Timing and efficacy of transmitter release at mossy fiber synapses in the hippocampal network
It is widely accepted that the hippocampus plays a major role in learning and memory. The mossy fiber synapse between granule cells in the dentate gyrus and pyramidal neurons in the CA3 region is a key component of the hippocampal trisynaptic circuit. Recent work, partially based on direct presynaptic patch-clamp recordings from hippocampal mossy fiber boutons, sheds light on the mechanisms of synaptic transmission and plasticity at mossy fiber synapses. A high Na+ channel density in mossy fiber boutons leads to a large amplitude of the presynaptic action potential. Together with the fast gating of presynaptic Ca2+ channels, this generates a large and brief presynaptic Ca2+ influx, which can trigger transmitter release with high efficiency and temporal precision. The large number of release sites, the large size of the releasable pool of vesicles, and the huge extent of presynaptic plasticity confer unique strength to this synapse, suggesting a large impact onto the CA3 pyramidal cell network under specific behavioral conditions. The characteristic properties of the hippocampal mossy fiber synapse may be important for pattern separation and information storage in the dentate gyrus-CA3 cell network.
KeywordsPresynaptic recording Mossy fiber boutons Mossy fiber synapses Hippocampus Autoassociative networks Episodic memory Synaptic efficacy
The mossy fiber synapse: a key connection in the hippocampal network
The nonmyelinated axons of the granule cells, the so-called mossy fibers, show several structural properties that distinguish them from the other synaptic pathways . They project to the CA3 region, mainly traveling within a narrow band termed “stratum lucidum.” Several (∼15) large “giant” boutons (∼3–5-μm diameter) emerge from a single mossy fiber axon, either arranged in an en passant manner or attached to the main axon via a short perpendicular axonal branch [1, 8, 29, 31] (Fig. 1c,d). A large mossy fiber bouton typically contacts only a single CA3 pyramidal neuron , and a mossy fiber axon contacts a given CA3 pyramidal neuron only once, as boutons are ∼150 μm apart. As the rat hippocampus contains ∼1 million granule cells and ∼300,000 CA3 pyramidal neurons (per hippocampus; ), and as a single granule cell contacts ∼15 CA3 pyramidal neurons, a single CA3 pyramidal neuron on average receives input from ∼15×1,000,000/300,000=50 granule cells. Thus, the mossy fiber–CA3 pyramidal neuron synapse forms a sparse connection.
In addition to the large boutons on CA3 pyramidal neurons, the mossy fiber forms three other types of synaptic terminals, contacting distinct postsynaptic target cells. (1) Large boutons emerging from hilar collaterals (∼15 per mossy fiber axon) form synapses on glutamatergic hilar mossy cells . Mossy cells, in turn, establish an excitatory feedback circuit by forming synapses on granule cells of the ipsilateral dentate gyrus and on granule cells of the contralateral side. (2) Filopodial extensions emerging from large mossy fiber boutons (∼2.5 filopodia per large bouton, ) form synapses on GABAergic interneurons in the stratum lucidum [1, 81] (Fig. 1d). In addition, large mossy fiber boutons can contact the dendritic shafts of GABAergic interneurons in the stratum lucidum directly . (3) Small boutons emerging from the main axon (∼15) or from hilar collaterals (∼150 per mossy fiber axon) form synapses on GABAergic interneurons in the stratum lucidum and the hilus . Thus, the total number of synapses formed on GABAergic interneurons (in CA3 and hilus) via filopodial extensions and small boutons greatly exceeds the total number of synapses formed on glutamatergic neurons (CA3 pyramidal neurons and mossy cells) via large boutons .
Presynaptic action potential and voltage-gated Na+ and K+ channels in hippocampal mossy fiber boutons
After the development of methods for patch-clamp recording in brain slices [26, 38], techniques were gradually refined to a point when it became possible to obtain recordings from presynaptic elements, including mossy fiber boutons [12, 28, 34]. The key steps of improvement were as follows: (1) the enhanced visibility provided by high-numerical aperture objectives and infrared illumination ; (2) the development of vibratomes with minimal blade vibration in the vertical direction, resulting in an improved preservation of superficial tissue layers , although mossy fiber terminal recordings with commercially available tissue slicers were also reported ; (3) the use of sucrose-based cutting solutions with reduced Na+ and Ca2+ concentrations, which further improves tissue preservation during cutting and maintenance of slices . Collectively, these refinements made it possible to routinely obtain patch-clamp recordings from large hippocampal mossy fiber boutons in the stratum lucidum [6, 27, 33, 37].
