DEFICITS IN NERVE GROWTH FACTOR RELEASE AND TYROSINE RECEPTOR KINASE PHOSPHORYLATION ARE ASSOCIATED WITH AGE-RELATED IMPAIRMENT IN LONG-TERM POTENTIATION IN THE DENTATE GYRUS A ´ . KELLY, C. MAGUIRE and M. A. LYNCH* Department of Physiology, Trinity College, Dublin 2, Ireland Abstract —Previous findings have indicated that nerve growth factor may play a role in the expression of long-term potentiation in perforant path–granule cell synapses and that nerve growth factor treatment restores the ability of aged rats to sustain long-term potentiation. In this study, we have attempted to analyse the changes which occur in nerve growth factor release and tyrosine receptor kinase phosphorylation following tetanization in tissue prepared from dentate gyrus of young rats, as well as aged rats which did or did not sustain long-term potentiation. We report that KCl-stimulated nerve growth factor release was significantly increased in slices of the dentate gyrus or whole hippocampus, but not in synaptosomes prepared from the dentate gyrus. KCl- induced nerve growth factor release was also significantly enhanced in slices prepared from tetanized, compared with untetanized, tissue obtained from young rats and aged rats which sustained long-term potentiation; this response was absent in tissue prepared from aged rats which failed to sustain long-term potentiation, perhaps due to the enhanced basal nerve growth factor release observed in this tissue. Tetanization increased tyrosine receptor kinase phosphorylation in the dentate gyrus of young rats and aged rats which sustained long-term potentiation. In parallel with the changes in nerve growth factor release, tyrosine receptor kinase phosphorylation was markedly increased in untetanized tissue, which may contribute to the lack of effect in tetanized tissue prepared from aged rats which failed to sustain long-term potentiation. We observed that nerve growth factor concentration and tyrosine receptor kinase expression were decreased in aged, compared with young, rats. The data suggest that deficits in nerve growth factor release and subsequent signalling may contribute to age-related deficits in long-term potentiation.  1999 IBRO. Published by Elsevier Science Ltd. Key words: long-term potentiation, NGF, trk phosphorylation, hippocampus. Several recent reports have emphasized the important role played by neurotrophins in synaptic plasticity. Particular attention has focussed on their modulatory role in the expres- sion of long-term potentiation (LTP), specifically in area CA1 of the hippocampus. For example, brain-derived neurotrophic factor and neurotrophin-3 induce a form of potentiation in area CA1 in vitro. 12,20,27 While NGF was shown to have no appreciable effect on LTP in CA1, 20,44 perhaps because of the relatively low tyrosine receptor kinase (trk) density in this area, 46 evidence suggests that NGF may play a role in the expression of LTP in perforant path–granule cell synapses, consistent with the relatively high expression of NGF 14 in the dentate gyrus. Thus, LTP in perforant path–granule cell synapses was shown to be associated with increased phosphorylation of trk, while trk phosphorylation was increased in vitro by NGF. 32 In addition, we have observed that LTP was impaired in the dentate gyrus of the genetically hypertensive (GH) strain of Wistar rats in which NGF concen- tration was decreased, while intraventricular injection of NGF reversed this deficit. 22 Further indirect support for a role of NGF in LTP is suggested by the observation that induction of LTP in the dentate gyrus increased NGF mRNA expression. 6,8 There is a good deal of evidence suggesting that LTP, in the dentate gyrus at least, relies on the presence of a retrograde messenger, since induction of LTP is considered to be a largely postsynaptic event, while increased release of gluta- mate is a feature of the maintenance phase of LTP. 30,31,34,35 Arachidonic acid is one candidate retrograde messenger which, following tetanic stimulation, appears to be released from the postsynaptic region 9 into the synaptic cleft 30 to stimulate glutamate release coincident with increased meta- botropic glutamate receptor activation. 19,33,34 It has been shown that the stimulation of glutamate release induced by arachidonic acid and (1S,3R)-1-amino-1,3-cyclopentane dicarboxylate (ACPD) requires arachidonic acid-induced activation of tyrosine kinase and the subsequent phosphory- lation and activation of phospholipase Cg, together with ACPD-induced activation of phospholipase Cb. 36 A number of observations have indicated that NGF mimics some of the effects of arachidonic acid; for example, it stimulates tyrosine kinase activation leading to phospholipase Cg activation 43 and interacts with ACPD to increase glutamate release. 22,23 Since this interaction is occluded by prior induction of LTP, it could be argued that NGF plays a role in LTP. Thoenen 45 has suggested that NGF may act as a retrograde messenger because it is released from granule cells of the dentate gyrus in response to appropriate stimuli, i.e. depolar- ization and/or glutamate receptor activation. Consistent with this are the observations (i) that depolarization stimulates NGF release from a primary culture of hippocampal neurons, 4 (ii) that NGF mRNA expression is increased following induc- tion of LTP in the dentate gyrus, 8 (iii) that trk phosphorylation is increased following induction of LTP 32 and (iv) that NGF has been shown to enhance glutamatergic transmission under certain conditions. 7,22,23,25 A role for NGF in LTP 359 359 Neuroscience Vol. 95, No. 2, pp. 359–365, 2000 Copyright  1999 IBRO. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 0306-4522/00 $20.00+0.00 PII: S0306-4522(99)00460-1 Pergamon www.elsevier.com/locate/neuroscience *To whom correspondence should be addressed. Tel.: + 353-1-608-1770; fax: + 353-1-679-3545. E-mail address: lynchma@tcd.ie (M. A. Lynch) Abbreviations: ACPD, (1S,3R)-1-amino-1,3-cyclopentane dicarboxylate; BSA, bovine serum albumin; EDTA, ethylenediaminetetra-acetate; EPSP, excitatory postsynaptic potential; HEPES, N-2-hydroxyethylpi- perazine-N 0 -2-ethanesulphonic acid; LTP, long-term potentiation; NGF, nerve growth factor; SDS, sodium dodecyl sulphate; trk, tyrosine receptor kinase.