Jaime Barral

Muscarinic presynaptic inhibition of neostriatal glutamatergic afferents is mediated by Q-type Ca2+ channels ☆

Cholinergic presynaptic inhibition was investigated on neostriatal glutamatergic transmission. Paired pulse facilitation (PPF) of orthodromic population spikes (PS) were used to construct a concentration-response relationship for muscarine on presynaptic inhibition. Muscarine had an effect proportional to its extracellular concentration with an EC50 (mean +/- standard estimation error) of: 2.5 +/- 1.5 nM, and a maximal effect (saturation) of 245 +/- 16%. Several peptidic toxins against some voltage-gated Ca2+-channels increased PPF indicating that the Ca2+-channels they block participate in transmitter release. However, neither 1 microM omega-conotoxin GVIA, a specific blocker of N-type Ca2+-channels, nor 10-30 nM omega-agatoxinTK, a selective blocker of P-type Ca2+-channels, were able to occlude muscarines effect on presynaptic inhibition. Nevertheless, 100-400 nM omega-agatoxinTK occluded muscarines action on PPF in a dose-dependent manner. These results are consistent with Q-type Ca2+-channels mediating muscarinic presynaptic inhibition of neostriatal afferents.

Gabaergic presynaptic inhibition of rat neostriatal afferents is mediated by Q-type Ca2+ channels

Population spikes associated with the paired pulse facilitation paradigm have been successfully used to measure presynaptic inhibition in several systems. In the present work, this paradigm was used to evaluate the action of baclofen on neostriatal glutamatergic transmission. Baclofen enhanced synaptic facilitation with an EC(50)=0.57 microM and a maximal effect of 457%. Selective antagonists for N-, P- and Q-type Ca(2+)-channels enhanced paired pulse facilitation; suggesting that these channel types participate in the release of transmitter. Nevertheless, neither 1 microM omega-conotoxin GVIA, nor 20 nM omega-agatoxinTK occluded the action of baclofen. Baclofens action was occluded only by 400 nM omega-agatoxinTK. These data suggest that Q-type Ca(2+)-channels mediate gamma-aminobutyric acid(B) presynaptic inhibition of neostriatal afferents.

5‐HT1A, 5‐HT2, and GABAB receptors interact to modulate neurotransmitter release probability in layer 2/3 somatosensory rat cortex as evaluated by the paired pulse protocol

Activation of γ‐aminobutyric acid B (GABAB) and 5‐hydroxytryptamine (5‐HT) receptors produces presynaptic inhibition at glutamatergic terminals in the rat neocortex. To evaluate interactions between these metabotropic receptors, field potentials were recorded in layer 2/3 of somatosensory cortex. In addition, the paired pulse (PP) protocol was used to measure changes in the ratio of the second/first extracellular synaptic potentials (S2/S1 ratio) as an index of glutamate release probability in the area. Lowering extracellular [Ca2+]o to 0.5 mM, increased the S2/S1 ratio by 318 ± 134%. 5‐HT (1 μM) increased the S2/S1 ratio by 61 ± 15%. In presence of the GABAA antagonist bicuculline (10 μM), 5‐HT increased the S2/S1 ratio by 98 ± 15%. This effect did not desensitize after two consecutive applications of the amine, and was dose dependent in the concentration range between 0.03–1 μM (EC50 = 2.36 × 10−7 mol/L). The increase of S2/S1 ratio induced by 5‐HT (1 μM) was blocked reversibly by the 5‐HT1A antagonist NAN‐190 (10–30 μM), but was unaffected by the selective GABAB antagonist CGP 52432 (1 μM). The action of 5‐HT was mimicked by the 5‐HT1A/7 agonist 8OH‐DPAT (10 μM), increasing the S2/S1 ratio by 84 ± 2%, a response that was unaffected by the 5‐HT2/7 antagonist ritanserin (2 μM). The 5‐HT1B agonist CP93129 (10 μM) had no effect. The GABAB agonist baclofen (1 μM) increased the S2/S1 ratio up to 308 ± 33%, which is similar to that produced by low [Ca2+]o. When the effect of baclofen was maximal, application of 5‐HT (1 μM) reversed the S2/S1 ratio back to 78 ± 27%, a result that was blocked by the 5‐HT2/7 antagonist ritanserin (100 nM). Notably, the interaction between the GABAB agonist and 5‐HT was order dependent, because enhancement of the S2/S1 ratio elicited by baclofen was not inhibited if 5‐HT was applied first. These results suggest a complex interaction between GABAB, 5‐HT1A, and 5‐HT2 receptors in layer 2/3 of rat somatosensory cortex. Activation of GABAB receptors induces PP facilitation (inhibits glutamate release) more efficiently than does activation of 5‐HT1A receptors. When the effect of GABAB receptor activation is maximal, however, the influence of 5‐HT changes to the opposite direction, inhibiting PP facilitation (increasing glutamate release) through activation of 5‐HT2 receptors.

