IN-VITRO DEVELOPMENT OF VERTEBRATE CENTRAL SYNAPSES

This article deals with basic determinants of synaptic efficacy during development of glutamatergic and GABAergic synaptic transmission: loc ation and number of release sites, release probability and single cell -activated (unitary) conductances. We hypothesize that both types of n euronal connections differ in major aspects of synaptogenesis. Disrega rding the fact that various test models and cell types could render di verging results, it can be observed that glutamatergic terminals displ ay a preference for dendrites, whereas GABAergic terminals select soma locations at initial stages of development. Glutamatergic synapses ar e characterised by receptor accumulation in the region of terminal app osition, whereas in GABAergic synapses receptor concentration is weak, if present at all. The expression of glutamate receptors (GluRs), but not GABA(A) receptors is under control of interneurons. Developmental changes in glutamatergic synaptic transmission have not yet been asse ssed by quantal analysis. For GABAergic synapses, first results are no w available from a culture preparation of the rat superior colliculus. In general terms, functional maturation seemed to lag behind the form ation of structurally differentiated release sites. Compound binomial analysis revealed that during in vitro development a considerable frac tion of GABAergic terminals remained in a low efficacy release state ( p < 0.2). A developmental increase in synaptic strength was reached by the appearance of singular highly effective release sites. Presynapti c maturation could be manipulated by long-term drug treatment. Additio n of GluR antagonists significantly increased amplitudes and decreased the coefficients of variations of evoked inhibitory postsyhaptic curr ents. Thus, the strength of inhibitory synaptic transmission could be influenced by the status of heteronymous synaptic input.