Membrane depolarization and carbamoylcholine stimulate phosphatidylinositol turnover in intact nerve terminals.

Synaptosomes, purified from rat cerebral cortex, were prelabeled with [3H]inositol to study phosphatidylinositol turnover in nerve terminals. Labeled synaptosomes were either depolarized with 40 mM K+ or exposed to carbamoylcholine (carbachol). K+ depolarization increased the level of inositol phosphates in a time-dependent manner. The inositol trisphosphate concentration increased rapidly and transiently, reaching maximum (250% of control) in less than 3 sec and returning to near basal levels by 30 sec. The inositol bisphosphate level also increased rapidly, but its elevated level (220% of control) was sustained during continued depolarization. The elevated level of inositol bisphosphate was reversed upon repolarization of the synaptosomes. The level of inositol monophosphate increased slowly to 120-130% of control. These effects of K+ depolarization depended on the presence of Ca2+ in the incubation medium. Carbachol stimulated the turnover of phosphatidylinositol in a dose- and time-dependent manner. The level of inositol trisphosphate increased only slightly (120-130% of control) during carbachol stimulation. The level of inositol bisphosphate increased to 210% of control, and this maximal response was seen from 15 to 60 min. Accumulation of inositol monophosphate (250% of control) was larger than that of inositol bisphosphate, but its time course was slower. Atropine and pirenzepine inhibited the carbachol effect with high affinities of 0.8 nM and 16 nM, respectively, indicating that the effect of carbachol was mediated by activation of a M1 muscarinic receptor. Incubation of synaptosomes in Ca2+-free buffer reduced the response to carbachol by 30%, and addition of EGTA abolished it. These data show that both Ca2+ influx and M1 muscarinic receptor activation stimulate phospholipase C activity in synaptosomes, suggesting that phosphatidylinositol turnover may be involved in regulating neurotransmitter release from nerve terminals.

Neurochemical basis for the photic control of circadian rhythms and seasonal reproductive cycles: role for acetylcholine.

A pharmacological approach was used to examine the role of acetylcholine in the photic control of circadian rhythms and seasonal reproductive cycles. The experimental protocol was designed to determine whether the administration of carbachol, a cholinergic agonist, could mimic the effects of brief light pulses on gonadal function and/or the circadian rhythm of wheel-running activity in golden hamsters. Intraventricular injections of carbachol, administered singularly at discrete phase points throughout the circadian cycle, induced phase-dependent shifts in the free-running rhythm of activity similar to those caused by a brief light exposure. Injections of carbachol once every 23.33 hr for 9 weeks entrained the activity rhythm and stimulated the neuroendocrine-gonadal axis in a manner similar to that observed after the presentation of 1-hr light pulses at this frequency. In contrast, the administration of carbachol once every 24 hr did not consistently provide an entraining signal for the activity rhythm and did not stimulate reproductive function. Importantly, the effects of carbachol on the seasonal reproductive response were correlated with the timing of the injections relative to the activity rhythm. These findings suggest that acetylcholine may play an important role in the mechanism by which light regulates circadian rhythms and seasonal reproductive cycles.

[Supersensitivity by reserpine and means of antagonizing it]. / Supersensitivität durch Reserpin und deren Beeinflussbarkeit.

Single or repeated reserpine application elicited in rats a supersensitivity which is measurable both by the rotation behavior and motor activity. This supersensitivity is accompanied by enhanced responsiveness to unspecific stimuli and increased responsiveness to receptor-active substances (dopamine, noradrenaline, serotonin, carbachol). Pretreatment with actinomycin D can transiently inhibit the development of supersensitivity, while repeated lithium application has no effect on it. The results give evidence for the unspecificity of reserpine-induced supersensitivity, which comprises both aminergic transmission systems and cholinergic systems and can be antagonized by inhibitors of protein synthesis.