Differential patterns of sympathetic responses to selective stimulation of nucleus tractus solitarius purinergic receptor subtypes

Cardiovascular and respiratory evaluation in adenosine A2A receptor knockout mice submitted to short-term sustained hypoxia

Sustained hypoxia (SH) in mice induces changes in the respiratory pattern and increase in the parasympathetic tone to the heart. Among adenosine G-protein-coupled receptors (GPCRs), the A2A receptors are especially important in mediating adenosine actions during hypoxia due to their expression in neurons involved with the generation and modulation of the autonomic and respiratory functions. Herein, we performed an in vivo evaluation of the baseline cardiovascular and respiratory parameters and their changes in response to SH in knockout mice for A2A receptors (A2A KO). SH produced similar and significant reductions in mean arterial pressure and heart rate in both wild-type (WT) and A2A KO mice when compared to their respective normoxic controls. Mice from WT and A2A KO groups submitted to normoxia or SH presented similar cardiovascular responses to peripheral chemoreflex activation (KCN). Under normoxic conditions A2A KO mice presented a respiratory frequency (fR ) significantly higher in relation to the WT group, which was reduced in response to SH. These data show that the lack of adenosine A2A receptors in mice does not affect the cardiovascular parameters and the autonomic responses to chemoreflex activation in control (normoxia) and SH mice. We conclude that the A2A receptors play a major role in the control of respiratory frequency and in the tachypnoeic response to SH in mice. NEW FINDINGS: What is the central question of this study? Are cardiovascular and respiratory parameters and their changes in response to sustained hypoxia (SH) altered in adenosine A2A receptor knockout mice? What is the main finding and its importance? Cardiovascular parameters and their changes in response to SH were not altered in A2A KO mice. The respiratory frequency in A2A KO was higher than in WT mice. In response to SH the respiratory frequency increased in WT, while it was reduced in A2A KO mice. A2A receptors play a major role in the modulation of respiratory frequency and in the tachypnoeic response to SH in mice.

Colocalization of A2a but not A1 adenosine receptors with GABA-ergic neurons in cardiopulmonary chemoreflex network in the caudal nucleus of the solitary tract

Adenosine operating in the nucleus of the solitary tract (NTS) may inhibit or facilitate neurotransmitter release from nerve terminals and directly inhibit or facilitate central neurons via A1 and A2a pre- and postsynaptic receptors, respectively. However, adenosine A2a receptors, may also activate GABA-ergic neurons/terminals which in turn inhibit glutamatergic transmission in the NTS network. Our previous studies showed that adenosine operating via both A1 (inhibitor) and A2a (activator) receptors powerfully inhibits the cardiopulmonary chemoreflex (CCR) at the level of the caudal NTS. A1 receptors most likely inhibit glutamate release in the CCR network, whereas A2a receptors facilitate NTS GABA-ergic mechanisms which in turn inhibit CCR glutamatergic transmission. Therefore, we hypothesized that A2a receptors are located on NTS GABA-ergic neurons/terminals whereas A1 receptors may be located on NTS glutamatergic neurons/terminals. We investigated this hypothesis using double immunofluorescent staining for A2a or A1 adenosine receptors and GABA synthesizing enzyme, GAD67, in 30 μm thick, floating, medullary rat sections. We found that A2a adenosine receptors are localized within the GABA-ergic cells in the caudal NTS, whereas A1 adenosine receptors are absent from these neurons. Instead, A1 receptors were located on non-GABA-ergic (likely glutamatergic) neurons/terminals in the caudal NTS. These data support our functional findings and the hypothesis that adenosine A2a, but not A1 receptors are located on GABA-ergic neurons.

NTS adenosine A2a receptors inhibit the cardiopulmonary chemoreflex control of regional sympathetic outputs via a GABAergic mechanism

Adenosine is a powerful central neuromodulator acting via opposing A1 (inhibitor) and A2a (activator) receptors. However, in the nucleus of the solitary tract (NTS), both adenosine receptor subtypes attenuate cardiopulmonary chemoreflex (CCR) sympathoinhibition of renal, adrenal, and lumbar sympathetic nerve activity and attenuate reflex decreases in arterial pressure and heart rate. Adenosine A1 receptors inhibit glutamatergic transmission in the CCR pathway, whereas adenosine A2a receptors most likely facilitate release of an unknown inhibitory neurotransmitter, which, in turn, inhibits the CCR. We hypothesized that adenosine A2a receptors inhibit the CCR via facilitation of GABA release in the NTS. In urethane-chloralose-anesthetized rats (n = 51), we compared regional sympathetic responses evoked by stimulation of the CCR with right atrial injections of the 5-HT3 receptor agonist phenylbiguanide (1-8 μg/kg) before and after selective stimulation of NTS adenosine A2a receptors [microinjections into the NTS of CGS-21680 (20 pmol/50 nl)] preceded by blockade of GABAA or GABAB receptors in the NTS [bicuculline (10 pmol/100 nl) or SCH-50911 (1 nmol/100 nl)]. Blockade of GABAA receptors virtually abolished adenosine A2a receptor-mediated inhibition of the CCR. GABAB receptors had much weaker but significant effects. These effects were similar for the different sympathetic outputs. We conclude that stimulation of NTS adenosine A2a receptors inhibits CCR-evoked hemodynamic and regional sympathetic reflex responses via a GABA-ergic mechanism.

Brain stem adenosine receptors modulate centrally mediated hypotensive responses in conscious rats: A review

Adenosine is implicated in the modulation of cardiovascular responses either at the peripheral or at central level in experimental animals. However, there are no dedicated reviews on the involvement of adenosine in mediating the hypotensive response of centrally administered clonidine in general and specifically in aortically barodenervated rats (ABD). The conscious ABD rat model exhibits surgically induced baroreflex dysfunction and exaggerated hypotensive response, compared with conscious sham-operated (SO) rats. The current review focuses on, the role of adenosine receptors in blood pressure (BP) regulation and their possible crosstalk with other receptors e.g. imidazoline (I₁) and alpha (α_2A) adrenergic receptor (AR). The former receptor is a molecular target for clonidine, whose hypotensive effect is enhanced approx. 3-fold in conscious ABD rats. We also discussed how the balance between the brain stem adenosine A₁ and A_2A receptors is regulated by baroreceptors and how such balance influences the centrally mediated hypotensive responses. The use of the ABD rat model yielded insight into the downstream signaling cascades following clonidine-evoked hypotension in a surgical model of baroreflex dysfunction.

