Decreased G-protein-mediated regulation and shift in calcium channel typeswith age in hippocampal cultures

The membrane density of L-type voltage-sensitive Ca2+ channels (L-VSCCs) of rat hippocampal neurons increases over age [days in vitro (DIV)] in long-t erm primary cultures, apparently contributing both to spontaneous cell deat h and to enhanced excitotoxic vulnerability. Similar increases in L-VSCCs o ccur during brain aging in vivo in rat and rabbit hippocampal neurons. Howe ver, unraveling both the molecular basis and the functional implications of these age changes in VSCC density will require determining whether the oth er types of high-threshold VSCCs (e.g., N, P/Q, and R) also exhibit altered density and/or changes in regulation, for example, by the important G-prot ein-coupled, membrane-delimited inhibitory pathway. These possibilities wer e tested here in long-term hippocampal cultures. Pharmacologically defined whole-cell currents were corrected for cell size differences over age by normalization with whole-cell capacitance. The Ca2 channel current density (picoamperes per picofarad), mediated by each Ca2 channel type studied here (L, N, and a combined P/Q + R component), increa sed through 7 DIV. Thereafter, however, only L-type current density continu ed to increase, at least through 21 DIV. Concurrently, pertussis toxin-sens itive G-protein-coupled inhibition of non-L-type Ca2+ channel current induc ed by the GABA(B) receptor agonist baclofen or by guanosine 5'-3-O-(thio)tr iphosphate declined dramatically with age in culture. Thus, the present stu dies identify selective and novel parallel mechanisms for the time-dependen t alteration of Ca2+ influx, which could importantly influence function and vulnerability during development and/or aging.