Endocannabinoid-Mediated Depolarization-Induced Suppression of Inhibition in Hilar Mossy Cells of the Rat Dentate Gyrus

Phosphodiesterase 2 and Its Isoform A as Therapeutic Targets in the Central Nervous System Disorders

Cyclic adenosine monophosphates (cAMP) and cyclic guanosine monophosphate (cGMP) are two essential second messengers, which are hydrolyzed by phosphodiesterase's (PDEs), such as PDE-2. Pharmacological inhibition of PDE-2 (PDE2A) in the central nervous system improves cAMP and cGMP signaling, which controls downstream proteins related to neuropsychiatric, neurodegenerative, and neurodevelopmental disorders. Considering that there are no specific treatments for these disorders, PDE-2 inhibitors' development has gained more attention in the recent decade. There is high demand for developing new-generation drugs targeting PDE2 for treating diseases in the central nervous and peripheral systems. This review summarizes the relationship between PDE-2 with neuropsychiatric, neurodegenerative, and neurodevelopmental disorders as well as its possible treatment, mainly involving inhibitors of PDE2.

Glucagon-like peptide-1 receptor differentially controls mossy cell activity across the dentate gyrus longitudinal axis

Understanding the role of dentate gyrus (DG) mossy cells (MCs) in learning and memory has rapidly evolved due to increasingly precise methods for targeting MCs and for in vivo recording and activity manipulation in rodents. These studies have shown MCs are highly active in vivo, strongly remap to contextual manipulation, and that their inhibition or hyperactivation impairs pattern separation and location or context discrimination. Less well understood is how MC activity is modulated by neurohormonal mechanisms, which might differentially control the participation of MCs in cognitive functions during discrete states, such as hunger or satiety. In this study, we demonstrate that glucagon-like peptide-1 (GLP-1), a neuropeptide produced in the gut and the brain that regulates food consumption and hippocampal-dependent mnemonic function, might regulate MC function through expression of its receptor, GLP-1R. RNA-seq demonstrated that most, though not all, Glp1r in hippocampal principal neurons is expressed in MCs, and in situ hybridization revealed strong expression of Glp1r in hilar neurons. Glp1r-ires-Cre mice crossed with Ai14D reporter mice followed by co-labeling for the MC marker GluR2/3 revealed that almost all MCs in the ventral DG expressed Glp1r and that almost all Glp1r-expressing hilar neurons were MCs. However, only ~60% of dorsal DG MCs expressed Glp1r, and Glp1r was also expressed in small hilar neurons that were not MCs. Consistent with this expression pattern, peripheral administration of the GLP-1R agonist exendin-4 (5 μg/kg) increased cFos expression in ventral but not dorsal DG hilar neurons. Finally, whole-cell patch-clamp recordings from ventral MCs showed that bath application of exendin-4 (200 nM) depolarized MCs and increased action potential firing. Taken together, this study adds to known MC activity modulators a neurohormonal mechanism that may preferentially affect ventral DG physiology and may potentially be targetable by several GLP-1R pharmacotherapies already in clinical use.

Endocannabinoid receptors contribute significantly to multiple forms of long-term depression in the rat dentate gyrus

Cannabinoid receptors are widely expressed throughout the hippocampal formation, but are particularly dense in the dentate gyrus (DG) subregion. We, and others, have shown in mice that cannabinoid type 1 receptors (CB1Rs) are involved in a long-term depression (LTD) that can be induced by prolonged 10 Hz stimulation of the medial perforant path (MPP)-granule cell synaptic input to the DG. Here, we extend this work to examine the involvement of CB1Rs in other common forms of LTD in the hippocampus of juvenile male and female Sprague–Dawley rats (Rattus norvegicus). We found, as in mice, that prolonged 10 Hz stimulation (6000 pulses) could reliably induce a form of LTD that was dependent upon CB1R activation. In addition, we also discovered a role for both CB1R and mGluR proteins in LTD induced with 1 Hz low-frequency stimulation (1 Hz-LTD; 900 pulses) and in LTD induced by bath application of the group I mGluR agonist (RS)-3,5-Dihydroxyphenylglycine (DHPG; DHPG-LTD). This study elucidates an essential role for endocannabinoid receptors in a number of forms of LTD in the rat DG, and identifies a novel role for CB1Rs as potential therapeutic targets for conditions that involve impaired LTD in the DG.

