Developmental increase in asynchronous GABA release in cultured hippocampal neurons

The role of the C2A domain of synaptotagmin 1 in asynchronous neurotransmitter release

Following nerve stimulation, there are two distinct phases of Ca²⁺-dependent neurotransmitter release: a fast, synchronous release phase, and a prolonged, asynchronous release phase. Each of these phases is tightly regulated and mediated by distinct mechanisms. Synaptotagmin 1 is the major Ca²⁺ sensor that triggers fast, synchronous neurotransmitter release upon Ca²⁺ binding by its C₂A and C₂B domains. It has also been implicated in the inhibition of asynchronous neurotransmitter release, as blocking Ca²⁺ binding by the C₂A domain of synaptotagmin 1 results in increased asynchronous release. However, the mutation used to block Ca²⁺ binding in the previous experiments (aspartate to asparagine mutations, syt^D-N) had the unintended side effect of mimicking Ca²⁺ binding, raising the possibility that the increase in asynchronous release was directly caused by ostensibly constitutive Ca²⁺ binding. Thus, rather than modulating an asynchronous sensor, syt^D-N may be mimicking one. To directly test the C₂A inhibition hypothesis, we utilized an alternate C₂A mutation that we designed to block Ca²⁺ binding without mimicking it (an aspartate to glutamate mutation, syt^D-E). Analysis of both the original syt^D-N mutation and our alternate syt^D-E mutation at the Drosophila neuromuscular junction showed differential effects on asynchronous release, as well as on synchronous release and the frequency of spontaneous release. Importantly, we found that asynchronous release is not increased in the syt^D-E mutant. Thus, our work provides new mechanistic insight into synaptotagmin 1 function during Ca²⁺-evoked synaptic transmission and demonstrates that Ca²⁺ binding by the C₂A domain of synaptotagmin 1 does not inhibit asynchronous neurotransmitter release in vivo.

Tetramethylenedisulfotetramine alters Ca²⁺ dynamics in cultured hippocampal neurons: mitigation by NMDA receptor blockade and GABA(A) receptor-positive modulation

Tetramethylenedisulfotetramine (TETS) is a potent convulsant that is considered a chemical threat agent. We characterized TETS as an activator of spontaneous Ca²⁺ oscillations and electrical burst discharges in mouse hippocampal neuronal cultures at 13-17 days in vitro using FLIPR Fluo-4 fluorescence measurements and extracellular microelectrode array recording. Acute exposure to TETS (≥ 2 µM) reversibly altered the pattern of spontaneous neuronal discharges, producing clustered burst firing and an overall increase in discharge frequency. TETS also dramatically affected Ca²⁺ dynamics causing an immediate but transient elevation of neuronal intracellular Ca²⁺ followed by decreased frequency of Ca²⁺ oscillations but greater peak amplitude. The effect on Ca²⁺ dynamics was similar to that elicited by picrotoxin and bicuculline, supporting the view that TETS acts by inhibiting type A gamma-aminobutyric acid (GABA(A)) receptor function. The effect of TETS on Ca²⁺ dynamics requires activation of N-methyl-D-aspartic acid (NMDA) receptors, because the changes induced by TETS were prevented by MK-801 block of NMDA receptors, but not nifedipine block of L-type Ca²⁺ channels. Pretreatment with the GABA(A) receptor-positive modulators diazepam and allopregnanolone partially mitigated TETS-induced changes in Ca²⁺ dynamics. Moreover, low, minimally effective concentrations of diazepam (0.1 µM) and allopregnanolone (0.1 µM), when administered together, were highly effective in suppressing TETS-induced alterations in Ca²⁺ dynamics, suggesting that the combination of positive modulators of synaptic and extrasynaptic GABA(A) receptors may have therapeutic potential. These rapid throughput in vitro assays may assist in the identification of single agents or combinations that have utility in the treatment of TETS intoxication.

