5-HT6 receptor antagonists reverse delay-dependent deficits in novel object discrimination by enhancing consolidation—an effect sensitive to NMDA receptor antagonism
Introduction
The 5-HT6 receptor is one of the most recent additions to the 5-HT receptor family and, like 5-HT4 and 5-HT7, is positively coupled to adenylyl cyclase via Gs-proteins. In the rat, 5-HT6 receptors are present almost exclusively within the central nervous system, and seem to be most abundant in regions associated with learning and memory, including the hippocampus, cerebral cortex and amygdala (Monsma et al., 1993, Ruat et al., 1993, Ward et al., 1995, Kohen et al., 1996, Gérard et al., 1996, Gérard et al., 1997).
5-HT6 receptors appear to be expressed on the soma and/or dendrites of neurones located postsynaptic to serotonergic nerve terminals (Gérard et al., 1997), and several functional studies indicate that they exert a tonic inhibitory effect on central cholinergic neurotransmission. Thus 5-HT6 receptor-directed antisense oligonucleotides (AO) and 5-HT6 receptor antagonists induce a centrally mediated behavioural syndrome that is attenuated by muscarinic receptor antagonists (Bourson et al., 1995, Bentley et al., 1999a). The 5-HT6 receptor antagonist 4-amino-N-(2,6-bis-methylamino-pyramidin-4-yl)-benzene sulphonamide (Ro 04-6790) also inhibits the muscarinic antagonist-induced ipsilateral rotations that occur following 6-hydroxydopamine lesions of the median forebrain bundle (Bourson et al., 1998), while an alternative antagonist 5-chloro-N-(4-methoxy-3-piperazin-1-yl-phenyl)-3-methyl-2-benzothiophene sulphonamide (SB-271046) potentiates acetylcholinesterase inhibitor-induced yawning in the rat (Routledge et al., 1999). Furthermore, microdialysis studies in the conscious rat show that 5-HT6 receptor antagonist administration elevates both extracellular ACh (Sleight et al., 1999, Shirazi-Southall et al., 2002, Reimer et al., 2003) and glutamate (Dawson et al., 2000, Dawson et al., 2001) in the hippocampus and frontal cortex. Since the increase in glutamate release is insensitive to muscarinic receptor blockade it would appear to be independent of alterations in central cholinergic neurotransmission (Dawson et al., 2001).
Given the fundamental role of both the cholinergic and glutamatergic systems in cognitive processes, and their disturbance in memory dysfunction (Bartus et al., 1982, Coyle et al., 1983, Collingridge et al., 1983, Greenamyre and Young, 1989) the 5-HT6 receptor has been implicated in the modulation of normal and/or dysfunctional memory (Tsai et al., 1999, but see Thome et al., 2001). Consistent with this suggestion, 5-HT6 receptor antagonists overcome muscarinic antagonist-induced deficits in object discrimination (Woolley et al., 2003), autoshaping (Meneses, 2001) Morris water maze and passive avoidance tests (Sleight et al., 1999, Bös et al., 2001, Reimer et al., 2003). In addition, 5-HT6 receptor antagonists administered alone improve performance of aged rats in operant and T-maze delayed alternation tasks (Rogers et al., 1999, Rogers et al., 2000) and enhance performance of healthy, unimpaired young adult rats in a food motivated autoshaping task (Meneses, 2001).
5-HT6 receptor AO or antagonist treatment also enhances performance of unimpaired adult rats in a spatial memory task in the Morris water maze. Thus acute treatment of rats with either i.c.v. AO (Bentley et al., 1997, Woolley et al., 2001) or systemic Ro 04-6790 (Woolley et al., 2001), SB-271046 (Rogers and Hagan, 2001) or N-(2,5-dibromo-3-fluorophenyl)-4-methoxy-3-piperazin-1-ylbenzene sulphonamide (SB-357134) (Rogers and Hagan, 2001, Stean et al., 2002) during training improves retention of the learned platform position as indicated by performance in subsequent probe trials. However, the lack of acute treatment effect on learning of the platform position during acquisition trials, and the failure of the acetylcholinesterase inhibitor Aricept (donepezil) to produce similar cognitive changes in the same studies (Rogers and Hagan, 2001), implies that the cognitive effects of acutely administered 5-HT6 receptor antagonists may be due to modulation of post-acquisition stages in the learning and memory process, via mechanisms independent of cholinergic neurotransmission.
The current study therefore employed the NOD task, a two-trial test of recognition memory (Ennaceur and Delacour, 1988) to investigate both the stage and neuropharmacological mechanism involved in the effect of 5-HT6 receptor antagonists on learning and memory. Following a short inter-trial interval (ITI), rats presented with two identical objects during the first (familiarisation) trial will preferentially spend longer exploring a novel than the familiar object during the subsequent choice trial. From this it can be inferred that they have a memory for the familiar object, the main neural substrates of which appear to be the perirhinal and entorhinal cortices, cortical association areas and globus pallidus following short (<10 min) ITI’s (Wiig and Bilkey, 1995, Ennaceur et al., 1996, Ennaceur et al., 1997, Ennaceur and Aggleton, 1997, Aggleton et al., 1997, Ennaceur, 1998), with additional participation of the hippocampus occurring after intermediate ITI’s (Wood et al., 1993, Clark et al., 2000). Longer ITI’s disrupt memory of the familiar object, and hence also object discrimination during the choice trial.
The effects of the selective 5-HT6 receptor antagonists Ro 04-6790 (Sleight et al., 1998) and SB-271046 (Bromidge et al., 1999) on such delay-induced deficits in object discrimination were determined, following administration at different times during the task in order to specifically examine modulatory effects on acquisition, consolidation, retrieval, and/or attentional processes. Secondly, the effect of the non-competitive NMDA receptor antagonist MK-801 on the ability of Ro 04-6790 to reverse delay-induced deficits in NOD was determined to assess the contribution of enhanced glutamatergic neurotransmission to the cognitive enhancing effects of this 5-HT6 receptor antagonist. Portions of this research have been published in abstract form (King et al., 2003).
Section snippets
Animals
Adult male Lister hooded rats (Charles River, UK or Biomedical Services Unit, University of Nottingham, derived from Charles River stock) weighing 210–330 g (experiment I) or 110–275 g (experiments II and III) were housed in groups of 3–4 on a 12 h light–dark cycle (lights on at 07.00 h) with food and water available ad libitum. Room temperature (21±1 °C) and humidity (60±5%) were kept constant. All experiments were performed in accordance with the UK Animals (Scientific Procedures) Act, 1986.
Drugs
Effect of varied ITI on NOD in drug-free animals
All groups of rats spent an equal amount of time exploring the two identical objects during the familiarisation trial (p>0.05 in each case) irrespective of the subsequent ITI, indicating a lack of any spatial preference for either of the two object positions (Fig. 1A). During the choice trial rats with ITI’s between 0.25 and 3 h spent significantly more time exploring the novel than the familiar object (0.25 h p<0.01; 1 h p<0.001; 2 h p<0.05; 3 h p<0.05) but the level of discrimination
Discussion
The present study utilised the NOD task, a two-trial test involving recognition memory, to investigate both the stage in the learning and memory process that 5-HT6 receptor antagonists modulate to enhance cognition following acute systemic administration, and the contribution of NMDA receptor-mediated glutamatergic neurotransmission to this effect.
The NOD task was selected because it is based on the spontaneous preference of rats for novelty rather than positive or negative reinforcement so the
Acknowledgements
This work was supported by F. Hoffmann La-Roche and the University of Nottingham.
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