Endocannabinoid long-term depression revealed at medial perforant path excitatory synapses in the dentate gyrus

The endocannabinoid system modulates synaptic plasticity in the hippocampus, but a link between long-term synaptic plasticity and the type 1 cannabinoid (CB₁) receptor at medial perforant path (MPP) synapses remains elusive. Here, immuno-electron microscopy in adult mice showed that ∼26% of the excitatory synaptic terminals in the middle 1/3 of the dentate molecular layer (DML) contained CB₁ receptors, and field excitatory postsynaptic potentials evoked by MPP stimulation were inhibited by CB₁ receptor activation. In addition, MPP stimulation at 10 Hz for 10 min triggered CB₁ receptor-dependent excitatory long-term depression (eCB-eLTD) at MPP synapses of wild-type mice but not on CB₁-knockout mice. This eCB-eLTD was group I mGluR-dependent, required intracellular calcium influx and 2-arachydonoyl-glycerol (2-AG) synthesis but did not depend on N-methyl-d-aspartate (NMDA) receptors. Overall, these results point to a functional role for CB₁ receptors with eCB-eLTD at DML MPP synapses and further involve these receptors in memory processing within the adult brain.

Pilocarpine-Induced Status Epilepticus Increases the Sensitivity of P2X7 and P2Y1 Receptors to Nucleotides at Neural Progenitor Cells of the Juvenile Rodent Hippocampus

Patch-clamp recordings indicated the presence of P2X7 receptors at neural progenitor cells (NPCs) in the subgranular zone of the dentate gyrus in hippocampal brain slices prepared from transgenic nestin reporter mice. The activation of these receptors caused inward current near the resting membrane potential of the NPCs, while P2Y1 receptor activation initiated outward current near the reversal potential of the P2X7 receptor current. Both receptors were identified by biophysical/pharmacological methods. When the brain slices were prepared from mice which underwent a pilocarpine-induced status epilepticus or when brain slices were incubated in pilocarpine-containing external medium, the sensitivity of P2X7 and P2Y1 receptors was invariably increased. Confocal microscopy confirmed the localization of P2X7 and P2Y1 receptor-immunopositivity at nestin-positive NPCs. A one-time status epilepticus in rats caused after a latency of about 5 days recurrent epileptic fits. The blockade of central P2X7 receptors increased the number of seizures and their severity. It is hypothesized that P2Y1 receptors after a status epilepticus may increase the ATP-induced proliferation/ectopic migration of NPCs; the P2X7 receptor-mediated necrosis/apoptosis might counteract these effects, which would otherwise lead to a chronic manifestation of recurrent epileptic fits.

Neuromodulation of the Feedforward Dentate Gyrus-CA3 Microcircuit

The feedforward dentate gyrus-CA3 microcircuit in the hippocampus is thought to activate ensembles of CA3 pyramidal cells and interneurons to encode and retrieve episodic memories. The creation of these CA3 ensembles depends on neuromodulatory input and synaptic plasticity within this microcircuit. Here we review the mechanisms by which the neuromodulators aceylcholine, noradrenaline, dopamine, and serotonin reconfigure this microcircuit and thereby infer the net effect of these modulators on the processes of episodic memory encoding and retrieval.

