Effects of omega-conotoxin GVIA on cardiac sympathetic nerve function

The organ-protective effect of N-type Ca(2+) channel blockade

The six subtypes of voltage-dependent Ca(2+) channels (VDCCs) mediate a wide range of physiological responses. N-type VDCCs (NCCs) were originally identified as a high voltage-activated Ca(2+) channel selectively blocked by omega-conotoxin (ω-CTX)-GVIA. Predominantly localized in the nervous system, NCCs are key regulators of neurotransmitter release. Both pharmacological blockade with ω-CTX-GVIA and, more recently, mice lacking CNCNA1B, encoding the α1B subunit of NCC, have been used to assess the physiological and pathophysiological functions of NCCs, revealing in part their significant roles in sympathetic nerve activation and nociceptive transmission. The evidence now available indicates that NCCs are a potentially useful therapeutic target for the treatment of several pathological conditions. Efforts are therefore being made to develop effective NCC blockers, including both synthetic ω-CTX-GVIA derivatives and small-molecule inhibitors. Cilnidipine, for example, is a dihydropyridine L-type VDCC blocking agent that also possesses significant NCC blocking ability. As over-activation of the sympathetic nervous system appears to contribute to the pathological processes underlying cardiovascular, renal and metabolic diseases, NCC blockade could be a useful approach to treating these ailments. In this review article, we provide an overview of what is currently known about the physiological and pathophysiological activities of NCCs and the potentially beneficial effects of NCC blockade in several disease conditions, in particular cardiovascular diseases.

Effects of Ca2+ channel antagonists on nerve stimulation-induced and ischemia-induced myocardial interstitial acetylcholine release in cats

Although an axoplasmic Ca2+ increase is associated with an exocytotic acetylcholine (ACh) release from the parasympathetic postganglionic nerve endings, the role of voltage-dependent Ca2+ channels in ACh release in the mammalian cardiac parasympathetic nerve is not clearly understood. Using a cardiac microdialysis technique, we examined the effects of Ca2+ channel antagonists on vagal nerve stimulation- and ischemia-induced myocardial interstitial ACh releases in anesthetized cats. The vagal stimulation-induced ACh release [22.4 nM (SD 10.6), n = 7] was significantly attenuated by local administration of an N-type Ca2+ channel antagonist ω-conotoxin GVIA [11.7 nM (SD 5.8), n = 7, P = 0.0054], or a P/Q-type Ca2+ channel antagonist ω-conotoxin MVIIC [3.8 nM (SD 2.3), n = 6, P = 0.0002] but not by local administration of an L-type Ca2+ channel antagonist verapamil [23.5 nM (SD 6.0), n = 5, P = 0.758]. The ischemia-induced myocardial interstitial ACh release [15.0 nM (SD 8.3), n = 8] was not attenuated by local administration of the L-, N-, or P/Q-type Ca2+ channel antagonists, by inhibition of Na+/Ca2+ exchange, or by blockade of inositol 1,4,5-trisphosphate [Ins( 1 , 4 , 5 )P3] receptor but was significantly suppressed by local administration of gadolinium [2.8 nM (SD 2.6), n = 6, P = 0.0283]. In conclusion, stimulation-induced ACh release from the cardiac postganglionic nerves depends on the N- and P/Q-type Ca2+ channels (with a dominance of P/Q-type) but probably not on the L-type Ca2+ channels in cats. In contrast, ischemia-induced ACh release depends on nonselective cation channels or cation-selective stretch activated channels but not on L-, N-, or P/Q type Ca2+ channels, Na+/Ca2+ exchange, or Ins( 1 , 4 , 5 )P3 receptor-mediated pathway.

