N-type voltage-dependent Ca 2؉ channels (VDCCs), predominantly localized in the nervous system, have been considered to play an essential role in a variety of neuronal functions, including neurotransmitter release at sympathetic nerve terminals. As a direct approach to elucidating the physiological significance of N-type VDCCs, we have generated mice genetically deficient in the ␣1B subunit (Cav 2.2). The ␣1B-deficient null mice, surprisingly, have a normal life span and are free from apparent behavioral defects. A complete and selective elimination of N-type currents, sensitive to -conotoxin GVIA, was observed without significant changes in the activity of other VDCC types in neuronal preparations of mutant mice. The baroreflex response, mediated by the sympathetic nervous system, was markedly reduced after bilateral carotid occlusion. In isolated left atria prepared from N-type-deficient mice, the positive inotropic responses to electrical sympathetic neuronal stimulation were dramatically decreased compared with those of normal mice. In contrast, parasympathetic nervous activity in the mutant mice was nearly identical to that of wild-type mice. Interestingly, the mutant mice showed sustained elevation of heart rate and blood pressure. These results provide direct evidence that N-type VDCCs are indispensable for the function of the sympathetic nervous system in circulatory regulation and indicate that N-type VDCC-deficient mice will be a useful model for studying disorders attributable to sympathetic nerve dysfunction. V oltage-dependent Ca 2ϩ channels (VDCCs) mediate calcium entry into cells, which is essential for a wide variety of physiological functions. Among three types of VDCCs, P͞Q-type, N-type, and R-type, predominantly expressed in neuronal cells (1, 2), N-type Ca 2ϩ channels have been recognized as high-voltage-activated Ca 2ϩ channels selectively sensitive to blockade by -conotoxin GVIA (-CTX), a 27-amino acid peptide isolated from the venom of the fish-hunting cone snail Conus geographus, but resistant to the L-type Ca 2ϩ channelspecific blockers such as dihydropyridines. Molecular studies have revealed that the ␣ 1B (Ca v 2.2) subunit gene encodes N-type Ca 2ϩ channels and is expressed widely in neuronal tissues (3-12). Experiments employing -CTX have demonstrated the physiological significance of the N-type Ca 2ϩ channels in the nervous system, where they have a specific developmental role in the migration of immature neurons before the establishment of their synaptic circuit (13). N-type Ca 2ϩ channels have been shown to be critically involved in the release of neurotransmitters, including glutamate (14-16), ␥-aminobutyric acid (14), acetylcholine (17), dopamine (18)(19)(20), and norepinephrine (21) in mammalian central neurons. For instance, in CA1 hippocampal neurons, the inhibitory GABAergic synaptic potentials are reduced by 85-90% by saturating concentrations of -CTX, whereas the excitatory glutaminergic transmission is blocked by 65-70% (16).In peripheral neurons such as autonomic neurons...

The role of N-type Ca(2+) channels in nociceptive transmission was examined in genetically engineered mice lacking the alpha(1B) subunit of N-type channels and in their heterozygote and wild-type littermates. In alpha(1B)-deficient mice, N-type channel activities in dorsal root ganglion neurons and spinal synaptoneurosomes were eliminated without compensation by other types of voltage-dependent Ca(2+) channels. The alpha(1B)-deficient mice showed a diminution in the phase 2 nociceptive responses more extensively than in the phase 1 nociceptive responses of the formalin test. The alpha(1B)-deficient mice exhibited significantly increased thermal nociceptive thresholds in the hot plate test, but failed to increase mechanical nociceptive thresholds in the tail pinch test. These results suggest a crucial role of N-type channels in nociceptive transmission, especially for persistent pain like phase 2 of the formalin test and for nociception induced by thermal stimuli.

Differential properties of voltage-dependent Ca2+ channels have been primarily ascribed to the alpha1 subunit, of which 10 different subtypes are currently known. For example, channels that conduct the N-type Ca2+ current possess the alpha1B subunit (Cav2.2), which has been localized, inter alia, to the piriform cortex, hippocampus, hypothalamus, locus coeruleus, dorsal raphe, thalamic nuclei, and granular layer of the cortex. Some of these regions have been previously implicated in metabolic and vigilance state control, and selective block of the N-type Ca2+ channel causes circadian rhythm disruption. In this study of Cav2.2-/- knock-out mice, we examined potential differences in feeding behavior, spontaneous locomotion, and the sleep-wake cycle. Cav2.2-/- mice did not display an overt metabolic phenotype but were hyperactive, demonstrating a 20% increase in activity under novel conditions and a 95% increase in activity under habituated conditions during the dark phase, compared with wild-type littermates. Cav2.2-/- mice also displayed vigilance state differences during the light phase, including increased consolidation of rapid-eye movement (REM) sleep and increased intervals between non-REM (NREM) and wakefulness episodes. EEG spectral power was increased during wakefulness and REM sleep and was decreased during NREM sleep in Cav2.2-/- mice. These results indicate a role of the N-type Ca2+ channel in activity and vigilance state control, which we interpret in terms of effects on neurotransmitter release.