PMA, 4-PMA, sphingosine and BIS were dissolved in DMSO, then diluted. effect of PMA was less at 10 mm[Ca2+]o than at 1 mm[Ca2+]o. When a train of action potentials was generated with a short interval, the EPSC was eventually depressed and reached a steady-state level. The recovery process followed a simple exponential relation with a rate constant of 0.132 0.029 s?1. PMA did not affect the recovery rate constant of EPSCs from tetanic depressive disorder. In addition, PMA did not affect the steady-state EPSC which should be proportional to the refilling rate of the readily releasable pool of vesicles. These results conflict with the hypothesis that PKC upregulates the size of the readily releasable pool or the number of release sites. PKC appears to upregulate the Ca2+ sensitivity of the process that controls the exocytotic fusion probability. Protein kinase C (PKC) has been implicated as having pivotal roles in the regulation of signal transduction (Nishizuka, 1992). Activation of PKC has been shown to be involved in the modulation of synaptic Rabbit Polyclonal to CDK10 transmission by a variety of signals (Tanaka & Nishizuka, 1994). It has been suggested that PKC may enhance synaptic transmission via a presynaptic mechanism. In hippocampal CA3 pyramidal neurones, the mossy fibre output is usually potentiated by phorbol esters in a PKC-dependent manner (Yamamoto 1987; Son & Carpenter, 1996). PKC also potentiates transmitter release from cholinergic nerve terminals of autonomic ganglia and neuromuscular junctions (Minota 1991; Bachoo 1992; Somogyi 1996; Redman 1997). The LY2365109 hydrochloride state- and/or time-dependent facilitation of transmitter release from sensory neurones is usually mediated by PKC (Byrne & Kandel, 1996). How does PKC potentiate transmitter release from nerve terminals? PKC may increase Ca2+ influx during the action potential either through activation of voltage-dependent Ca2+ channels (Doerner 1990; Schroeder 1990; Swartz, 1993; Zhu & Ikeda, 1994; Stea 1995) or through suppression of K+ channels (Bowlby & Levitan, 1995). This is consistent with the observation that PKC-dependent potentiation is usually often accompanied by a reduction in paired-pulse facilitation (PPF) (Zalutsky & Nicoll, 1990). Alternatively, PKC may LY2365109 hydrochloride directly modulate the exocytosis of synaptic vesicles downstream of Ca2+ entry (Redman 1997). Phorbol esters increase the frequency of spontaneous miniature inhibitory postsynaptic currents in CA3 pyramidal neurones through a Ca2+-impartial mechanism (Capogna 1995), whereas in CA1 pyramidal neurones they increase the frequency of miniature excitatory postsynaptic currents (EPSCs) through both Ca2+-dependent and -impartial mechanisms (Parfitt & Madison, 1993). One objective of the present study was to determine whether the PKC-dependent potentiation of nerve-evoked transmitter LY2365109 hydrochloride release is usually accompanied by an increase in Ca2+ influx in the giant presynaptic terminal of chick ciliary ganglion. To this end, the intraterminal Ca2+ concentration ([Ca2+]i) was measured directly (Yawo & Chuhma, 1993, 1994). If PKC activation increases the LY2365109 hydrochloride evoked transmitter release without altering [Ca2+]i, this would indicate that PKC acts on an exocytotic mechanism other than Ca2+ influx, buffering and removal. The results of this paper indicate that this is usually indeed the case. In order to further elucidate the underlying mechanism of PKC-dependent modulation of exocytosis, the effect of [Ca2+]o on PKC-dependent potentiation, the effect of PKC around the recovery rate of EPSCs from depressive disorder after a high frequency train of stimuli, and the correlation between the EPSC potentiation and steady-state EPSC during a train were investigated. The present results exclude the notion that PKC upregulates the size of the readily releasable pool or the number of release sites..