The enigmatic mossy cell of the dentate gyrus

Mossy cells comprise a large fraction of the cells in the hippocampal dentate gyrus, suggesting that their function in this region is important. They are vulnerable to ischaemia, traumatic brain injury and seizures, and their loss could contribute to dentate gyrus dysfunction in such conditions. Mossy cell function has been unclear because these cells innervate both glutamatergic and GABAergic neurons within the dentate gyrus, contributing to a complex circuitry. It has also been difficult to directly and selectively manipulate mossy cells to study their function. In light of the new data generated using methods to preferentially eliminate or activate mossy cells in mice, it is timely to ask whether mossy cells have become any less enigmatic than they were in the past.

Expression of c-fos in hilar mossy cells of the dentate gyrus in vivo

Granule cells (GCs) of the dentate gyrus (DG) are considered to be quiescent--they rarely fire action potentials. In contrast, the other glutamatergic cell type in the DG, hilar mossy cells (MCs) often have a high level of spontaneous activity based on recordings in hippocampal slices. MCs project to GCs, so activity in MCs could play an important role in activating GCs. Therefore, we investigated whether MCs were active under basal conditions in vivo, using the immediate early gene c-fos as a tool. We hypothesized that MCs would exhibit c-fos expression even if rats were examined randomly, under normal housing conditions. Therefore, adult male rats were perfused shortly after removal from their home cage and transfer to the laboratory. Remarkably, most c-fos immunoreactivity (ir) was in the hilus, especially temporal hippocampus. C-fos-ir hilar cells co-expressed GluR2/3, suggesting that they were MCs. C-fos-ir MCs were robust even when the animal was habituated to the investigator and laboratory where they were euthanized. However, c-fos-ir in dorsal MCs was reduced under these circumstances, suggesting that ventral and dorsal MCs are functionally distinct. Interestingly, there was an inverse relationship between MC and GC layer c-fos expression, with little c-fos expression in the GC layer in ventral sections where MC expression was strong, and the opposite in dorsal hippocampus. The results support the hypothesis that a subset of hilar MCs are spontaneously active in vivo and provide other DG neurons with tonic depolarizing input.

Marijuana, endocannabinoids, and epilepsy: potential and challenges for improved therapeutic intervention

Phytocannabinoids isolated from the cannabis plant have broad potential in medicine that has been well recognized for many centuries. It is presumed that these lipid soluble signaling molecules exert their effects in both the central and peripheral nervous system in large part through direct interaction with metabotropic cannabinoid receptors. These same receptors are also targeted by a variety of endogenous cannabinoids including 2-arachidonoyl glycerol and anandamide. Significant effort over the last decade has produced an enormous advance in our understanding of both the cellular and the synaptic physiology of endogenous lipid signaling systems. This increase in knowledge has left us better prepared to carefully evaluate the potential for both natural and synthetic cannabinoids in the treatment of a variety of neurological disorders. In the case of epilepsy, long standing interest in therapeutic approaches that target endogenous cannabinoid signaling systems are, for the most part, not well justified by available clinical data from human epileptics. Nevertheless, basic science experiments have clearly indicated a key role for endogenous cannabinoid signaling systems in moment to moment regulation of neuronal excitability. Further it has become clear that these systems can both alter and be altered by epileptiform activity in a wide range of in vitro and in vivo models of epilepsy. Collectively these observations suggest clear potential for effective therapeutic modulation of endogenous cannabinoid signaling systems in the treatment of human epilepsy, and in fact, further highlight key obstacles that would need to be addressed to reach that goal.

