N and P/Q-type calcium channels

Cypros Pharm. Corp. describes the use of polyguanidino derivatives as presynaptic N- and P/Q-type calcium channel blockers for i.v. (or i.c.v.) administration (Marangos et al. (Cypros Pharmaceutical Corp.), W09836743). Compound 5 was administered to gerbils (7.5 mg/kg i.v.) prior to bilateral carotid occlusion. After 72 h the animals were sacrificed. Brains were perfusion-fixed and sections were stained to enable quantitative cell counts of live and dead neurons. The number of damaged neurons in the subiculum was 91.5 compared to 214 for a control treated with saline. It has been claimed that this compound can be used for the treatment of neuropathic pain and for the protection of neurons from excitatory damage under conditions of cerebral hypoxia. 

It has been demonstrated that cannabinoids act to suppress action potential-evoked calcium rises in the presynaptic terminal, thereby decreasing transmitter release. The action potential-evoked rise in intraterminal calcium was decreased by postsynaptic depolarization. This postsynaptic depolarization induced reduction of presynaptic calcium was prevented by application of antagonists to the CBi receptor (Kreitzer and Regehr, 2001). Cannabinoid-induced decreases in synaptic transmission have been shown to result from an inhibition of N- and P/Q-type calcium channels, the subtypes through which calcium influx occurs during evoked transmitter release (Twitchell et al., 1997). 

Twitchell, W., Brown, S., Mackie, K. Cannabinoids inhibit N- and P/Q-type calcium channels in cultured rat hippocampal neurons, J. Neurophysiol. 1997, 78, 43-50. 

Bayliss DA, Li YW, Talley EM. Effects of serotonin on caudal raphe neurons Inhibition of N- and P/Q-type calcium channels and the afterhyperpolarization. J Neurophysiol 1997 77 1362-1374. 

It is well established that activation of G protein-coupled receptors (GPCRs), in particular those that couple to the Got / subunit, can result in potent inhibition of both N-type and P/Q-type calcium channels (Figure 2) (Beech et al. 1992 Bemheim et al. 1991 Caulfield et al. 1994 Dunlap and Fischbach 1981 Golardand Siegelbaum 1993 Ikeda 1992 Ikeda and Schofield 1989 Lipscombe et al. 1989 Mintz and Bean 1993 Shapiro and Hille 1993 Zhu and Ikeda 1993). The physio-... 

The activity of N-type and P/Q-type calcium channels is also regulated by proteins that form part of the synaptic vesicle release machinery (Figure 3). These channel subtypes contain a specific synaptic protein interaction site (termed synprint) in the... 

Two types of cannabinoid receptor have so far been identified (reviewed in Howlett et al. 2002). These are CBi, cloned in Tom Bonner s laboratory in the USA in 1990, and CB2, cloned by Sean Munro in the UK in 1993. Both these receptors are coupled through Gi/o proteins, negatively to adenylate cyclase and positively to mitogen-activated protein kinase. CBi receptors are also coupled through Gi/o proteins, positively to A-type and inwardly rectifying potassium channels and negatively to N-type and P/Q-type calcium channels and to D-type potassium channels. In addition, there are reports that CBi and CB2 receptors can enhance intracellular free Ga concentrations (Fan and Yazulla 2003 Rubovitch et al. 2002 Sugiura et al. 1996, 1997, 2000). It is unclear whether this enhancement is Gi/o... 

Caulfield MP, Jones S, Vallis Y, Buckley NJ, Kim GD, Milligan G, Brown DA (1994) Muscarinic M-current inhibition via G alpha q/11 and alpha-adrenoceptor inhibition of Ca2+ current via G alpha o in rat sympathetic neurones. J Physiol 477 Pt 3 415-22 Charvin N, L Eveque C, Walker D, Berton F, Raymond C, KataokaM, Shoji-Kasai Y, Takahashi M, De Waard M, Seagar MJ (1997) Direct interaction of the calcium sensor protein synaptotagmin I with a cytoplasmic domain of the alphal A subunit of the P/Q-type calcium channel. Embo J 16 4591-6... 

Cribbs LL, Lee JH, Yang J, Satin J, Zhang Y, Daud A, Barclay J, Williamson MP, Fox M, Rees M, Perez-Reyes E (1998) Cloning and characterization of alphalH from human heart, a member of the T-type Ca2+ channel gene family. Circ Res 83 103-9 Currie KP, Fox AP (1997) Comparison of N- and P/Q-type voltage-gated calcium channel current inhibition. J Neurosci 17 4570-9... 

Ishikawa T, Kaneko M, Shin HS, Takahashi T (2005) Presynaptic N-type and P/Q-type Ca2+ channels mediating synaptic transmission at the calyx of Held of mice. J Physiol 568 199-209 Jarvis SE, Barr W, Feng ZP, Hamid J, Zamponi GW (2002) Molecular determinants of syntaxin 1 modulation of N-type calcium channels. J Biol Chem 277 44399 407 Jarvis SE, Magga JM, Beedle AM, Braun JE, Zamponi GW (2000) G protein modulation of N-type calcium channels is facilitated by physical interactions between syntaxin 1A and Gbetagamma. J Biol Chem 275 6388-94... 

Using CB, knockout animals, a study in hippocampal slices definitively showed that activation of the CB, receptor was responsible for the downstream inhibition of presynaptic VDCC in cells in which DSI was produced (Wilson et al., 2001). The investigators also showed that a specific subtype of GABAergic intemeuron is targeted by endocannabinoids. In this case, activation of CB, receptors led to the specific inhibition of N- but not P/Q- type calcium channels, and this resulted in decreased release of neurotransmitter. 

The notion that N-type and P/Q-type channel knockout mice are phenotypically distinct suggests that these two calcium channel subtypes contribute to different aspects of synaptic function. Indeed, is has been reported that P/Q-type channels are more frequently linked to excitatory transmission, whereas N-type channels may contribute more often to inhibitory transmission (Potier et al. 1993). This is an important point to consider in the context of channel modulation as outlined below. 

Overall, the regulation of presynaptic calcium channels by different types of calcium binding proteins may provide for mechanisms by which neurons can fine tune the amount of calcium entering presynaptic nerve terminals, shift the relative contributions of N-type and P/Q-type channels to calcium entry, and thus regulate the amount of neurotransmitter that can be released from the synapse. 

Arikkath J, Campbell KP (2003) Auxiliary subunits essential components of the voltage-gated calcium channel complex. Curr Opin Neurobiol 13 298-307 Amot MI, Stotz SC, Jarvis SE, Zamponi GW (2000) Differential modulation of N-type IB and P/Q-type 1A calcium channels by different G protein subunit isoforms. J Physiol 527 Pt 2 203-12... 

L-type calcium channels are the primary trigger for excitation-contraction (EC) coupling in cardiac, skeletal, and smooth muscles (Bean, 1989). They are also found in most central and peripheral neurons where they in part control calcium-dependent gene expression, as well as in endocrine cells and many types of non-excitable cells where they contribute to a variety of processes including exocytotic release. Unlike most synapses in the brain and spinal cord that rely on P/Q- and N-type calcium channels for neurotransmitter release, (Wheeler et al., 1994), the presynaptic terminals in photoreceptor cells rely on the Cav1.4 (a1F) L-type calcium channel for mediating glutamate release (Tachibana et al., 1993 Nachman-Clewner et al., 1999). Photoreceptor neurotransmission is atypical first,... 

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