Synergy between membrane depolarization and muscarinic receptor activation leads to potentiation of neurotransmitter release (II)

Amplification of muscarinic agonist-induced [³H]noradrenaline ([³H]NE)-release by depolarizing agents, was studied in rat brain cortical slices. [³H]NE basal outflow was enhanced by either K⁺ (25 mM) or veratridine (2 μM) in a Ca²⁺-dependent manner and was potentiated beyond additivity in the presence of muscarinic agonists. Facilitation of the [³H]NE-induced release by the simultaneous presence of muscarinic agonists and depolarizing agents is calcium-dependent with a maximal effective concentration of 0.6-0.8 mM. The efficacy of muscarinic agonists to induce basal outflow of [³H]NE is as follows: CCh > ACh > arecoline > oxotremorine M > bethanechol > pilocarpine, which is similar to their potentiatory effects observed in the presence of depolarizing agents. Potentiation of muscarinic agonist-induced release of [³H]NE by elevated K⁺ is more pronounced (up to 7-fold) in comparison to potentiation by veratridine (up to 4-fold), irrespective of the various muscarinic agonists. The sequential presence of muscarinic agonists followed by depolarizing agents is not sufficient for eliciting a synergy of [³H]NE outflow, whether receptor activation was initiated prior to depolarization or depolarization was initiated prior to receptor activation. Receptors which do not mediate phosphatidyl inositol (PI) turnover such as nicotine induced [³H]NE-release which is not affected by the presence of depolarizing agents and yielded in their presence additive fractional release only. In this report was establish synergy of [³H]NE release by muscarinic agonists under depolarizing conditions, similar to synergism of inositol phosphate (IP) production which was observed by muscarinic agonists and depolarization agents. Facilitation of [³H]NE release which correlates with facilitation of IP-production suggests that the common site of action of depolarizing agents and mAChR agonists is phospholipase C which hydrolyses phosphatidyl inositol bisphosphate to generate the two intracellular messengers, 1,4,5-inositol trisphosphate and diacylglycerol. These results further confirm recent studies which showed tight correlation between muscarinic induced [³H]NE release and muscarinic activation of PI turnover. Dissociation of the stimulation period into two consecutive intervals reveals no further release by CCh in the second period of stimulation. On the other hand, potentiation of release by CCh in the presence of K⁺ persists in both periods of stimulation. These results point to the onset of activation of release initiated by CCh and K⁺ together, which continues even though the CCh-induced release is desensitized in the second period of stimulation. It is thus proposed that membrane depolarization in combination with receptor-activated hydrolysis of the PI cycle induces a change in synaptic transmission and hence can be regarded as a possible in vitro model for long-term potentiation.