Molecular and morphological configuration for 2-arachidonoylglycerol-mediated retrograde signaling at mossy cell-granule cell synapses in the dentate gyrus

2-Arachidonoylglycerol (2-AG) is the endocannabinoid that mediates retrograde suppression of neurotransmission in the brain. In the present study, we investigated the 2-AG signaling system at mossy cell (MC)-granule cell (GC) synapses in the mouse dentate gyrus, an excitatory recurrent circuit where endocannabinoids are thought to suppress epileptogenesis. First, we showed by electrophysiology that 2-AG produced by diacylglycerol lipase α (DGLα) mediated both depolarization-induced suppression of excitation and its enhancement by group I metabotropic glutamate receptor activation at MC-GC synapses, as they were abolished in DGLα-knock-out mice. Immunohistochemistry revealed that DGLα was enriched in the neck portion of GC spines forming synapses with MC terminals, whereas cannabinoid CB(1) receptors accumulated in the terminal portion of MC axons. On the other hand, the major 2-AG-degrading enzyme, monoacylglycerol lipase (MGL), was absent at MC-GC synapses but was expressed in astrocytes and some inhibitory terminals. Serial electron microscopy clarified that a given GC spine was innervated by a single MC terminal and also contacted nonsynaptically by other MC terminals making synapses with other GC spines in the neighborhood. MGL-expressing elements, however, poorly covered GC spines, amounting to 17% of the total surface of GC spines by astrocytes and 4% by inhibitory terminals. Our findings provide a basis for 2-AG-mediated retrograde suppression of MC-GC synaptic transmission and also suggest that 2-AG released from activated GC spines is readily accessible to nearby MC-GC synapses by escaping from enzymatic degradation. This molecular-anatomical configuration will contribute to adjust network activity in the dentate gyrus after enhanced excitation.

Cannabinoid receptor agonists potentiate action potential-independent release of GABA in the dentate gyrus through a CB1 receptor-independent mechanism

We report a novel excitatory effect of cannabinoid agonists on action potential-independent GABAergic transmission in the rat dentate gyrus. Specifically, we find that both WIN55,212-2 and anandamide increase the frequency of miniature IPSCs (mIPSCs)recorded from hilar mossy cells without altering event amplitude, area, rise time, or decay. The effect of WIN55,212-2 on mIPSCs is insensitive to AM251 and preserved in CB1 −/− animals,indicating that it does not depend on activation of CB1 receptors. It is also insensitive to AM630 and unaffected by capsazepine suggesting that neither CB2 nor TRPV1 receptors are involved. Further, it is blocked by pre-incubation in suramin and by a selective protein kinase A inhibitor (H-89), and is mimicked (and occluded) by bath application of forskolin. Similar CB1 receptor-independent facilitation of exocytosis is not apparent when recording evoked IPSCs in the presence of AM251, suggesting that the exocytotic mechanism that produces WIN55,212-2 sensitive mIPSCs is distinct from that which produces CB1 sensitive and action potential-dependent release. Despite clear independence from action potentials, WIN55,212-2 mediated facilitation of mIPSCs requires calcium, and yet is insensitive to chelation of calcium in the postsynaptic cell. Finally, we demonstrate that both bath application of 2-arachidonoylglycerol(2-AG) and depolarization-induced release of endogenous cannabinoids have minimal effect on mIPSC frequency. Cumulatively, our results indicate that cannabinoid ligands can selectively facilitate action potential-independent exocytosis of GABA in the rat dentate gyrus, and further emphasize that this new cannabinoid sensitive signalling system is distinct from previously described CB1 receptor-dependent systems in numerous respects.

Two distinct and activity-dependent mechanisms contribute to autoreceptor-mediated inhibition of GABAergic afferents to hilar mossy cells

