Epilepsia, 47(4):773–780, 2006 Blackwell Publishing, Inc. C  2006 International League Against Epilepsy Differential Actions of NPY on Seizure Modulation via Y1 and Y2 Receptors: Evidence from Receptor Knockout Mice ∗ †En-Ju Deborah Lin, ∗ Deborah Young, ∗ Kristin Baer, †Herbert Herzog, and ∗ Matthew J. During ∗ Department of Molecular Medicine and Pathology, University of Auckland, Auckland, New Zealand; and †Neurobiology Program, Garvan Institute of Medical Research, Sydney, Australia Summary: Purpose: Neuropeptide Y (NPY) has been shown to modulate seizure activities. To provide further understand- ing of the involvement of two of the most abundantly expressed NPY receptors, Y1 and Y2, we assessed the effect of Y1 and Y2 gene deletion on systemic kainic acid–induced seizures. We also examined the effect of rAAV-mediated hippocampal NPY over- expression on seizure susceptibility in these receptor knockout mice. Methods: Recombinant adeno-associated viral vector overex- pressing NPY (rAAV-NPY) or an empty vector control (rAAV- Empty) was injected into the hippocampus of adult C57BL/6– 129/SvJ wild-type male mice and mice deficient of Y1 or Y2 receptors on the same background. Four weeks after vector in- jection, mice were subjected to systemic kainic acid–induced seizures, and the seizure behaviors were scored. Results: The rAAV-mediated hippocampal overexpression of NPY led to a twofold reduction in seizures induced by systemic kainic acid in wild-type mice and Y1 receptor knockout mice but not in mice deficient of Y2 receptors. A differential action by the receptors was observed in the seizure-induced mortality rate, with increased fatality in Y2 −/− mice. In addition, although NPY overexpression did not significantly affect the mortality rate in Y2 −/− and wild-type mice, it abolished KA-induced mortality in Y1 −/− mice. Conclusions: This study shows for the first time an al- tered susceptibility to chemically induced seizures in Y1 and Y2 knockout mice and demonstrates a differential seizure modulation mediated by these receptors via a ge- netic approach. Key Words: Epilepsy—Neuropeptides— Gene therapy—AAV—Gene knockout. Since its discovery in 1982 (1), neuropeptide Y (NPY) has attracted significant interest because of its abun- dance in the brain and its central role in modulating a diverse range of physiologic functions including energy homeostasis, neuroendocrine and cardiovascular func- tions, cognition, and anxiety (2,3). More recently, NPY has been implicated in mediating neuronal excitability, seizure modulation, and neuroprotection (4–6). Substan- tial changes in the expression of NPY and its receptors occur in brain regions critically involved in the initiation and propagation of epileptiform activity, such as the hip- pocampus, in experimental seizure models and in patients with intractable temporal lobe epilepsy (6,7). Compelling evidence for an anticonvulsive role of NPY also comes from the suppression of epileptiform activity by intracere- bral infusion of NPY, its analogues, or receptor agonists in various models of epilepsy (6,8). NPY mediates its various effects by binding to G protein–coupled receptors, of which five subtypes have Accepted December 15, 2005. Address correspondence and reprint requests to Dr. E.-J.D. Lin at Neurobiology Program, The Garvan Institute of Medical Research, 384 Victoria Street, Darlinghurst, Sydney, NSW2010, Australia. E-mail: d.lin@garvan.org.au been cloned to date, denoted Y1, Y2, Y4, Y5, and Y6 (9,10), with Y1 and Y2 being the predominant receptor subtypes in the hippocampus (9). Despite extensive study in the seizure modulatory role of NPY, the involvement of each receptor subtype remains controversial. Although in vitro studies have long suggested a critical role for the Y2 receptors in mediating the seizure-inhibitory ef- fects of NPY (11,12), little is known about its effect in vivo. Similarly, data for the role of Y1 also are sparse and unclear. Studies demonstrating anticonvulsive prop- erties of Y1 antagonists suggested a proconvulsive action by this receptor subtype (13,14) and may explain the ob- servation of a direct excitatory effect of NPY in dentate granule neurons by Brooks et al. (15). However, Y1 re- ceptors also have been shown to inhibit glutamate release from rat hippocampal synaptosomes (16) and to reduce seizure-like activity in both hippocampal neuron cultures and hippocampal slices (17,18). In addition, it was shown that Y1 is the predominant receptor subtype mediating the anticonvulsive actions of NPY in rat frontal cortex (18). The observation that these Y1 antagonists (BIBP3226 and BIBO3304) also bind to neuropeptide FF receptors (19) further complicates the interpretation and raises the pos- sibility that activation of neuropeptide FF receptors may 773