Participation of noradrenergic neurotransmission in the suppression by substance P of alpha 2-adrenoceptors at the nucleus reticularis gigantocellularis involved in central cardiovascular regulation in the rat

Alpha 2-adrenoceptor modulates the release of acetylcholine from the rostral ventrolateral medulla in response to morphine

The present study examined the role of the noradrenergic system in the modulation of acetylcholine (ACh) release in the rostral ventrolateral medulla (RVLM) using in vivo microdialysis of morphine. The basal level of ACh was 325.0 +/- 21.1 fmol/20 microl/15 min in the presence of neostigmine (10 microM). Intraperitoneal (i.p.) administration of 5 and 10 mg/kg morphine significantly increased ACh release by the RVLM. This enhancement was reversed by naloxone (1 mg/kg, i.p.). In addition, pretreatment with yohimbine (0.5 mg/kg, i.p.) or prazosin (0.2 mg/kg, i.p.) attenuated the systemic morphine-induced release of ACh in the RVLM. However, propranolol (0.2 mg/kg, i.p.) did not affect the morphine-induced ACh release. The addition of morphine (10(-4) M) to the perfusion medium increased the ACh release by 72.4% of the predrug values. The increased ACh release induced by local application of morphine was attenuated by pretreatment with yohimbine, but not prazosin. These findings suggest that morphine exerts an indirect stimulatory effect on the release of ACh by the RVLM and that the morphine-induced increase in ACh release is modulated by alpha2-adrenoceptors in freely moving rats.

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.

Parabrachial nucleus induces suppression of baroreflex bradycardia by the release of glutamate in the rostral ventrolateral medulla of the rat

The involvement of glutamatergic neurotransmission in the rostral ventrolateral medulla (RVLM) in the suppression of baroreflex bradycardia by the parabrachial nucleus (PBN) was investigated. Repeated electrical activation of the PBN increased the concentration of glutamate in the dialysate collected from the RVLM. The same stimulation also suppressed baroreflex bradycardia in response to transient hypertension evoked by phenylephrine (5 microg/kg, intravenously). Microinfusion of L-glutamate (10, 50 or 100 microM) via the microdialysis probe into the RVLM dose-dependently elicited a significant inhibition of baroreflex bradycardia that paralleled the concentration and time course of the PBN-elicited elevation in extracellular glutamate in the RVLM. The suppression of baroreflex bradycardia elicited by microinjection of L-glutamate (1 nmol) into the RVLM was appreciably reversed by coinjection of the NMDA receptor antagonist, dizocilpine (500 pmol), or the non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2, 3-dione (50 pmol). These results suggest that an increase in the extracellular concentration of glutamate and activation of both NMDA and non-NMDA receptors in the RVLM may mediate the suppression of baroreflex bradycardia by activation of the PBN.

Noradrenergic neurotransmission at PVN in locus ceruleus-induced baroreflex suppression in rats

We investigated the role of ascending noradrenergic projections from the locus ceruleus (LC) to the paraventricular nucleus (PVN) of the hypothalamus in LC-induced suppression of the baroreceptor reflex (BRR) response in adult Sprague-Dawley rats maintained under pentobarbital anesthesia. On the basis of in vivo microdialysis and high-performance liquid chromatography-electrochemical detection, microinjection of L-glutamate (5 nmol) into the LC resulted in a site-specific increase in norepinephrine (NE) concentration in the dialysate collected from the parvocellular subnucleus of the PVN. The temporal course of this increase in extracellular NE concentration in the PVN coincided with the time course of inhibition elicited by the LC on the BRR response. Microinfusion of NE (10, 50, or 100 nM) into the parvocellular subnucleus of the PVN by reverse microdialysis also promoted a parallel increase in NE at the PVN and a reduction in the BRR response. Inhibition of the BRR response induced by microinjection into the PVN of the alpha 1-adrenoceptor agonist phenylephrine (10 nmol) or chemical activation of the LC was reversed by bilateral PVN microinjection of prazosin (100 pmol). However, local application to the PVN of the alpha 2- or beta-adrenoceptor agonist guanabenz (10 nmol) or isoproterenol (10 nmol) was ineffective. Our results suggest that NE released from the LC-PVN noradrenergic projection may participate in LC-induced suppression of the BRR response by activating the alpha 1-adrenoceptors at the parvocellular subnucleus of the PVN.

Participation of presynaptic noradrenergic fibers in the suppression of alpha 2-adrenoceptor activity by substance P at the nucleus reticularis gigantocellularis of the rat

We applied reverse microdialysis and high performance liquid chromatography (HPLC) analysis to evaluate the participation of presynaptic noradrenergic neurotransmission in the suppression by substance P (SP) of alpha 2-adrenoceptor activity at the nucleus reticularis gigantocellularis (NRGC) in Sprague-Dawley rats anesthetized with pentobarbital sodium. Microinfusion of SP (600 microM/min) into the NRGC through a stereotaxically positioned microdialysis probe attenuated the hypotensive and bradycardiac actions of the alpha 2-adrenoceptor agonist, guanabenz (100 micrograms/kg, i.v.). This inhibitory effect correlated positively with the time course of elevation in the estimated extracellular concentrations of SP and norepinephrine (NE) in the NRGC. Direct microinfusion of NE (50 nM/min) into the NRGC also lessened the cardiosuppressant effects of guanabenz. These circulatory and NE responses to SP were, however, significantly blunted in rats in which the noradrenergic innervation in the NRGC was depleted with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine hydrochloride (DSP4) pretreatment. Microinfusion of NE into the NRGC of DSP4-pretreated animals restored the attenuation of guanabenz-induced cardiovascular suppression. These results suggest that SP may depress the activity of alpha 2-adrenoceptor at the NRGC that are involved in circulatory regulation by increasing the extracellular concentration of NE via a presynaptic modulation of noradrenergic neurotransmission.