We report that bath application of 3 mum carbachol (CCh), a muscarinic acetylcholine receptor agonist, reduces evoked IPSC amplitude recorded from hilar mossy cells in the rat dentate gyrus through a presynaptic mechanism. While CCh has been shown to inhibit evoked IPSCs in other systems, this effect is intriguing in that it does not require inhibitory action of either presynaptic muscarinic receptors or presynaptic cannabinoid receptors. Previous work from our lab has shown that identical application of CCh produces an action potential-dependent increase in ambient GABA in this system; however, inhibition of evoked IPSCs produced by both 3 and 10 mum CCh is insensitive to the GABA(B) antagonist CGP52432. Therefore we hypothesized that CCh-mediated inhibition of evoked IPSCs might be produced by activity-dependent increases in ambient GABA and subsequent activation of presynaptic GABA(A) receptors. Consistent with that hypothesis, we report that CCh-mediated inhibition of evoked IPSCs appears to be well correlated with CCh-mediated facilitation of spontaneous IPSCs and that CCh does not affect GABA(B)-mediated IPSCs recorded in the presence of the GABA(A) receptor antagonist picrotoxin. Intriguingly, however, we found that bath application of the GAT-1 transport blocker NO-711 (1 mum) produces inhibition of evoked IPSCs that is reversed by CGP52432, and that lower doses of CCh produce inhibition with greater CGP52432 sensitivity. These observations, combined with subsequent work on multiple pulse depression, reveal that feedback inhibition of GABAergic afferents to hilar mossy cells is governed by a complex relationship between two distinct and activity-dependent mechanisms.

Modulation of NMDA and AMPA-mediated synaptic transmission by CB1 receptors in frontal cortical pyramidal cells

Although the endogenous cannabinoid system modulates a variety of physiological and pharmacological processes, the specific role of cannabinoid CB1 receptors in the modulation of glutamatergic neurotransmission and neural plasticity is not well understood. Using whole-cell patch clamp recording techniques, evoked or spontaneous excitatory postsynaptic currents (eEPSCs or sEPSCs) were recorded from visualized, layer II/III pyramidal cells in frontal cortical slices from rat brain. Bath application of the CB1 receptor agonist, WIN 55212-2 (WIN), reduced the amplitude of NMDA receptor-mediated EPSCs in a concentration-dependent manner. When co-applied with the specific CB1 antagonists, AM251 or AM281, WIN did not suppress NMDA receptor-mediated EPSCs. WIN also reduced the amplitude of evoked AMPA receptor-mediated EPSCs, an effect that was also reversed by AM251. Both the frequency and amplitude of spontaneous AMPA receptor-mediated EPSCs were significantly reduced by WIN. In contrast, WIN reduced the frequency, but not the amplitude of miniature EPSCs, suggesting that the suppression of glutamatergic activity by CB1 receptors in the frontal neocortex is mediated by a presynaptic mechanism. Taken together, these data indicate a critical role for endocannabinoid signaling in the regulation of excitatory synaptic transmission in frontal neocortex, and suggest a possible neuronal mechanism whereby THC regulates cortical function.

Muscarinic receptor activation modulates the excitability of hilar mossy cells through the induction of an afterdepolarization

In the present study we used electrophysiological techniques in an in vitro preparation of the rat dentate gyrus to examine the effect of muscarinic acetylcholine receptor activation on the intrinsic excitability of hilar neurons. We found that bath application of muscarine caused a direct depolarization in approximately 80% of mossy cells tested, and also produced a clear afterdepolarization (ADP) in nearly 100% of trials. The ADP observed in hilar mossy cells is produced by the opening of a Na(+) permeant and yet largely TTX insensitive ion channel. It requires an increase in postsynaptic calcium for activation, and is blocked by flufenamic acid, an antagonist of a previously identified calcium activated non-selective cation channel (I(CAN)). Further, we demonstrate that induction of an ADP in current clamp causes release of cannabinoids, and subsequent depression of GABAergic transmission that is comparable to that produced in the same cells by a more conventional 5s depolarization in voltage clamp. By contrast, other types of hilar neurons were less strongly depolarized by bath application of muscarinic agonists, and uniformly lacked a similar muscarinic ADP. Overall, the data presented here extend our understanding of the specific mechanisms through which muscarinic agonists are likely to modulate neuronal excitability in the hilar network, and further reveal a mechanism that could plausibly promote endocannabinoid mediated signaling in vivo.

mGluR-mediated and endocannabinoid-dependent long-term depression in the hilar region of the rat dentate gyrus