PACAP modulation of the colon-inferior mesenteric ganglion reflex in the guinea pig

We investigated the effect of pituitary adenylate cyclase activating peptide (PACAP) on the colon-inferior mesenteric ganglion (IMG) reflex loop in vitro. PACAP27 and PACAP38 applied to the IMG caused a prolonged depolarization and intense generation of fast EPSPs and action potentials in IMG neurones. Activation of PACAP-preferring receptors (PAC1-Rs) with the selective agonist maxadilan or vasoactive intestinal peptide (VIP)/PACAP (VPAC) receptors with VIP produced similar effects whereas prior incubation of the IMG with selective PAC1-R antagonists PACAP6-38 and M65 inhibited the effects of PACAP. Colonic distension evoked a slow EPSP in IMG neurones that was reduced in amplitude by prolonged superfusion of the IMG with either PACAP27, maxidilan, PACAP6-38, M65 or VIP. Activation of IMG neurones by PACAP27 or maxadilan resulted in an inhibition of ongoing spontaneous colonic contractions. PACAP-LI was detected in nerve trunks attached to the IMG and in varicosities surrounding IMG neurones. Cell bodies with PACAP-LI were present in lumbar 2-3 dorsal root ganglia and in colonic myenteric ganglia. Colonic distension evoked release of PACAP peptides in the IMG as measured by radioimmunoassay. Volume reconstructed images showed that a majority of PACAP-LI, VIP-LI and VAChT-LI nerve endings making putative synaptic contact onto IMG neurones and a majority of putative receptor sites containing PAC1-R-LI and nAChR-LI on the neurones were distributed along secondary and tertiary dendrites. These results suggest involvement of a PACAP-ergic pathway, operated through PAC1-Rs, in controlling the colon-IMG reflex.

Effects of Ca2+ channel antagonists on acetylcholine and catecholamine releases in the in vivo rat adrenal medulla

To elucidate the types of voltage-dependent Ca(2+) channels controlling ACh and catecholamine releases in the in vivo adrenal medulla, we implanted microdialysis probes in the left adrenal medulla of anesthetized rats and investigated the effects of Ca(2+) channel antagonists on ACh, norepinephrine, and epinephrine releases induced by nerve stimulation. The dialysis probes were perfused with Ringer solution containing a cholinesterase inhibitor, neostigmine. The left splanchnic nerves were electrically stimulated at 2 and 4 Hz before and after intravenous administration of Ca(2+) channel antagonists. omega-Conotoxin GVIA (an N-type Ca(2+) channel antagonist, 10 microg/kg) inhibited ACh release at 2 and 4 Hz by approximately 40%, norepinephrine release at 4 Hz by approximately 50%, and epinephrine release at 2 and 4 Hz by approximately 45%. A fivefold higher dose of omega-conotoxin GVIA (50 microg/kg) did not further inhibit these releases. omega-Conotoxin MVIIC (a P/Q-type Ca(2+) channel antagonist, 50 microg/kg) inhibited ACh and epinephrine releases at 4 Hz by approximately 30%. Combined omega-conotoxin GVIA (50 microg/kg) and MVIIC (250 microg/kg) inhibited ACh release at 2 and 4 Hz by approximately 70% and norepinephrine and epinephrine releases at 2 and 4 Hz by approximately 80%. Nifedipine (an L-type Ca(2+) channel antagonist, 300 and 900 microg/kg) did not change ACh release at 2 and 4 Hz; however, nifedipine (300 microg/kg) inhibited epinephrine release at 4 Hz by 20%, and nifedipine (900 microg/kg) inhibited norepinephrine and epinephrine releases at 4 Hz by 30%. In conclusion, both N- and P/Q-type Ca(2+) channels control ACh release on preganglionic splanchnic nerve endings while L-type Ca(2+) channels do not. L-type Ca(2+) channels are involved in norepinephrine and epinephrine releases on chromaffin cells.

Effects of ketamine on exocytotic and non-exocytotic noradrenaline release

To characterise ketamine-induced sympathomimetic action, we examined the effects of ketamine on in vivo cardiac sympathetic nerve endings function. Using adult cats given anaesthesia with pentobarbital, dialysis probes were implanted in the left ventricular myocardium and dialysate noradrenaline (NA) concentrations were measured as an indicator of NA output at the cardiac sympathetic nerve endings. Ketamine was locally administered through the dialysis probe, and dialysate NA responses were obtained in the following conditions. (1) In the resting state, ketamine (10 mM) increased dialysate NA concentration. This increase in dialysate NA was not altered by addition of omega-conotoxin GVIA (N-type Ca(2+) channel blocker) or desipramine (membrane NA uptake blocker). (2) Sympathetic activation by electrical stimulation of the stellate ganglia (ES-SG; exocytotic NA release): ES-SG caused an increase in dialysate NA, which was further augmented by addition of desipramine. During co-administration of desipramine and ketamine, dialysate NA response to ES-SG was smaller than with desipramine alone. Further, there was no significant difference in the dialysate NA response to ES-SG between ketamine and ketamine + desipramine. These data suggested that both exocytosis and NA uptake function were impaired by ketamine. (3) Non-exocytotic NA release by ouabain: ouabain caused increases in dialysate NA. These increases in dialysate NA were suppressed by ketamine, which impaired the membrane outward NA transport evoked by ouabain. We conclude that ketamine impaired exocytotic and non-exocytotic NA release. However, ketamine spontaneously evoked NA efflux that was independent of exocytosis and insensitive to NA transporter.