Alpha2-adrenoceptor and adenosine A1 receptor within the nucleus tractus solitarii in hypertension development

Alpha2-adrenoceptor and A1 adenosine receptor systems within the nucleus tractus solitarii (NTS) play an important role in cardiovascular control. Deregulation of these systems may result in an elevated sympathetic tone, one of the root causes of neurogenic hypertension. The dorsomedial/dorsolateral and subpostremal NTS subnuclei of spontaneously hypertensive rats (SHR) show density changes in both receptors, even at 15 days of age, prior to the onset of hypertension. In addition, adenosine A1 receptors have been specifically reported to modulate alpha2-adrenoceptors in several brain regions, including the NTS, via a PLC-dependent pathway involving cross regulation between sympathetic neurons and astrocytes. The physiological cross talk between these receptor systems is also deregulated in SHR suggesting that alpha2-adrenoceptor and A1 adenosine receptor might be germane to the development of hypertension. In this review, we will focus on these systems within the NTS during development, pointing out some interesting modulations in processes, and chemical changes within specific subnuclei of NTS circuitry, that might have implications for neurogenic hypertension.

Central activation of the A1 adenosine receptor (A1AR) induces a hypothermic, torpor-like state in the rat

Since central activation of A1 adenosine receptors (A1ARs) plays an important role in the induction of the hypothermic and hypometabolic torpid state in hibernating mammals, we investigated the potential for the A1AR agonist N6-cyclohexyladenosine to induce a hypothermic, torpor-like state in the (nonhibernating) rat. Core and brown adipose tissue temperatures, EEG, heart rate, and arterial pressure were recorded in free-behaving rats, and c-fos expression in the brain was analyzed, following central administration of N6-cyclohexyladenosine. Additionally, we recorded the sympathetic nerve activity to brown adipose tissue; expiratory CO2 and skin, core, and brown adipose tissue temperatures; and shivering EMGs in anesthetized rats following central and localized, nucleus of the solitary tract, administration of N6-cyclohexyladenosine. In rats exposed to a cool (15°C) ambient temperature, central A1AR stimulation produced a torpor-like state similar to that in hibernating species and characterized by a marked fall in body temperature due to an inhibition of brown adipose tissue and shivering thermogenesis that is mediated by neurons in the nucleus of the solitary tract. During the induced hypothermia, EEG amplitude and heart rate were markedly reduced. Skipped heartbeats and transient bradycardias occurring during the hypothermia were vagally mediated since they were eliminated by systemic muscarinic receptor blockade. These findings demonstrate that a deeply hypothermic, torpor-like state can be pharmacologically induced in a nonhibernating mammal and that recovery of normothermic homeostasis ensues upon rewarming. These results support the potential for central activation of A1ARs to be used in the induction of a hypothermic, therapeutically beneficial state in humans.

Nucleus tractus solitarii A(2a) adenosine receptors inhibit cardiopulmonary chemoreflex control of sympathetic outputs

Previously we have shown that stimulation of inhibitory A1 adenosine receptors located in the nucleus tractus solitarii (NTS) attenuates cardiopulmonary chemoreflex (CCR) evoked inhibition of renal, adrenal and lumbar sympathetic nerve activity and reflex decreases in arterial pressure and heart rate. Activation of facilitatory A2a adenosine receptors, which dominate over A1 receptors in the NTS, contrastingly alters baseline activity of regional sympathetic outputs: it decreases renal, increases adrenal and does not change lumbar nerve activity. Considering that NTS A2a receptors may facilitate release of inhibitory transmitters we hypothesized that A2a receptors will act in concert with A1 receptors differentially inhibiting regional sympathetic CCR responses (adrenal>lumbar>renal). In urethane/chloralose anesthetized rats (n=38) we compared regional sympathetic responses evoked by stimulation of the CCR with right atrial injections of serotonin 5HT3 receptor agonist, phenylbiguanide, (1-8μg/kg) before and after selective stimulation, blockade or combined blockade and stimulation of NTS A2a adenosine receptors (microinjections into the NTS of CGS-21680 0.2-20pmol/50nl, ZM-241385 40pmol/100nl or ZM-241385+CGS-21680, respectively). We found that stimulation of A2a adenosine receptors uniformly inhibited the regional sympathetic and hemodynamic reflex responses and this effect was abolished by the selective blockade of NTS A2a receptors. This indicates that A2a receptor triggered inhibition of CCR responses and the contrasting shifts in baseline sympathetic activity are mediated via different mechanisms. These data implicate that stimulation of NTS A2a receptors triggers unknown inhibitory mechanism(s) which in turn inhibit transmission in the CCR pathway when adenosine is released into the NTS during severe hypotension.

Activation of NTS A(1) adenosine receptors inhibits regional sympathetic responses evoked by activation of cardiopulmonary chemoreflex

Previously we have shown that adenosine operating via the A(1) receptor subtype may inhibit glutamatergic transmission in the baroreflex arc within the nucleus of the solitary tract (NTS) and differentially increase renal (RSNA), preganglionic adrenal (pre-ASNA), and lumbar (LSNA) sympathetic nerve activity (ASNA>RSNA≥LSNA). Since the cardiopulmonary chemoreflex and the arterial baroreflex are mediated via similar medullary pathways, and glutamate is a primary transmitter in both pathways, it is likely that adenosine operating via A(1) receptors in the NTS may differentially inhibit regional sympathetic responses evoked by activation of cardiopulmonary chemoreceptors. Therefore, in urethane-chloralose-anesthetized rats (n = 37) we compared regional sympathoinhibition evoked by the cardiopulmonary chemoreflex (activated with right atrial injections of serotonin 5HT(3) receptor agonist phenylbiguanide, PBG, 1-8 μg/kg) before and after selective stimulation of NTS A(1) adenosine receptors [microinjections of N(6)-cyclopentyl adenosine (CPA), 0.033-330 pmol/50 nl]. Activation of cardiopulmonary chemoreceptors evoked differential, dose-dependent sympathoinhibition (RSNA>ASNA>LSNA), and decreases in arterial pressure and heart rate. These differential sympathetic responses were uniformly attenuated in dose-dependent manner by microinjections of CPA into the NTS. Volume control (n = 11) and blockade of adenosine receptor subtypes in the NTS via 8-(p-sulfophenyl)theophylline (8-SPT, 1 nmol in 100 nl) (n = 9) did not affect the reflex responses. We conclude that activation of NTS A(1) adenosine receptors uniformly inhibits neural and cardiovascular cardiopulmonary chemoreflex responses. A(1) adenosine receptors have no tonic modulatory effect on this reflex under normal conditions. However, when adenosine is released into the NTS (i.e., during stress or severe hypotension/ischemia), it may serve as negative feedback regulator for depressor and sympathoinhibitory reflexes integrated in the NTS.