We report that bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) causes acute inhibition of evoked IPSCs recorded from hilar mossy cells, and that significant long-term depression (LTD) of synaptic transmission remains following washout of DHPG. Subsequent experiments using minimal stimulation techniques revealed that expression of both acute and long-term effects of DHPG are restricted to a subset of GABAergic afferents that are also sensitive to depolarization-induced suppression of inhibition (DSI). Experiments with a selective CB1 antagonist and with transgenic animals lacking CB1 receptors indicate that all effects of DHPG, like DSI, depend on activation of CB1 receptors. Further work with selective mGluR antagonists suggests a direct involvement of mGluR1 receptors. Interestingly, we also report that induction of LTD under our experimental conditions does not require prior direct somatic depolarization via the patch pipette and does not appear to depend critically on the level of activity in incoming GABAergic afferents. Collectively, these results represent the first characterization of mGluR-mediated and endocannabinoid-dependent LTD in the hilar region of the dentate gyrus. The dentate gyrus is thus one of relatively few areas where this mechanism has clearly been demonstrated to induce long-term modulation of inhibitory synaptic transmission.

Endocannabinoid-mediated control of synaptic transmission

The discovery of cannabinoid receptors and subsequent identification of their endogenous ligands (endocannabinoids) in early 1990s have greatly accelerated research on cannabinoid actions in the brain. Then, the discovery in 2001 that endocannabinoids mediate retrograde synaptic signaling has opened up a new era for cannabinoid research and also established a new concept how diffusible messengers modulate synaptic efficacy and neural activity. The last 7 years have witnessed remarkable advances in our understanding of the endocannabinoid system. It is now well accepted that endocannabinoids are released from postsynaptic neurons, activate presynaptic cannabinoid CB(1) receptors, and cause transient and long-lasting reduction of neurotransmitter release. In this review, we aim to integrate our current understanding of functions of the endocannabinoid system, especially focusing on the control of synaptic transmission in the brain. We summarize recent electrophysiological studies carried out on synapses of various brain regions and discuss how synaptic transmission is regulated by endocannabinoid signaling. Then we refer to recent anatomical studies on subcellular distribution of the molecules involved in endocannabinoid signaling and discuss how these signaling molecules are arranged around synapses. In addition, we make a brief overview of studies on cannabinoid receptors and their intracellular signaling, biochemical studies on endocannabinoid metabolism, and behavioral studies on the roles of the endocannabinoid system in various aspects of neural functions.

Excitatory afferents to CA3 pyramidal cells display differential sensitivity to CB1 dependent inhibition of synaptic transmission

Recent advances in immunohistochemical techniques have, contrary to earlier reports, positively identified CB1 receptors on glutamatergic terminals in the hippocampus. Further work has implicated these receptors in modulation of susceptibility to kainic acid induced seizures. Based on these results, the current study was designed to test the hypothesis that both exogenous and endogenous cannabinoids can selectively modulate glutamatergic afferents to CA3 pyramidal cells, and that such modulation is mediated by cannabinoid type 1 (CB1) receptors. Towards that end we employed either conventional or two-photon guided minimal stimulation techniques to isolate mossy fiber and/or associational/commissural (A/C) inputs to CA3 pyramidal cells. We report that bath application of WIN55,212-2 selectively inhibits minimally evoked A/C inputs to CA3 pyramidal cells, without significantly altering simultaneously recorded mossy fiber inputs. Further, we find that WIN55,212-2 mediated inhibition of A/C inputs is completely blocked by the CB1 selective antagonist AM-251 and absent in CB1(-/-) animals, suggesting a dependence on CB1 receptors. Finally, we demonstrate that depolarization of CA3 pyramidal cells leads to calcium dependent release of endogenous cannabinoids that transiently inhibit A/C mediated responses, and that this effect is also sensitive to both AM-251 and the muscarinic acetylcholine receptor antagonist atropine. To our knowledge this represents the first demonstration of depolarization induced suppression of excitation in area CA3 of the hippocampus. Collectively, these results provide new information relevant to developing a thorough understanding of how ECs modulate excitatory transmission in an area that is both essential for the acquisition of new memories and intimately involved in epileptogenesis.

