ESTUDIOS EMPÍRICOS DE LOS DETERMINANTES DEL DESEMPLEO

A ctivity dependent synaptic plasticity is thought to be im portant for certain form s of learnin g and m em ory (review ed in M artin et al., 2000). M o st studies o f neuronal p la sticity have fo cu sed on F T P and LT D o f g lu ta m a te rg ic sy n ap ses (B liss and C ollingridge, 1993; N icoll and M alenka, 1995). H ow ever, studies have also reported LTP and LT D at G A B A ergic synapses in a num ber of brain regions including hippocam pus (M cLean et al., 1996; Gaillard, 1999a,b) cortex (K om atsu, 1994) and cerebellum (K ano et al., 1995). It is cle a r th a t such changes in the stren g th o f G A B A erg ic synaptic transm ission could be im portant in m odulating neuronal plasticity since this could have an im portant effect on the input-output relationship o f a neurone. F urtherm o re, LT D of G A B A ergic inhibition has recently been shown to underlie the increased excitability of C A l neurones associated with LT P (Lu et al., 2000). In this case, G A B A ergic LTD was

found to be im portant for increasing the likelihood in the postsynaptic cell o f an EPSP discharging an action potential (Bliss and Lom o, 1973; Linden, 1999; Lu et al., 2000).

L T P and L T D o f synaptic transm ission can be ach iev ed by m o d u latin g the efficacy o f the postsynaptic response to transm itter release. T his can be achieved by m odifying receptor channel properties, for instance by phosph orylatio n. A lternatively, under saturating conditions of neurotransm itter, increasing or decreasing the num ber of receptors in the postsynaptic m em brane w ould be expected to have a significant effect on synaptic transm ission. In the case of A M PA receptors, both receptor phosphorylation and receptor turnover at postsynaptic sites have been found to effect glutam atergic synaptic strength and plasticity (O ’Brien et al., 1998; L uscher et al., 1999; M an et al., 2000; W ang and Linden, 2000; Lee et al., 2000). Sim ilarly, receptor p ho sphorylation can m odulate GABA^ receptor channel properties (M oss et al., 1992; M oss et al., 1995; M oss and Sm art, 1996; M cD onald et al., 1998). Im portantly, there is ev id en ce to suggest that regulating the num ber o f postsynaptic GABA^ receptors can also have a significant effect on inhibitory synaptic transm ission. In cerebellar stellate cells, differences in postsynaptic receptor num ber have been found to determ ine G AB A m ini am plitu de (N usser et al., 1997). F urtherm ore, activity dependent increases in G ABA ^ receptor num ber have been found to underlie potentiation at hippocam pal inhibitory synapses (N usser et al., 1998a). The m echanism s behind changes in synaptic receptor num ber at inhibitory synapses are unknow n but could be due to an increased rate of postsynaptic receptor insertion and / or decreased rate o f receptor removal.

T he results presented here suggest that the regu lation o f p o stsy n ap tic G ABA ^ rece p to r cy clin g co u ld be an effectiv e m eans o f re g u la tin g in h ib ito ry sy n ap tic transm ission. In neurones, GABA^ receptors were found to constitutively endocytose by a clathrin d ep en d en t m ech anism , via an association w ith the ad ap tin co m p lex A P2.

GAB A A recep to r m IPSC . T his occurred on a relativ ely rap id tim e scale (over 50% increase in m IPSC am plitude over 20 m in) im plying that the rates o f in sertion and rem oval o f receptors are relatively fast at G A BA ergic synapses. Im portantly, this im plies that m odulating the turnover o f receptor num ber, by changing the rates o f either insertion or rem oval o f receptors could significantly effect the strength o f G A B A ergic synaptic transm ission providing a m echanism for G A BA ergic LT P and LTD.

This thesis also reports a potential role for N SF in G ABA ^ receptor trafficking via an asso ciatio n betw een N SF and G A B A R A P. In tere stin g ly , N SF has alread y been im plicated in the trafficking of both m etabotropic (M cD onald et al., 1999) and ionotropic receptors (N ishim une et al., 1998; Song et al., 1998; O sten et al., 1998) as w ell as in the generation of glutam atergic synaptic plasticity (Luscher et al., 1999; Luthi et al., 1999). In ag reem en t w ith this G A B A R A P was found lo calised to in tracellu lar co m p artm en ts in clud in g sub sy naptic cisternae. These m em brane com p artm en ts could be part o f a trafficking route taken by receptors cycling betw een the cell surface and internal pools. An in tra c e llu la r pool o f G A BA ^ recep to rs has been d e te c te d by im m u n o eletro n m icroscopy (Sur et al., 1995) and GABA^ receptors have been show n to translocate to the cell surface from internal storage com partm ents upon som e signals (W ang et al., 1997). By p lay in g a role in G A BA ^ receptor postsynaptic m em brane insertion or rem oval, G A B A R A P via an associatio n with N SF could serve as p art o f a pro tein com plex im portant for regulating the postsynaptic cycling of receptors.