The characteristic properties of the presynaptic action potential in hippocampal mossy fiber boutons are conferred by the expression of a distinct set of voltage-gated ion channels. Voltage-clamp analysis revealed that several types of voltage-gated channels are expressed in hippocampal mossy fiber boutons. First, these boutons contain a high density of voltage-gated Na+ channels . Estimates based on outside-out patch recordings suggested that the Na+ conductance density was ∼50 mS cm−2, corresponding to ∼40 channels μm−2 and ∼2,000 channels per typically sized bouton. The Na+ channel density in hippocampal mossy fiber boutons is comparable to previously reported Na+ channel densities in peripheral axons [41, 42]. Thus, mossy fiber boutons have axon-like properties. Immunocytochemical studies showed a high level of Nav1.2 immunoreactivity in the mossy fiber tract . Furthermore, Nav1.2 and Nav1.6 immunoreactivity are often colocalized in nonmyelinated axons throughout the entire brain . This suggests that native presynaptic channels in mossy fiber boutons are assembled from either Nav1.2 or Nav1.2/Nav1.6 alpha subunits.
Mossy fiber boutons also express functionally specialized voltage-gated K+ channels . These channels show low activation threshold, relatively fast inactivation, and are blocked by external tetraethylammonium ions (TEA) and dendrotoxin (a toxin from the green mamba, which blocks K+ channels assembled from Kv1.1, 1.2, and 1.6 subunits). These functional and pharmacological properties of the presynaptic Kv channels greatly constrain their subunit composition. The low activation threshold and the sensitivity to TEA and dendrotoxin suggest that the presynaptic channel is assembled from Kv1 subunits. Furthermore, the fast inactivation suggests that the presynaptic channel contains either Kv1.4 subunits or Kvbeta subunits. Consistent with this hypothesis, Kv1.1, Kv1.4, and Kvbeta immunoreactivity is found in the mossy fiber tract [22, 69]. Thus, the results suggest that the native K+ channel in mossy fiber buotons is a Kv1.1/Kv1.4 heteromer or a Kv1.1 alpha/beta subunit heteromer .
Are hippocampal mossy fibers involved in information processing ? Or do they follow the rule, “The axon does not think. It only ax.” ? Experimental evidence suggests that the propagation of action potentials in mossy fiber axons is reliable, in both the orthodromic  and the antidromic direction [70, 72]. Furthermore, simulation of action potential propagation in mossy fiber boutons showed that the experimentally determined Na+ conductance density of 50 mS cm−2, if homogeneously implemented in boutons and axons, provides a high safety factor of action potential propagation . These findings suggest that mossy fiber axons are reliable transmission devices.
However, two lines of evidence suggest that mossy fiber axons are also capable of information processing. First, the duration of the presynaptic action potential is not constant, but shows broadening during repetitive stimulation . Action potential broadening is much more pronounced in mossy fiber boutons than in the calyx of Held in the auditory brainstem , implying that broadening is a specific property of cortical presynaptic terminals. This may suggest that information in the mossy fiber system is not only coded by action potential frequency and timing but also as action potential duration . Second, recent experiments revealed that subthreshold excitatory postsynaptic potentials (EPSPs) can be passively propagated from the somatodendritic domain of granule cells to mossy fiber boutons . This may suggest that information in the mossy fiber system is coded by both action potential (“digital”) and subthreshold (“analogue”) signals. Both spike duration coding and analogue coding may enrich the computational repertoire of mossy fiber axons.
Action potential–exocytosis coupling in hippocampal mossy fiber boutons
As in many other synapses, exocytosis at hippocampal mossy fiber boutons requires the opening of voltage-gated presynaptic Ca2+ channels. Voltage-clamp analysis of Ca2+ currents in hippocampal mossy fiber boutons revealed that the presynaptic Ca2+ channels show a high activation threshold . Ca2+ currents recorded in mossy fiber boutons are largely blocked by the selective Ca2+ channel blockers ω-agatoxin IVa and ω-conotoxin GVIa, suggesting that they are of the P/Q-type (Cav2.1) and N-type (Cav2.2; ) Evoked mossy fiber–CA3 pyramidal neuron excitatory postsynaptic currents (EPSCs) are also reduced by ω-agatoxin and ω-conotoxin, demonstrating that both P/Q-and N-type channels couple to glutamate release . Thus, converging evidence suggests that synaptic transmission at the mossy fiber–CA3 synapse is mediated by P/Q- and N-type Ca2+ channels. Ca2+-imaging experiments recently suggested the presence of R-type Ca2+ channels in a subset of hippocampal mossy fiber boutons . It remains to be shown whether these findings can be confirmed in direct recordings and what role R-type Ca2+ channels play in mossy fiber synaptic transmission [16, 32].
