How do G proteins directly control neuronal Ca2+ channel function?

Presynaptic Ca2+ channels--integration centers for neuronal signaling pathways

Calcium influx into presynaptic nerve terminals via voltage-gated Ca2+ channels is an essential step in neurotransmitter release. The predominant Ca2+ channel species in synaptic nerve terminals are P/Q-type and N-type channels, with their relative levels of expression varying across the nervous system. The different distributions of these two channel subtypes are reflected in their distinct physiological and pathological roles, yet their activity is regulated by common mechanisms and both function as part of larger signaling complexes that enable their precise regulation and subcellular targeting. Here, we provide a broad overview of molecular and cellular mechanisms that regulate Ca2+ channels, and how these cellular signaling pathways are integrated at the level of the channel protein.

Voltage-Gated Calcium Channels and Idiopathic Generalized Epilepsies

The idiopathic generalized epilepsies encompass a class of epileptic seizure types that exhibit a polygenic and heritable etiology. Advances in molecular biology and genetics have implicated defects in certain types of voltage-gated calcium channels and their ancillary subunits as important players in this form of epilepsy. Both T-type and P/Q-type channels appear to mediate important contributions to seizure genesis, modulation of network activity, and genetic seizure susceptibility. Here, we provide a comprehensive overview of the roles of these channels and associated subunits in normal and pathological brain activity within the context of idiopathic generalized epilepsy.

Decreased mu-opioid receptor signalling and a reduction in calcium current density in sensory neurons from chronically morphine-treated mice

Sensory neurons are a major site of opioid analgesic action, but the effect of chronic morphine treatment (CMT) on mu-opioid receptor function in these cells is unknown. We examined mu-opioid receptor modulation of calcium channel currents (I(Ca)) in small trigeminal ganglion (TG) neurons from mice chronically treated with morphine and measured changes in mu-opioid receptor mRNA levels in whole TG. Mice were injected subcutaneously with 300 mg kg(-1) of morphine base in a slow release emulsion three times over 5 days, or with emulsion alone (vehicles). CMT mice had a significantly reduced response to the acute antinociceptive effects of 30 mg kg(-1) morphine compared with controls (P=0.035).Morphine inhibited I(Ca) in neurons from CMT (EC(50) 300 nM) and vehicle (EC(50) 320 nM) mice with similar potency, but morphine's maximum effect was reduced from 36% inhibition in vehicle to 17% in CMT (P<0.05). Similar results were observed for the selective mu-opioid agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol enkephalin (DAMGO). Inhibition of I(Ca) by the GABA(B) agonist baclofen was unaffected by CMT. In neurons from CMT mice, there were significant reductions in P/Q-type (P=0.007) and L-type (P=0.002) I(Ca) density.mu-Opioid receptor mRNA levels were not altered by CMT. These data demonstrate that CMT produces a significant reduction of the effectiveness of mu-opioid agonists to inhibit I(Ca) in TG sensory neurons, accompanied by a reduction in I(Ca) density. Thus, adaptations in sensory neurons may mediate some of the tolerance to the antinociceptive effects of morphine that develop during systemic administration.

Molecular mechanisms underlying the modulation of exocytotic noradrenaline release via presynaptic receptors

The release of noradrenaline from nerve terminals is modulated by a variety of presynaptic receptors. These receptors belong to one of the following three receptor superfamilies: transmitter-gated ion channels, G protein-coupled receptors (GPCR), and membrane receptors with intracellular enzymatic activities. For representatives of each of these three superfamilies, receptor activation has been reported to cause either an enhancement or a reduction of noradrenaline release. As these receptor classes display greatly diverging structures and functions, a multitude of different molecular mechanisms are involved in the regulation of noradrenaline release via presynaptic receptors. This review gives a short overview of the presynaptic receptors on noradrenergic nerve terminals and summarizes the events involved in vesicle exocytosis in order to finally delineate the most important signaling cascades that mediate the modulation via presynaptic receptors. In addition, the interactions between the various presynaptic receptors are described and the underlying molecular mechanisms are elucidated. Together, these presynaptic signaling mechanisms form a sophisticated network that precisely adapts the amount of noradrenaline being released to a given situation.