NMDA and mGluR1 receptor subtypes as major players affecting depotentiation in the hippocampal CA1-region

Neurons have the ability to modify their structure and function which ultimately serves for learning (Abraham and Bear, 1996). Dendritic events provide a major contribution to such modifications. For example, natural and artificial patterns of afferent activation have been shown to induce persistent forms of synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD) at distinct dendritic synapses. LTP and LTD are both assumed to occur during the physiological processes of learning and memory formation and to sustain the latter (Abraham, 2008). In recent years, there has been a burgeoning interest in the understanding of metaplasticity, which refers to the plasticity of synaptic plasticity (Abraham and Bear, 1996). In particular, depotentiation (DP) is the mechanism by which synapses that have recently undergone LTP can reverse their synaptic strengthening in response to low frequency stimulation (LFS; Abraham, 2008). Typically, DP is thought to prevent the saturation of synaptic potentiation by resetting synapses into a more efficient state to store new information. The detailed mechanisms that underlie DP still remain unclear. Bortolotto et al. (1994) first identified metabotropic glutamate receptors (mGluRs) as being involved in DP. Experimental evidence indicates that both subtypes of group I mGluRs (mGluR1 and mGluR5) have distinct functions in synaptic plasticity in the hippocampal CA1 region (Gladding et al., 2008). However, their role in DP was not addressed yet in detail and appear to be distinct from those involved in NMDAR-dependent DP (Zho et al., 2002). Therefore, we investigated the precise mechanisms responsible for NMDAR and mGluR-dependent DP by combining electrophysiological recordings in vitro and pharmacological approach. Transverse hippocampal slices (400 µm thick) were prepared from the right hippocampus with a tissue chopper and placed into a submerged-type chamber, where they were continuously perfused with artificial cerebrospinal fluid (ACSF) saturated with carbogen (95 % O2 / 5% CO2). For recording of field excitatory postsynaptic potentials (fEPSPs), a glass electrode (filled with ACSF, 3–7MΩ) was placed in the stratum radiatum opposite the stimulating electrode. The time course of the fEPSP was measured as the descending slope function for all experiments. After input /output curves had been established, the stimulation strength was adjusted to elicit a fEPSP-slope of 35% of the maximum and kept constant throughout the experiment. During baseline recording, 3 single stimuli (0.1 ms pulse width; 10 s interval) were measured every 5 min and averaged for the 45 min fEPSP values. When we applied a standard DP protocol which consisted of LTP induced by a single theta burst stimulation (TBS, 100 Hz-2s) followed by DP (5Hz-2 min; see Balschun et al., 1999) we found that the NMDAR competitive antagonist (D-AP5) and the group I mGluRs antagonists (YM 298198 and MPEP) failed to affect DP. To check whether more robust DP protocols required a substantial NMDAR and mGluR activation, we tested 5Hz-LFS protocols of 3, 5 and 8 minute duration, applied 6 min after TBS-LTP induction. Ultimately, we established that the 8 min DP protocol yielded a pronounced DP (see figure 1). Therefore, this protocol was used as the standard for all subsequent measurements. Drugs or test substances were applied from 8 minutes prior to and until 22 min after the first LFS train. Bath-application of the competitive NMDAR antagonist D-AP5 (50µM) immediately after the LTP-inducing TBS episode led to a significant impairment of DP. Application of the mGluR5 antagonist MPEP (40µM) did not alter the expression of DP, excluding a role of mGluR5 in DP. In contrast, the selective mGluR1 antagonist, YM 298198 (1µM) caused a marked enhancement of DP. Hence, our results point to a distinct stimulation-strength dependent involvement of NMDAR and mGluR1 in DP. While DP triggered by 2 min of 5 Hz is independent of the activation of NMDAR and group I mGluRs, DP induced by a strong 8 min protocol requires activation of NMDARs and is enhanced by concomitant inactivation of mGluR1. Interestingly, 1xTBS LTP was dependent of mGluR5 activation but did not require activation of mGluR1. Thus, our data suggest a reciprocal involvement of mGluR1 and mGluR5 in LTP and DP. Future work will disentangle the detailed interaction of NMDARs and group I mGluRs in these forms of synaptic plasticity.