Differential activation of adrenal, renal, and lumbar sympathetic nerves following stimulation of the rostral ventrolateral medulla of the rat

Under acute and chronic conditions, the sympathetic nervous system can be activated in a differential and even selective manner. Activation of the rostral ventrolateral medulla (RVLM) has been implicated in differential control of sympathetic outputs based on evidence primarily in the cat. Although several studies indicate that differential control of sympathetic outflow occurs in other species, only a few studies have addressed whether the RVLM is capable of producing varying patterns of sympathetic activation in the rat. Therefore, the purpose of the present study was to determine whether activation of the RVLM results in simultaneous and differential increases in preganglionic adrenal (pre-ASNA), renal (RSNA), and lumbar (LSNA) sympathetic nerve activities. In urethane-chloralose anesthetized rats, pre-ASNA, RSNA, and LSNA were recorded simultaneously in all animals. Microinjections of selected concentrations and volumes of glutamate increased pre-ASNA, RSNA, and LSNA concurrently and differentially. Pre-ASNA and RSNA (in most cases) exhibited greater increases compared with LSNA on a percentage basis. By varying the volume or location of the glutamate microinjections, we also identified individual examples of differential and selective activation of these nerves. Decreases in arterial pressure or bilateral blockade of RVLM GABA(A) receptors also revealed differential activation, with the latter having a 3- to 4-fold greater effect on sympathetic activity. Our data provide evidence that activation of the rat RVLM increases renal, lumbar, and preganglionic adrenal sympathetic nerve activities concurrently, differentially, and, in some cases, selectively.

Sympatho-vagal interaction in the recovery phase of exercise

Reciprocal autonomic regulation occurs during incremental exercise. We hypothesized that sympatho-vagal interplay may become altered after exercise because of the differences in recovery patterns of autonomic arms. The cardiac vagal activity was assessed by measurement of beat-to-beat R-R interval oscillations using a Poincaré plot method (SD1), and muscle sympathetic nervous activity (MSNA) was measured from peroneus nerve by a microneurography technique during and after exercise in 16 healthy subjects. Autonomic regulation was compared between the rest and after exercise (3·5 ± 1·0 min after exercise) at equal heart rates (HR). SD1 was at the equal level at the recovery phase (40 ± 21 ms) compared to the resting condition (38 ± 16 ms, P = ns) at comparable HR (57 ± 10 for both). MSNA was higher at the recovery phase (40 ± 19 burst per 100 heartbeats) than at rest (25 ± 13 burst per 100 heartbeats, P<0·0001). The difference of MSNA activity between rest and late recovery phase had a strong positive correlation with the difference in SD1 (r = 0·78, P<0·001) at equal HRs. Subjects who have a higher sympathetic activity in the recovery phase of exercise have a more augmented cardiac vagal activity resulting in an accentuated sympatho-vagal outflow. The altered autonomic interaction observed here may partly explain the clustering of various cardiovascular events to the recovery phase of exercise.

Neural and humoral control of regional vascular beds via A1 adenosine receptors located in the nucleus tractus solitarii

Our previous studies showed that stimulation of adenosine A(1) receptors located in the nucleus of the solitary tract (NTS) exerts counteracting effects on the iliac vascular bed: activation of the adrenal medulla and β-adrenergic vasodilation vs. sympathetic and vasopressinergic vasoconstriction. Because NTS A(1) adenosine receptors inhibit baroreflex transmission in the NTS and contribute to the pressor component of the HDR, we hypothesized that these receptors also contribute to the redistribution of blood from the visceral to the muscle vasculature via prevailing sympathetic and vasopressinergic vasoconstriction in the visceral (renal and mesenteric) vascular beds and prevailing β-adrenergic vasodilation in the somatic (iliac) vasculature. To test this hypothesis, we compared the A(1) adenosine-receptor-mediated effects of each vasoactive factor triggered by NTS A(1) adenosine receptor stimulation [N(6)-cyclopentyladenosine (CPA), 330 pmol in 50 nl] on the regional vascular responses in urethane/chloralose-anesthetized rats. The single-factor effects were separated using adrenalectomy, β-adrenergic blockade, V(1) vasopressin receptor blockade, and sinoaortic denervation. In intact animals, initial vasodilation was followed by large, sustained vasoconstriction with smaller responses observed in renal vs. mesenteric and iliac vascular beds. The initial β-adrenergic vasodilation prevailed in the iliac vs. mesenteric and renal vasculature. The large and sustained vasopressinergic vasoconstriction was similar in all vascular beds. Small sympathetic vasoconstriction was observed only in the iliac vasculature in this setting. We conclude that, although A(1) adenosine-receptor-mediated β-adrenergic vasodilation may contribute to the redistribution of blood from the visceral to the muscle vasculature, this effect is overridden by sympathetic and vasopressinergic vasoconstriction.

Vasopressin is a major vasoconstrictor involved in hindlimb vascular responses to stimulation of adenosine A(1) receptors in the nucleus of the solitary tract

Our previous study showed that stimulation of adenosine A(1) receptors located in the nucleus of the solitary tract (NTS) exerts counteracting effects on the iliac vascular bed: activation of the adrenal medulla and beta-adrenergic vasodilation versus vasoconstriction mediated by neural and unknown humoral factors. In the present study we investigated the relative contribution of three major potential humoral vasoconstrictors: vasopressin, angiotensin II, and norepinephrine in this response. In urethane-chloralose anesthetized rats we compared the integral changes in iliac vascular conductance evoked by microinjections into the NTS of the selective A(1) receptor agonist N(6)-cyclopentyladenosine (CPA; 330 pmol in 50 nl) in intact (Int) animals and following: V(1) vasopressin receptor blockade (VX), angiotensin II AT(1) receptor blockade (ATX), bilateral adrenalectomy + ganglionic blockade (ADX + GX; which eliminated the potential increases in circulating norepinephrine and epinephrine), ADX + GX + VX and ADX + GX + VX + ATX. In Int animals, stimulation of NTS A(1) adenosine receptors evoked typical variable responses with prevailing pressor and vasoconstrictor effects. VX reversed the responses to depressor ones. ATX did not significantly alter the responses. ADX + GX accentuated pressor and vasoconstrictor responses, whereas ADX + GX + VX and ADX + GX + VX + ATX virtually abolished the responses. Stimulation of NTS A(1) adenosine receptors increased circulating vasopressin over fourfold (26.4 + or - 10.4 vs. 117.0 + or - 19 pg/ml). These data strongly suggest that vasopressin is a major vasoconstrictor factor opposing beta-adrenergic vasodilation in iliac vascular responses triggered by stimulation of NTS A(1) adenosine receptors, whereas angiotensin II and norepinephrine do not contribute significantly to the vasoconstrictor responses.

