Synergistic control of protein kinase Cgamma activity by ionotropic and metabotropic glutamate receptor inputs in hippocampal neurons

Isoform specificity of protein kinase Cs in synaptic plasticity

Protein kinase Cs (PKCs) are implicated in many forms of synaptic plasticity. However, the specific isoform(s) of PKC that underlie(s) these events are often not known. We have used Aplysia as a model system in order to investigate the isoform specificity of PKC actions due to the presence of fewer isoforms and a large number of documented physiological roles for PKC in synaptic plasticity in this system. In particular, we have shown that distinct isoforms mediate distinct types of synaptic plasticity induced by the same neurotransmitter: The novel calcium-independent PKC Apl II is required for actions mediated by serotonin (5-HT) alone, while the classical calcium-dependent PKC Apl I is required for actions mediated when 5-HT is coupled to activity. We will discuss the reasons for PKC isoform specificity, assess the tools used to uncover isoform specificity, and discuss the implications of isoform specificity for understanding the roles of PKC in regulating synaptic plasticity.

Persistent activation of protein kinase Calpha is not necessary for expression of cerebellar long-term depression

Protein kinase Calpha (PKCalpha) plays a major role in the induction of long-term depression (LTD) in a cerebellar Purkinje cell (PC). The sequential activation model for classical PKC states that PKCalpha translocates to the plasma membrane by binding Ca(++) and then becomes fully activated by binding diacylglycerol (DAG), which enables estimation of the activity by monitoring its localization. Here, we performed simultaneous electrophysiological recording and fluorescence imaging in a cultured PC expressing GFP-tagged PKCalpha. When a PC was depolarized, PKCalpha transiently translocated to the plasma membrane in a Ca(++)-dependent manner. Application of membrane permeable DAG or the blocker of DAG lipase prolonged the translocation. These results suggest that the sequential activation model is applicable to PCs. Conjunctive applications of glutamate and depolarization pulse induced LTD, but did not prolong the translocation. Thus, our results imply that persistent activation of PKCalpha is not necessary for the expression of LTD.

Nicotinic receptor-mediated enhancement of long-term potentiation involves activation of metabotropic glutamate receptors and ryanodine-sensitive calcium stores in the dentate gyrus

Little is known about the mechanisms underlying the enhancement of long-term potentiation (LTP) by nicotine. In the present study, the mechanisms of nicotinic enhancement of LTP were investigated in the rat dentate gyrus in vitro. Acute application of nicotine enhanced LTP induction, an action requiring activation of alpha7 nicotinic acetylcholine receptors (nAChRs), as it was blocked by the nAChR antagonist methyl-lycaconitine, mimicked by the acetylcholine receptor agonist choline and absent in mutant mice null for alpha7 nAChR. Nicotinic enhancement of LTP was both dependent on N-methyl-D-aspartate receptor activation, as no LTP was induced in the presence of nicotine and an N-methyl-D-aspartate receptor antagonist, and expressed post-synaptically, as no change in paired-pulse ratio accompanied nicotinic enhancement of LTP. The nicotinic-enhanced component of LTP, unlike control LTP, was dependent on activation of metabotropic glutamate receptors (mGluRs), being inhibited by the group I/II antagonist LY341495 and the mGluR5 antagonist MPEP, and also dependent on influx of Ca via L-type Ca channels and release from ryanodine (RyR)-sensitive intracellular stores, being prevented by nifedipine and RyR, respectively. It is suggested that nicotinic activation of the Ca-permeable alpha7 nAChRs fills RyR Ca stores and release of Ca from such stores by high-frequency stimulation via Ca-induced Ca release and activation of mGluRs induces an additional component of LTP which summates with control LTP. Chronic application of nicotine in vivo also enhanced LTP induction in slices and was dependent on activation of mGluRs and Ca release from RyR-sensitive intracellular stores, although acutely applied nicotine was not required for such enhanced LTP.