Acid-sensing ion channels (ASICs) regulate synaptic activities and play important roles in neurodegenerative diseases. We found that these channels can be activated in neurons of the medial nucleus of the trapezoid body (MNTB) of the auditory system in the CNS. A drop in extracellular pH induces transient inward ASIC currents (I_ASICs) in postsynaptic MNTB neurons from wild-type mice. The inhibition of I_ASICs by psalmotoxin-1 (PcTx1) and the absence of these currents in knock-out mice for ASIC-1a subunit (ASIC1a^−/−) suggest that homomeric ASIC-1as are mediating these currents in MNTB neurons. Furthermore, we detect ASIC1a-dependent currents during synaptic transmission, suggesting an acidification of the synaptic cleft due to the corelease of neurotransmitter and H⁺from synaptic vesicles. These currents are capable of eliciting action potentials in the absence of glutamatergic currents. A significant characteristic of these homomeric ASIC-1as is their permeability to Ca²⁺. Activation of ASIC-1a in MNTB neurons by exogenous H⁺induces an increase in intracellular Ca²⁺. Furthermore, the activation of postsynaptic ASIC-1as during high-frequency stimulation (HFS) of the presynaptic nerve terminal leads to a PcTx1-sensitive increase in intracellular Ca²⁺in MNTB neurons, which is independent of glutamate receptors and is absent in neurons from ASIC1a^−/−mice. During HFS, the lack of functional ASICs in synaptic transmission results in an enhanced short-term depression of glutamatergic EPSCs. These results strongly support the hypothesis of protons as neurotransmitters and demonstrate that presynaptic released protons modulate synaptic transmission by activating ASIC-1as at the calyx of Held–MNTB synapse.SIGNIFICANCE STATEMENTThe manuscript demonstrates that postsynaptic neurons of the medial nucleus of the trapezoid body at the mouse calyx of Held synapse express functional homomeric Acid-sensing ion channel-1a (ASIC-1as) that can be activated by protons (coreleased with neurotransmitter from acidified synaptic vesicles). These ASIC-1as contribute to the generation of postsynaptic currents and, more relevant, to calcium influx, which could be involved in the modulation of presynaptic transmitter release. Inhibition or deletion of ASIC-1a leads to enhanced short-term depression, demonstrating that they are concerned with short-term plasticity of the synapse. ASICs represent a widespread communication system with unique properties. We expect that our experiments will have an impact in the neurobiology field and will spread in areas related to neuronal plasticity.

Serotonin receptors are targets of drug therapies for a variety of neuropsychiatric and neurodegenerative disorders. Cocaine inhibits the re‐uptake of serotonin (5‐HT), dopamine, and noradrenaline, whereas caffeine blocks adenosine receptors and opens ryanodine receptors in the endoplasmic reticulum. We studied how 5‐HTand adenosine affected spontaneousGABAergic transmission from thalamic reticular nucleus. We combined whole‐cell patch clamp recordings of miniature inhibitory post‐synaptic currents (mIPSCs) in ventrobasal thalamic neurons during local (puff) application of 5‐HTin wild type (WT) or knockout mice lacking 5‐HT_2Areceptors (5‐HT_2A−/−). Inhibition of mIPSCs frequency by low (10 μM) and high (100 μM) 5‐HTconcentrations was observed in ventrobasal neurons from 5‐HT_2A−/− mice. InWTmice, only 100 μM 5‐HTsignificantly reduced mIPSCs frequency. In 5‐HT_2A−/− mice,NAN‐190, a specific 5‐HT_1Aantagonist, prevented the 100 μM 5‐HTinhibition while blocking H‐currents that prolonged inhibition during post‐puff periods. The inhibitory effects of 100 μM 5‐HTwere enhanced in cocaine binge‐treated 5‐HT_2A−/− mice. Caffeine binge treatment did not affect 5‐HT‐mediated inhibition. Our findings suggest that both 5‐HT_1Aand 5‐HT_2Areceptors are present in pre‐synaptic thalamic reticular nucleus terminals. Serotonergic‐mediated inhibition ofGABArelease could underlie aberrant thalamocortical physiology described after repetitive consumption of cocaine. image Our findings suggest that both 5‐HT_1A, 5‐HT_2Aand A1 receptors are present in pre‐synaptic TRN terminals. 5‐HT_1Aand A1 receptors would down‐regulate adenylate cyclase, whereas 5‐HT_1Awould also increase the probability of the opening of G‐protein‐activated inwardly rectifying K⁺channels (GIRK). Sustained opening of GIRK channels would hyperpolarize pre‐synaptic terminals activating H‐currents, resulting in less GABA release. 5‐HT_2A‐would activate PLC and IP₃, increasing intracellular [Ca²⁺] and thus facilitating GABA release.

Methylphenidate (MPH) is widely used to treat children and adolescents diagnosed with attention deficit/hyperactivity disorder. AlthoughMPHshares mechanistic similarities to cocaine, its effects onGABAergic transmission in sensory thalamic nuclei are unknown. Our objective was to compare cocaine andMPHeffects onGABAergic projections between thalamic reticular and ventrobasal (VB) nuclei. Mice (P18‐30) were subjected tobinge‐like cocaine andMPHacuteandsub‐chronicadministrations. Cocaine andMPHenhanced hyperlocomotion, althoughsub‐chroniccocaine‐mediated effects were stronger thanMPHeffects. Cocaine andMPHsub‐chronicadministration altered paired‐pulse and spontaneousGABAergic input differently. The effects of cocaine on evoked paired‐pulseGABA‐mediated currents changed from depression to facilitation with the duration of the protocols used, whileMPHinduced a constant increase throughout the administration protocols. Thalamic reticular nucleusGAD67 andVBCa_V3.1 protein levels were measured using western blot to better understand their link to increasedGABArelease. Both proteins were increased bysub‐chronicadministration of cocaine.MPHshowed effects onGABAergic transmission that seems less disruptive than cocaine. Unique effects of cocaine on postsynapticVBcalcium currents might explain deleterious cocaine effects on sensory thalamic nuclei. These results suggest that cocaine andMPHproduced distinct presynaptic alterations onGABAergic transmission.

Acid-sensing ion channels (ASIC) open in response to extracellular acidosis. ASIC1a, a particular subtype of these channels, has been described to have a postsynaptic distribution in the brain, being involved not only in ischemia and epilepsy, but also in fear and psychiatric pathologies. High-frequency stimulation of skeletal motor nerve terminals (MNTs) can induce presynaptic pH changes in combination with an acidification of the synaptic cleft, known to contribute to muscle fatigue. Here, we studied the role of ASIC1a channels on neuromuscular transmission. We combined a behavioral wire hanging test with electrophysiology, pharmacological, and immunofluorescence techniques to compare wild-type and ASIC1a lacking mice (ASIC1a ^−/− knockout). Our results showed that 1) ASIC1a ^−/− female mice were weaker than wild type, presenting shorter times during the wire hanging test; 2) spontaneous neurotransmitter release was reduced by ASIC1a activation, suggesting a presynaptic location of these channels at individual MNTs; 3) ASIC1a-mediated effects were emulated by extracellular local application of acid saline solutions (pH = 6.0; HEPES/MES-based solution); and 4) immunofluorescence techniques revealed the presence of ASIC1a antigens on MNTs. These results suggest that ASIC1a channels might be involved in controlling neuromuscular transmission, muscle contraction and fatigue in female mice.