Adenosine modulates alpha2-adrenergic receptors through a phospholipase C pathway in brainstem cell culture of rats

Adenosine acts in the nucleus tractus solitarii (NTS), one of the main brain sites related to cardiovascular control. In the present study we show that A(1) adenosine receptor (A(1R)) activation promotes an increase on alpha(2)-adrenoceptor (Alpha(2R)) binding in brainstem cell culture from newborn rats. We investigated the intracellular cascade involved in such modulatory process using different intracellular signaling molecule inhibitors as well as calcium chelators. Phospholipase C, protein kinase Ca(2+)-dependent, IP(3) receptor and intracellular calcium were shown to participate in A(1R)/Alpha(2R) interaction. In conclusion, this result might be important to understand the role of adenosine within the NTS regarding autonomic cardiovascular control.

Activation of NTS A2a adenosine receptors differentially resets baroreflex control of renal vs. adrenal sympathetic nerve activity

The role of nucleus of solitary tract (NTS) A(2a) adenosine receptors in baroreflex mechanisms is controversial. Stimulation of these receptors releases glutamate within the NTS and elicits baroreflex-like decreases in mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA), whereas inhibition of these receptors attenuates HR baroreflex responses. In contrast, stimulation of NTS A(2a) adenosine receptors increases preganglionic adrenal sympathetic nerve activity (pre-ASNA), and the depressor and sympathoinhibitory responses are not markedly affected by sinoaortic denervation and blockade of NTS glutamatergic transmission. To elucidate the role of NTS A(2a) adenosine receptors in baroreflex function, we compared full baroreflex stimulus-response curves for HR, RSNA, and pre-ASNA (intravenous nitroprusside/phenylephrine) before and after bilateral NTS microinjections of selective adenosine A(2a) receptor agonist (CGS-21680; 2.0, 20 pmol/50 nl), selective A(2a) receptor antagonist (ZM-241385; 40 pmol/100 nl), and nonselective A(1) + A(2a) receptor antagonist (8-SPT; 1 nmol/100 nl) in urethane/alpha-chloralose anesthetized rats. Activation of A(2a) receptors decreased the range, upper plateau, and gain of baroreflex-response curves for RSNA, whereas these parameters all increased for pre-ASNA, consistent with direct effects of the agonist on regional sympathetic activity. However, no resetting of baroreflex-response curves along the MAP axis occurred despite the marked decreases in baseline MAP. The antagonists had no marked effects on baseline variables or baroreflex-response functions. We conclude that the activation of NTS A(2a) adenosine receptors differentially alters baroreflex control of HR, RSNA, and pre-ASNA mostly via non-baroreflex mechanism(s), and these receptors have virtually no tonic action on baroreflex control of these sympathetic outputs.

Neonatal caffeine induces sex-specific developmental plasticity of the hypoxic respiratory chemoreflex in adult rats

Caffeine is widely used to treat apneas of prematurity during the neonatal period; however, the potential consequences of administering a neonatal caffeine treatment (NCT) during a critical period for respiratory control development are unknown. The present study therefore determined whether NCT in rats alters the hypoxic respiratory chemoreflex measured at adulthood. Newborn rats received either caffeine (15 mg/kg) or water (control) each day from postnatal day 3 to 12. The ventilatory response to a hypoxic challenge (inspired O(2) fraction = 0.12) was first evaluated in awake adult female and male rats using whole body plethysmography. Results showed that NCT increased the initial phase of the breathing frequency response to hypoxia in males only. This result was confirmed in anesthetized and artificially ventilated adult male rats where NCT also increased the phrenic burst frequency response to hypoxia. RT-PCR assessment of mRNA encoding for adenosine A(1A) and A(2A) receptors, dopamine D(2) receptors, and tyrosine hydroxylase in the rat carotid bodies showed that NCT enhanced mRNA expression levels of adenosine A(2A), dopamine D(2) receptors, and tyrosine hydroxylase of males but not females. Subsequent experiments on awake male rats showed that injection of the adenosine A(2A) receptor antagonist ZM2413855 (1 mg/kg ip) before ventilatory measurements abolished, in NCT rats, the enhanced respiratory frequency response observed during the early phase of hypoxia. We propose that NCT elicits a sex-specific increase in the hypoxic respiratory chemoreflex, which is related, at least partially, to an enhancement in adenosine A(2A) receptors in the rat carotid body.

Age-dependent changes in adenosine A1 receptor distribution and density within the nucleus tractus solitarii of normotensive and hypertensive rats

This study shows the distribution and density of adenosine A1 receptor (A1R) within the nucleus tractus solitarii (NTS) of Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) from birth to adulthood (1, 15, 30 and 90 days old). The NTS shows heterogeneous distribution of A1R in dorsomedial/dorsolateral, subpostremal and medial/intermediate subnuclei. A1R decrease from rostral to caudal within dorsomedial/dorsolateral subnucleus in 15-, 30- and 90-day-old WKY and SHR. A1R increase from rostral to caudal subpostremal subnucleus in 30- and 90-day-old WKY, and in 15-, 30- and 90-day-old SHR. Furthermore, A1Rs are increased in SHR as compared with WKY within dorsomedial/dorsolateral in 30- and 90-day-old and within subpostremal of 15-, 30- and 90-day-old rats. Finally, A1Rs increase from 1- to 30-day-old rats. Medial/intermediate did not show any changes in A1R from rostral to caudal levels, age or strain. In summary, our result highlights the importance of A1 adenosine system regarding the neural control of blood pressure and the development of hypertension.

Stimulation of NTS A1 adenosine receptors differentially resets baroreflex control of regional sympathetic outputs

