Calcium Channel Subtypes in Lamprey Sensory and Motor Neurons

El Manira, A. and N. Bussières. Calcium channel subtypes in different modulators require further study. Characterization lamprey sensory and motor neurons. J. Neurophysiol. 78: 1334– of the types and properties of calcium channels in identified 1340, 1997. Pharmacologically distinct calcium channels have spinal cord neurons is necessary to provide insight into their been characterized in dissociated cutaneous sensory neurons and specific roles and the functional significance of these chanmotoneurons of the larval lamprey spinal cord. To enable cell nels for neuromodulation. identification, sensory dorsal cells and motoneurons were selecIn mammals, biophysical and pharmacological studies tively labeled with fluorescein-coupled dextran amine in the intact have documented the existence of both LVA and HVA calspinal cord in vitro before dissociation. Calcium channels present cium channels. The latter can be subdivided into the L, N, in sensory dorsal cells, motoneurons, and other spinal cord neurons P, and Q subtypes and have been characterized with the use were characterized with the use of whole cell voltage-clamp recordings and specific calcium channel agonist and antagonists. The of specific blockers (Fox et al. 1987; Llinás et al. 1992; results show that a transient low-voltage-activated (LVA) calcium Mintz et al. 1992; Nowycky et al. 1985; Pearson et al. 1995; current was present in a proportion of sensory dorsal cells but not Randall and Tsien 1995). Furthermore, molecular cloning in motoneurons, whereas high-voltage-activated (HVA) calcium has allowed the identification of additional calcium channels currents were seen in all neurons recorded. The different compothat are widely distributed in the nervous system (see Birnnents of HVA current were dissected pharmacologically and simibaumer et al. 1994; Snutch and Reiner 1992; Tsien et al. lar results were obtained for both dorsal cells and motoneurons. 1991). These different calcium channels contribute to variThe N-type calcium channel antagonist v-conotoxin-GVIA ous physiological functions and they can be differentially (v-CgTx) blocked ú70% of the HVA current. A large part of the controlled by specific neuromodulators (Bean 1989; Tsien v-CgTx block was reversed after washout of the toxin. The L-type et al. 1988). calcium channel antagonist nimodipine blocked Ç15% of the total HVA current. The dihydropyridine agonist ({)-BayK 8644 markCharacterization and analysis of the relative importance edly increased the amplitude of the calcium channel current. The of the different calcium channels have been performed in BayK-potentiated current was not affected by v-CgTx, indicating spinal cord neurons in different vertebrate species. In rat that the reversibility of the v-CgTx effect is not due to a blockade spinal cord neurons, N-type channels contribute Ç50% and of L-type channels. Simultaneous application of v-CgTx and niL-type channels represent 20–30% of the total calcium curmodipine left Ç15% of the HVA calcium channel current, a small rent (Regan et al. 1991). In chick embryo dorsal root ganpart of which was blocked by the P/Q-type channel antagonist vglion neurons and sympathetic neurons of both rat and frog, agatoxin-IVA. In the presence of the three antagonists, the persisthe calcium current is mediated largely through N-type chantent residual current (Ç10%) was completely blocked by cadmium. nels (Boland et al. 1994; Cox and Dunlap 1992). Spinal Our results provide evidence for the existence of HVA calcium cord neurons in the Xenopus embryo also possess v-conochannels of the N, L, and P/Q types and other HVA calcium channels in lamprey sensory neurons and motoneurons. In addition, toxin-GVIA (v-CgTx)-sensitive calcium channels, but they certain types of neurons express LVA calcium channels. do not exhibit any dihydropyridine-sensitive L-type channels (Barish 1991; Wall and Dale 1994). The relative contribution of the different calcium channel subtypes to the total I N T R O D U C T I O N calcium current in spinal cord neurons thus varies between the species and the cell type studied. Activation of voltage-gated calcium channels controls a In the lamprey, which is a lower vertebrate, calcium entry variety of neuronal processes, including neurotransmitter rethrough voltage-activated channels can regulate the firing lease and ion channel activation or inactivation (see Hille properties of single neurons through activation of calcium1992). In the lamprey locomotor network, the intracellular dependent potassium channels and modulate the overall accalcium level in spinal neurons phasically fluctuates during tivity of the locomotor networks (see Grillner et al. 1995). locomotor activity (Bacskai et al. 1995). Both low-voltageTo understand the relative roles of the different calcium activated (LVA) and high-voltage-activated (HVA) calcium channels, it is necessary to define their pharmacological prochannels are present in lamprey spinal cord neurons (Matfile as well as their relative contribution to the total calcium sushima et al. 1993). These can be modulated by different current in different types of spinal neurons. This will provide transmitters, for example g-aminobutyric acid, dopamine, information on whether the same pharmacological classifiand serotonin (El Manira et al. 1997; Matsushima et al. cation of calcium channels applies to lower vertebrates. In 1993; Schotland et al. 1995), resulting in changes in firing the present study we utilized whole cell patch-clamp techfrequency and the strength of synaptic transmission (see niques on identified cutaneous sensory neurons, dorsal cells, Grillner et al. 1995). Although the modulation of calcium and motoneurons to determine the different types of calcium channels has been studied in some detail, the specific types channels present. Using specific agonists and antagonists, of LVA and HVA calcium channels present in different lamprey spinal neurons and their relative responsiveness to we provide evidence for the existence of N-, L-, and P/Q-