Tetanus-induced asynchronous GABA release in cultured hippocampal neurons

Different types of retinal inhibition have distinct neurotransmitter release properties

Neurotransmitter release varies between neurons due to differences in presynaptic mechanisms such as Ca(2+) sensitivity and timing. Retinal rod bipolar cells respond to brief dim illumination with prolonged glutamate release that is tuned by the differential release of GABA and glycine from amacrine cells in the inner retina. To test if differences among types of GABA and glycine release are due to inherent amacrine cell release properties, we directly activated amacrine cell neurotransmitter release by electrical stimulation. We found that the timing of electrically evoked inhibitory currents was inherently slow and that the timecourse of inhibition from slowest to fastest was GABAC receptors > glycine receptors > GABAA receptors. Deconvolution analysis showed that the distinct timing was due to differences in prolonged GABA and glycine release from amacrine cells. The timecourses of slow glycine release and GABA release onto GABAC receptors were reduced by Ca(2+) buffering with EGTA-AM and BAPTA-AM, but faster GABA release on GABAA receptors was not, suggesting that release onto GABAA receptors is tightly coupled to Ca(2+). The differential timing of GABA release was detected from spiking amacrine cells and not nonspiking A17 amacrine cells that form a reciprocal synapse with rod bipolar cells. Our results indicate that release from amacrine cells is inherently asynchronous and that the source of nonreciprocal rod bipolar cell inhibition differs between GABA receptors. The slow, differential timecourse of inhibition may be a mechanism to match the prolonged rod bipolar cell glutamate release and provide a way to temporally tune information across retinal pathways.

Tyrosine 402 phosphorylation of Pyk2 is involved in ionomycin-induced neurotransmitter release

Protein tyrosine kinases, which are highly expressed in the central nervous system, are implicated in many neural processes. However, the relationship between protein tyrosine kinases and neurotransmitter release remains unknown. In this study, we found that ionomycin, a Ca²⁺ ionophore, concurrently induced asynchronous neurotransmitter release and phosphorylation of a non-receptor protein tyrosine kinase, proline-rich tyrosine kinase 2 (Pyk2), in clonal rat pheochromocytoma PC12 cells and cerebellar granule cells, whereas introduction of Pyk2 siRNA dramatically suppressed ionomycin-induced neurotransmitter release. Further study indicated that Tyr-402 (Y402) in Pyk2, instead of other tyrosine sites, underwent rapid phosphorylation after ionomycin induction in 1 min to 2 min. We demonstrated that the mutant of Pyk2 Y402 could abolish ionomycin-induced dopamine (DA) release by transfecting cells with recombinant Pyk2 and its mutants (Y402F, Y579F, Y580F, and Y881F). In addition, Src inhibition could prolong phosphorylation of Pyk2 Y402 and increase DA release. These findings suggested that Pyk2 was involved in ionomycin-induced neurotransmitter release through phosphorylation of Y402.

Molecular mechanisms for synchronous, asynchronous, and spontaneous neurotransmitter release

Most neuronal communication relies upon the synchronous release of neurotransmitters, which occurs through synaptic vesicle exocytosis triggered by action potential invasion of a presynaptic bouton. However, neurotransmitters are also released asynchronously with a longer, variable delay following an action potential or spontaneously in the absence of action potentials. A compelling body of research has identified roles and mechanisms for synchronous release, but asynchronous release and spontaneous release are less well understood. In this review, we analyze how the mechanisms of the three release modes overlap and what molecular pathways underlie asynchronous and spontaneous release. We conclude that the modes of release have key fusion processes in common but may differ in the source of and necessity for Ca(2+) to trigger release and in the identity of the Ca(2+) sensor for release.

Synapsin II desynchronizes neurotransmitter release at inhibitory synapses by interacting with presynaptic calcium channels

In the central nervous system, most synapses show a fast mode of neurotransmitter release known as synchronous release followed by a phase of asynchronous release, which extends over tens of milliseconds to seconds. Synapsin II (SYN2) is a member of the multigene synapsin family (SYN1/2/3) of synaptic vesicle phosphoproteins that modulate synaptic transmission and plasticity, and are mutated in epileptic patients. Here we report that inhibitory synapses of the dentate gyrus of Syn II knockout mice display an upregulation of synchronous neurotransmitter release and a concomitant loss of delayed asynchronous release. Syn II promotes γ-aminobutyric acid asynchronous release in a Ca²⁺-dependent manner by a functional interaction with presynaptic Ca²⁺ channels, revealing a new role in synaptic transmission for synapsins.

The arrival of action potentials at nerve terminals often leads to synchronous neurotransmitter release. Medrihan and colleagues use electrophysiology on mouse hippocampal neurons to show that the vesicle protein Synapsin II promotes GABAergic asynchronous release by interacting with calcium channels.