Previously we showed that pressor and differential regional sympathoexcitatory responses (adrenal > renal >/= lumbar) evoked by stimulation of A(1) adenosine receptors located in the nucleus of the solitary tract (NTS) were attenuated/abolished by baroreceptor denervation or blockade of glutamatergic transmission in the NTS, suggesting A(1) receptor-elicited inhibition of glutamatergic transmission in baroreflex pathways. Therefore we tested the hypothesis that stimulation of NTS A(1) adenosine receptors differentially inhibits/resets baroreflex responses of preganglionic adrenal (pre-ASNA), renal (RSNA), and lumbar (LSNA) sympathetic nerve activity. In urethane-chloralose-anesthetized male Sprague-Dawley rats (n = 65) we compared baroreflex-response curves (iv nitroprusside and phenylephrine) evoked before and after bilateral microinjections into the NTS of A(1) adenosine receptor agonist (N(6)-cyclopentyladenosine, CPA; 0.033-330 pmol/50 nl). CPA evoked typical dose-dependent pressor and differential sympathoexcitatory responses and similarly shifted baroreflex curves for pre-ASNA, RSNA, and LSNA toward higher mean arterial pressure (MAP) in a dose-dependent manner; the maximal shifts were 52.6 +/- 2.8, 48.0 +/- 3.6, and 56.8 +/- 6.7 mmHg for pre-ASNA, RSNA, and LSNA, respectively. These shifts were not a result of simple baroreceptor resetting because they were two to three times greater than respective increases in baseline MAP evoked by CPA. Baroreflex curves for pre-ASNA were additionally shifted upward: the maximal increases of upper and lower plateaus were 41.8 +/- 16.4% and 45.3 +/- 8.7%, respectively. Maximal gain (%/mmHg) measured before vs. after CPA increased for pre-ASNA (3.0 +/- 0.6 vs. 4.9 +/- 1.3), decreased for RSNA (4.1 +/- 0.6 vs. 2.3 +/- 0.3), and remained unaltered for LSNA (2.1 +/- 0.2 vs. 2.0 +/- 0.1). Vehicle control did not alter the baroreflex curves. We conclude that the activation of NTS A(1) adenosine receptors differentially inhibits/resets baroreflex control of regional sympathetic outputs.

Involvement of guanylate cyclase in the cardiovascular response induced by adenosine A2B receptor stimulation in the posterior hypothalamus of the anesthetized rats

Cardiovascular inhibitory effects induced by the posterior hypothalamic adenosine A(2) receptors were suggested by our previous reports. In this experiment, we examined the influence of the posterior hypothalamic adenosine A(2B) receptors on central cardiovascular regulation of blood pressure (BP) and heart rate (HR). Posterior hypothalamic injection of drugs was performed in anesthetized, artificially ventilated male Sprague-Dawley rats. Injection of 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA; 2 nmol), an adenosine A(2) receptor agonist, showed the decrease of arterial blood pressure and heart rate, and the alloxazine, an adenosine A(2B) receptor antagonist, partially blocked the depressor and bradycardiac effects of CPCA (2 nmol). To examine the role of adenosine A(2B) receptors among the adenosine A(2) subtypes, we applied the 5'-N-Ethylcarboxamidoadenosine (NECA), an adenosine A(2B) receptor agonist, to the posterior hypothalamus. Injection of NECA (1, 4 and 8 nmol) produced a dose-dependent decrease of arterial blood pressure and HR. Pretreatment with alloxazine (5 nmol) partially blocked the depressor and bradycardiac effects of NECA (4 nmol). Also, pretreatment with LY-83,583 (5 nmol), a soluble guanylate cyclase inhibitor, attenuated the depressor and bradycardiac effects of NECA (4 nmol). However, pretreatment with MDL-12,330 (10 nmol), an adenylate cyclase inhibitor, did not affect these effects of NECA (4 nmol). These results suggest that adenosine A(2B) receptor in the posterior hypothalamus plays an inhibitory role in central cardiovascular regulation, and that guanylate cyclase mediates the depressor and bradycardiac actions of adenosine A(2B) receptors.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Adenosine receptors located in the NTS contribute to renal sympathoinhibition during hypotensive phase of severe hemorrhage in anesthetized rats

Stimulation of nucleus of the solitary tract (NTS) A(2a)-adenosine receptors elicits cardiovascular responses quite similar to those observed with rapid, severe hemorrhage, including bradycardia, hypotension, and inhibition of renal but activation of preganglionic adrenal sympathetic nerve activity (RSNA and pre-ASNA, respectively). Because adenosine levels in the central nervous system increase during severe hemorrhage, we investigated to what extent these responses to hemorrhage may be due to activation of NTS adenosine receptors. In urethane- and alpha-chloralose-anesthetized male Sprague-Dawley rats, rapid hemorrhage was performed before and after bilateral nonselective or selective blockade of NTS adenosine-receptor subtypes [A(1)- and A(2a)-adenosine-receptor antagonist 8-(p-sulfophenyl)theophylline (1 nmol/100 nl) and A(2a)-receptor antagonist ZM-241385 (40 pmol/100 nl)]. The nonselective blockade reversed the response in RSNA (-21.0 +/- 9.6 Delta% vs. +7.3 +/- 5.7 Delta%) (where Delta% is averaged percent change from baseline) and attenuated the average heart rate response (change of -14.8 +/- 4.8 vs. -4.4 +/- 3.4 beats/min). The selective blockade attenuated the RSNA response (-30.4 +/- 5.2 Delta% vs. -11.1 +/- 7.7 Delta%) and tended to attenuate heart rate response (change of -27.5 +/- 5.3 vs. -15.8 +/- 8.2 beats/min). Microinjection of vehicle (100 nl) had no significant effect on the responses. The hemorrhage-induced increases in pre-ASNA remained unchanged with either adenosine-receptor antagonist. We conclude that adenosine operating in the NTS via A(2a) and possibly A(1) receptors may contribute to posthemorrhagic sympathoinhibition of RSNA but not to the sympathoactivation of pre-ASNA. The differential effects of NTS adenosine receptors on RSNA vs. pre-ASNA responses to hemorrhage supports the hypothesis that these receptors are differentially located/expressed on NTS neurons/synaptic terminals controlling different sympathetic outputs.

Vasopressin V1 receptors contribute to hemodynamic and sympathoinhibitory responses evoked by stimulation of adenosine A2a receptors in NTS

Activation of adenosine A2a receptors in the nucleus of the solitary tract (NTS) decreases mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA), whereas increases in preganglionic adrenal sympathetic nerve activity (pre-ASNA) occur, a pattern similar to that observed during hypotensive hemorrhage. Central vasopressin V1 receptors may contribute to posthemorrhagic hypotension and bradycardia. Both V1 and A2a receptors are densely expressed in the NTS, and both of these receptors are involved in cardiovascular control; thus they may interact. The responses elicited by NTS A2a receptors are mediated mostly via nonglutamatergic mechanisms, possibly via release of vasopressin. Therefore, we investigated whether blockade of NTS V1 receptors alters the autonomic response patterns evoked by stimulation of NTS A2a receptors (CGS-21680, 20 pmol/50 nl) in α-chloralose-urethane anesthetized male Sprague-Dawley rats. In addition, we compared the regional sympathetic responses to microinjections of vasopressin (0.1–100 ng/50 nl) into the NTS. Blockade of V1 receptors reversed the normal decreases in MAP into increases (−95.6 ± 28.3 vs. 51.4 ± 15.7 ∫Δ%), virtually abolished the decreases in HR (−258.3 ± 54.0 vs. 18.9 ± 57.8 ∫Δbeats/min) and RSNA (−239.3 ± 47.4 vs. 15.9 ± 36.1 ∫Δ%), and did not affect the increases in pre-ASNA (279.7 ± 48.3 vs. 233.1 ± 54.1 ∫Δ%) evoked by A2a receptor stimulation. The responses partially returned toward normal values ∼90 min after the blockade. Microinjections of vasopressin into the NTS evoked dose-dependent decreases in HR and RSNA and variable MAP and pre-ASNA responses with a tendency toward increases. We conclude that the decreases in MAP, HR, and RSNA in response to NTS A2a receptor stimulation may be mediated via release of vasopressin from neural terminals in the NTS. The differential effects of NTS V1 and A2a receptors on RSNA versus pre-ASNA support the hypothesis that these receptor subtypes are differentially located/expressed on NTS neurons/neural terminals controlling different sympathetic outputs.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Central Adenosine Signaling Plays a Key Role in Centrally Mediated Hypotension in Conscious Aortic Barodenervated Rats

