Faculty Bibliography

Long-term potentiation (LTP) was investigated in the mammalian entorhinal cortex by using two in vitro preparations, the isolated brain and the entorhinal cortex slice. Hebbian and non-Hebbian types of LTP appear to be present in layer II entorhinal cortex cells as demonstrated using two protocols: (i) tetanic stimulation of the piriform-entorhinal cortex afferent pathway to generate homosynaptic potentiation and (ii) postsynaptic subthreshold rhythmic membrane potential manipulation not paired to presynaptic activation, which gives rise to non-Hebbian LTP. The induction and the expression of both types of LTP were found to be dependent on activation of N-methyl-D-aspartate receptors as shown by their sensitivity to the receptor agonist D-2-amino-5-phosphonovalerate. This is in contrast to LTP in the hippocampus [Zalutsky, R. A. & Nicoll, R. A. (1990) Science 248, 1619-1624], where LTP is expressed by quisqualate receptors. Since, in the entorhinal cortex, LTP is linked to a selective increase of the N-methyl-D-aspartate-receptor-mediated synaptic responses, this enhancement is most likely due to postsynaptic factors

The molecular events that control synaptic vesicle availability in chemical synaptic junctions have not been fully clarified. Among the protein molecules specifically located in presynaptic terminals, synapsin I and calcium/calmodulin-dependent protein kinase II (CaM kinase II) have been shown to modulate evoked transmitter release in the squid giant synapse. In the present study, analysis of synaptic noise in this chemical junction was used to determine whether these proteins also play a role in the control of spontaneous and enhanced spontaneous transmitter release. Injections of dephosphorylated synapsin I into the presynaptic terminal reduced the rate of spontaneous and enhanced quantal release, whereas injection of phosphorylated synapsin I did not modify such release. By contrast CaM kinase II injection increased enhanced miniature release without affecting spontaneous miniature frequency. These results support the view that dephosphorylated synapsin I 'cages' synaptic vesicles while CaM kinase II, by phosphorylating synapsin I, 'decages' these organelles and increases their availability for release without affecting the release mechanism itself

Real-time visualization of intracellular calcium concentration ([Ca2+]i) changes in mammalian Purkinje cells in vitro, utilizing the dye Fura-2, indicates that calcium action potentials are generated in the dendritic tree and follow a particular activation sequence. During spontaneous oscillations or after direct current injection, dendritic spikes are initiated as slow and graded plateau potentials at the level of the tertiary or spiny branchlets of the dendrite. As the plateau potentials become sufficiently high to reach the firing threshold for full dendritic spike generation, calcium entry is observed at the more proximal branches of the dendritic tree. These action potentials are then conducted orthodromically toward the soma and may invade other branches in the arbor antidromically. Simultaneous recording of the intracellular electrical activity and the Fura-2 fluorescent signal indicates that the intracellular calcium transients are accompanied by a very rapid increase in intracellular calcium concentration. This increase in [Ca2+]i exhibits an almost equally fast return to baseline after the termination of the action potential, indicating the presence of very efficient calcium sequestering and extruding mechanisms in the dendrites. Iontophoretic application of glutamate at the dendritic level provided a further demonstration of the spatial separation of plateau potentials from dendritic spikes and gives further insights into the details of dendritic integration in this neuron

Injection of rat brain mRNA into Xenopus oocytes has been shown to induce a calcium current (ICa) that is insensitive to dihydropyridine and omega-conotoxin. We examined the effect of funnel-web spider venom on two aspects of this expressed ICa: (i) the calcium-activated chloride current [ICl(Ca)] and (ii) the currents carried by barium ions through calcium channels (IBa). In the presence of 1.8 mM extracellular calcium, ICl(Ca) tail current became detectable between -30 and -40 mV from a holding potential of -80 mV and reached a maximal amplitude between 0 and +10 mV. Total spider venom partially (83%) and reversibly blocked the calcium-activated chloride current without changing its voltage sensitivity. A chromatographic toxin fraction from the venom also blocked this current (64%). The venom had a minimal effect on INa and IK. Direct investigation of inward current mediated by calcium channels was carried out in high-barium solution. IBa had a higher threshold of activation (-30 to -20 mV) and reached its maximal amplitude at about +20 mV. Total venom or a partly purified chromatographic toxic fraction blocked IBa partially and reversibly without changing its current-voltage characteristics. Furthermore, the extent of the total venom block depended on the concentration of extracellular barium. Only 35% of the IBa was blocked in 60 mM Ba2+, whereas the block increased to 65% and 71%, respectively, for 40 and 20 mM Ba2+. On the basis of these results, we propose that the calcium channels expressed from rat brain mRNA in Xenopus oocytes is similar to the recently discovered P-type channels