Localization of cholinergic neurons involved in the cardiovascular response to intrathecal injection of carbachol

Differential muscarinic receptor gene expression levels in the ventral medulla of spontaneously hypertensive and Wistar-Kyoto rats: role in sympathetic baroreflex function

OBJECTIVE

We demonstrated previously that central muscarinic cholinergic receptor (mAChR) activation increased splanchnic sympathetic nerve activity and sympathetic baroreflex function via activation of mAChR in the rostral ventrolateral medulla (RVLM), and we found that some RVLM bulbospinal neurons contain muscarinic M2R mRNA. Here, we examined the gene expression, cellular distribution and functional role of muscarinic receptors in the RVLM in spontaneously hypertensive rats (SHR) compared with Wistar-Kyoto (WKY) rats.

METHOD AND RESULTS

Using the sensitive technique of quantitative real time reverse transcriptase-PCR, M2R mRNA level was elevated two-fold (P<0.05) and M4R mRNA was downregulated two-fold (P<0.001), with all other receptors expressed at similar levels, in the rostral ventral medulla of SHR compared with WKY. Bulbospinal, but not catecholaminergic neurons, in the RVLM expressed M2R mRNA (M2RR), and similar numbers were found in the RVLM of SHR and WKY. Could elevated M2R within individual neurons or enhanced presynaptic activity reflects enhanced cholinergic effects in the RVLM? Activation of central mAChR using oxotremorine evoked a larger increase in mean arterial pressure in SHR compared with WKY (P<0.01); however, oxotremorine-induced increases in splanchnic sympathetic nerve activity, and sympathetic baroreflex function were similar in SHR and WKY.

CONCLUSION

These data indicate that enhanced pressor responses in SHR, following centrally mediated mAChR activation, are not associated with RVLM-mediated constriction of the splanchnic circulation or effects on the sympathetic baroreflex, but could reflect modified mAChR gene expression elsewhere. RVLM-dependent splanchnic sympathetic nerve activity effects, evoked by mAChR activation, are not mediated by the differential M2/M4 receptor mRNA levels identified in SHR compared with WKY.

Agatoxin-IVA-sensitive calcium channels mediate the presynaptic and postsynaptic nicotinic activation of cardiac vagal neurons

Whole cell currents and miniature glutamatergic synaptic events (minis) were recorded in vitro from cardiac vagal neurons in the nucleus ambiguus using the patch-clamp technique. We examined whether voltage-dependent calcium channels were involved in the nicotinic excitation of cardiac vagal neurons. Nicotine evoked an inward current, increase in mini amplitude, and increase in mini frequency in cardiac vagal neurons. These responses were inhibited by the nonselective voltage-dependent calcium channel blocker Cd (100 microM). The P-type voltage-dependent calcium channel blocker agatoxin IVA (100 nM) abolished the nicotine-evoked responses. Nimodipine (2 microM), an antagonist of L-type calcium channels, inhibited the increase in mini amplitude and frequency but did not block the ligand gated inward current. The N- and Q-type voltage-dependent calcium channel antagonists conotoxin GVIA (1 microM) and conotoxin MVIIC (5 microM) had no effect. We conclude that the presynaptic and postsynaptic facilitation of glutamatergic neurotransmission to cardiac vagal neurons by nicotine involves activation of agatoxin-IVA-sensitive and possibly L-type voltage-dependent calcium channels. The postsynaptic inward current elicited by nicotine is dependent on activation of agatoxin-IVA-sensitive voltage-dependent calcium channels.

