Cellular mechanism of nicotine-mediated intracellular calcium homeostasis in primary culture of mouse cerebral granule cells

Intracellular calcium concentration ([Ca²⁺](i)) may play a crucial role in a variety of neuronal functions. Here we report that in primary culture of mouse cerebellar granule cells nicotinic acetylcholine receptors (nAChRs) are expressed in a specific developmental stage and involved in the regulation of intracellular calcium homeostasis. Nicotine-mediated calcium responses were measured using ⁴⁵Ca²⁺ or fluorometrically using the calcium-sensitive fluorescent dye fura-2. Maximal uptake of ⁴⁵Ca²⁺ evoked by nicotine in mouse cerebellar granule cells were revealed 8 ~ 12 days in culture. In contrast, nicotine did not alter the basal ⁴⁵Ca²⁺ uptake in cultured glial cells. In cerebellar granule cells nicotine-evoked ⁴⁵Ca²⁺ uptake was largely blocked by the NMDA receptor antagonists. Glutamate pyruvate transaminase (GPT), which removes endogenous glutamate, also prevented nicotine effects, implying the indirect involvement of glutamate in nicotine mediated calcium responses. Fluorometric studies using fura-2 showed two phases of nicotine-evoked [Ca²⁺](i) rises: the initial rising phase and the later plateau phase. Interestingly, the NMDA receptor antagonists and GPT appeared to inhibit only the later plateau phase of nicotine-evoked [Ca²⁺](i) rises. The present results imply that nicotine mediated ⁴⁵Ca²⁺ uptake and [Ca²⁺](i) rises are attributed to the calcium fluxes through both nAChRs and NMDA receptors in a time-dependent manner. Consequently, nAChRs may play an important role in neuronal development by being expressed in a specific developmental stage and regulating the intracellular calcium homeostasis.