The selective orexin 1 receptor antagonist SB-334867-A impairs acquisition and consolidation but not retrieval of spatial memory in Morris water maze

Abstract

The novel neuropeptides orexin-A and orexin-B derive from a common 130-amino acid precursor molecule (prepro-orexin), are mainly localized to neurons within and around the lateral hypothalamus, and exhibit high affinity to the closely related G-Protein-coupled receptors orexin 1 and 2 receptor (OX₁R, OX₂R). Orexinergic neurons send their axons to the hippocampal formation (CA1, CA2 and dentate gyrus), which expresses OX₁Rs. Recent studies have shown that central administration of orexin-A and orexin-B have effects on learning and memory but literature concerning the role of orexinergic system in cognition remains controversial. More recently, antagonists have been described. The most potent and selective is SB-334867-A, which has an affinity of 40 nM at OX₁R which is at least 50-fold selective over OX₂R. It is likely that the intracerebroventricular (i.c.v.) administration may block OX₁Rs in many brain regions. Previously we have shown that intra-CA1 injection of SB-334867-A impairs acquisition, consolidation and retrieval of spatial memory in MWM task. In the present study, the effect of pre-training, post-training and pre-probe of trial intra-DG (dentate gyrus) administration of SB-334867-A (1.5, 3, 6 μg/0.5 μl) on acquisition, consolidation and retrieval in a single-day testing version of MWM (Morris water maze) task was examined. Our results show impaired acquisition and consolidation of MWM task for SB-334867-A as compared with the control group. However, SB-334867-A had no effect on retrieval in spatial memory. Also, this antagonist had no effect on escape latency of a non-spatial visual discrimination task. Therefore, it seems that endogenous orexin-A and orexin-B, through DG OX₁Rs, play an important role in spatial learning and memory in the rat.

Introduction

A specific population of neurons located in the lateral and dorsomedial hypothalamus synthesize two neuropeptides, orexin-A (hypocretin-1) and orexin-B (hypocretin-2) [10], [26], [35], [36]. Both orexin-A and orexin-B are encoded by a single gene composed of two exons. The prepro-orexin messenger RNA (mRNA) encodes a 130-residue (rodent) or 131-residue (human) polypeptide, which has a typical secretory sequence and is cleaved to form mature orexin-A and orexin-B peptides [35], [37]. Orexinergic neurons widely project to the cerebral cortex, olfactory bulb, amygdala, septum, diagonal band of Broca, hippocampus, etc. [9], [10], [26], [30], [35]. The actions of these peptides are mediated by two membrane receptors, orexin 1 (OX₁R), and orexin 2 (OX₂R), that are coupled with G-proteins [35], [51]. While orexin-A has equal affinity at OX₁R and OX₂R, orexin-B has an appreciably greater (approximately 10-fold) affinity to OX₂R [35], [41], [42]. In situ hybridization studies have shown that orexin receptors are expressed in some regions, in which dense orexin neuronal projections are observed. High levels of OX₁R mRNA occur in the hippocampal formation (CA₁, CA2, DG), tenia tecta, dorsal raphe nucleus and most prominently locus coeruleus (LC), while OX₂R mRNA was found mainly in the cerebral cortex, nucleus accumbens, and subthalamic and paraventricular thalamic nuclei [22], [51]. The widespread projections of orexin containing neurons, coupled with the regional distribution of OX₁R and OX₂R, suggests that the endogenous orexins have diverse physiological functions including food intake [15], [35], arousal [9], sleep–waking cycle [31], [48], sleep disorder [17], drinking behavior [18], nociception [7], neuroendocrine functions [13], [19], [32], [45] and learning and memory [2], [4], [16], [46], [47]. Reports about the effect of orexins on learning and memory are controversial. Aou et al. have reported that i.c.v. administered orexin-A has produced memory impairment in water maze performance in a 2-day training protocol [4] but other reports show that i.c.v. administration of orexin-A and orexin-B facilitates avoidance learning in rats [46], [47]. Likewise, orexin-A improved learning and memory in SAMP8 mice: “an animal model of Alzheimer's disease” [16]. Also, it has been shown that orexin-A enhances normal long-term potentiation (LTP) in perforant path-dentate granule cell synapses in anesthetized rats [52]. Furthermore, our previous study has shown that blockade of CA1 region OX1Rs impaired memory processing in MWM task [2]. It has been reported that stimulation of the lateral hypothalamus area, the major source of orexinergic signaling, as well as various targets of orexinergic fibers such as the amygdala, LC and septum, modulates synaptic plasticity in the hippocampus [12], [29], [39], [53], a widely accepted cellular and molecular correlate of learning and memory [8], [10], [21].

The most potent and selective OX₁R antagonist is SB-334867-A, which is at least 50-fold selective over OX₂R [43]. Furthermore, this compound reaches maximal plasma and brain levels 0.5 h post-dosing and has a terminal elimination with half-life of approximately 4 h following systemic administration [14]. Therefore, SB-334867-A is increasingly being used as a tool to study not only receptor mediation of responses to administered peptides exogenously but also (when administered alone) to assess the physiological and behavioral significance of endogenous orexins [5], [33], [44]. However, it is likely that the systemic or i.c.v. administration of SB-334867-A may antagonize OX₁Rs in many brain regions. Therefore, it cannot precisely assess the functions of specific region OX₁Rs involved in learning and memory like dentate gyrus (DG), an integral neural substrate for spatial memory [20], [34], [49], [50]. Thus, in the present study, in order to avoid the nonspecific effects of systemic administration and extend the attempt for a better understanding of the functions of DG OX₁Rs, we assessed the effects of intra-DG SB-334867-A treatment in three different memory processing stages on MWM task.