Inhibitory coupling between inhibitory interneurons in the spinal cord dorsal horn

Local inhibitory interneurons in the dorsal horn play an important role in the control of excitability at the segmental level and thus determine how nociceptive information is relayed to higher structures. Regulation of inhibitory interneuron activity may therefore have critical consequences on pain perception. Indeed, disinhibition of dorsal horn neuronal networks disrupts the balance between excitation and inhibition and is believed to be a key mechanism underlying different forms of pain hypersensitivity and chronic pain states. In this context, studying the source and the synaptic properties of the inhibitory inputs that the inhibitory interneurons receive is important in order to predict the impact of drug action at the network level. To address this, we studied inhibitory synaptic transmission in lamina II inhibitory interneurons identified under visual guidance in spinal slices taken from transgenic mice expressing enhanced green fluorescent protein (EGFP) under the control of the GAD promoter. The majority of these cells fired tonically to a long depolarizing current pulse. Monosynaptically evoked inhibitory postsynaptic currents (eIPSCs) in these cells were mediated by both GABAA and glycine receptors. Consistent with this, both GABAA and glycine receptor-mediated miniature IPSCs were recorded in all of the cells. These inhibitory inputs originated at least in part from local lamina II interneurons as verified by simultaneous recordings from pairs of EGFP+ cells. These synapses appeared to have low release probability and displayed potentiation and asynchronous release upon repeated activation. In summary, we report on a previously unexamined component of the dorsal horn circuitry that likely constitutes an essential element of the fine tuning of nociception.

Imaging of Ca2+ responses mediated by presynaptic L-type channels on GABAergic boutons of cultured hippocampal neurons

We have previously demonstrated that L-type Ca(2+) channels are involved in post-tetanic potentiation (PTP) of GABAergic IPSCs in cultured hippocampal neurons. Here we have used intracellular Fluo-3 to detect [Ca(2+)](i) in single GABAergic boutons in response to stimulation that evokes PTP. During control stimulation of the presynaptic GABAergic neuron at 40 Hz for 1-2 s, DeltaF/F(0) increased rapidly to a peak value and started to decline shortly after the train ended, returning to baseline within 10-20 s. The L-type channel blocker, isradipine (5 microM), had no significant effect on the amplitude or kinetics of the Ca(2+) signal. Following blockade of N- and P/Q-type Ca(2+)-channels, the amplitude was reduced by 52.9+/-3%. Isradipine caused a reduction of the remaining response (by 26.6+/-5%, P<0.01), that was fully reversible on washing. The L-type channel "agonist", BayK 8644 (8 microM), caused a significant enhancement of the peak (by 18.7%+/-7%, P<0.05). The rising phase of the Ca(2+) signal, which is related to the rate of entry of Ca(2+) into the bouton, was decreased by isradipine (by 25.5+/-6%, P<0.05) and enhanced by BayK 8644 (by 45.2%+/-16%, P<0.05). These Ca(2+) imaging experiments support the putative role of L-type channels in PTP of GABAergic synapses on cultured hippocampal neurons. We expect L-channels to be few in number, although they may couple strongly to intracellular signalling cascades that could amplify a signal that regulates synaptic vesicle turnover in the GABAergic boutons.

Adaptation reduces spike-count reliability, but not spike-timing precision, of auditory nerve responses

Sensory systems use adaptive coding mechanisms to filter redundant information from the environment to efficiently represent the external world. One such mechanism found in most sensory neurons is rate adaptation, defined as a reduction in firing rate in response to a constant stimulus. In auditory nerve, this form of adaptation is likely mediated by exhaustion of release-ready synaptic vesicles in the cochlear hair cell. To better understand how specific synaptic mechanisms limit neural coding strategies, we examined the trial-to-trial variability of auditory nerve responses during short-term rate-adaptation by measuring spike-timing precision and spike-count reliability. After adaptation, precision remained unchanged, whereas for all but the lowest-frequency fibers, reliability decreased. Modeling statistical properties of the hair cell-afferent fiber synapse suggested that the ability of one or a few vesicles to elicit an action potential reduces the inherent response variability expected from quantal neurotransmitter release, and thereby confers the observed count reliability at sound onset. However, with adaptation, depletion of the readily releasable pool of vesicles diminishes quantal content and antagonizes the postsynaptic enhancement of reliability. These findings imply that during the course of short-term adaptation, coding strategies that employ a rate code are constrained by increased neural noise because of vesicle depletion, whereas those that employ a temporal code are not.