Differential involvement of conotoxin-sensitive mechanisms in neurogenic vasodilatation responses: effects of age

During aging there is a decline in sensory nerve function that is associated with reduced neurogenic inflammation and poor wound repair. The cellular mechanism(s) responsible for this decline in function with age is not well understood. We previously reported that sensory nerves in aged rats release sensory neuropeptides preferentially in response to low-frequency (5 Hz) as compared with higher-frequency (15 Hz) antidromic electrical stimulation, and that low-frequency transcutaneous electrical nerve stimulation accelerates wound healing. The present study investigates possible mechanisms for this preferential response. Using laser Doppler techniques, we have measured changes in blood flow in the base of vacuum-induced blisters induced in the rat hind footpad of young and old animals in response to low-frequency (5 Hz) or high-frequency (15 Hz) electrical stimulation (20 V, 2 ms for 1 minute) of the sciatic nerve. The relative contributions of the sensory neuropeptides, substance P and calcitonin gene-related peptide (CGRP), and of N-type voltage-gated calcium channels to the vascular responses were assessed by using the specific receptor antagonists RP67580, which is 2-(1-imino-2-(2 methoxy phyenyl) ethyl)-7,7 diphenyl-4 perhydroisoindolone-(3aR, 7aR); CGRP(8-37); and omega-conotoxin GVIA (Conus geographus), respectively. The results showed a greater involvement of substance P at high-frequency electrical stimulation and of CGRP at low-frequency stimulation. Our finding that omega-conotoxin-sensitive N-type calcium channel function was preserved with age and was only involved in the vascular response to low-frequency electrical stimulation could explain our previous report demonstrating beneficial effects of low-frequency transcutaneous electrical nerve stimulation to wound repair in aged animals. The current results have important practical implications for improving tissue repair in the aged.

Functional disorders of the sympathetic nervous system in mice lacking the alpha 1B subunit (Cav 2.2) of N-type calcium channels

N-type voltage-dependent Ca(2+) channels (VDCCs), predominantly localized in the nervous system, have been considered to play an essential role in a variety of neuronal functions, including neurotransmitter release at sympathetic nerve terminals. As a direct approach to elucidating the physiological significance of N-type VDCCs, we have generated mice genetically deficient in the alpha(1B) subunit (Ca(v) 2.2). The alpha(1B)-deficient null mice, surprisingly, have a normal life span and are free from apparent behavioral defects. A complete and selective elimination of N-type currents, sensitive to omega-conotoxin GVIA, was observed without significant changes in the activity of other VDCC types in neuronal preparations of mutant mice. The baroreflex response, mediated by the sympathetic nervous system, was markedly reduced after bilateral carotid occlusion. In isolated left atria prepared from N-type-deficient mice, the positive inotropic responses to electrical sympathetic neuronal stimulation were dramatically decreased compared with those of normal mice. In contrast, parasympathetic nervous activity in the mutant mice was nearly identical to that of wild-type mice. Interestingly, the mutant mice showed sustained elevation of heart rate and blood pressure. These results provide direct evidence that N-type VDCCs are indispensable for the function of the sympathetic nervous system in circulatory regulation and indicate that N-type VDCC-deficient mice will be a useful model for studying disorders attributable to sympathetic nerve dysfunction.

