Extracellular levels of NPY in the dorsal hippocampus of freely moving rats are markedly elevated following a single electroconvulsive stimulation, irrespective of anticonvulsive Y1 receptor blockade

Abstract

Neuropeptide Y (NPY) has been proposed to play a role in the pathophysiology of depression and also to act as an endogenous anticonvulsant. Repeated administration of electroconvulsive stimulations (ECS) has been shown to induce a long-term increase in hippocampal NPY neurotransmission, while the effects of single ECS are largely unexplored. In this study, we assessed extracellular levels of NPY in the dorsal hippocampus of freely moving rats following a single ECS. We also studied the effect of locally administered BIBP3226, a selective NPY Y1 receptor antagonist with reported anticonvulsant properties, on the duration of the ECS-induced seizure and NPY release in freely moving animals. Our data demonstrate that a single ECS increases extracellular NPY-like immunoreactivity (LI) levels in the dorsal hippocampus, reaching statistical significance 2 h following the treatment. KCl transiently and calcium-dependently increased extracellular levels of NPY, suggesting that the measured NPY-LI is derived from functional neurons. Local BIBP3226 perfusion essentially abolished the ECS-induced seizure but had no effect on the basal NPY-LI outflow or on the ECS-induced rise in extracellular NPY levels. Our data are in line with the hypothesis that one mechanism of action of ECS is to release NPY in the hippocampus and suggest that the increase is in itself not associated with anticonvulsant activity but may represent other properties of NPY.

Introduction

Although electroconvulsive therapy (ECT) has been used in the treatment of psychiatric and neurological disorders for more than 50 years, its therapeutic mechanism(s) of action has not been identified. Neuropeptide Y (NPY), is a highly conserved 36 amino acid peptide (Tatemoto et al., 1982), proposed to play a role in affective and anxiety-related disorders (Widerlöv et al., 1988), stress-susceptibility (Thorsell et al., 2000) and limbic epileptic seizures (Woldbye et al., 1997). Specifically, NPY has been suggested to act as an endogenous anticonvulsant since NPY knock-out and knock-down mice display an increased vulnerability to seizures (DePrato et al., 2000; Baraban et al., 1997; Erickson et al., 1996). In rats, a series of electroconvulsive stimulations (ECS), an experimental model of ECT, is known to increase hippocampal levels of preproNPY mRNA, tissue concentrations of NPY-like immunoreactivity (LI) as well as extracellular levels of NPY-LI, while NPY-sensitive binding sites are down-regulated (Husum et al., 2000; Madsen et al., 2000; Mathé et al., 1998; Greisen et al., 1997; Zachrisson et al., 1995; Mikkelsen et al., 1994; Stenfors et al., 1989). Surprisingly, the effects of single ECS on NPY activity in the hippocampus are largely unexplored.

Five NPY receptors, the Y1 (Herzog et al., 1992), Y2 (Rose et al., 1995), Y4 (Lundell et al., 1996), Y5 (Gerald et al., 1996), and Y6 (Gregor et al., 1996), have been identified and cloned and are all, except Y6, present in rat brain. In the hippocampus, Y1, Y2, and Y5 receptors have been implicated in the modulation of seizure activity. For example, NPY can inhibit the release of glutamate acting via an Y2 receptor (Weiser et al., 2000; Colmers and Bleakman, 1994; Greber et al., 1994; Klapstein and Colmers, 1993; Colmers et al., 1991), while the Y5 receptor has been implicated in the inhibition of kainic acid-induced seizures in rats (Woldbye et al., 1997). On the other hand, excitatory effects of NPY in cultured dentate granule cells have also been reported (Brooks et al., 1987) and intrahippocampal injections of BIBP3226, a selective non-peptide Y1 receptor antagonist (Rudolf et al., 1994), attenuated kainic acid and penetylenetetrazole-induced seizure activity in rats (Vezzani et al., 2000a; Vezzani et al., 2000b; Gariboldi et al., 1998). These studies suggest a possible excitatory effect of NPY mediated by the Y1 receptor subtype, which may contribute to the excessive hippocampal activity occurring during seizures. Indeed, the Y1 receptor was recently suggested to modulate glutamate release from a presynaptic site and to function as an autoreceptor on NPY expressing neurons in the hippocampus (St-Pierre et al., 2000). Consequently, the aim of the present study was to investigate, in the dorsal hippocampus of freely moving rats, the effects of (1) potassium stimulation and calcium-free conditions on extracellular levels of NPY, (2) a single ECS on extracellular levels of NPY-LI in the dorsal hippocampus, (3) BIBP3226 pretreatment on NPY outflow during and after a single ECS and on the duration of the ECS-induced seizure.