We tested the hypothesis that clonidine-evoked hypotension is dependent on central adenosinergic pathways. Five groups of male, conscious, aortic baroreceptor-denervated (ABD) rats received clonidine (10 microg/kg i.v.) 30 min after i.v. 1) saline, 2) theophylline (10 mg/kg), or 3) 8-(p-sulfophenyl)theophylline (8-SPT) (2.5 mg/kg) or 1 h after i.p. 4) dipyridamole (5 mg/kg) or 5) an equal volume of sesame oil. Blockade of central (theophylline) but not peripheral (8-SPT) adenosine receptors abolished clonidine hypotension. In contrast, dipyridamole substantially enhanced the bradycardic response to clonidine. In additional groups, intracisternal (i.c.) dipyridamole (150 microg) and 8-SPT (10 microg) enhanced and abolished, respectively, clonidine (0.6 microg i.c.)-evoked hypotension. Because clonidine is a mixed I1/alpha2 agonist, we also investigated whether adenosine signaling is linked to the I1 or the alpha2A receptor by administering the selective I1 (rilmenidine, 25 microg) or alpha2A [alpha-methylnorepinephrine (alpha-MNE), 4 microg] agonist 30 min after central adenosine receptor blockade (8-SPT; 10 microg i.c.) or artificial cerebrospinal fluid. The hypotensive response elicited by rilmenidine or alpha-MNE was abolished in 8-SPT-pretreated rats. To delineate the role of the adenosine A2A receptor in clonidine-evoked hypotension, i.c. clonidine (0.6 microg) was administered 30 min after central adenosine receptor A2A blockade [5-amino-7-(2-phenylethyl)-2-(2-furyl)-pyrazolo[4,3-epsilon]-1,2,4-triazolo[1,5-c]-pyrimidine (SCH58261); 150 microg i.c.]. The latter virtually abolished the hypotensive and bradycardic responses elicited by clonidine. In conclusion, central adenosine A2A signaling plays a key role in clonidine-evoked hypotension in conscious aortic barodenervated rats.

Influence of short-term versus prolonged cardiopulmonary receptor stimulation on renal and preganglionic adrenal sympathetic nerve activity in rats

Renal and preganglionic adrenal sympathetic nerve activities (RSNA, ASNA) are regulated differentially. Various cardiopulmonary receptor (CPR) stimulation procedures were performed to distinguish short-term and prolonged as well as mechanical and chemical stimulatory effects on RSNA and ASNA. In anesthetized male Sprague-Dawley rats blood pressure, heart rate, left ventricular end-diastolic pressure (LVEDP), RSNA and ASNA were recorded. CPRs were stimulated as follows: Short-term mechanical: LVEDP changes (+/-4, +/-6, +/-8 mmHg) via aortic and caval vein occlusion; Short-term chemical: phenylbiguanide (PBG-bolus, 0.1, 1, 10 microg IV); Prolonged mechanical (15 min): volume expansion (0.9% NaCl, 5% body weight) and hemorrhage, to modulate LVEDP; Prolonged chemical: PBG infusion (32 microg/min IV, for 15 min); Stimulations were done with 1) all afferents intact, 2) bilateral cervical vagotomy (VX), 3) VX + SAD (sino-aortic denervation; short-term protocols and hemorrhage).1) Short-term mechanical stimuli decreased RSNA (-52 +/- 12%) and ASNA (-37 +/- 13%). 2) PBG-bolus decreased RSNA (-54 +/- 12%) but increased ASNA (+40 +/- 13%). 3) Volume expansion decreased RSNA (-55 +/- 7%), ASNA was unaffected. 4) PBG infusion persistently decreased RSNA (-60 +/- 6%) but just shortly increased ASNA (+120 +/- 15%); VX abolished all responses. 5) Hypotensive hemorrhage decreased RSNA (-39 +/- 9%) but increased ASNA (+42 +/- 9%). VX abolished RSNA response; ASNA response only disappeared with VX + SAD.Short-term mechanical CPR stimulation uniformly decreased sympathetic activities, whereas chemical stimulation had opposing effects on renal and adrenal sympathetic responses. All prolonged stimuli decreased RSNA, whereas ASNA was virtually unaffected: Sympathetic out.ow is differentially controlled not only with regard to target organs or afferent receptors but also stimulus time pattern.

Stimulation of NTS A1 adenosine receptors evokes counteracting effects on hindlimb vasculature

Our previous studies concluded that stimulation of the nucleus of the solitary tract (NTS) A2a receptors evokes preferential hindlimb vasodilation mainly via inducing increases in preganglionic sympathetic nerve activity (pre-ASNA) directed to the adrenal medulla. This increase in pre-ASNA causes the release of epinephrine and subsequent activation of β-adrenergic receptors that are preferentially located in the skeletal muscle vasculature. Selective activation of NTS A1 adenosine receptors evokes variable, mostly pressor effects and increases pre-ASNA, as well as lumbar sympathetic activity, which is directed to the hindlimb. These counteracting factors may have opposite effects on the hindlimb vasculature resulting in mixed vascular responses. Therefore, in chloralose-urethane-anesthetized rats, we evaluated the contribution of vasodilator versus vasoconstrictor effects of stimulation of NTS A1 receptors on the hindlimb vasculature. We compared the changes in iliac vascular conductance evoked by microinjctions into the NTS of the selective A1 receptor agonist N6-cyclopentyladenosine (330 pmol in 50 nl volume) in intact animals with the responses evoked after β-adrenergic blockade, bilateral adrenalectomy, bilateral lumbar sympathectomy, and combined adrenalectomy + lumbar sympathectomy. In intact animals, stimulation of NTS A1 receptors evoked variable effects: increases and decreases in mean arterial pressure and iliac conductance with prevailing pressor and vasoconstrictor effects. Peripheral β-adrenergic receptor blockade and bilateral adrenalectomy eliminated the depressor component of the responses, markedly potentiated iliac vasoconstriction, and tended to increase the pressor responses. Lumbar sympathectomy tended to decrease the pressor and vasoconstrictor responses. After bilateral adrenalectomy plus lumbar sympathectomy, a marked vasoconstriction in iliac vascular bed still persisted, suggesting that the vasoconstrictor component of the response to stimulation of NTS A1 receptors is mediated mostly via circulating factors (e.g., vasopressin, angiotensin II, or circulating catecholamines released from other sympathetic terminals). These data strongly suggest that stimulation of NTS A1 receptors exerts counteracting effects on the iliac vascular bed: activation of the adrenal medulla and β-adrenergic vasodilation versus vasoconstriction mediated by neural and humoral factors.

