Different effects of baclofen and GTP gamma S on voltage-activated Ca2+ currents in rat hippocampal neurons in vitro

Cholesterol perturbing agents inhibit NMDA-dependent calcium influx in rat hippocampal primary culture

The present study was carried out to investigate the potential involvement of cholesterol-rich membrane microdomains in the mobilization of calcium induced by NMDA-receptors (NMDA-R). We herein provide evidence that agents interfering with plasma membrane cholesterol (namely, filipin and methyl-beta-cyclodextrin (Cdex)) inhibit the NMDA-stimulated influx of calcium in hippocampal cells in culture. Filipin-treated cells maintained their morphology and were able to respond with a calcium influx to high K(+) challenge, whereas Cdex altered both cellular parameters. These results suggest that the NMDA-R can be located in cholesterol-rich membrane microdomains or alternatively that the mechanisms coupling their dynamics in the post-synaptic membrane are dependent on the integrity of the microdomains.

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

The membrane density of L-type voltage-sensitive Ca(2+) channels (L-VSCCs) of rat hippocampal neurons increases over age [days in vitro (DIV)] in long-term primary cultures, apparently contributing both to spontaneous cell death and to enhanced excitotoxic vulnerability. Similar increases in L-VSCCs occur during brain aging in vivo in rat and rabbit hippocampal neurons. However, unraveling both the molecular basis and the functional implications of these age changes in VSCC density will require determining whether the other 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-protein-coupled, membrane-delimited inhibitory pathway. These possibilities were 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 Ca(2+) channel current density (picoamperes per picofarad), mediated by each Ca(2+) channel type studied here (L, N, and a combined P/Q + R component), increased through 7 DIV. Thereafter, however, only L-type current density continued to increase, at least through 21 DIV. Concurrently, pertussis toxin-sensitive G-protein-coupled inhibition of non-L-type Ca(2+) channel current induced by the GABA(B) receptor agonist baclofen or by guanosine 5'-3-O-(thio)triphosphate declined dramatically with age in culture. Thus, the present studies identify selective and novel parallel mechanisms for the time-dependent alteration of Ca(2+) influx, which could importantly influence function and vulnerability during development and/or aging.

Modulation of voltage-gated calcium channels by orphanin FQ in freshly dissociated hippocampal neurons

Orphanin FQ (OFQ) has recently been reported to be an endogenous ligand for the opioid-like LC132 receptor. The effect of OFQ on high voltage-gated calcium channels (VGCCs) was examined in freshly dissociated rat pyramidal neurons using the whole-cell configuration of the patch-clamp technique. High-threshold Ba2+ currents were reversibly inhibited by OFQ. The depression of the currents was associated with a slowed rate of activation and a change in the activation I-V relationship at step potentials higher than +30 mV. In concentration-response experiments, a mean (+/-SEM) pEC50 value of 7.0 +/- 0.07 and a Hill coefficient of 1.5 +/- 0.08 (n = 5) were obtained. The near-maximum inhibition of the Ba2+ currents by OFQ (1 microM) amounted to 31 +/- 2.2% of control (n = 15). Opioid receptors could not account for the effects of OFQ on VGCCs, because naloxone, a broad spectrum mu-, delta-, and kappa-receptor antagonist, did not reduce the effectiveness of OFQ. When GTP-gamma-S was included in the pipette, the depression of the currents by OFQ was irreversible, whereas currents from neurons preincubated with pertussis toxin were not inhibited by OFQ, consistent with the involvement of a PTX-sensitive G-protein. When selective blockers of VGCCs were used, it was demonstrated that all subtypes of VGCCs were affected by OFQ. In conclusion, the effect of OFQ on VGCCs expressed in hippocampal CA3 and CA1 neurons may play an important role in the regulation of hippocampal cell excitability and neurotransmitter release.