By B. Halasz, K. Fuxe, Agnati. L. F., M. Kalia, M. Goldstein, K. Anderson, Harfs
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These so-called small intensely fluorescent (SIF) cells are thought to be intemeurones, which give rise to processes that impinge on the postganglionic cell body (Fig. 5). It has been proposed that dopamine, released by preganglionic muscarinic excitation of the SIF cells, acts on a receptor which in tum activates a specific adenylate cyclase in the postganglionic membrane. Accumulation of adenosine 3' : 5' -monophosphate (cAMP) brings about transient hyperpolarization of the ganglion cell resulting in inhibition of transmission of impulses arising from the preganglionic neurone [18, 28, 40].
In view of the complex actions of dopamine, this ambiguity is not surprising. The vasodilator effects of dopamine resulting from actions on renal vascular receptors would tend to reduce renin activity, but this could well be offset by increases in renin activity mediated by simultaneous stimulation of fJ-adrenoceptors on cells in the juxtaglomerular apparatus; vasoconstriction resulting from a-adrenoceptor or 5-HT receptor stimulation would have a similar effect. In the intact animal, the situation becomes even more complex since dopamine-induced changes in blood pressure, cardiac output and regional blood flow would all modify renin secretion indirectly.
If dopamine is released from these stores as the result of physiological stimuli, a dopaminergic component in the normal regulation of renal blood flow and renin release may exist . Some of the evidence presented in support of a peripheral neurotransmitter role for dopamine is circumstantial (which is not unusual in neuroscience), and the morphological data is limited. There is, however, sufficient experimental data to propose that dopamine serves an indepen- dent role in peripheral neurotransmission.