Purinergic mechanisms of the nucleus of the solitary tract and neural cardiovascular control

OBJECTIVES

This review addresses the role of central purinergic receptors in the operation of the cardiovascular reflexes.

METHODS

Potential physiological role of purinergic receptors operating in the nucleus of the solitary tract (NTS) was assessed via comparison of the regional patterns of hemodynamic and sympathetic responses evoked by selective stimulation/inhibition of NTS purinergic receptor subtypes, with the patterns evoked by stimulation and unloading of arterial baroreceptors, and other known patterns of autonomic responses. The effects of sino-aortic denervation plus vagotomy and ionotropic glutamatergic blockade of NTS mechanisms on the patterns of the responses were also considered.

RESULTS

Selective stimulation of NTS A1 receptors with CPA evoked a pattern of regional autonomic responses consistent with inhibition of baroreflex mechanisms and facilitation/ disinhibition of chemoreflex mechanisms. Selective stimulation of NTS A(2a) receptors with CGS 21680-evoked pattern of the responses different than that evoked by stimulation of baroreflex afferents what remains in contrast to previous reports suggesting that NTS A2a receptors facilitate baroreflex transmission. The pattern of the responses was similar to that observed during hypotensive hemorrhage. Preferential, b -adrenergic iliac vasodilation evoked by stimulation of adenosine A2a receptors and preferential activation of sympathetic output to the adrenal medulla by both adenosine A1 and A2a receptors are consistent with contribution of these receptors to the defense response, stress and exercise. These observations support previous findings that NTS A1 receptors contribute to the hypothalamic defense response. The effects of stimulation and blockade of NTS P2x receptors with alpha, beta-methylene ATP and suramin, respectively, suggested that neuronally-released ATP operating via P2x receptors may be a crucial co-transmitter with glutamate in mediating baroreflex responses.

DISCUSSION

The above observations strongly suggest that purinergic receptor subtypes operating in NTS circuitry are linked to specific afferent and descending mechanisms primarily integrated in the NTS.

Differential role of nitric oxide in regional sympathetic responses to stimulation of NTS A2a adenosine receptors

Our previous studies showed that preganglionic adrenal (pre-ASNA), renal (RSNA), lumbar, and postganglionic adrenal sympathetic nerve activities (post-ASNA) are inhibited after stimulation of arterial baroreceptors, nucleus of the solitary tract (NTS), and glutamatergic and P2x receptors and are activated after stimulation of adenosine A1 receptors. However, stimulation of adenosine A2a receptors inhibited RSNA and post-ASNA, whereas it activated pre-ASNA. Because the effects evoked by NTS A2a receptors may be mediated via activation of nitric oxide (NO) mechanisms in NTS neurons, we tested the hypothesis that NO synthase (NOS) inhibitors would attenuate regional sympathetic responses to NTS A2a receptor stimulation, whereas NO donors would evoke contrasting responses from pre-ASNA versus RSNA and post-ASNA. Therefore, in chloralose/urethane-anesthetized rats, we compared hemodynamic and regional sympathetic responses to microinjections of selective A2a receptor agonist (CGS-21680, 20 pmol/50 nl) after pretreatment with NOS inhibitors Nomega-nitro-L-arginine methyl ester (10 nmol/100 nl) and 1-[2-(trifluoromethyl)phenyl]imidazole (100 pmol/100 nl) versus pretreatment with vehicle (100 nl). In addition, responses to microinjections into the NTS of different NO donors [40 and 400 pmol/50 nl sodium nitroprusside (SNP); 0.5 and 5 nmol/50 nl 3,3-bis(aminoethyl)-1-hydroxy-2-oxo-1-triazene (DETA NONOate, also known as NOC-18), and 2 nmol/50 nl 3-(2-hydroxy-2-nitroso-1-propylhydrazino)-1-propanamine (PAPA NONOate, also known as NOC-15)], the NO precursor L-arginine (10-50 nmol/50 nl), and sodium glutamate (500 pmol/50 nl) were evaluated. SNP, DETA NONOate, and PAPA NONOate activated pre-ASNA and inhibited RSNA and post-ASNA, whereas l-arginine and glutamate microinjected into the same site of the NTS inhibited all these sympathetic outputs. Decreases in heart rate and depressor or biphasic responses accompanied the neural responses. Pretreatment with NOS inhibitors reversed the normal depressor and sympathoinhibitory responses to stimulation of NTS A2a receptors into pressor and sympathoactivatory responses and attenuated the heart rate decreases; however, it did not change the increases in pre-ASNA. We conclude that NTS NO mechanisms differentially affect regional sympathetic outputs and differentially contribute to the pattern of regional sympathetic responses evoked by stimulation of NTS A2a receptors.

ATP- and adenosine-mediated signaling in the central nervous system: synaptic purinoceptors: the stage for ATP to play its "dual-role"

The studies aiming to understand the function of purinoceptors in the central nervous system (CNS), which has been explored mostly in isolated and cultured cell systems, are now at the stage of identifying their physiological and pathophysiological significance in the native organs, tissues, and whole animals. The results of our recent studies made in brain slice preparations are not in full accordance with what have been demonstrated in isolated cells, mostly due to strong interplay between ATP receptors, adenosine receptors, and ecto-nucleotidases. This suggests that these proteins form coordinated regulation systems in the native tissue, controlling the local network behaviors through regulating the balance between the effects of ATP and adenosine on synaptic transmissions. We propose that this tripartite regulation system by extracellular purines may be an important target of CNS drugs.

