Inhibition of mGluR1 and IP₃Rs impairs long-term memory formation in young chicks

Abstract

Calcium (Ca²⁺) is involved in a myriad of cellular functions in the brain including synaptic plasticity. However, the role of intracellular Ca²⁺ stores in memory processing remains poorly defined. The current study explored a role for glutamate-dependent intracellular Ca²⁺ release in memory processing via blockade of metabotropic glutamate receptor subtype 1 (mGluR1) and inositol (1,4,5)-trisphosphate receptors (IP₃Rs). Using a single-trial discrimination avoidance task developed for the young chick, administration of the specific and potent mGluR1 antagonist JNJ16259685 (500 nM, immediately post-training, ic), or the IP₃R antagonist Xestospongin C (5 μM, immediately post-training, ic), impaired retention from 90 min post-training. These findings are consistent with mGluR1 activating IP₃Rs to release intracellular Ca²⁺ required for long-term memory formation and have been interpreted within an LTP2 model. The consequences of different patterns of retention loss following ryanodine receptor (RyR) and IP₃R inhibition are discussed.

Introduction

Calcium (Ca²⁺) is an ubiquitous signal transduction molecule, active in a diverse range of cellular functions in the brain including the regulation of neuronal excitability, neurotransmitter release and gene transcription (Berridge, 1998). Many studies have revealed that Ca²⁺ has an integral role in memory processing (Bauer et al., 2002, Blackwell and Alkon, 1999, Deyo et al., 1992, Gibbs et al., 1979, Quevedo et al., 1998, Woodside et al., 2004). A vital role of Ca²⁺ can also be identified in long-term potentiation (LTP) and long-term depression (LTD), the putative cellular correlates of learning and memory (Bliss and Collingridge, 1993, Cavazzini et al., 2005, Lynch, 2004).

The capacity of Ca²⁺ to regulate cellular functions is dependent on levels in the cytosol. Intracellular Ca²⁺ levels can be raised by Ca²⁺ influx from the extracellular environment or through release from intracellular stores. The mechanisms of Ca²⁺ influx though plasma membrane channels are well established (Catterall, 2000). Ca²⁺ can also be released from intracellular stores in the endoplasmic reticulum upon activation of either ryanodine receptors (RyRs) or inositol (1,4,5)-trisphosphate (IP₃) receptors (IP₃Rs; Berridge, 2002, Meldolesi, 2001), but less is known about the role of such stores in memory processing. Nevertheless, a number of studies using both genetic knockout techniques and pharmacological methods have demonstrated that RyR activation contributes to LTP and LTD (Balschun et al., 1999, Futatsugi et al., 1999, O’Mara et al., 1995, Obenaus et al., 1989, Reyes and Stanton, 1996, Wang et al., 1996). In particular, RyRs appear to be involved in LTP1, a form of LTP induced by weak stimulation (Raymond and Redman, 2002, Raymond and Redman, 2006). Several behavioural studies have also demonstrated that the inhibition of RyRs impairs retention for spatial (Ohnuki & Nomura, 1996) and discrimination avoidance learning (Edwards and Rickard, 2006, Salinska et al., 2001). In addition, mice with genetic knockout of RyR subtype 3 also display deficits for contextual fear conditioning and spatial memory (Balschun et al., 1999, Kouzu et al., 2000).

IP₃R-dependent calcium stores have received much less attention with respect to their role in memory processing. Interestingly, inhibition studies with the drug Xestospongin C (5 μM) suggest that IP₃Rs appear to underlie LTP2, a translation-dependent, but transcription-independent form of LTP induced by moderate stimulation (Raymond, 2007, Raymond and Redman, 2002, Raymond and Redman, 2006). Behavioural studies have tended to investigate the receptors or enzymes that lead to IP₃ production rather than via direct blockade of IP₃Rs. As such only broad statements about the possible role of IP₃Rs in memory processing can be made. For example, group I metabotropic glutamate receptors (mGluRs), which include subtype 1 (mGluR1) and 5 (mGluR5), are coupled to the enzyme phospholipase C (Pin & Duvoisin, 1995). Stimulation of group I mGluRs leads to the formation of IP₃ and diacylglycerol (Hermans & Challiss, 2001), and in turn intracellular Ca²⁺ release (Cui et al., 2007, Kawabata et al., 1996, Kawabata et al., 1998, Power and Sah, 2007). There is a substantial body of evidence supporting a role for group I mGluRs in synaptic plasticity (Anwyl, 1999, Bashir et al., 1993, Ugolini et al., 1997, Volk et al., 2006, Yang et al., 1998) and long-term memory processing (Aiba et al., 1994, Balschun and Wetzel, 2002, Conquet et al., 1994, Riedel et al., 2003, Simonyi et al., 2005, Simonyi et al., 2007), and by broad inference IP₃Rs. Interestingly, the retention loss produced by group I mGluR inhibition is consistent with the findings of Buckley and Caldwell, 2004, Weeber and Caldwell, 2004 in which phospholipase C activity was observed in the hippocampus and medial frontal cortex of rats during long-term memory formation of a fear conditioning task. Nevertheless, these findings provide only limited and indirect support for a role for IP₃Rs in memory processing.

The present study sought to directly investigate the activation of IP₃Rs in memory processing using a single-trial discrimination avoidance task developed for the young chick. This task has several advantages for studying the role of IP₃Rs in memory formation. The task is considered to be ecologically valid and is temporally precise (Gibbs & Ng, 1977). In addition, previous studies using this task have yielded relevant findings related to intracellular Ca²⁺ release. For example, inhibition of RyR-dependent intracellular Ca²⁺ release with dantrolene (5 mM, ic) resulted in a persistent retention loss from 40 min post-training (Edwards & Rickard, 2006). In addition, Rickard and Ng (1995) demonstrated that 500 μM (RS)-α-methyl-4-carboxyphenylglycine (MCPG), a non-specific group I mGluR antagonist, administered intracranially 5 min post-training, impaired retention during long-term memory formation from 90 min post-training. In a non-discrimination variant of this task, Salinska (2006) demonstrated that the inhibition of mGluR5 using 8 mM (ic) 2-methyl-6-(phenyl)-pirydyne (MPEP), administered immediately post-training, resulted in an impairment of retention evident from 15 min post-training. Given that group I mGluRs are likely to activate IP₃Rs, but that mGluR5 inhibition does not coincide with long-term memory, the current study explored the possibility that memory processing requires mGluR1-activated Ca²⁺ release from IP₃Rs.