Peripheral chemoreflex activation with potassium cyanide (KCN) in awake rats or in the working heart-brainstem preparation (WHBP) produces: (a) a sympathoexcitatory/pressor response; (b) bradycardia; and (c) an increase in the frequency of breathing. Our main aim was to evaluate neurotransmitters involved in mediating the sympathoexcitatory component of the chemoreflex within the nucleus tractus solitarii (NTS). In previous studies in conscious rats, the reflex bradycardia, but not the pressor response, was reduced by antagonism of either ionotropic glutamate or purinergic P2 receptors within the NTS. In the present study we evaluated a possible dual role of both P2 and NMDA receptors in the NTS for processing the sympathoexcitatory component (pressor response) of the chemoreflex in awake rats as well as in the WHBP. Simultaneous blockade of ionotropic glutamate receptors and P2 receptors by sequential microinjections of kynurenic acid (KYN, 2 nmol (50 nl) −1 ) and pyridoxalphosphate-6-azophenyl-2 ,4 -disulphonate (PPADS, 0.25 nmol (50 nl) −1 ) into the commissural NTS in awake rats produced a significant reduction in both the pressor (+38 ± 3 versus +8 ± 3 mmHg) and bradycardic responses (−172 ± 18 versus −16 ± 13 beats min −1 ; n = 13), but no significant changes in the tachypnoea measured using plethysmography (270 ± 30 versus 240 ± 21 cycles min −1 , n = 7) following chemoreflex activation in awake rats. Control microinjections of saline produced no significant changes in these reflex responses. In WHBP, microinjection of KYN (2 nmol (20 nl) −1 ) and PPADS (1.6 nmol (20 nl) −1 ) into the commissural NTS attenuated significantly both the increase in thoracic sympathetic activity (+52 ± 2% versus +17 ± 1%) and the bradycardic response (−151 ± 17 versus −21 ± 3 beats min −1 ) but produced no significant changes in the increase of the frequency of phrenic nerve discharge (+0.24 ± 0.02 versus +0.20 ± 0.02 Hz). The data indicate that combined microinjections of PPADS and KYN into the commissural NTS in both awake rats and the WHBP are required to produce a significant reduction in the sympathoexcitatory response (pressor response) to peripheral chemoreflex activation. We conclude that glutamatergic and purinergic mechanisms are part of the complex neurotransmission system of the sympathoexcitatory component of the chemoreflex at the level of the commissural NTS. There is both anatomical and functional evidence that the peripheral chemoreceptor afferents make their first synapses in the nucleus tractus solitarii (NTS) terminating mainly in the commissural NTS (Donoghue et al.

Changes in mean arterial pressure (MAP), heart rate (HR), and vascular resistance (hindquarter and mesenteric territories) in response to electrical stimulation (ES) of the aortic depressor nerve (ADN) were evaluated in conscious freely moving rats. Platinum electrodes were implanted into the ADN of all rats studied, and some of these animals were also implanted with miniaturized Doppler probes around the superior mesenteric artery and inferior abdominal aorta (hindquarter). In both groups, the femoral artery and vein were catheterized one day before the experiments. In the first group of rats ( n = 7), the control ES of the ADN in the range from 0.5 to 3.0 V (50 Hz, 10 ms) produced bradycardia and hypotension in an intensity-dependent manner, and treatment with methylatropine (intravenously) blocked the bradycardia but produced no significant changes in the hypotensive response. In a second group ( n = 6), ES of the ADN was performed with the intensity fixed at 3 V and the frequency of the stimuli varying from 10 to 50 Hz. In this group, the hypotensive response was frequency dependent, whereas the bradycardic response was not. In a third group of rats ( n = 6), ES of the ADN (2.5 V) produced hypotension (−35 ± 4 mmHg), minor changes in the mesenteric (+5 ± 14%), and vasodilation in hindquarter (−32 ± 6%) vascular beds. The data show that 1) ES of the ADN produces a fall in pressure, bradycardia, vasodilation in the hindquarter, and no changes in the mesenteric vascular resistance, 2) methylatropine blocked the bradycardia and produced no effect on the hypotensive response to ES of the ADN, and 3) the baroreceptor afferent fibers involved in the hypotensive response to ES of ADN are sensitive to the variation of the frequency of the stimuli, whereas the fibers involved in the bradycardic response are not.