Post-tetanic potentiation of GABAergic IPSCs in cultured hippocampal neurons is exclusively time-dependent

We have previously shown that post-tetanic potentiation (PTP) of GABAergic IPSCs in cultured hippocampal neurons involves activation of L-type Ca(2+) channels. Although there is little Ca(2+) entry by this route, it is possible that L-type Ca(2+) channels mediate an increase in probability of release (Pr) by a mechanism that remains dormant in the absence of stimulation. We have tested this hypothesis in the present study using dual whole-cell patch clamp recordings. IPSCs were evoked by low-frequency stimulation (LFS; 0.2 Hz) of presynaptic GABAergic neurons. Run-down was corrected by linear regression. Following tetanic stimulation (80 pulses at 40 Hz), the presence of PTP was probed by resuming LFS after various post-tetanic intervals (PTI). To control for possible effects associated with LFS, the train and PTI were replaced by corresponding pauses. Following pauses >or=16 s, the first IPSC was significantly increased by 20-25% (P<0.01, paired t-test). These post-pause responses were subtracted from IPSCs following tetanic stimulation. Following correction, PTP was greatest ( approximately 50%) after the shortest PTI (4 s) and IPSC amplitudes declined back to the baseline value over 1-2 min. With a PTI of 16 s, the first IPSC was potentiated to the same level as that to which PTP with a PTI of 4 s had decayed with continued LFS. There was no significant PTP with PTIs of 64 and 128 s. Since PTP decays entirely in the absence of stimulation, it is concluded that the process(es) mediating the increase in vesicular Pr appear to be time-dependent, but not use-dependent.

Autonomous function of synaptotagmin 1 in triggering synchronous release independent of asynchronous release

Ca(2+) triggers neurotransmitter release in at least two principal modes, synchronous and asynchronous release. Synaptotagmin 1 functions as a Ca(2+) sensor for synchronous release, but its role in asynchronous release remains unclear. We now show that in cultured cortical neurons stimulated at low frequency (<or.1 Hz), deletion of synaptotagmin 1 blocks synchronous GABA and glutamate release without significantly increasing asynchronous release. At higher stimulation frequencies (>or Hz), deletion of synaptotagmin 1 also alters only synchronous, not asynchronous, release during the stimulus train, but dramatically enhances "delayed asynchronous release" following the stimulus train. Thus synaptotagmin 1 functions as an autonomous Ca(2+) sensor independent of asynchronous release during isolated action potentials and action potential trains, but restricts asynchronous release induced by residual Ca(2+) after action potential trains. We propose that synaptotagmin 1 occupies release "slots" at the active zone, possibly in a Ca(2+)-independent complex with SNARE proteins that are freed when action potential-induced Ca(2+) influx activates synaptotagmin 1.

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.

Synapse density regulates independence at unitary inhibitory synapses

Neurotransmitter transporters may promote synapse specificity by limiting spillover between release sites. At GABAergic synapses, transport block prolongs synaptic responses when many inputs are activated, yet it is unclear whether transporters alter signaling by single axons. We found that unitary IPSCs generated by paired recordings between hippocampal interneurons and granule cells could be either prolonged or totally unaffected by block of GABA transporters. This variability was explained by the density of active release sites rather than the number of active sites. Prolongation by transport block required release from multiple sites and was enhanced by repetitive activation. Furthermore, transport-sensitive unitary IPSCs were accelerated when the release probability was reduced, indicating that cross talk prolonged the time course of IPSCs even when transport was intact. Our results suggest that the release site density regulates the degree of cross talk as well as the contribution of transporters to GABA clearance. Thus, interplay between release site density and transporter action determines the independence of unitary inhibitory synapses.

Intraterminal Ca2+ concentration and asynchronous transmitter release at single GABAergic boutons in rat collicular cultures

Neurotransmitter release in response to a single action potential has a precise time course. A significant fraction of the releasable vesicles is exocytosed synchronously, within a few milliseconds after the arrival of an action potential. If repeatedly activated, stimulus-locked phasic synchronous release declines, but synaptic transmission can be maintained through tonic asynchronous transmitter release. The desynchronisation of release during repetitive activation is generally attributed to a build-up of intraterminal Ca2+ concentration. However, the precise relationship between presynaptic Ca2+ level and asynchronous release rate at small central synapses has remained unclear. Here we characterise this relationship for single GABAergic terminals in rat collicular cultures. In the presence of tetrodotoxin, inhibitory postsynaptic currents (IPSCs) and presynaptic Ca2+ transients were recorded in response to direct presynaptic depolarisation of individual boutons. Repetitive stimulation indeed resulted in a shift from phasic to asynchronous neurotransmitter release. A clear dominance of the asynchronous release mode was observed after 10 pulses. The steady-state asynchronous release rate showed a third-power dependency on the presynaptic Ca2+ concentration, which is similar to that of evoked release. The Ca2+ sensor for asynchronous release exhibited a high affinity for Ca2+ and was far from saturation. These properties of the Ca2+ sensor should make the asynchronous release very sensitive to any modification of presynaptic Ca2+ concentration, including those resulting from changes in presynaptic activity patterns. Thus, asynchronous release represents a powerful but delicately regulated mechanism that ensures the maintenance of appropriate inhibition when the readily releasable pool of vesicles is depleted.