Spinal muscarinic cholinergic and nitric oxide systems in cardiovascular regulation

Pharmacological activation of muscarinic receptors located in the thoracic spinal cord evokes a marked increase in blood pressure and heart rate. We have previously demonstrated that the cardiovascular response to stimulation of spinal cord muscarinic cholinergic receptors is dependent upon a pharmacologically described ascending spino-bulbar pathway. The purpose of the study was to determine whether the blood pressure and heart rate responses to intrathecal (i.t.) injection of the muscarinic cholinergic receptor agonist carbachol are mediated by a local nitric oxide (NO)-generating system. Freely moving rats were previously prepared with chronic indwelling i.t. and intra-arterial catheters. Both the pressor and tachycardic responses produced by i.t. injection of carbachol were inhibited in a dose-dependent manner by i.t. pre-treatment with the NO synthase inhibitor N-omega-Nitro-L-arginine methylester (L-NAME). To confirm the site of action of the drugs employed in conscious rats, a separate group of rats was anesthetized, and using surgical procedures previously developed in this laboratory, drug distribution was limited specifically to the lower thoracic spinal cord. When carbachol was administered by i.t. injection and localized to the lower thoracic area, muscarinic cholinergic receptor stimulation again produced a marked pressor response, but without the accompanying tachycardia. The ability of N-omega-Nitro-L-arginine methylester (L-NAME) to inhibit the pressor response to carbachol in conscious rats was confirmed in anesthetized rats, although higher doses of L-NAME than those employed in conscious rats were required. L-NAME-induced inhibition of the carbachol-evoked pressor response was reversed by the L-, but not the D-isomer, of arginine. Moreover, i.t. pre-treatment with Methylene blue, that interferes with NO production and function, effectively inhibited the expression of the pressor response to i.t. injection of carbachol. The 'anti-muscarinic' action of L-NAME was not due to a direct interaction with spinal muscarinic receptors, as L-NAME did not significantly displace [3H]methyl-scopolamine from spinal cord membranes in vitro. The results of this study support the hypothesis that spinal muscarinic cholinergic receptors participate in a sympathoexcitatory pathway that interacts either directly or indirectly with a local NO-generating system involved in the regulation of blood pressure.

Spinal NMDA receptor--nitric oxide mediation of the expression of morphine withdrawal symptoms in the rat

Previous studies in this laboratory have demonstrated that cholinergic receptors within the spinal cord play an important role in the expression of naloxone-precipitated withdrawal symptoms in the morphine-dependent rat. Related cardiovascular studies in non-dependent animals have demonstrated that this spinal cholinergic system is linked to a glutamatergic, NMDA pressor pathway which also involves the participation of a nitric oxide (NO) generating system. The purpose of this study was to determine whether spinal NMDA receptors and/or NO are involved in the expression of morphine withdrawal symptoms. Rats bearing previously implanted intrathecal (IT) catheters were dependent on morphine following chronic i.a. infusion of increasing doses over 5 days. Naloxone (0.5 mg/kg) was administered via the i.a. line to precipitate withdrawal; and both cardiovascular and behavioral symptoms were recorded over 60 min. Pretreatment 20 min before naloxone with IT injection of either of the NMDA receptor antagonists, MK-801 or AP-7 (100-200 nmol), produced a significant reduction in the expression of both the cardiovascular and behavioral symptoms of up to about 60%. IT pretreatment with the NO synthase inhibitor L-NAME--a methyl ester derivative of L-arginine, also produced a dose-dependent, L-arginine reversible inhibition of the cardiovascular (mainly the pressor) component of withdrawal, but had no significant effect on the expression of behavioral signs. In contrast, IT pretreatment with L-NOARG and L-NMMA, non-ester analogs of L-arginine, significantly inhibited the expression of the behavioral signs of withdrawal but did not alter the pressor component. A combined pretreatment with L-NAME and L-NOARG resulted in suppression of both pressor and behavioral components of withdrawal. The anti-withdrawal actions of either class of NO synthase inhibitor could not be attributed to blockade of local muscarinic receptors. These findings are consistent with a role for both spinal NMDA receptors and a NO generating system in the expression of both the behavioral and autonomic components of naloxone-precipitated withdrawal. They also suggest that different structural analogs of L-arginine have different profiles of activity in this regard--opening the possibility that different isozymes of NO synthase located within the same spinal region mediate different physiological or behavioral functions.