Positive modulation of δ-subunit containing GABA(A) receptors in mouse neurons

δ-subunit containing extrasynaptic GABA(A) receptors are potential targets for modifying neuronal activity in a range of brain disorders. With the aim of gaining more insight in synaptic and extrasynaptic inhibition, we used a new positive modulator, AA29504, of δ-subunit containing GABA(A) receptors in mouse neurons in vitro and in vivo. Whole-cell patch-clamp recordings were carried out in the dentate gyrus in mouse brain slices. In granule cells, AA29504 (1 μM) caused a 4.2-fold potentiation of a tonic current induced by THIP (1 μM), while interneurons showed a potentiation of 2.6-fold. Moreover, AA29504 (1 μM) increased the amplitude and prolonged the decay of miniature inhibitory postsynaptic currents (mIPSCs) in granule cells, and this effect was abolished by Zn²⁺ (15 μM). AA29504 (1 μM) also induced a small tonic current (12.7 ± 3.2 pA) per se, and when evaluated in a nominally GABA-free environment using Ca²⁺ imaging in cultured neurons, AA29504 showed GABA(A) receptor agonism in the absence of agonist. Finally, AA29504 exerted dose-dependent stress-reducing and anxiolytic effects in mice in vivo. We propose that AA29504 potentiates δ-containing GABA(A) receptors to enhance tonic inhibition, and possibly recruits perisynaptic δ-containing receptors to participate in synaptic phasic inhibition in dentate gyrus.

Neuromagnetic imaging of movement-related cortical oscillations in children and adults: age predicts post-movement beta rebound

We measured visually-cued motor responses in two developmentally separate groups of children and compared these responses to a group of adults. We hypothesized that if post-movement beta rebound (PMBR) depends on developmentally sensitive processes, PMBR will be greatest in adults and progressively decrease in children performing a basic motor task as a function of age. Twenty children (10 young children 4-6 years; 10 adolescent children 11-13 years) and 10 adults all had MEG recorded during separate recordings of right and left index finger movements. Beta band (15-30 Hz) event-related desynchronization (ERD) of bi-lateral sensorimotor areas was observed to increase significantly from both contralateral and ipsilateral MI with age. Movement-related gamma synchrony (60-90 Hz) was also observed from contralateral MI for each age group. However, PMBR was significantly reduced in the 4-6 year group and, while more prominent, remained significantly diminished in the adolescent (11-13 year) age group as compared to adults. PMBR measures were weak or absent in the youngest children tested and appear maximally from bilateral MI in adults. Thus PMBR may reflect an age-dependent inhibitory process of the primary motor cortex which comes on-line with normal development. Previous studies have shown PMBR may be observed from MI following a variety of movement-related tasks in adult participants - however, the origin and purpose of the PMBR is unclear. The current study shows that the expected PMBR from MI observed from adults is increasingly diminished in adolescent and young children respectively. A reduction in PMBR from children may reflect reduced motor cortical inhibition. Relatively less motor inhibition may facilitate neuronal plasticity and promote motor learning in children.

Maturation of inhibitory and excitatory motor cortex pathways in children

OBJECTIVE

To study intracortical inhibition and facilitation with paired-pulse transcranial magnetic stimulation in children, adolescents and adults.

METHODS

Paired-pulse transcranial magnetic stimulation (interstimulus intervals (ISI): 1, 3, 5, 10 and 20 ms) was applied over the primary motor cortex (M1) in 30 healthy subjects (range 6-30 years, median age 15 years and 8 months, SD 7,9) divided in three groups: adults (>or=18 years), adolescents (> 10 and < 18 years) and children (<or=10 years).

RESULTS

We observed significantly less intracortical inhibition (SICI) in children's M1 compared to that of adults. Adolescents showed significantly less SICI at the 5 ms interval than did adults. No significant differences were apparent in intracortical facilitation (ICF).

CONCLUSION

We postulate that, as in adults, the maturing M1 possesses horizontal glutamatergic cross-links that represent the neuronal substrate of excitatory intracortical pathways. GABAergic interneurons, the neuronal substrate of inhibitory intracortical pathways, mature between childhood and adulthood. Reduced GABAergic inhibition may facilitate neuronal plasticity and motor learning in children.

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.

Discharge of the readily releasable pool with action potentials at hippocampal synapses

A readily releasable pool (RRP) of synaptic vesicles has been identified at hippocampal synapses with application of hypertonic solution. RRP size correlates with important properties of synaptic function such as release probability. However, a discrepancy in RRP size has been reported depending on the method used to evoke synaptic release. This study was undertaken to determine quantitative relationships between the RRP defined with hypertonic solution and that released with trains of action potentials. We find that asynchronous release at cell culture synapses contributes significantly to the discharge of the RRP with trains of action potentials and that RRP size is the same when elicited by either nerve stimuli or hypertonic challenge.

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.