GLUTAMATERGIC AND GABAERGIC SYNAPTIC CURRENTS IN GANGLION-CELLS FROM ISOLATED RETINAE OF PIGMENTED RATS DURING POSTNATAL-DEVELOPMENT

This study was aimed at characterizing the earliest phases of synaptog enesis in the mammalian retina. Spontaneous activity of ganglion cells in the isolated superfused retina was used as an indicator for the fu nctionality of synaptic connections. Retinal ganglion neurons (RGNs) w ere identified by location of their somata in the ganglion cell layer (GCL) and by their ability to generate large (> 500 pA) voltage-activa ted sodium currents. Spontaneous spiking was found in many RGNs prior to cell perfusion. Between postnatal day (P) 1 and 18, a total of 195 RGNs was tested for light-induced currents, conductance changes in res ponse to exogenous glutamate (Glu) and gamma-aminobutyric acid (GABA), and depolarizing or hyperpolarizing synaptic activity. The vast major ity of the material was derived from RGNs at day P5. Whole-cell ion cu rrents were always sampled at somatic sites, using either conventional or perforated patch whole-cell recordings. On day P5, 5% of tested RG Ns (n = 73) were already responsive to light stimulation. A higher per centage of cells (23%, n = 187) generated spontaneous depolarizing cur rents that were regarded as glutamatergic excitatory postsynaptic curr ents (EPSCs), since (1) they were blocked by Glu antagonists, (2) they conformed to the Na+/Cs+ equilibrium potential, (3) and they displaye d a time course characteristic of glutamatergic EPSCs. The mean EPSC a mplitude was 19.0 pA (S.D. 11.83 pA). Amplitude distributions were fit ted by multiple Gaussian equations rendering a quantal size of 6.6 to 9.1 pA at a holding voltage (V(h)) of -70 mV (driving force about 70 m V). Spontaneous EPSCs were never observed under condition of Ca2+-free solutions, but they persisted in the presence of tetrodotoxin. Bath a pplication of quisqualate (500 mu M) consistently increased EPSC frequ encies. In contrast to the relatively high percentage of RGNs generati ng spontaneous EPSCs, very few RGNs at P5 (3%, n = 187) displayed inhi bitory postsynaptic currents (IPSCs), although by that time all tested RGNs (n = 14) were responsive to both exogenous Glu and GABA. These r esults indicate that in the postnatal rat retina development of excita tory synapses precedes the maturation of inhibitory afferents. Excitat ory inputs to RGNs were to some extent functional before the animals o pened their eyes. Glutamatergic synaptic activity may, thus, play an i mportant role in shaping visual connections in the absence of visual e xperience.