Either desipramine or TMB-8 suppresses cyanide-induced norepinephrine efflux from in vivo cardiac sympathetic nerves of cats

To investigate the effect of hypoxia on endogenous norepinephrine (NE) release from cardiac sympathetic nerve ending, we administered sodium cyanide (NaCN) for 30 min into the myocardial interstitial space through a dialysis probe and measured dialysate NE levels. During the NaCN perfusion, a marked and concentration-dependent increase in dialysate NE was observed. This cyanide-induced NE response was suppressed by pretreatment with despiramine (a membraneous NE transport inhibitor). Furthermore, the cyanide-induced NE response was suppressed by pretreatment with TMB-8 (intracellular Ca(2+) antagonist) but unaffected by omega-conotoxin GVIA (NE releasing inhibitor). Our data suggest that two (desipramine or TMB-8 suppressive) mechanisms contributed to the amount of NE efflux induced by cyanide in in vivo cardiac sympathetic nerve.

Tyramine-induced endogenous noradrenaline efflux from in situ cardiac sympathetic nerve ending in cats

With the use of dialysis technique, the effects of tyramine on in situ cardiac sympathetic nerve endings were examined in anaesthetized cats. Dialysis probes were implanted in the left ventricular myocardium, and the concentration of dialysate noradrenaline (NA) served as an indicator of NA output at the cardiac sympathetic nerve ending. Locally applied tyramine (600 microM) increased dialysate NA levels from 17 +/- 1 (pg mL-1) to 3466 +/- 209 (pg mL-1). Pretreatment with reserpine (vesicle transport NA blocker 1 microM) did not affect tyramine-induced NA efflux. The tyramine-induced NA efflux was augmented by pretreatment with pargyline (1 mM) but suppressed by pargyline (10 mM). Pretreatment with alpha-methyl-tyrosine suppressed NA efflux evoked by tyramine. These pretreatments did not affect the time course of NA efflux but only altered peak height of NA efflux. The efflux of NA evoked by tyramine was not associated with any reduction of dihydroxyphenylglycol (DHPG). In contrast, in the pretreatment with reserpine, the efflux of NA was associated with a reduction of DHPG. This result suggests that NA graduation between axoplasm and stored vesicle contributes to maintaining the axoplasmic NA level during carrier-mediated outward NA transport. The tyramine-induced NA efflux provides a close reflection of the NA content at the nerve ending. With the use of dialysis, this experimental model is suitable for studying the mechanism of sympathomimetic amine-induced neurotransmitter efflux.

Effects of ouabain on in situ cardiac sympathetic nerve endings

Using dialysis technique, the effects of ouabain on in situ cardiac sympathetic nerve endings were examined in anesthetized cats. Dialysis probes were implanted in the left ventricular myocardium, and the concentration of dialysate norepinephrine (NE) was used as an indicator of NE output at the cardiac sympathetic nerve ending. Locally applied ouabain dose-dependently (1, 10, 100 microM) increased dialysate NE levels. This finding suggested that ouabain causes an increase in NE efflux without any requirement for prior mobilization of NE from vesicular stores. Transection of sympathetic nerves innervating the heart, was without effect on the ouabain (100 microM)-induced increase in NE efflux. Pretreatment with a Ca2+-channel blocker, omega-conotoxin GVIA (10 microg/kg i.v.) suppressed the ouabain-induced NE efflux. These data suggested that ouabain opened N-type calcium channels coupled to NE release without centrally mediated neural transmission. Furthermore, ouabain-induced NE efflux was suppressed by pretreatment with desipramine (neuronal NE uptake inhibitor, 100 microM). Our data suggest that the two mechanisms (exocytosis and carrier-mediated outward transport), to the same extent, contributed to the amount of NE efflux evoked by ouabain in in situ cardiac sympathetic nerve endings.

Norepinephrine efflux evoked by potassium chloride in cat sympathetic nerves: dual mechanism of action

Using a dialysis technique, prominent efflux of norepinephrine (NE) from cardiac sympathetic nerve endings was observed under local administration of potassium chloride (KCl, 100 mM). KCl induced NE efflux was suppressed by omega-conotoxin GVIA or desipramine but residual efflux of NE was still detectable. In the presence of omega-conotoxin GVIA, KCl induced efflux of NE was augmented by pretreatment with reserpine, indicating that this efflux of NE was derived from axoplasma with neurotransporter. These data suggest that a KCl induced brisk increase in dialysate NE levels might occur as a consequence of exocytotic NE release and carrier mediated outward NE transport from nerve endings.