The coupling between Ca2+ channels and Ca2+ sensor of exocytosis at the mossy fiber synapse has not been analyzed at a quantitative level. However, the presynaptic Ca2+ current evoked by an action potential has a large amplitude, with an average value of ∼170 pA . Assuming a single channel current of ∼0.2 pA at physiological divalent concentrations [15, 35], this would correspond to ∼850 Ca2+ channels open at the peak in a typical terminal. Further assuming that P/Q- and N-type Ca2+ channels are restricted to active zones and that the number of release sites is 37 in a large bouton , we estimate that ∼23 Ca2+ channels per release site are open at the peak. Thus, although the estimate is rough, the number of Ca2+ channels per release site appears to be large. Early studies suggested that the slow Ca2+ chelator ethylene glycol bis-2-aminoethyl ether-N,N,N′,N′-tetraacetic acid (EGTA) applied extracellularly in a membrane-permeant form (200-μM EGTA-AM) reduces the peak amplitude of mossy fiber–CA3 pyramidal neuron EPSCs by ∼40% . Likewise, the fast Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) applied intracellularly (10 mM) completely blocks transmission at granule cell autapses . Thus, the distance between Ca2+ source and Ca2+ sensor might be relatively large. In conclusion, the available data are consistent with the view that transmitter release at hippocampal mossy fiber synapses is triggered by a large number of presynaptic Ca2+ channels, which might result in a precisely timed presynaptic Ca2+ signal even in the presence of stochastic single channel gating. However, the distance between Ca2+ source and Ca2+ sensor may allow modulation of the amplitude of the presynaptic Ca2+ transient by endogenous Ca2+ buffers, such as calbindin-D28k .
Differential synaptic transmission at mossy fiber–pyramidal neuron and mossy fiber–interneuron synapses
Recent work revealed substantial differences between mossy fiber synapses on CA3 pyramidal neurons via large boutons and on stratum lucidum interneurons via filopodial extensions and small boutons [55, 80]. First, the release probability at mossy fiber synapses on stratum lucidum interneurons is larger, while the number of release sites is smaller than at mossy fiber–CA3 pyramidal neuron synapses . Second, significant differences in frequency facilitation are observed. In the mossy fiber–CA3 pyramidal neuron synapse, an increase in stimulation frequency from 0.05 to 1 Hz results in an ∼600% increase in EPSC amplitude ([71, 80]—although facilitation is less pronounced in mature synapses, ). In contrast, in stratum lucidum interneurons the extent of frequency facilitation is much smaller (∼200%, ). Finally, marked differences in plasticity were observed. In CA3 pyramidal neurons, high-frequency stimulation (HFS) leads to a large posttetanic potentiation (∼800%) and long-term potentiation (∼200%; [36, 68, 71, 88, 89]). In contrast, in stratum lucidum interneurons HFS induces synaptic depression rather than potentiation [50, 67]. Likewise, activation of the cAMP pathway by forskolin leads to a large chemical potentiation at mossy fiber–CA3 pyramidal neuron synapses [45, 82], but has only minimal effects on monosynaptic transmission at mossy fiber-stratum lucidum interneurons . As synaptic plasticity at mossy fiber synapses is believed to be expressed presynaptically , these results suggest that the release properties of filopodial extensions and small boutons in the CA3 region differ substantially from those of large mossy fiber boutons.
However, the properties of small boutons emerging from axonal collaterals in the hilus are more similar to those of the large boutons. At the mossy fiber synapses on fast-spiking basket cells in the dentate gyrus, HFS induces both PTP and LTP , but the extent of plasticity at these synapses appears to be smaller than at mossy fiber–CA3 pyramidal neuron synapses . For example, the extent of LTP (300 stimuli) at the mossy fiber-basket cell synapse is 133% , whereas it is 236% in mossy fiber–CA3 pyramidal neuron synapses . Furthermore, both forskolin and phorbolesters induce a chemical potentiation of mossy fiber-dentate gyrus basket cell EPSCs , but again the extent of potentiation is less [239% by 50-μM forskolin; 217% by 1-μM phorbol-12,13-diacetate (PDA)] than that at the mossy fiber–CA3 pyramidal neuron synapse (418 and 670%; ).
What is the function of differential synaptic transmission and plasticity? If granule cells fire at low frequency, the net output onto the CA3 region will be inhibitory [1, 13, 14, 60]. In contrast, if granule cells fire high-frequency bursts, differential synaptic dynamics will shift the balance between monosynaptic excitation and disynaptic inhibition towards excitation. This may implement a highly nonlinear gating mechanism, by which only the granule cells with the highest activity levels relay excitation to the CA3 network.
