Further studies on the blockade of 5-HT depolarizations of rabbit vagal afferent and sympathetic ganglion cells by MDL 72222 and other antagonists

Abstract

The blocking action of MDL 72222 (1αH, 3α, 5αH-tropan-3-yl-3, 5-dichlorobenzoate at 5-hydroxytryptamine (5-HT) receptors on nodose (NG) and superior cervical ganglia (SCG) has been investigated further. The sucrose-gap technique was used to record potential changes from populations of neurones. The surmountable blockade induced by small concentrations of the antagonist was quantified and the blocking potency compared with that of a number of other compounds. In nodose ganglia three 4–5 point dose-response (DR) curves were established, using bolus injections of 5-HT (5–80 nmol). The mean amplitude of the response to 80 nmol was 4.18 ± 0.53 mV and the ED 50 was 18.2 nmol. Second and 3rd dose-response curves showed small displacements to the right, indicating a slight reduction in sensitivity. In superior cervical ganglia responsiveness was less. Amounts of 5-HT ranging from 20 to 320 nmol evoked dose-related depolarizations. The mean amplitude of the response evoked by 320 nmol 5-HT was 1.7 ± 0.14 mV. Three 4–5 point dose-response curves could be elicited from a single ganglion. The ED 50 was 55.8 nmol. Initial, 2nd and 3rd dose-response curves could be superimposed, there being no significant rightward shift. The results confirm that MDL 72222 is a potent, selective antagonist at 5-HT receptors in nodose and superior cervical ganglia. In the nodose ganglion, after equilibration for 1 hr with 10 −8 or 10 −7 M MDL 72222, dose-response curves for 5-HT showed rightward, parallel shifts. In contrast, 10 −6 M MDL 72222 or prolonged exposure (3–4 hr) to 10 −8, 10 −7 or 10 −6 M caused larger rightward shifts of the dose-response curves and depressed the maximum responses. In the superior cervical ganglion, equilibration for 1 hr with concentrations of 10 −8 or 10 −7 M produced effects on the dose-response curves similar to those seen in the nodose ganglion, but longer exposures (3–4 hr) did not depress the maximum. Apparent pA 2 values were determined from individual experiments on both the nodose and superior cervical ganglia, where MDL 72222 (10 −7 M or less, for 1 hr) caused parallel or near parallel shifts of dose-response curves. In the nodose ganglion the apparent pA 2 was 7.7 ± 0.1, while in the superior cervical ganglion it was 7.8 ± 0.1 (means ± SEM). The nature of the blockade induced by prolonged exposures or by concentrations greater than 10 −7 M is discussed. The selectivity of action of MDL 72222 was confirmed. Exposure to the antagonist for 1 hr had no effect on responses to γ-aminobutyric acid (GABA) or on the dose-response curves to 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP) in the nodose ganglion nor any effect on the dose-response curves to DMPP in superior cervical ganglion. Metoclopramide (10 −7, 10 −6 M) induced a surmountable blockade of responses to 5-HT in both the nodose and superior cervical ganglion, equilibration with this antagonist being apparently more rapid. The apparent pA 2 in the nodose ganglion was 7.1 ± 0.1 and in the superior cervical ganglion was 7.2 ± 0.2 (means ± SEM). Quipazine (10 −8, 10 −7 M) also induced a surmountable blockade of responses to 5-HT in both ganglia. The apparent pA 2 was 7.5 ± 0.1 in the nodose ganglion and 7.6 ± 0.1 in the superior cervical ganglion (means ± SEM). m-Chlorophenylpiperazine ( mCPP) had some blocking action at concentrations greater than 10 −7 M, but the slope of the dose-response curves was substantially depressed. Neither ketanserin (10 −7 M) nor methiothepin (10 −6 M) had any effect on responses to 5-HT in the nodose ganglion. It is concluded that the 5-HT receptors on cell bodies in the nodose and superior cervical ganglion cannot be differentiated using these antagonists, are likely to be homologous with receptors on the axons and sensory endings of afferent neurones, but are probably not homologous with excitatory receptors on the nerve terminals of post-ganglionic sympathetic neurones. These two sub-types of receptor, and the receptor on neurones in the ileum, constitute three kinds of 5-HT M receptor.