The role of Ca2+ in transmitter release and long-term potentiation at hippocampal mossy fiber synapses

Mossy fiber synaptic transmission: communication from the dentate gyrus to area CA3

Communication between the dentate gyrus (DG) and area CA3 of the hippocampus proper is transmitted via axons of granule cells--the mossy fiber (MF) pathway. In this review we discuss and compare the properties of transmitter release from the MFs onto pyramidal neurons and interneurons. An examination of the anatomical connectivity from DG to CA3 reveals a surprising interplay between excitation and inhibition for this circuit. In this respect it is particularly relevant that the major targets of the MFs are interneurons and that the consequence of MF input into CA3 may be inhibitory or excitatory, conditionally dependent on the frequency of input and modulatory regulation. This is further complicated by the properties of transmitter release from the MFs where a large number of co-localized transmitters, including GABAergic inhibitory transmitter release, and the effects of presynaptic modulation finely tune transmitter release. A picture emerges that extends beyond the hypothesis that the MFs are simply "detonators" of CA3 pyramidal neurons; the properties of synaptic information flow from the DG have more subtle and complex influences on the CA3 network.

Metabotropic glutamate receptor antagonist AIDA blocks induction of mossy fiber-CA3 LTP in vivo

Metabotropic glutamate receptors (mGluR) are implicated in long-term memory storage. mGluR-I and mGluR-II antagonists impede various forms of learning and long-term potentiation (LTP) in animals. Despite the evidence linking mGluR to learning mechanisms, their role in mossy fiber-CA3 long-term potentiation (LTP) is not yet clear. To explain the involvement of mGluR-I in memory mechanisms, we examined the function of the mGluR-I antagonist 1-aminoindan-1, 5-dicarboxylic acid (AIDA) on the induction of mossy fiber-CA3 LTP in vivo in male Sprague Dawley and Fischer 344 (F344) rats. Acute extracellular mossy fiber (MF) responses were evoked by stimulation of the MF bundle and recorded in the stratum lucidum of CA3. The excitatory postsynaptic potential (EPSP) magnitude was measured by using the initial slope of the field EPSP slope measured 2-3 ms after response onset. After collection of baseline MF-CA3 responses at 0.05 Hz, animals received either ((+/-))-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (N-methyl-D-aspartate-R antagonist, 10 mg/kg ip), naloxone (opioid-R antagonist, 10 mg/kg ip), or AIDA (mGluR antagonist, 1 mg/kg ip or 37.5 nmol ic). LTP was induced by two 100-Hz trains at the intensity sufficient to evoke 50% of the maximal response. Responses were collected for an additional 1 h. AIDA blocked induction of LTP in the mossy fiber pathway (P < 0.05) in both strains of rats after systemic and in Sprague Dawley rats after intrahippocampal injection.

L-type voltage-gated calcium channels are involved in the in vivo and in vitro expression of fear conditioning

Fear conditioning, a behavioral model of fear learning and cue-related anxiety, causes enhanced neuronal transmission in the thalamic to lateral amygdala pathway.(1,2) In the expression phase of learned fear, this increased transmission recorded in vitro is revealed in increased amplitudes of excitatory postsynaptic currents (EPSCs) and occlusion of paired-pulse facilitation (PPF) implicating a presynaptic increase in transmitter release. Here we examined the contribution of L-type calcium channels in fear conditioning. We measured the effect of nimodipine (Nim, 1.5-20 mg/kg), an L-type calcium channel antagonist, on fear-potentiated startle in which startle was assessed in animals receiving paired or unpaired tone and foot shock. Nim administered intraperitoneally blocked fear-potentiated startle but not baseline startle in a dose-dependent manner. We also analyzed the effect of Nim (10 micro M) in vitro on synaptic facilitation of EPSCs and PPF in slices from naïve control, unpaired control, and fear-conditioned animals. In neurons from naïve control animals, Nim had no effect on EPSC amplitude or PPF, but in slices from fear-conditioned rats, Nim reduced EPSC amplitude, suggesting the recruitment of L-type calcium channels within the fear-conditioning pathway. Nim increased PPF in slices from fear-conditioned animals, suggesting that L-type calcium channels may contribute to increased probability of release in fear conditioning. In slices from unpaired animals, Nim decreased synaptic transmission but had little effect on PPF, suggesting that stress or contextual fear learning may induce L-type channel activity in fear-conditioned and unpaired control animal groups. We also analyzed protein expression of the alpha(1C) and alpha(1D) L-type calcium channel subunits isolated from the amygdala and found that alpha(1C) protein was significantly increased in fear-conditioned animals. These findings suggest that L-type calcium channels play a role in the amygdala in cued fear conditioning and have important implications in the treatment of anxiety and in emotional learning and plasticity.