The mossy fiber synapse—a teacher synapse for completion, separation, and linkage of patterns in the dentate gyrus–CA3 cell network
Several lines of evidence suggest that the hippocampus is involved in the processing, storage, and recall of spatial information . A large proportion of granule cells in the dentate gyrus and pyramidal neurons in the CA3 and CA1 region are “place cells,” showing a firing rate that is modulated by the animal’s position in space . It is thought that the hippocampus processes spatial information by both pattern completion (restoration of original patterns from degraded or incomplete inputs; ) and pattern separation (amplification of differences between input patterns; [51, 52]). Experimental evidence for pattern completion is that CA3 pyramidal neurons generate consistent activity patterns after small changes in the environment . Experimental evidence for pattern separation is that CA3 cells show abrupt and coordinated place field remapping when the environment is changed to a larger extent [51, 77, 85]. Finally, it is believed that the hippocampus is able to store sequences of places. Experimental support for this hypothesis is the replay of firing sequences during sleep  and the phenomenon of phase precession . As an animal moves along a linear track, action potentials shift from late to early time points within a theta cycle, which has been interpreted as a cued and time-compressed recall of previously stored spatial information [53, 54].
How could mossy fiber synapses assist in these complex network functions? If a granule cell fires at low frequency (<0.5 Hz, ), e.g., if the animal is located outside the corresponding place field, the cell is likely to have minimal impact on the CA3 network because the mossy fiber–CA3 synapse is unfacilitated and inhibition will dominate over excitation [1, 13, 14, 60]. In contrast, when the animal is located in a place field center (Fig. 5a; [44, 73]) or during delayed nonmatch-to-sample tasks , granule cells will fire at higher frequencies (∼50 and ∼10 Hz, respectively), and the differential synaptic dynamics will lead to conditions in which unitary mossy fiber synaptic events trigger spikes in postsynaptic CA3 pyramidal neurons, as suggested by both in vitro and in vivo experiments [39, 40, 47, 48] (Figs. 4 and 5). Likewise, activation of the direct perforant path input onto CA3 cells (presumably signaling information about context, ) may lead to a scenario in which unitary mossy fiber synaptic events discharge postsynaptic CA3 pyramidal neurons, as suggested by in vitro recordings  (Fig. 4b). Thus, mossy fiber synapses can be viewed as “conditional detonators,” discharging their postsynaptic CA3 pyramidal targets under certain conditions. Remarkably, action potentials in CA3 cells are generated within a narrow time window under these conditions  (Fig. 5b,c), showing that detonation is conditional, but temporally precise.
Additionally or alternatively, the mossy fiber pathway could contribute to pattern separation [66, 78]. The sparse connectivity between dentate gyrus granule cells and CA3 pyramidal neurons  and the differential dynamics of mossy fiber synapses on pyramidal neurons, which facilitate markedly, and those on interneurons, which facilitate less or even depress , may implement a “winner takes all” mechanism in which only the most active granule cells relay excitation to the CA3 network. Computational studies suggested that pattern separation can be implemented by this mechanism . In such a model, regulation of the strength of the mossy fiber–CA3 pyramidal neuron synapse, e.g., by non-Hebbian frequency facilitation, PTP, or LTP, could set the balance between pattern completion and pattern separation .
Finally, mossy fiber synapses may also help to establish heteroassociative links between individual items in a sequence, such as series of places  (Fig. 6). It was proposed that heteroassociative links are established in the dentate gyrus . To link two consecutive items by spike timing-dependent plasticity, the action potentials encoding the first item need to be “played back” to the granule cells at a time point matching the activity period corresponding to the second item. This would require a connection among granule cells, e.g., by the polysynaptic excitatory pathway granule cell → CA3 pyramidal neuron → mossy cell → granule cell  (Fig. 6). If action potentials corresponding to subsequent items in a sequence were separated by a time interval corresponding to a gamma oscillation cycle (∼20 ms; ), the polysynaptic granule cell → CA3 pyramidal neuron → mossy cell → granule cell pathway would need to relay activity back precisely within 20 ms. The unique anatomical and functional properties of the mossy fiber axon, including reliable action potential propagation, precisely tuned presynaptic Ca2+ signaling and transmitter release, and large impact on target cells under certain conditions, appears to be an ideal design to implement such a reliable and temporally precise delay line .
We thank Beat Gähwiler, Liyi Li, and Chris McBain for comments on an earlier version of the manuscript. This work was supported by grants of Deutsche Forschungsgemeinschaft (SFB 505/C9 to J.B., SFB 505/C5, C12, and Leibniz program to P.J.) and the Bundesministerium für Bildung und Forschung (OI GQ 0420).
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