Chronic exposure to intermittent hypoxia (CIH) has been used in animals to mimic the arterial hypoxemia that accompanies sleep apnea. Humans with sleep apnea and animals exposed to CIH have elevated blood pressures and augmented sympathetic nervous system responses to acute exposures to hypoxia. To test the hypothesis that exposure to CIH alters neurons within the nucleus of the solitary tract (NTS) that integrate arterial chemoreceptor afferent inputs, we measured whole cell currents induced by activation of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors in enzymatically dispersed NTS neurons from normoxic (NORM) and CIH-exposed rats (alternating cycles of 3 min at 10% O2 followed by 3 min at 21% O2 between 8 AM and 4 PM for 7 days). To identify NTS neurons receiving carotid body afferent inputs the anterograde tracer 4- (4-(dihexadecylamino)styryl-N-methylpyridinum iodide (DiA) was placed onto the carotid body 1 wk before exposure to CIH. AMPA dose-response curves had similar EC50 but maximal responses increased in neurons isolated from DiA-labeled CIH (20.1 +/- 0.8 microM, n = 9) compared with NORM (6.0 +/- 0.3 microM, n = 8) rats. NMDA dose-response curves also had similar EC50 but maximal responses decreased in CIH (8.4 +/- 0.4 microM, n = 8) compared with NORM (19.4 +/- 0.6 microM, n = 9) rats. These results suggest reciprocal changes in the number and/or conductance characteristics of AMPA and NMDA receptors. Enhanced responses to AMPA receptor activation could contribute to enhanced chemoreflex responses observed in animals exposed to CIH and humans with sleep apnea.

ATP (0.31, 0.62, 1.25, and 2.5 nmol/50 nl) into the nucleus tractus solitarii (NTS) produced a dose-dependent pressor response. Prazosin abolished the pressor response and produced no change in the bradycardic response to ATP. Microinjection of pyridoxal phosphate-6-azophenyl-2Ј,4Ј-disulfonic acid (0.25 nmol/50 nl), a nonselective P2 receptor antagonist into the NTS, reduced the bradycardic response but had no effect on the pressor response to microinjection of ATP (1.25 nmol/50 nl) into the NTS. Microinjection of suramin (2 nmol/50 nl), another nonselective P2 receptor antagonist, had no effect on the pressor and bradycardic responses to microinjection of ATP (1.25 nmol/50 nl) into the NTS. Antagonism of A1 receptors of adenosine with 1,3-dipropyl-8-cyclopentylxanthine also produced no changes in the cardiovascular responses to microinjection of ATP into the NTS. The involvement of excitatory amino acid (EAA) receptors in the pressor and bradycardic responses to microinjection of ATP into the NTS was also evaluated. Microinjection of kynurenic acid, a nonselective EAA receptor antagonist (10 nmol/50 nl), into the NTS reduced the bradycardic response and had no effect on the pressor response to microinjection of ATP into the NTS. The data show that 1) microinjection of ATP into the NTS of awake rats produced pressor and bradycardic responses by independent mechanisms, 2) the activation of parasympathetic component may involve an interaction of P2 and EAA receptors in the NTS, and 3) the sympathoexcitatory response to microinjection of ATP into the NTS was not affected by the blockade of P2, A1, or EAA receptors. adenosine 5Ј-triphosphate; P2 purinergic receptors; kynurenic acid; suramin; pyridoxalphosphate-6-azophenyl-2Ј,4Ј-disulfonic acid; 8-cyclopentyl-1,3-dipropylxanthine THE CONCEPT THAT ATP may act as a nonadrenergic noncholinergic neurotransmitter on peripheral nerves was established by Burnstock (4), and several recent studies indicate that ATP and adenosine can act as neurotransmitters or neuromodulators in the central nervous system, including areas involved in cardiovascular regulation, such as the nucleus tractus solitarii (NTS) (1-3, 7, 8, 13, 19, 20, 24, 26 -28, 33). Immunohistochemical studies documented the presence of different subtypes of P2 receptors in the NTS of rats (11,15,34,35). ATP interacts with two distinct families of receptors, P2X and P2Y, which differ in structure and signal transduction mechanism (9, 21). There is evidence indicating that the available P2 receptor antagonists such as pyridoxal phosphate-6-azophenyl-2Ј,4Ј-disulfonic acid (PPADS) and suramin produced no blockade of P2X4 and P2X6 receptor subtypes (25).Studies performed in anesthetized rats documented that microinjection of ATP into the NTS produced a fall in arterial pressure and bradycardia, which were altered in different manner by suramin (8). These cardiovascular responses to ATP in anesthetized rats exhibited a typical pattern of fast transmitter similar to the responses to microinjection of L-glutamate into th...

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