Ion channels of importance for the locomotor pattern generation in the lamprey brainstem-spinal cord

Abstract

The intrinsic function of the spinal network that generates locomotion can be studied in the isolated brainstem-spinal cord of the lamprey, a lower vertebrate. The motor pattern underlying locomotion can be elicited in the isolated spinal cord. The network consists of excitatory glutamatergic and inhibitory glycinergic interneurones with known connectivity. The current review addresses the different subtypes of ion channels that are present in the cell types that constitute the network. In particular the roles of the different subtypes of Ca2+ channels and potassium channels that regulate integrated neuronal functions, like frequency regulation, spike frequency adaptation and properties that are important for generating features of the motor pattern (e.g. burst termination), are reviewed. By knowing the role of an ion channel at the cellular level, we also, based on previous knowledge of network connectivity, can understand which effect a given ion channel may exert at the different levels from molecule and cell to network and behaviour.

Figure 1. Imaging of calcium fluctuations in motoneurone dendrites

A, during network activity, periodic fluctuations in calcium fluorescence can be detected in distal motoneurone dendrites. Pseudocolour images of the same dendritic region at five different time points are shown. Blue-green depicts low fluorescence, i.e. low calcium levels, while red indicates spots of high calcium fluorescence. B, calcium response in a distal motoneurone dendrite during subthreshold synaptic stimulation, recorded as a fluorescence increase (arbitrary units, a.u.) concomitant with a compound EPSP (the latter not shown). Upon blockade of NMDA channels with APV, about 50 % of the response remains. C, fluorescence measurements from a small region of the dendrite in A. Numbers correspond to images in A. The intra-dendritic calcium level fluctuations are time-locked to the fictive locomotor rhythm, recorded from the ipsilateral ventral root. Modified from Bacskai et al. (1995).

Figure 2. Ca²⁺ channel subtypes present in spinal cord neurones

A, calcium current was elicited in a spinal cord neurone by voltage steps to -20 mV from a holding potential of -90 mV. The N-type Ca²⁺ channel antagonist ω-conotoxin (ω-CgTx) markedly reduced the amplitude of the current, the L-type antagonist nimodipine (Nimo) decreased the amplitude of the current further, and the P/Q-type blocker ω-agatoxin (ω-Aga) had a further small effect on the current. The resistant current was blocked by cadmium. B, average traces showing the effect of the different Ca²⁺ channel antagonists. V_t, test potential; V_h, holding potential. C, the amount of current mediated by N-, L- and P/Q-type channels. Modified from El Manira & Bussières (1997).

Figure 3. Importance of K_Ca channels for spike frequency regulation and NMDA receptor-induced plateau potentials

The types of Ca²⁺ channel involved in the generation of the slow AHP. A, blockade of N-type Ca²⁺ channels by ω-conotoxin GVIA (1 μm) reduced the amplitude of the slow AHP by 76.2 ± 14.9 % (mean ±s.d.). Resting membrane potential of cell illustrated, -64 mV. B, the P/Q-type channel antagonist ω-agatoxin IVA (200-400 nm) also reduced the amplitude of the AHP in 9 out of 11 cells tested (20.3 ± 10.4 %). Resting membrane potential of cell illustrated, -67 mV. C, blockade of L-type Ca²⁺ channels by nimodipine had no effect on the slow AHP. D, the amplitude of the slow AHP (sAHP) determines whether one or several action potentials will occur during the phase of synaptic excitation in the locomotor cycle. A large and long-lasting sAHP will make locomotor bursts shorter. E, K_Ca channels not only cause the sAHP but will also promote the termination of NMDA receptor-induced plateau potentials. The control plateau is markedly prolonged in the presence of the K_Ca channel blocker apamin. A-C modified from Wickström & El Manira (1998).

Figure 4. Different factors contributing to the initiation of the depolarising phase, its maintenance and termination

In addition to conventional synaptic excitation from e.g. excitatory (E) interneurones, voltage-dependent NMDA receptors and LVA Ca²⁺ channels are activated. Ca²⁺ will enter the cell through these channels, cause activation of K_Ca channels and thereby a progressive hyperpolarisation leading to closure of the NMDA channels. The initiation of the depolarising phase is facilitated by activation of ipsilateral excitatory stretch receptor neurones (SR-E), while the termination of the depolarising phase is partially the result of activation of contralateral inhibitory stretch receptor neurones (SR-I).