Asynchronous GABA Release Research Articles

Asynchronous GABA release generates long-lasting inhibition at a hippocampal interneuron–principal neuron synapse

Hippocampal GABAergic interneurons show diverse molecular and morphological properties. The functional significance of this diversity for information processing is poorly understood. Here we show that cholecystokinin (CCK)-expressing interneurons in rat dentate gyrus release GABA in a highly asynchronous manner, in contrast to parvalbumin (PV) interneurons. With a gamma-frequency burst of ten action potentials, the ratio of asynchronous to synchronous release is 3:1 in CCK interneurons but is 1:5 in parvalbumin interneurons. N-type channels trigger synchronous and asynchronous release in CCK interneuron synapses, whereas P/Q-type Ca(2+) channels mediate release at PV interneuron synapses. Effects of Ca(2+) chelators suggest that both a long-lasting presynaptic Ca(2+) transient and a large distance between Ca(2+) source and sensor of exocytosis contribute to the higher ratio of asynchronous to synchronous release in CCK interneuron synapses. Asynchronous release occurs at physiological temperature and with behaviorally relevant stimulation patterns, thus generating long-lasting inhibition in the brain.

β‐Adrenoceptor‐mediated long‐term up‐regulation of the release machinery at rat cerebellar GABAergic synapses

Properly regulated interactions among excitatory and inhibitory synapses are critical for brain function. Compared to excitatory synapses, much less is known about the gain control mechanisms at inhibitory synapses. Herein we report a mechanism of noradrenergic long-term potentiation (LTP) at inhibitory synapses following presynaptic beta-adrenoceptor activation. Stimulation of beta-adrenoceptors elicited LTP of GABA release from terminals of cerebellar interneurones. This action was dependent on the cAMP/protein kinase A signalling cascade and independent of the beta-adrenoceptor-mediated acceleration of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel. Furthermore, the beta-adrenoceptor- and protein kinase A-mediated LTP was triggered by enhancement of the Ca2+ sensitivity of the release machinery and increase in the readily releasable pool. beta-Adrenoceptor activation also accelerated the recruitment of GABA into the releasable pool and enhanced synchronous and asynchronous release of GABA from the presynaptic terminal. Thus, the up-regulation of GABA release machinery mediated by noradrenaline and beta-adrenoceptor activation provides a likely mechanism of feedforward inhibition of the cerebellar output neurone Purkinje cell, leading to a profound effect on motor control and learning associated with the cerebellum.

Developmental increase in asynchronous GABA release in cultured hippocampal neurons

Developmental changes in GABAergic synaptic transmission were examined in cultured hippocampal neurons using patch-clamp recordings and Ca 2+ imaging. In paired recordings, tetanization of the presynaptic GABAergic neuron with 80 pulses at either 40 or 80 Hz was accompanied by tetanic depression of inhibitory postsynaptic responses. In neurons that had been cultured for more than two weeks, asynchronous inhibitory postsynaptic currents often appeared during the tetanus and continued for several seconds following stimulation. There was little asynchronous activity in neurons that had been cultured for shorter times. However, no age-related changes were observed in the amplitude of single synchronous inhibitory postsynaptic currents, paired-pulse depression or post-tetanic potentiation of inhibitory postsynaptic currents. Following equimolar replacement of extracellular Ca 2+ with strontium ions (Sr 2+), single autaptic inhibitory postsynaptic currents were depressed in amplitude and asynchronous inhibitory postsynaptic currents were present on the decaying phase. Sr 2+-induced asynchronous inhibitory postsynaptic currents showed no dependence on age in culture. Imaging of Ca 2+ in single GABAergic boutons was performed by including Fluo-3 in the patch pipette. During action potential firing induced by stimulating at 80 Hz for 1 s, intracellular calcium [Ca 2+] i increased rapidly in individual boutons. Following the stimulus, [Ca 2+] i decayed back to baseline within 10–15 s. The half-time of decay increased from 1.7±0.2 s at 15 days in vitro to 4.0±0.2 s at 30 days in vitro ( P<0.05), with a developmental profile that closely matched the increase in asynchronous inhibitory postsynaptic currents. We propose that the increase in tetanus-induced asynchronous GABA-release during the first month of synapse maturation in vitro is caused by a slowing of the Ca 2+-clearing mechanisms in the GABAergic boutons. This results in larger and more prolonged elevations of [Ca 2+] i during tetanic stimulation, which leads to enhanced asynchronous transmitter release. We propose that the results of this study demonstrate a potentially important aspect of synapse maturation during development, and also imply that GABA release is up-regulated in conditions of decreased Ca 2+ buffering and clearing.

Tetanus-induced asynchronous GABA release in cultured hippocampal neurons

Asynchronous GABA release was studied in cultured hippocampal neurons using paired whole-cell recordings. Tetanization of the presynaptic GABAergic neuron was accompanied by a train of IPSCs which showed tetanic depression. Asynchronous IPSCs (asIPSCs) also developed during the train and continued for 1.85±0.3 s after the stimulation. The threshold frequency for evoking asIPSCs was 10 Hz, while maximal asynchronous activity was achieved at 40 Hz. Perfusion with EGTA-AM blocked asIPSCs. The elevation of [Ca 2+] i that accompanies presynaptic action potential firing triggers asynchronous release of GABA vesicles, thereby counteracting tetanic depression of synchronous IPSCs.

Sr2+ supports depolarization-induced suppression of inhibition and provides new evidence for a presynaptic expression mechanism in rat hippocampal slices.

1. We studied the transient suppression of evoked GABAA ergic inhibitory postsynaptic currents (eIPSCs) that follows brief membrane depolarization in rat CA1 hippocampal pyramidal cells, a process called depolarization-induced suppression of inhibition (DSI). We used whole-cell patch electrodes filled with a CsCl-based solution to voltage clamp the currents. All experiments were done in the presence of 50 microM 2-amino-5-phosphonovaleric acid (APV) and 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) to block ionotropic glutamate-induced currents and polysynaptic transmission in the slice preparation. 2. Substituting strontium (Sr2+) for extracellular calcium (Ca2+) led to the appearance of numerous 'asynchronous' small IPSCs following an eIPSC. These asynchronous IPSCs were indistinguishable from TTX-insensitive quantal IPSCs. 3. Although somewhat less effective than Ca2+, Sr2+ was capable of supporting DSI, and both asynchronous and synchronous IPSCs were blocked by the DSI process. 4. During DSI, quantal content of eIPSCs, but not quantal size, was significantly reduced. 5. Sr2+ converted paired-pulse depression (PPD) of eIPSCs to a paired-pulse facilitation (PPF), presumably by altering the probability of release at inhibitory nerve terminals. DSI had no effect on either PPD or PPF. 6. The results show that Sr2+ induces asynchronous release of GABA as it does of other neurotransmitters and changes the probability of release at GABAA ergic terminals as well. Most importantly, the results support the hypothesis that, despite being induced postsynaptically, DSI is expressed presynaptically as a decrease in GABA release, possibly by acting at a site other than the Ca(2+)-dependent release step.