NEUROPEPTIDE Y REGULATES RECURRENT MOSSY FIBER SYNAPTIC TRANSMISSION LESS EFFECTIVELY IN MICE THAN IN RATS: CORRELATION WITH Y2 RECEPTOR PLASTICITY B. TU, a,b1 Y. JIAO, a,b H. HERZOG c AND J. V. NADLER a,b * a Department of Pharmacology and Cancer Biology, Duke University Medical Center, PO Box 3813, 100B Research Park 2, Research Drive, Durham, NC 27710, USA b Department of Neurobiology, Duke University Medical Center, Durham, NC 27710, USA c Neurobiology Program, Garvan Institute of Medical Research, Darlin- ghurst, Sydney, NSW 2010, Australia Abstract—A unique feature of temporal lobe epilepsy is the formation of recurrent excitatory connections among gran- ule cells of the dentate gyrus as a result of mossy fiber sprouting. This novel circuit contributes to a reduced threshold for granule cell synchronization. In the rat, ac- tivity of the recurrent mossy fiber pathway is restrained by the neoexpression and spontaneous release of neuropep- tide Y (NPY). NPY inhibits glutamate release tonically through activation of presynaptic Y2 receptors. In the present study, the effects of endogenous and applied NPY were investigated in C57Bl/6 mice that had experienced pilocarpine-induced status epilepticus and subsequently developed a robust recurrent mossy fiber pathway. Whole cell patch clamp recordings made from dentate granule cells in hippocampal slices demonstrated that, as in rats, applied NPY inhibits recurrent mossy fiber synaptic trans- mission, the Y2 receptor antagonist (S)-N 2 -[[1-[2-[4-[(R,S)- 5,11-dihydro-6(6H)-oxodibenz[b,e]azepin-11-yl]-1-piperazi- nyl]-2-oxoethyl]cyclopentyl]acetyl]-N-[2-[1,2-dihydro-3,5 (4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]-arginin- amide (BIIE0246) blocks its action and BIIE0246 enhances synaptic transmission when applied by itself. Y5 receptor ago- nists had no significant effect. Thus spontaneous release of NPY tonically inhibits synaptic transmission in mice and its effects are mediated by Y2 receptor activation. However, both NPY and BIIE0246 were much less effective in mice than in rats, despite apparently equivalent expression of NPY in the recur- rent mossy fibers. Immunohistochemistry indicated greater ex- pression of Y2 receptors in the mossy fiber pathway of normal mice than of normal rats. Pilocarpine-induced status epilepticus markedly reduced the immunoreactivity of mouse mossy fibers, but increased the immunoreactivity of rat mossy fibers. Mossy fiber growth into the inner portion of the dentate molecular layer was associated with increased Y2 receptor immunoreactivity in rat, but not in mouse. These contrasting receptor changes can explain the quantitatively different effects of endogenously re- leased and applied NPY on recurrent mossy fiber transmission in mice and rats. © 2006 IBRO. Published by Elsevier Ltd. All rights reserved. Key words: neuropeptide Y, mossy fiber, Y2 receptor, hip- pocampus, epilepsy. Mossy fibers are axons of the hippocampal dentate gran- ule cells. The mossy fiber pathway normally projects to the pyramidal cells of area CA3 and to inhibitory interneurons of the CA3 area and dentate gyrus, but makes very few recurrent synapses onto granule cells. Thus the pathway drives monosynaptic excitation and feedforward inhibition to CA3 pyramidal cells, but only feedback inhibition to granule cells. A unique feature of temporal lobe epilepsy is the sprouting of mossy fibers within the dentate gyrus, which results in formation of novel monosynaptic recurrent excitatory circuitry (Nadler, 2003). The development of recurrent excitation, coupled with the loss of certain inhib- itory neurons (Obenaus et al., 1993; Buckmaster and Jon- gen-Rêlo, 1999), may enhance the participation of granule cells in seizures. Studies of hippocampal slices support this view (Tauck and Nadler, 1985; Cronin et al., 1992; Patrylo and Dudek, 1998; Hardison et al., 2000; Okazaki and Nadler, 2001; Gabriel et al., 2004; Santhakumar et al., 2005), although the ability of the recurrent mossy fiber pathway to synchronize granule cell discharge depends on stimulus frequency (Feng et al., 2003), [K + ] o (Patrylo and Dudek, 1998; Hardison et al., 2000) and coreleased trans- mitters/modulators (Tu et al., 2005). The ability of mossy fibers to drive the CA3 pyramidal cell population is similarly constrained (Weisskopf et al., 1993; Henze et al., 2002). Glutamate pathways, including the mossy fibers, do not normally express neuropeptide Y (NPY). In the pilo- carpine model of temporal lobe epilepsy, however, NPY immunoreactivity appears de novo in the mossy fiber path- way, including the recurrent projection (Lurton and Caval- heiro, 1997; Scharfman et al., 2000; Tu et al., 2005). Spontaneous release of NPY from recurrent mossy fiber terminals in rats reduces the probability of glutamate re- lease from those terminals by activating presynaptic Y2 receptors. Thus endogenous NPY impedes the ability of recurrent mossy fibers to synchronize granule cell dis- 1 Present address: National Institute of Environmental Health Sci- ences, Research Triangle Park, NC 27709, USA. *Correspondence to: J. V. Nadler, Department of Pharmacology and Cancer Biology, Duke University Medical Center, PO Box 3813, 100B Research Park 2, Research Drive, Durham, NC 27710, USA. Tel: +1-919-684-5317; fax: +1-919-681-8609. E-mail address: nadle002@acpub.duke.edu (J. V. Nadler). Abbreviations: AANPY, [ala 31 ,aib 32 ]-neuropeptide Y; ACSF, artificial cerebrospinal fluid; BIIE0246, (S)-N 2 -[[1-[2-[4-[(R,S)-5,11-dihydro-6(6H)- oxodibenz[b,e]azepin-11-yl]-1-piperazinyl]-2-oxoethyl]cyclopentyl]acetyl]-N- [2-[1,2-dihydro-3,5(4H)-dioxo-1,2-diphenyl-3H-1,2,4-triazol-4-yl]ethyl]- argininamide; D-AP5, D-2-amino-5-phosphonopropanoate; EGTA, ethylene glycol-bis(2-aminoethylether)-N,N,N=,N=-tetraacetic acid; Hepes, 4-(2-hy- droxyethyl)piperazine-1-ethanesulfonic acid; NBQX, 2,3-dihydroxy-6-nitro-7- sulfonyl-benzo[F]quinoxaline; NPY, neuropeptide Y; PBS, phosphate-buff- ered saline; PBS-T, phosphate-buffered saline with 0.1% (v/v) Triton X-100; QX-314, N-ethyllidocaine; S.E.M., standard error of the mean. Neuroscience 143 (2006) 1085–1094 0306-4522/06$30.00+0.00 © 2006 IBRO. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.neuroscience.2006.08.036 1085