Differential effects of caffeine on the antidepressant-like effect of amitriptyline in female rat subpopulations with low and high immobility in the forced swimming test.

The interaction of caffeine (1 mg/kg) and amitriptyline (15 mg/kg) on the immobility time (IT) during Porsolts forced swimming test (FST) was investigated in female Wistar rats. Akaikes Information Criterion indicated that the ITs recorded from 142 rats during the first day of the FST followed a bimodal distribution. Hence, the median (125.5 s) was used to classify the animals in subpopulations with low (<125.5 s, LI-rats) or high (>125.5 s, HI-rats) immobility. The paired t-test was used to compare the change of ITs between the first and second swimming sessions. Vehicle-treated animals had a significant increase of ITs during the second day of the test, either in LI-rats (77+/-12 s vs. 196+/-8 s, P<0.0001, n=6) or HI-rats (150+/-8 s vs. 201+/-10 s, P<0.02, n=6). In LI-rats amitriptyline only prevented the increase of ITs during the second session (74+/-27 s vs. 97+/-42 s, n=12), whereas in HI-rats the antidepressant produced a significant decrease of ITs during the second session (161+/-22 s vs. 118+/-32 s, n=7, P<0.02). While caffeine alone prevented the increase of ITs in both groups, the methylxanthine abolished the effect of amitriptyline in HI-rats (165+/-23 s vs. 165+/-46 s, n=9), leaving the antidepressant action unaffected in LI-rats (87+/-23 s vs. 96+/-58 s, n=9). These results suggest that the anti-immobility effect of amitriptyline in HI-rats is mediated in part by endogenous adenosine.

The presynaptic modulation of corticostriatal afferents by μ-opioids is mediated by K+ conductances

Population spikes associated with the paired pulse ratio protocol were used to measure the presynaptic inhibition of corticostriatal transmission caused by mu-opioid receptor activation. A 1 microM of [D-Ala(2), N-MePhe(4), Gly-ol(5)]-enkephalin (DAMGO), a selective mu-opioid receptor agonist, enhanced paired pulse facilitation by 44+/-8%. This effect was completely blocked by 2 nM of the selective mu-receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-NH (CTOP). Antagonists of N- and P/Q-type Ca(2+) channels inhibited, whereas antagonists of potassium channels enhanced, synaptic transmission. A 1 microM of omega-conotoxin GVIA, a blocker of N-type Ca(2+) channels, had no effect on the action of DAMGO, but 400 nM omega-agatoxin TK, a blocker of P/Q-type Ca(2+)-channels, partially blocked the action of this opioid. However, 5 mM Cs(2+) and 400 microM Ba(2+), unselective antagonists of potassium conductances, completely prevented the action of DAMGO on corticostriatal transmission. These data suggest that presynaptic inhibition of corticostriatal afferents by mu-opioids is mediated by the modulation of K(+) conductances in corticostriatal afferents.

High-affinity inhibition of glutamate release from corticostriatal synapses by ω-agatoxin TK

Abstract To know which Ca2+ channel type is the most important for neurotransmitter release at corticostriatal synapses of the rat, we tested Ca2+ channel antagonists on the paired pulse ratio. ω-Agatoxin TK was the most effective Ca2+ channel antagonist (IC50=127 nM; maximal effect=211% (with >1 μM) and Hill coefficient=1.2), suggesting a single site of action and a Q-type channel profile. Corresponding parameters for Cd2+ were 13 μM, 178% and 1.2. The block of L-type Ca2+ channels had little impact on transmission, but we also tested facilitation of L-type Ca2+ channels. The L-type Ca2+ channel agonist, s-(−)-1,4 dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)phenyl]-3-pyridine carboxylic acid methyl ester (Bay K 8644 (5 μM)), produced a 45% reduction of the paired pulse ratio, suggesting that even if L-type channels do not participate in the release process, they may participate in its modulation.

