Previous studies from our laboratory have suggested a role for nerve growth factor (NGF), and its receptor, TrkA, in long-term potentiation (LTP) in the rat dentate gyrus (Kelly et al. 1998, 2000). It therefore seems likely that intracellular signalling pathways activated downstream of the Trk receptor may play a role in the expression of LTP. This study was designed to investigate a possible role for one such pathway, the PI3K-Akt pathway, in LTP. PI3K is an enzyme involved in phospholipid signalling which has been shown to modulate calcium concentration, glutamate release and LTP (Kelly & Lynch 2000) and it is known in several cell types to activate Akt. Both enzymes have recently been suggested to play a role in synaptic plasticity and fear conditioning in the rat amygdala (Lin et al. 2001).

In this study we have used the PI3K inhibitor LY294002 to investigate the role of this kinase and its substrate, Akt, in LTP. Male Wistar rats (250-300 g) were anaesthetized with urethane (1.5 g kg^-1 I.P.). LTP was induced unilaterally in dentate gyrus by tetanic stimulation (3 trains of stimuli of 250 Hz in 50 ms; 30 s inter-train interval) of the perforant path. Test shocks (1/30 s) were delivered to the perforant path for 15 min preceding and 45 min following tetanic stimulation. Intracerebroventricular injection of LY294002 (10 µl; 60 µg ml^-1 in dimethylsulphoxide; 1 % v/v) 30 min before high-frequency stimulation (HFS) blocked LTP. The mean percentage change in EPSP slope in the last 5 min of the experiment compared with the mean EPSP slope in the 5 min immediately prior to tetanic stimulation was 100.93 ± 4.2 % (n = 7; data expressed as means ± S.E.M.) compared with vehicle-injected controls (116.75 ± 1.244 %, n = 6). At the end of the recording period, rats were humanely killed by decapitation and untetanized and tetanized dentate gyrii were dissected free, homogenized and stored at -20 °C for analysis. Western immunoblotting using phosphospecific antibodies revealed an increased activation of PI3K and Akt in tetanized compared with untetanized dentate gyrus of control rats; these increases were blocked by prior treatment with LY294002. LY294002 also blocked an LTP-associated activation of mitogen-activated protein kinase (MAPK), suggesting that PI3K may be acting upstream of MAPK.

The data demonstrate that LTP in the rat dentate gyrus is associated with increased activation of PI3K, Akt and MAPK and suggest that inhibition of LTP by LY294002 is a consequence of blockade of activation of these protein kinases.

This work was supported by Health Research Board, Ireland.