Lack of adenosine A1 and dopamine D2 receptor-mediated modulation of the cardiovascular effects of the adenosine A2A receptor agonist CGS 21680

Some behavioral and biochemical effects of the systemically administered adenosine A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine (CGS 21680) in rats are potentiated by adenosine A(1) receptor agonists and counteracted by dopamine D2 receptor agonists. In the present study we compared potentiating and antagonistic interactions between CGS 21680 and adenosine A(1) and dopamine D2 receptor agonists on motor activity and on cardiovascular responses (arterial blood pressure and heart rate). The motor-depressant effects produced by CGS 21680 (0.5 mg/kg, i.p.) were potentiated by the adenosine A(1) receptor agonist N(6)-cyclopentyladenosine (CPA, 0.3 mg/kg, i.p.) and counteracted by the dopamine D2 receptor agonist quinpirole (0.5 mg/kg, i.p.). In contrast, neither CPA nor quinpirole significantly modified the decrease in arterial pressure or the increase in heart rate induced by CGS 21680. However, the adenosine A(2A) receptor antagonist 3-(3-hydroxypropyl)-8-(m-methoxystyryl)-7-methyl-1-propargylxanthine phosphate disodium salt (MSX-3, 3 mg/kg, i.p.) counteracted both the motor-depressant and cardiovascular effects of CGS 21680. Therefore, the effects of the systemically administered adenosine A(2A) receptor agonist CGS 21680 on cardiovascular function, in contrast to its effects on motor behavior, appear to be independent of the effects of adenosine A(1) and dopamine D2 receptor activity.

Action potential-independent release of glutamate by Ca2+ entry through presynaptic P2X receptors elicits postsynaptic firing in the brainstem autonomic network

P2X receptors are ATP-gated channels permeable to cations including Ca(2+). In acute slices containing the nucleus of the solitary tract, in which neuronal ATP release and ATP-elicited physiological responses are demonstrated in vivo, we recorded spontaneous action potential-independent EPSCs [miniature EPSCs (mEPSCs)]. Activation of presynaptic P2X receptors with alpha,beta-methylene ATP (alphabetamATP) triggered Ca(2+)-dependent glutamate release that was resistant to blockade of voltage-dependent calcium channels but abolished by P2X receptor antagonists. mEPSCs elicited with alphabetamATP were of larger amplitude than basal mEPSCs and resulted in postsynaptic firing caused by temporal summation of miniature events. The large-amplitude mEPSCs provoked by alphabetamATP were likely to result from highly synchronized multivesicular release of glutamate at single release sites. Neither alphabetamATP nor ATP facilitated GABA release. We conclude that this facilitated release and consequent postsynaptic firing underlie the profound autonomic responses to activation of P2X receptors observed in vivo.

Neuroendocrine control of body fluid metabolism

Mammals control the volume and osmolality of their body fluids from stimuli that arise from both the intracellular and extracellular fluid compartments. These stimuli are sensed by two kinds of receptors: osmoreceptor-Na+ receptors and volume or pressure receptors. This information is conveyed to specific areas of the central nervous system responsible for an integrated response, which depends on the integrity of the anteroventral region of the third ventricle, e.g., organum vasculosum of the lamina terminalis, median preoptic nucleus, and subfornical organ. The hypothalamo-neurohypophysial system plays a fundamental role in the maintenance of body fluid homeostasis by secreting vasopressin and oxytocin in response to osmotic and nonosmotic stimuli. Since the discovery of the atrial natriuretic peptide (ANP), a large number of publications have demonstrated that this peptide provides a potent defense mechanism against volume overload in mammals, including humans. ANP is mostly localized in the heart, but ANP and its receptor are also found in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation. Blood volume expansion acts not only directly on the heart, by stretch of atrial myocytes to increase the release of ANP, but also on the brain ANPergic neurons through afferent inputs from baroreceptors. Angiotensin II also plays an important role in the regulation of body fluids, being a potent inducer of thirst and, in general, antagonizes the actions of ANP. This review emphasizes the role played by brain ANP and its interaction with neurohypophysial hormones in the control of body fluid homeostasis.

Mechanisms mediating regional sympathoactivatory responses to stimulation of NTS A(1) adenosine receptors

Selective activation of adenosine A(1) and A(2a) receptors in the subpostremal nucleus tractus solitarius (NTS) increases and decreases mean arterial pressure (MAP), respectively, and decreases heart rate (HR). We have previously shown that the decreases in MAP evoked by NTS A(2a) receptor stimulation were accompanied with differential sympathetic responses in renal (RSNA), lumbar (LSNA), and preganglionic adrenal sympathetic nerve activity (pre-ASNA). Therefore, now we investigated whether stimulation of NTS A(1) receptors via unilateral microinjection of N(6)-cyclopentyladenosine (CPA) elicits differential activation of the same sympathetic outputs in alpha-chloralose-urethane-anesthetized male Sprague-Dawley rats. CPA (0.33-330.0 pmol in 50 nl) evoked dose-dependent increases in MAP, variable decreases in HR, and differential increases in all recorded sympathetic outputs: upward arrow pre-ASNA >> upward arrow RSNA > or = upward arrow LSNA. Sinoaortic denervation + vagotomy abolished the MAP and LSNA responses, reversed the normal increases in RSNA into decreases, and significantly attenuated increases in pre-ASNA. NTS ionotropic glutamatergic receptor blockade with kynurenate sodium (4.4 nmol/100 nl) reversed the responses in MAP, LSNA, and RSNA and attenuated the responses in pre-ASNA. We conclude that afferent inputs and intact glutamatergic transmission in the NTS are necessary to mediate the pressor and differential sympathoactivatory responses to stimulation of NTS A(1) receptors.

Differential control of sympathetic outflow

With advances in experimental techniques, the early views of the sympathetic nervous system as a monolithic effector activated globally in situations requiring a rapid and aggressive response to life-threatening danger have been eclipsed by an organizational model featuring an extensive array of functionally specific output channels that can be simultaneously activated or inhibited in combinations that result in the patterns of autonomic activity supporting behavior and mediating homeostatic reflexes. With this perspective, the defense response is but one of the many activational states of the central autonomic network. This review summarizes evidence for the existence of tissue-specific sympathetic output pathways, which are likely to include distinct populations of premotor neurons whose target specificity could be assessed using the functional fingerprints developed from characterizations of postganglionic efferents to known targets. The differential responses in sympathetic outflows to stimulation of reflex inputs suggest that the circuits regulating the activity of sympathetic premotor neurons must have parallel access to groups of premotor neurons controlling different functions but that these connections vary in their ability to influence different sympathetic outputs. Understanding the structural and physiological substrates antecedent to premotor neurons that mediate the differential control of sympathetic outflows, including those to noncardiovascular targets, represents a challenge to our current technical and analytic approaches.