Dopaminergic Modulation of Spiny Neurons in the Turtle Striatum

Intracellular recordings were obtained from brain slice preparation in neurons of the striatum of the turtle Trachemys scripta elegans, analogous to the mammalian striatum in its topographic organization, synaptic connectivity, cytoarchitecture, and neurochemistry. Here we show that these similarities extend to the electrophysiological properties of its neurons. Biocytin staining revealed that 85% of the recorded neurons were medium spiny neurons while 15% were aspiny neurons. Spiny neurons of the turtle resembled those found in the mammalian and avian striatum and express dopaminergic D1 and D2 class receptors. Because the striatum of the turtle receives a dense dopaminergic innervation from tegmental dopaminergic neurons we investigated the postsynaptic actions of selective dopamine receptor agonists in the excitability of spiny neurons. As in mammals and birds, activation of D1-receptors enhances, whereas activation of D2-receptors decreases the evoked discharge. Apparently, actions of dopamine agonists occur via the modulation of L-type (CaV1) Ca2+-conductances. Strong cellular evidence suggests that the role of dopamine in the modulation of motor networks is preserved along vertebrate evolution.

δ Opioids reduce the neurotransmitter release probability by enhancing transient (KV4) K+-currents in corticostriatal synapses as evaluated by the paired pulse protocol

Field recordings were used to determine the influence of delta-opioid receptor activation on corticostriatal synaptic transmission. Application of the selective delta-opioid receptor agonist, [Tyr-D-Pen-Gly-Phe-D-Pen]-enkephalin (DPDPE, 1 microM), decreased the amplitude of the field-excitatory synaptic potential and at the same time increased the paired pulse ratio (PPR) suggesting a presynaptic site of action. This response reversed rapidly when DPDPE was washed and blocked by 1 nM of the selective delta-receptor antagonist naltrindole. Neither omega-conotoxin GVIA (1 microM) nor omega-agatoxin TK (400 nM), blockers of N- and P/Q-type Ca2+-channels, respectively, nor TEA (1 mM), blocker of some classes of K+-channels, occluded the effects of DPDPE. Instead, 1 mM 4-AP or 400 microM Ba2+ occluded completely the effects of DPDPE. Therefore, the results suggest that the modulation by delta opioids at corticostriatal terminals is mediated by transient (KV4) K+-conductances.

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

Presynaptic modulation has been associated mainly with calcium channels but recent data suggests that inward rectifier potassium channels (KIR) also play a role. In this work we set to characterize the role of presynaptic KIR channels in corticostriatal synaptic transmission. We elicited synaptic potentials in striatum by stimulating cortical areas and then determined the synaptic responses of corticostriatal synapsis by using paired pulse ratio (PPR) in the presence and absence of several potassium channel blockers. Unspecific potassium channels blockers Ba2+ and Cs+ reduced the PPR, suggesting that these channels are presynaptically located. Further pharmacological characterization showed that application of tertiapin‐Q, a specific KIR3 channel family blocker, also induced a reduction of PPR, suggesting that KIR3 channels are present at corticostriatal terminals. In contrast, exposure to Lq2, a specific KIR1.1 inward rectifier potassium channel, did not induce any change in PPR suggesting the absence of these channels in the presynaptic corticostriatal terminals. Our results indicate that KIR3 channels are functionally expressed at the corticostriatal synapses, since blockage of these channels result in PPR decrease. Our results also help to explain how synaptic activity may become sensitive to extracellular signals mediated by G‐protein coupled receptors. A vast repertoire of receptors may influence neurotransmitter release in an indirect manner through regulation of KIR3 channels. Synapse 69:446–452, 2015. 2015 Wiley Periodicals, Inc.

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 GABAA and GABAB 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. GABAA agonist, muscimol, decreased the amplitude of synaptic responses whereas the facilitation produced with the PP protocol did not change, suggesting a postsynaptic action for GABAA receptors. GABAB agonist, baclofen, enhanced paired pulse ratio indicating a presynaptic modulation through the GABAB 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 GABAB in the corticostriatal synapse of the turtle. Synapse 63:855–862, 2009.