Studies on norepinephrine-containing afferents to Purkinje cells of rat cerebellum. III. Evidence for mediation of norepinephrine effects by cyclic 3′, 5′-adenosine monophosphate

Abstract

The relationship between cerebellar adenyl cyclase and norepinephrine (NE) receptivity of cerebellar Purkinje cells was explored by recording their spontaneous discharge during microiontophoretic application of various substances. The depression in firing rate of single Purkinje cells produced by NE was usually mimicked by adenosine 3′5′-monophosphate (cyclic AMP). The synthetic dibutyryl derivative of cyclic AMP, however, was no more potent than the parent compound. Two other adenine nucleotides, adenosine triphosphate (ATP) and 5′-AMP generally accelerated discharge rate or had no effect. The excitatory effects of the nucleotides may be related to chelation of divalent cations, since EDTA and citrate were universally found to accelerate unit firing while calcium ions always slowed or blocked spontaneous activity. Further evidence for a link between the effects of NE and cyclic AMP was seen with the administration of the methyl xanthines, which inhibit the enzymatic breakdown of cyclic AMP by phosphodiesterase. Theophylline and aminophylline, whether parenterally or iontophoretically administered, markedly potentiated the depressant effects of both NE and cyclic AMP on Purkinje cells. Methyl xanthines usually transformed the infrequent speeding effects of cyclic AMP to those of slowing, or brought about an inhibitory response in cells previously non-responsive to cyclic AMP. Furthermore, iontophoresis of prostaglandins E 1 and E 2 (but not F 1α, F 2α, linoleic or linolenic acids), and of nocotinate, reported to reduce cyclic AMP levels in peripheral neuro-effector systems, also block the action of iontophoretically applied NE in rat cerebellum. To determine if the locus of the action of the various iontophoretically applied substances was exerted presynaptically on the proposed adrenergic terminals on Purkinje cells, 6-hydroxydopamine (6-HODA) treated animals were studied. Under these conditions the effects of all the drugs and their interactions appeared identical to those of normal cerebella, in spite of biochemical and histochemical evidence for complete degeneration of adrenergic nerve terminals. These studies provide indirect evidence for mediation of NE effects by cyclic AMP in a living single cell system. Thus, they parallel in vitro studies with cerebellar slices which show an increase in cyclic AMP levels with NE, as well as histamine. Taken with the histochemical data for norepinephrine nerves terminating on Purkinje cells, the findings point to a possible transsynaptic modulation of adenyl cyclase activity.