Properties and plasticity of paired-pulse depression at a central synapse

Synaptic depression was studied at the axo-axonic connection between the goldfish Mauthner axon and identified cranial relay interneurons using simultaneous presynaptic and postsynaptic recordings and a paired-pulse stimulus paradigm. We used interstimulus intervals (ISIs) ranging from 10 msec to 1 sec and a cycle time of approximately 5 sec. Depression (Delta EPSP/EPSP1) was maximal at the shorter intervals (80%) and decreased exponentially with a tau approximately 400 msec (360 +/- 107 msec, mean +/- SD). We found the amplitudes of the first and second EPSP were not correlated, indicating the magnitude of depression does not depend on the amount of transmitter released by the conditioning stimulus. At short ISIs, the latency of EPSP2 was 23% longer than that of EPSP1 and recovered to control with tau approximately 400 msec, whereas rise time and decay time were not altered significantly. The latency distribution, which is determined by the timing of the first quantum released each trial, was used to derive alpha(t), the rate of evoked exocytosis after an action potential. alpha(t) was biphasic, and both components were consistently delayed during depression. Presynaptic manipulations of putative intracellular regulatory pathways, such as Ca(2+) and GTPgammaS injections, preferentially affected the amplitude of EPSP1 or EPSP2. These results are not consistent with simple depletion of the available pool of synaptic vesicles as the major mechanism underlying depression. They rather suggest that it is attributable to a modification or refractoriness of the release process and that there may be multiple pathways subserving evoked exocytosis.

Metaplasticity of mossy fiber synaptic transmission involves altered release probability

Activity-dependent synaptic plasticity is a fundamental feature of CNS synapses. Intriguingly, the capacity of synapses to express plastic changes is itself subject to considerable activity-dependent variation, or metaplasticity. These forms of higher order plasticity are important because they may be crucial to maintain synapses within a dynamic functional range. In this study, we asked whether neuronal activity induced in vivo by application of kainate can induce lasting changes in mossy fiber short- and long-term plasticity. Several weeks after kainate-induced status epilepticus, the mossy fiber, but not the associational-commissural pathway, exhibits a marked loss of paired-pulse facilitation, augmentation, and long-term potentiation (LTP). Because the adenylyl cyclase-protein kinase A cascade is involved in mossy fiber LTP induction, we have tested the integrity of this key pathway by pharmacological activation of either adenylyl cyclase or protein kinase A. These treatments resulted in LTP in control, but not in kainate-treated animals, indicating that status-induced changes occur downstream of protein kinase A. To test whether altered neurotransmitter release might account for these changes, we measured the size of the releasable pool of glutamate in mossy fiber terminals. We find that the size of the releasable pool of glutamate was significantly increased in kainate-treated rats, indicating an increased release probability at the mossy fiber-CA3 synapse. Therefore, we suggest that lasting changes in neurotransmitter release probability caused by neuronal activity may be a powerful mechanism for metaplasticity that modulates both short- and long-term plasticity in the mossy fiber-CA3 synapse after status epilepticus.