Bradykinin inhibits M current via phospholipase C and Ca2+ release from IP3-sensitive Ca2+ stores in rat sympathetic neurons

A variety of intracellular signaling pathways can modulate the properties of voltage-gated ion channels. Some of them are well characterized. However, the diffusible second messenger mediating suppression of M current via G protein-coupled receptors has not been identified. In superior cervical ganglion neurons, we find that the signaling pathways underlying M current inhibition by B2 bradykinin and M1 muscarinic receptors respond very differently to inhibitors. The bradykinin pathway was suppressed by the phospholipase C inhibitor U-73122, by blocking the IP3 receptor with pentosan polysulfate or heparin, and by buffering intracellular calcium, and it was occluded by allowing IP3 to diffuse into the cytoplasm via a patch pipette. By contrast, the muscarinic pathway was not disrupted by any of these treatments. The addition of bradykinin was accompanied by a [Ca2+]i rise with a similar onset and time to peak as the inhibition of M current. The M current inhibition and the rise of [Ca2+]i were blocked by depletion of Ca2+ internal stores by thapsigargin. We conclude that bradykinin receptors inhibit M current of sympathetic neurons by activating phospholipase C and releasing Ca2+ from IP3-sensitive Ca2+ stores, whereas muscarinic receptors do not use the phospholipase C pathway to inhibit M current channels.

Toxoplasma invasion: the parasitophorous vacuole is formed from host cell plasma membrane and pinches off via a fission pore.

Most intracellular pathogens avoid lysing their host cells during invasion by wrapping themselves in a vacuolar membrane. This parasitophorous vacuole membrane (PVM) is often retained, serving as a critical transport interface between the parasite and the host cell cytoplasm. To test whether the PVM formed by the parasite Toxoplasma gondii is derived from host cell membrane or from lipids secreted by the parasite, we used time-resolved capacitance measurements and video microscopy to assay host cell surface area during invasion. We observed no significant change in host cell surface area during PVM formation, demonstrating that the PVM consists primarily of invaginated host cell membrane. Pinching off of the PVM from the host cell membrane occurred after an unexpected delay (34-305 sec) and was seen as a 0.219 +/- 0.006 pF drop in capacitance, which corresponds well to the predicted surface area of the entire PVM (30-33 microns2). The formation and closure of a fission pore connecting the extracellular medium and the vacuolar space was detected as the PVM pinched off. This final stage of parasite entry was accomplished without any breach in cell membrane integrity.

Brain-derived neurotrophic factor rapidly enhances synaptic transmission in hippocampal neurons via postsynaptic tyrosine kinase receptors.

Although neurotrophins are primarily associated with long-term effects on neuronal survival and differentiation, recent studies have shown that acute changes in synaptic transmission can also be produced. In the hippocampus, an area critically involved in learning and memory, we have found that brain-derived neurotrophic factor (BDNF) rapidly enhanced synaptic efficacy through a previously unreported mechanism--increased postsynaptic responsiveness via a phosphorylation-dependent pathway. Within minutes of BDNF application to cultured hippocampal neurons, spontaneous firing rate was dramatically increased, as were the frequency and amplitude of excitatory postsynaptic currents. The increased frequency of postsynaptic currents resulted from the change in presynaptic firing. However, the increased amplitude was postsynaptic in origin because it was selectively blocked by intracellular injection of the tyrosine kinase receptor (Ntrk2/TrkB) inhibitor K-252a and potentiated by injection of the phosphatase inhibitor okadaic acid. These results suggest a role for BDNF in the modulation of synaptic transmission in the hippocampus.