Presynaptic inhibition of excitatory afferents to hilar mossy cells

The hippocampus contains one very strong recurrent excitatory network formed by associational connections between CA3 pyramidal cells and another that depends largely on a disynaptic excitatory pathway between dentate granule cells. The recurrent excitatory network in CA3 has long been considered a possible location of autoassociative memory storage, whereas changes in the level and arrangement of recurrent excitation between granule cells are strongly implicated in epileptogenesis. Hilar mossy cells are likely to receive collateral input from CA3 pyramidal cells and they are key intermediaries (by mossy fiber inputs) in the recurrent excitatory network between granule cells. The current study uses minimal stimulation techniques in an in vitro preparation of the rat dentate gyrus to examine presynaptic modulation of both mossy fiber and non-mossy fiber inputs to hilar mossy cells. We report that both mossy fiber and non-mossy fiber inputs to hilar mossy cells express presynaptic gamma-aminobutyric acid type B (GABA(B)) receptors that are subject to tonic inhibition by ambient GABA. We further find that only non-mossy fiber inputs express presynaptic muscarinic acetylcholine receptors, but that bath application of cholinergic agonists produces action potential-dependent increases in ambient GABA that can indirectly inhibit mossy fiber inputs. Finally, we demonstrate that mossy cells express high-affinity postsynaptic GABA(A) receptors that are also capable of detecting changes in ambient GABA produced by cholinergic agonists. Our results are among the first to directly characterize these important collateral inputs to hilar mossy cells and may help facilitate informed comparison between primary and collateral projections in two major excitatory pathways.

Opposing Modifications in Intrinsic Currents and Synaptic Inputs in Post-Traumatic Mossy Cells: Evidence for Single-Cell Homeostasis in a Hyperexcitable Network

Recent experimental and modeling results demonstrated that surviving mossy cells in the dentate gyrus play key roles in the generation of network hyperexcitability. Here we examined if mossy cells exhibit long-term plasticity in the posttraumatic, hyperexcitable dentate gyrus. Mossy cells 1 wk after fluid percussion head injury did not show alterations in their current-firing frequency ( I-F) and current-membrane voltage ( I-V) relationships. In spite of the unchanged I-F and I-V curves, mossy cells showed extensive modifications in Na+, K+ and h-currents, indicating the coordinated nature of these opposing modifications. Computational experiments in a realistic large-scale model of the dentate gyrus demonstrated that individually, these perturbations could significantly affect network activity. Synaptic inputs also displayed systematic, opposing modifications. Miniature excitatory postsynaptic current (EPSC) amplitudes were decreased, whereas miniature inhibitory postsynaptic current (IPSC) amplitudes were increased as expected from a homeostatic response to network hyperexcitability. In addition, opposing alterations in miniature and spontaneous synaptic event frequencies and amplitudes were observed for both EPSCs and IPSCs. Despite extensive changes in synaptic inputs, cannabinoid-mediated depolarization-induced suppression of inhibition was not altered in posttraumatic mossy cells. These data demonstrate that many intrinsic and synaptic properties of mossy cells undergo highly specific, long-term alterations after traumatic brain injury. The systematic nature of such extensive and opposing alterations suggests that single-cell properties are significantly influenced by homeostatic mechanisms in hyperexcitable circuits.

Endocannabinoids in the dentate gyrus

Recent years have produced rapid and enormous growth in our understanding of endocannabinoid-mediated signaling in the CNS. While much of the recent progress has focused on other areas of the brain, a significant body of evidence has developed that indicates the presence of a robust system for endocannabinoid-mediated signaling in the dentate gyrus. This chapter will provide an overview of our current understanding of that system based on available anatomical and physiological data.