Cannabinoid CB1 receptor-mediated inhibition of hippocampal acetylcholine release is preserved in aged mice

The cannabinoid CB1 receptor inverse agonist/antagonist SR 141716 increases acetylcholine release in rodent hippocampus and improves memory in some experimental paradigms. Since drugs like SR 141716 may represent a novel class of cognition-enhancing drugs, we wanted to check whether the function of the CB1 receptor is preserved during ageing. Hippocampal and striatal slices from 2- to 3- and 24- to 28-month-old C57BL/6J mice were preincubated with [3H]-choline or [3H]-noradrenaline ([3H]-NA) and superfused. The cannabinoid receptor agonist WIN 55,212-2 inhibited, and SR 141716 facilitated, the electrically (3 Hz) evoked tritium overflow in hippocampal slices (preincubated with [3H]-choline) from young and aged mice to the same extent. The evoked overflow per se was less by 33% in slices from aged animals. WIN 55,212-2 and SR 141716 did not affect, but the muscarinic receptor agonist oxotremorine inhibited, the evoked (3 Hz) overflow in striatal slices (preincubated with [3H]-choline) from young and aged mice to the same extent. The evoked overflow per se tended to be less in slices from aged animals. The evoked (0.3 Hz) overflow in hippocampal slices (preincubated with [3H]-NA) was not affected by WIN 55,212-2 and SR 141716, but was inhibited by histamine (via H3 receptors) in slices from young mice and, to a somewhat less extent, in slices from aged mice. The evoked overflow per se did not differ between age groups. In conclusion, the function of the CB1 receptor involved in the tonic inhibition of hippocampal acetylcholine release is preserved in aged mice. Keywords: Ageing, acetylcholine release, noradrenaline release, hippocampus, striatum, cannabinoid CB1 receptor, muscarinic receptors, histamine H3 receptor, C57BL/6J mouse, SR 141716 Introduction Hashish/marijuana and agonists at cannabinoid CB1 receptors impair learning and memory in humans and animals (for review, see Ameri, 1999; Sullivan, 2000). Two lines of evidence suggest that the endocannabinoid system is involved in cognitive processes. First, the CB1 receptor inverse agonist/antagonist SR 141716 facilitates memory tasks in some behavioural paradigms (Terranova et al., 1996; Lichtman, 2000). Second, in each of the three CB1 receptor knockout mice, marked alterations of learning were observed (Reibaud et al., 1999; Marsicano et al., 2002; Varvel & Lichtman, 2002). CB1 receptors are frequently located presynaptically on nerve endings, where their activation results in inhibition of release of the respective neurotransmitter (for review, see Schlicker & Kathmann, 2001; Howlett et al., 2002). CB1 receptor activation also causes inhibition of acetylcholine release in the hippocampus of rodents, whereas SR 141716 or CB1 receptor deficiency increases the release of this transmitter (for review, see Schlicker & Kathmann, 2001). Thus, the possibility has to be considered that the septohippocampal cholinergic system, the morphological and functional integrity of which is important for learning and memory (Dutar et al., 1995), is implicated in the detrimental effects of cannabinoids on cognitive processes. Even if this should not hold true, the fact that CB1 receptor inverse agonists/antagonists (e.g. SR 141716), like the reversible cholinesterase inhibitors (e.g. donepezil; for review, see Fodero & Small, 2002), increase synaptic acetylcholine levels might mean that CB1 receptor inverse agonists/antagonists might represent a new class of cognition-enhancing agents for use in humans. In this context, it is of interest that CB1 receptor-mediated inhibition of acetylcholine release and its facilitation by SR 141716 have recently also been shown in superfused slices of the human brain (Steffens et al., 2002). Since cognitive decline in humans most frequently occurs in aged individuals, the question has to be addressed whether the molecular substrates for potential cognition-enhancing drugs retain their function during the ageing process. With respect to CB1 receptors, this aspect has so far been studied only rarely. An age-induced reduction of cannabinoid receptor binding and mRNA levels has been shown in the striatum and other extrapyramidal brain regions (Mailleux & Vanderhaeghen, 1992; Romero et al., 1998), but is less marked in other brain regions including the cerebral cortex or the hippocampus (Berrendero et al., 1998). In the study of Romero et al. (1998), a functional parameter, the (CB1 receptor-mediated) stimulation of [35S]-guanylyl-5′-O-(γ-thio)-triphosphate binding by WIN 55,212-2, was also studied in extrapyramidal areas and was found to be decreased in some brain regions. The aim of the present study was to check whether the function of the CB1 receptors involved in the inhibition of hippocampal acetylcholine release is preserved in aged mice of the C57BL/6 strain (frequently used for studies of the ageing brain; for review, see Jucker & Ingram, 1997). We examined the effects of cannabinoid receptor ligands on the electrically evoked tritium overflow from superfused hippocampal slices preincubated with [3H]-choline. For the sake of comparison, the electrically evoked tritium overflow from striatal slices preincubated with [3H]-choline and from hippocampal slices preincubated with [3H]-noradrenaline and its modulation via presynaptic receptors were studied as well.