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

Abstract

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 Ca(2+)-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 omega-conotoxin GVIA (1 microM), but not by the blockage of P/Q-type Ca2+ channels with omega-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.

Dopaminergic Modulation of Spiny Neurons in the Turtle Striatum

“…The mammalian basal ganglia architecture is highly conserved in birds and reptiles, including circuitry localization of many transmitters and synapses (Parent1976; Reiner et al 1980Reiner et al , 1984Reiner et al , 1998Brauth et al1983;Reiner 1987;Greenberg et al 1988;Marin et al 1998;Medina and Smeets 1991;Fowler et al 1999;Smeets and Gonzalez 2000;Smeets et al , 2003Sánchez-Mejorada et al 2009). Despite slight differences in cell shape, the main cellular type in the striatal part of the basal ganglia is called ''medium spiny neurons'' with spiny dendrites in all amniotes.…”

G‐protein‐coupled inward rectifier potassium channels involved in corticostriatal presynaptic modulation

“…Using the paired pulse protocol, presynaptic regulation by potassium channels is expected to change the PPR (Barral et al, ; Jose et al, ; Sanchez‐Mejorada et al, ). Where PPR is equal to S 2 / S 1 , being S 1 and S 2 the first and second orthodromic responses to the stimuli, respectively; the percentage of change was then calculated as follows: …”

“…Likewise an increase of S 2 indicates paired pulse facilitation (S 1 < S 2 ) rendering PPR > 1. Presynaptic regulation by potassium channels is expected to change the paired pulse ratio (PPR) towards paired pulse depression (Barral et al, 2000(Barral et al, , 2001(Barral et al, , 2003Jose et al, 2007;Sanchez-Mejorada et al, 2009). Control stimuli was adjusted to PPR 1 in order to better visualize reduction of PPR by the action of the K 1 channel blockers.…”