Glucose-induced excitation in molluscan central neurons producing insulin-related peptides

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Abstract
The light green cells (LGCs) are a group of identified central neurons in the pond snail, Lymnaea stagnalis, that produce a number of insulin-related peptides. Freshly dissociated LGCs are activated by physiological concentrations of extracellular glucose. The response to glucose consists of a slow depolarization, which, at concentrations of 1 mM or more, rapidly induces regular spiking activity. The response persists during prolonged application of glucose but is completely reversed upon washing. The threshold concentration is 0.2 mM; the maximal effect occurs at 5 mM. In LGCs in central nervous system preparations kept in organ culture for 16–24 h, glucose causes a similar depolarization, which may lead to spiking activity. In freshly isolated preparations, which have very inexcitable LGCs, no direct response to glucose was seen. The response is specific to the LGCs: no other central neurons in Lymnaea showed consistent responses. The glucose response is evoked by d-glucose and the non-metabolized analogue 2-deoxy-d-glucose, but not by related hexoses, including l-glucose, nor pentoses or disaccharides. The response is not affected by interfering with the glucose metabolism, nor is the response mimicked by the metabolite d-glyceraldehyde or by injection of glucose. This suggests that glucose metabolites are not involved in the response. The glucose response depends on the presence of extracellular Na+ and is blocked by phlorizin, which specifically inhibits Na+-coupled glucose transport. This suggests that the response is due to activation of an electrogenic Na+-coupled glucose transporter.