Neuropeptide Y (NPY) potently inhibits glutamate release and seizure activity in rodent hippocampus in vitro and in vivo, but the nature of the receptor(s) mediating this action is controversial. In hippocampal slices from rats and several wild-type mice, a Y2-preferring agonist mimicked, and the Y2-specific antagonist BIIE0246 blocked, the NPY-mediated inhibition both of glutamatergic transmission and of epileptiform discharges in two different slice models of temporal lobe epilepsy, stimulus train-induced bursting (STIB) and 0-Mg2+ bursting. Whereas Y5 receptor-preferring agonists had small but significant effects in vitro, they were blocked by BIIE0246, and a Y5 receptor-specific antagonist did not affect responses to any agonist tested in any preparation. In slices from mice, NPY was without effect on evoked potentials or in either of the two slice seizure models. In vivo, intrahippocampal injections of Y2- or Y5-preferring agonists inhibited seizures caused by intrahippocampal kainate, but again the Y5 agonist effects were insensitive to a Y5 antagonist. Neither Y2- nor Y5-preferring agonists affected kainate seizures in mice. A Y5-specific antagonist did not displace the binding of two different NPY ligands in WT or mice, whereas all NPY binding was eliminated in the mouse. Thus, we show that Y2 receptors alone mediate all the anti-excitatory actions of NPY seen in the hippocampus, whereas our findings do not support a role for Y5 receptors either in vitro or in vivo. The results suggest that agonists targeting the Y2 receptor may be useful anticonvulsants.

1 Neuropeptide Y (NPY) has been shown to suppress synaptic excitation in rat hippocampus by a presynaptic action. The Y 2 (Y 2 R) and the Y 5 (Y 5 R) receptors have both been implicated in this action. We used the non-peptide, Y 2 R-selective antagonist, BIIE0246, to test the hypothesis that the Y 2 R mediates both the presynaptic inhibitory and anti-epileptic actions of NPY in rat hippocampus in vitro. 2 NPY and the Y 2 R-selective agonist, [ahx 5-24 ]NPY, both inhibited the population excitatory postsynaptic potential (pEPSP) evoked in area CA1 by stratum radiatum stimulation in a concentration-dependent manner. BIIE0246 suppressed the inhibitory e ects of both agonists, suppressing the maximal inhibition without causing a change in the agonist EC 50 , in a manner inconsistent with competitive antagonism. 3 BIIE0246 washed out from hippocampal slices extremely slowly. Application of agonist at high concentrations (1 ± 3 mM) for prolonged periods did not alter the rate of washout, but did partially overcome the antagonism, inconsistent with an insurmountable antagonism by BIIE0246. 4 In the stimulus train-induced bursting (STIB) model of ictal activity in hippocampal slices, both NPY and [ahx [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] ]NPY inhibited primary afterdischarge (18AD) activity. BIIE0246 (100 nM) completely suppressed the actions of NPY and [ahx [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] ]NPY in this model. In contrast, the potent Y 5 R-selective agonist, Ala 31 Aib 32 NPY, a ected neither 18AD activity in the presence of BIIE0246, nor, by itself, even the pEPSP in CA1. 5 BIIE0246 potently suppresses NPY actions in rat hippocampus, suggesting a dominant role for Y 2 R there. The apparently insurmountable antagonism observed may result from the lipophilic nature of the antagonist.

No abstract

It is a central tenet of the epilepsy field that seizures result from the imbalance of excitation over inhibition (1). The bulk of excitation is mediated by the neurotransmitter glutamate, whereas inhibition results mainly from the actions of ␥ -aminobutyric acid (GABA).europeptide Y (NPY) is a 36-amino acid peptide made by neurons throughout the brain and by other secretory cells of the body. NPY has been associated with a number of physiologic processes in the brain, including the regulation of energy balance, memory and learning, and epilepsy. In the hippocampus and neocortex, NPY is made by neurons that almost all express ␥ -aminobutyric acid (GABA). Many NPYcontaining interneurons also coexpress somatostatin (3). NPY receptors are densely concentrated in the strata radiatum and oriens of Ammon's horn of rats (3) and humans (4).Early investigations focused on the actions of NPY in the hippocampus. We (5-7,) and others (8) observed that application of NPY to freshly prepared slices of rat hippocampus maintained in vitro potently and selectively reduced synaptic excitation mediated by glutamate release (Fig. 1A). Further work showed that the action of NPY was highly selective, inhibiting only excitatory inputs onto pyramidal cell throughout Ammon's horn in the rat, but not affecting either synaptic inhibition (7,8) or inputs to dentate granule cells (9), despite the very high levels of NPY in the molecular layer of the dentate (10). The receptor or receptors involved in these actions belong to the G protein-coupled superfamily (11; see later).Detailed studies of the action of NPY were consistent with an entirely presynaptic site in rat hippocampus. Specifically, in neurons whose glutamatergic inputs were suppressed by NPY, the peptide did not affect the postsynaptic responses to glutamate application (6,12). Consistent with this, the release of glutamate from hippocampal slices was shown to be suppressed by NPY (13). Further studies showed that NPY suppressed N-type Ca 2 ϩ currents at presynaptic terminals in neuronal cultures (14) and N-, P/Q-, and other types of Ca 2 ϩ currents in presynaptic terminals of freshly prepared hippocampal slices (15). Once the sites and mechanisms of action were clear, the next question was what role NPY actually played in the physiology and pathophysiology of the hippocampus.It appears that elevated activity in hippocampal circuitry, such as that accompanying epileptiform discharges, results in increased NPY expression in interneurons and dentate granule cells, but that prolonged overstimulation in some epilepsy models and in humans ablates NPY (and other) interneurons. Thus several laboratories determined that NPY peptide and messenger RNA (mRNA) expression was increased in epilepsy models in vivo (16)(17)(18). At about the same time, it was reported that NPY/GABA interneurons in the hippocampus were selectively ablated in rat epilepsy models in vivo (19,20) and in epilepsy patients (21), although it was recently questioned whether NPY cells are indeed selectively vulnerable in...