Studies on the location of catecholamine receptors in canine sympathetic ganglia

Pharmacological evidence that dopamine inhibits the cardioaccelerator sympathetic outflow via D2-like receptors in pithed rats

It has been suggested that N,N-di-n-propyl-dopamine (dopamine analogue) decreased heart rate in rats through stimulation of dopamine receptors. Nevertheless, the role of prejunctional dopamine D1/2-like receptors or even α2-adrenoceptors to mediate cardiac sympatho-inhibition induced by dopamine remains unclear. Hence, this study identified the pharmacological profile of the cardiac sympatho-inhibition to dopamine in pithed rats. Male Wistar rats were pithed and prepared to stimulate the cardiac sympathetic outflow or to receive i.v. bolus of exogenous noradrenaline. I.v. continuous infusions of dopamine (endogenous ligand) or quinpirole (D2-like agonist) dose-dependently inhibited the tachycardic responses to sympathetic stimulation, but not those to exogenous noradrenaline. In contrast, SKF-38393 (100 μg/kg∙min, D1-like agonist) failed to modify both of these responses. The sympatho-inhibition to dopamine (1.8 μg/kg∙min) or quinpirole (100 μg/kg∙min): i) remained unaltered after saline or the antagonists SCH-23390 (D1-like, 300 μg/kg) and rauwolscine (α2-adrenoceptors, 300 μg/kg); and ii) was significantly antagonized by raclopride (D2-like, 300 μg/kg). These antagonists, at the above doses, failed to modify the sympathetically-induced tachycardic responses. The above results suggest that the inhibition of the cardiac sympathetic outflow to dopamine and quinpirole is primarily mediated by prejunctional D2-like receptors but not D1-like receptors or α2-adrenoceptors.

The dopamine receptors mediating inhibition of the sympathetic vasopressor outflow in pithed rats: pharmacological correlation with the D(2) -like type

This study investigated in pithed rats whether dopamine can inhibit the sympathetic vasopressor outflow and analysed the pharmacological profile of the receptors involved. Male Wistar pithed rats were pre-treated with intravenous (i.v.) bolus injections of gallamine (25 mg/kg) and desipramine (50 μg/kg). The vasopressor responses to electrical stimulation of the sympathetic vasopressor outflow (0.03-3 Hz; 50 V and 2 msec.) were analysed before and during i.v. continuous infusions of the agonists dopamine (endogenous ligand), SKF-38393 (D(1) -like) or quinpirole (D(2) -like). If inhibition was produced by any agonist, then its capability to inhibit the vasopressor responses to i.v. bolus injections of exogenous noradrenaline (0.03-3 μg/kg) was also investigated. Dopamine (3-100 μg/kg min.) inhibited the vasopressor responses to both electrical stimulation and noradrenaline. In contrast, SKF-38393 (10-100 μg/kg min.) failed to inhibit the vasopressor responses to electrical stimulation; whereas quinpirole (0.1-30 μg/kg min.) inhibited the vasopressor responses to electrical stimulation but not those to noradrenaline. The sympatho-inhibition by quinpirole (1 μg/kg min.) remained unaltered after i.v. SCH 23390 (300 and 1000 μg/kg; D(1) -like receptor antagonist), but was abolished after i.v. raclopride (1000 μg/kg; D(2) -like receptor antagonist). These doses of antagonists did not modify per se the sympathetically-induced vasopressor responses. In conclusion, quinpirole-induced inhibition of the sympathetic vasopressor outflow is primarily mediated by activation of dopamine D(2) -like receptors.

The mechanism of alpha2-adrenergic inhibition of sympathetic ganglionic transmission

Alpha2-adrenergic agonists produce analgesia and reduce hemodynamic stress through central and peripheral mechanisms, but the effect of adrenergic agonists on pre- and postganglionic sites has not yet been clarified. In this study, we examined the effects of dexmedetomidine (DMT), an alpha2-agonist, on neural conduction and neurotransmitter release in sympathetic ganglia. The stellate ganglia from 48 mongrel dogs were isolated, desheathed, and superfused with Krebs' solution. Compound action potentials were evoked, and chromatography was used to detect acetylcholine released by preganglionic stimulation in the presence or absence of DMT. To further elucidate the mechanism of alpha2 effects, DMT was applied in combination with the alpha2-antagonist atipamezole (AT) or the imidazoline antagonist idazoxan (ID). In other experiments, DMT was applied in the presence of exogenous nicotinic stimulation with 1,1-dimethyl-4-phenylpiperazinium iodide or muscarinic stimulation with (+)cis-dioxolane. DMT dose-dependently inhibited synaptic transmission with a 50% effective dose of 71.6 (26.0-174.3) microM. Neurotransmitter release was reduced 25% by 70 microM DMT during low-frequency (0.4 Hz) stimulation, but this effect was abolished at higher frequency (5 Hz) stimulation. AT but not ID blocked the inhibitory action of DMT. DMT inhibited the excitatory postsynaptic response to exogenous muscarinic stimulation but not nicotinic stimulation. These results indicate that alpha2-receptor activation depresses ganglionic transmission through postsynaptic inhibition of muscarinic stimulation, although reduction of neurotransmitter release through a presynaptic autofeedback mechanism is also involved.

IMPLICATIONS

This article provides novel insights into the mechanism of drug action of alpha2-receptor agonists in the sympathetic ganglia of dogs by directly measuring the relative contribution of pre- and postganglionic receptors. Our study indicates that the central sympatholytic effects of alpha2-adrenoceptor stimulation are augmented by peripheral inhibition of ganglionic transmission.