GABA(B) receptor activation causes a depression of low- and high-voltage- activated Ca²⁺ currents, postinhibitory rebound, and postspike afterhyperpolarization in lamprey neurons

1. Activation of γ-aminobutyric acid-B (GABA(B)) receptors during N- methyl-D-aspartate (NMDA)-induced fictive locomotor activity in the lamprey spinal cord reduces the burst frequency and changes the intersegmental coordination. Presynaptic inhibition of both the excitatory and inhibitory synaptic transmission from spinal premotor interneurons occurs through GABA(B) receptor activation. To further analyze the cellular mechanisms underlying the GABA(B)ergic modulation of the locomotor network, the present study investigates somatodendritic effects of GABA(B) receptor activation on interneurons and motoneurons in the lamprey spinal cord in vitro using single-electrode current- and voltage-clamp techniques. 2. High- (HVA) and low- (LVA) voltage-activated calcium currents were studied with single- electrode voltage clamp when Na⁺ and K^- currents were blocked-using tetrodotoxin, tetraethylammonium (TEA), and CsCl electrodes-after substituting Ca²⁺ with Ba²⁺. Cobalt-sensitive inward barium currents, activated at -50 mV, became larger when the holding potential was set to a more hyperpolarized level, thus suggesting the existence of an LVA calcium current. The presence of cobalt-sensitive inward barium currents activated at -30 and -10 mV suggests the existence of an HVA calcium current. GABA(B) receptor activation (baclofen) reduced the peak amplitude of both the LVA and HVA Ca²⁺ component. 3. The late phase of the afterhyperpolarization (AHP), which follows the action potential, was reduced in amplitude by cobalt, thus lending further support to the notion that the Ca²⁺ influx, and the subsequent activation of Ca²⁺-dependent K⁺ channels (K(Ca²⁺)), constitutes the major part of the AHP generation. Application of the GABA(B) agonist baclofen also reduced the peak amplitude of the AHP in interneurons and motoneurons, and this reduction was counteracted by the GABA(B) antagonist 2(OH)saclofen. Baclofen reduced the duration of action potentials broadened by TEA, thus suggesting that the Ca²⁺ inflow was reduced. Intracellular injection of the GTP analogue GTPγS also reduced the duration of the action potential and the peak amplitude of the AHP in TEA, thus supporting the notion that a GTP-binding protein (G-protein)-mediated GABA(B) receptor activation reduced the calcium inflow, leading to less activation of K(Ca) channels and, consequently, to a smaller peak amplitude of the AHP. 4. Baclofen suppressed the subthreshold depolarization induced by a depolarizing current pulse injection without affecting either the spike threshold or the resting membrane conductance. When the synaptic potentials occurring during fictive locomotion were mimicked with a sinusoidal current injection, baclofen and also cobalt caused a delay of the onset of the first action potential and reduced the number of action potentials during the depolarized phase. 5. Several neurons fired on the rebound depolarization that followed a hyperpolarizing current pulse, with a dependence on holding potential, negative current strength, and pulse duration. Baclofen reversibly depressed these rebound depolarizing responses. These results indicate that the LVA calcium current constitutes at least a part in the generation of the postinhibitory rebound, which may have an important role in the locomotor network. 6. These data suggest that the baclofen-sensitive, LVA current- mediated effect on the subthreshold and rebound depolarization would be of particular importance for neurons mediating the reciprocal inhibition between the hemisegments in the lamprey spinal cord. GABA(B) receptor activation would then result in a later spike onset, leading to a longer hyperpolarized phase and therefore also to a slower alteration between the hemisegments and, consequently, to a lower burst frequency. In addition, the GABA(B) receptor- mediated reduction of the HVA current, leading to a reduced AHP, could result in a longer burst duration for the hemisegment and thus in a lower burst frequency. Thus the somatodendritic effects of GABA(B) receptor activation reported here, taken together with the previously described GABA(B)ergic presynaptic modulation, may account for the effects of GABA(B) receptor activation on the fictive locomotion in the lamprey.