Phorbol ester uncouples adenosine inhibition of presynaptic Ca2+ transients at hippocampal synapses

Slow intracellular accumulation of GABA(A) receptor delta subunit is modulated by brain-derived neurotrophic factor

GABA(A) receptors composed of the gamma2 and delta subunits have distinct properties, functions and subcellular localization, and pathological conditions differentially modulate their surface expression. Recent studies demonstrate that acute seizure activity accelerated trafficking of the gamma2 and beta2/3 subunits but not that of the delta subunit. The trafficking of the gamma2 and beta2/3 subunits is relatively well understood but that of the delta subunit has not been studied. We compared intracellular accumulation of the delta and gamma2 subunits in cultured hippocampal neurons using an antibody feeding technique. Intracellular accumulation of the delta subunit peaked between 3 and 6 h, whereas, maximum internalization of the gamma2 subunit took 30 min. In the organotypic hippocampal slice cultures internalization of the delta subunit studied using a biotinylation assay revealed highest accumulation between 3 and 5 h and that of the gamma2 subunit between 15 and 45 min. The surface half-life of the delta subunit was 171 min in cultured hippocampal neurons and 102 min in the organotypic hippocampal slice cultures. In the subsequent studies, internalization of the delta subunit was found to be dependent on network activity but independent of ligand-binding. Brain-derived neurotrophic factor (BDNF) reduced buildup of the delta subunit in the cytoplasmic compartments and increased its surface expression, and this BDNF effect was independent of network activity. BDNF effect was mediated by activation of TrkB receptors, PLCgamma and PKC. Increase in the basal PKC activity augmented cell surface stability of the delta subunit. These results suggest that rate of intracellular accumulation of the delta subunit was distinct and modulated by BDNF.

Impact of protein kinase C activation on epileptiform activity in the hippocampal slice

There is evidence suggesting that protein kinase C (PKC) activation can prevent the enhanced network excitability associated with status epilepticus and group I metabotropic glutamate receptor (mGluR)-induced epileptogenesis. However, we observed no suppression of mGluR-induced burst prolongation in the guinea pig hippocampal slice when applied in the presence of the PKC activator phorbol-12,13-dibutyrate (PDBu). Furthermore, PDBu alone converted picrotoxin-induced interictal bursts into ictal-length discharges ranging from 2 to 6s in length. This effect could not be elicited by the inactive analog 4-alpha-PDBu and was suppressed with the PKC inhibitor chelerythrine, indicating PKC dependence. PKC activation can enhance neurotransmitter release, and both glutamate and acetylcholine are capable of eliciting similar prolonged synchronized discharges. However, neither mGluR1 nor NMDA receptor antagonist suppressed PDBu-driven burst prolongation, suggesting that increased glutamate release alone is unlikely to account for the PKC-induced expression of ictaform discharges. Similarly, atropine, a broad-spectrum muscarinic receptor antagonist, had no effect on PKC-induced burst prolongation. By contrast, AMPA/kainate receptor antagonist abolished PKC-induced burst prolongation, and mGluR5 antagonist significantly blunted the maximum burst length induced by PKC. These data suggest that PKC-induced prolongation of epileptiform bursts is dependent on changes specific to mGluR5 and AMPA/kainate receptors and not mediated simply by a generalized increase in transmitter release.

Transient recovery of synaptic transmission is related to rapid energy depletion during hypoxia

Transient recovery (TR) of evoked synaptic potential during the late stage of hypoxic hypoglycemia (HH) insult was investigated in rat hippocampal slices using extracellular recording methods. TR was observed in association with a rapid deterioration of antidromic population spikes (aPSs) following HH insult. TR was not elicited in normoglycemic hypoxia (NH), in which a gradual and delayed deterioration of aPSs was noted. TR was not modulated by either Ca(2+)- or PKC-dependent processes. When a glycolytic inhibitor was added, NH resulted in a rapid deterioration of aPSs and prompted appearance of TR. TR was also seen in slices using lactate to generate energy via oxidative phosphorylation, when hypoxic conditions were subsequently created. Other pharmacological interventions that aimed to cause rapid deterioration of aPSs without depleting energy stores failed to reproduce TR. The evidence thus suggests that the underlying mechanisms of TR appearance during HH insult are highly correlated with rapid energy depletion.