N‐type calcium channels mediate a GABAB presynaptic modulation in the corticostriatal synapse in turtle's paleostriatum augmentatum

Spikes population evoked by a paired pulse protocol were used to assess the influence of GABA_A and GABA_B receptors agonists and antagonists on the synaptic potentials and in the S₂/S₁ ratio in a paired pulse (PP) protocol in the cortico‐paleostriatum augmentatum synapses of the turtle. GABA_A agonist, muscimol, decreased the amplitude of synaptic responses whereas the facilitation produced with the PP protocol did not change, suggesting a postsynaptic action for GABA_A receptors. GABA_B agonist, baclofen, enhanced paired pulse ratio indicating a presynaptic modulation through the GABA_B receptor. Selective antagonists for N‐ and P/Q‐type Ca²⁺‐channels also enhanced paired pulse ratio, suggesting that any of these channel types may be involved in neurotransmitter release. However, the strong paired pulse facilitation produced by baclofen was occluded by blocking the N‐type Ca²⁺ channels with ω‐conotoxin GVIA (1 μM), but not by the blockage of P/Q‐type Ca²⁺ channels with ω‐agatoxin TK (400 nM). These data suggest that N and P/Q channels participate in the neurotransmitter release, whereas only N‐type Ca²⁺ channels are involved in the presynaptic modulation of GABA_B in the corticostriatal synapse of the turtle. Synapse 63:855–862, 2009. © 2009 Wiley‐Liss, Inc.