus (NTS). To determine whether AVP influences the activity of NTS neurons receiving cardiovascular afferent input, we used single-unit extracellular recording combined with local microinjection to test the effects of AVP and V1-receptor antagonists (antAVP) on spontaneously active NTS neurons in anesthetized rats. Phenylephrine-induced increases in arterial pressure were used to identify neurons receiving baroreceptor input. Phenylbiguanide was used to stimulate chemosensitive cardiopulmonary receptors. AVP excited 31 of 81 NTS neurons tested and inhibited 15 of 81 neurons. AntAVP had independent effects on NTS neurons: in addition to blocking the effects of AVP, antAVP inhibited 26 of 72 neurons but excited only 13. Eighty-two percent of NTS neurons receiving excitatory or inhibitory baroreceptor inputs responded to AVP; 61% of these were excited by AVP. Fifty-eight percent of neurons receiving cardiopulmonary receptor input responded to AVP. These results suggest that AVP in rat NTS has a tonic, predominantly excitatory influence on a significant proportion of baroreceptor-related neurons.