Modulation of buccal muscle contractility by serotonergic metacerebral cells in Aplysia: evidence for a role of cyclic adenosine monophosphate

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Abstract
Experiments were performed to test the hypothesis that cyclic (c)AMP may mediate the effects of serotonin and the metacerebral cell (MCC) on contractility of the accessory radula closer muscle (ARC) of Aplysia. A cell-free homogenate of the ARC muscle showed a dose-related increase in the rate of accumulation of cAMP in the presence of serotonin. Serotonin did not reduce phosphodiesterase activity in the muscle homogenate. The ARC apparently contains a serotonin-sensitive adenylate cyclase. The intact ARC muscle showed a dose-related increase in the synthesis (accumulation of radioactive precursor) and total levels of cAMP in the presence of serotonin. Firing of a single MCC cell (mean, 297 spikes) produced a statistically significant increase of the synthesis of cAMP. The mean synthesis was 3.5 times higher in experimental compared to paired control muscles from the same animal. This represents an 8% increase per MCC spike. Two eighth position analogs of cAMP (8-BT cAMP and 8-PCPT (phenylchlorothio) cAMP) enhanced ARC muscle contractions elicited by fixed bursts of motor neuron spikes. The potentiation occurred in spite of the fact that the cAMP analogs reduced the size of the EJPs in the muscle. The effect of the MCC in enhancing contractions of ARC muscle was enhanced in magnitude and duration by the phosphodiesterase inhibitor RO 20-1724 [4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone]. The hypothesis that the modulatory action of serotonin at buccal muscle is mediated by cAMP is supported.