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

Abstract

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.

Introduction

Endogenous cannabinoids (endocannabinoids, eCB) regulate neurotransmission in the CNS primarily via retrograde activation of presynaptic metabotropic type 1 cannabinoid receptors (CB1Rs) (Kano et al., 2009, Katona et al., 1999, Kawamura et al., 2006, Wilson et al., 2001). Depending on a variety of factors, endocannabinoids can affect either a short-term or long-term synaptic depression. The most commonly observed forms of short-term plasticity, depolarization-induced suppression of inhibition (DSI) or excitation (DSE), are initiated by direct depolarization of postsynaptic neurons and depend heavily on subsequent calcium-dependent synthesis of eCBs. By contrast, eCB-mediated long-term depression (LTD) often requires activation of postsynaptic metabotropic glutamate receptors (and not just depolarization) as a key signal promoting eCB production. Metabotropic glutamate receptor-mediated, and CB1 receptor-dependent, forms of LTD have now been well characterized for excitatory synapses in the striatum and nucleus accumbens (Gerdeman et al., 2002, Robbe et al., 2002), and for inhibitory synapses in the hippocampus, basolateral amygdala, and dorsal striatum (Adermark and Lovinger, 2009, Azad et al., 2004, Chevaleyre and Castillo, 2003, Marsicano et al., 2002). In the hippocampus, this type of plasticity has been shown to have lower dependence on postsynaptic calcium influx and to clearly outlast the presence of a CB1 agonist. In addition to mGluR activation, other mechanisms potentially involved in enabling long-term as opposed to short-term eCB-mediated depression include the recent history of excitation in the postsynaptic neuron, the duration of time that agonist is available to CB1 receptors, and the level of activity of presynaptic neurons during that time (for examples from hippocampal studies see: Chevaleyre and Castillo, 2003, Edwards et al., 2008, Heifets et al., 2008). Overall, this field is intriguing in part because it is revealing what may be one of the major forms of postsynaptically induced yet presynaptically expressed long-term plasticity in the CNS, and in part because it has exciting potential roles in both synaptic recovery and metaplasticity (for recent and excellent review see, Chevaleyre et al., 2006, Lovinger, 2008).

Despite the likely importance of this mechanism relatively little is known about the potential for mGluR and eCB-dependent LTD in the dentate gyrus. While significant effort has gone into an examination of LTD as produced by low frequency stimulation of perforant path inputs to dentate granule cells (e.g. Huang et al., 1999, Naie et al., 2007, Trommer et al., 1996), a recent study suggests that eCB-dependent mechanisms in this area are likely restricted to mossy cell axon terminals in the inner molecular layer, and that even these lack eCB-dependent LTD as described in CA1 (Chiu and Castillo, 2008). Similarly, despite the fact that mossy cells also have been shown to produce robust DSI of CB1 positive GABAergic afferents in the hilus (Hofmann et al., 2006, Howard et al., 2007), the potential of CB1-dependent LTD has not previously been examined at these terminals. In fact, although DSI has been well characterized throughout the brain, to date mGluR-mediated and eCB-dependent LTD of GABAergic terminals has only been described at three distinct inhibitory synapses: stratum radiatum interneurons to CA1 pyramidal cells in the hippocampus (Chevaleyre and Castillo, 2003, Chevaleyre and Castillo, 2004, Edwards et al., 2006, Edwards et al., 2008, Heifets et al., 2008), interneurons to principal cells in the central and basolateral amygdala (Azad et al., 2004, Marsicano et al., 2002), and intrinsic afferents to medium spiny neurons in the striatum (Adermark and Lovinger, 2009). The present study provides an initial description of mGluR-induced and eCB-dependent LTD of CB1 positive GABAergic afferents to hilar mossy cells.