GABA And Glycine Receptors On Carp Retinal Ganglion Cells

Many lines of evidence indicate that GABAergic and glycinergic inhibitoryinputs into retinal ganglion cells (RGCs) from amacrine cell (AC) and/orinterplexiform cell (IPC) play many important roles in visual information precessingof RGCs, including organization of receptive fields. On freshly dissociated carpRGCs identified by retrograde labeling, we characterized GABA and glycine (GLY)receptors of these neurons and modulation of these receptors by endogenousneuromodulator Zn2+, using whole-cell patch clamp recording techniques combinedwith a Rapid Solution Changer (RSC-100). A fraction of (10/24) solitory carp RGCs fired action potentials (APs)spontaneously. Under whole-cell current clamp conditions, some of them firedrandomly (n = 5), others fired regularly at a frequency of ~10 Hz (n = 3), and stillothers fired in burst (n = 2). With current injection (10~100 pA), all identified RGCsfired APs (n = 24). In most of them (n = 18), the depolarization-induced firing wasphasic. That is, APs fired only in the early phase of depolarization. Moreover, thelarger the current injected, the shorter the duration of the APs. Some RGCs only fireda single AP at the beginning of depolarization, no matter how intense the current was ---7 ---英文摘要(n = 4). A few RGCs fired APs all along the depolarization, and the frequency offiring increased almost linearly with the current intensity (n = 2). On almost all carp RGCs tested, 100 μM GABA induced a large inward current(1~3 nA), which decayed remarkably after reaching a peak (desensitization), and itstime course was fitted very well with monoexponential equation, yielding a timeconstant (τ) of 1.35 ± 0.17 s (n = 14). These GABA responses were completelyblocked by co-application of 100 μM bicuculline (BIC) (n = 5), indicating that theywere mediated exclusively by GABAA receptors. The single channel conductance ofthis GABAA receptor was estimated to be ~10 pS using non-stationary varianceanalysis (NSVA). Dose-response curve of GABA was determined, yielding an EC50of 25.2 ± 0.17 μM and a Hill coefficiency of 1.64 ± 0.02 (n = 5). We also studiedmodulation of GABA receptors by Zn2+. Zn2+ surppressed the GABA response in adose-dependent manner: 10 μM Zn2+ reduced the current response to 100 μM GABAto 64.5 ± 12.1% (n = 6) of the control level, and 100 μM Zn2+ further reduced it to34.3 ± 13.2% (n = 6) of control. Similar dose-dependent inhibition was observed for30 μM GABA induced current responses. On most of carp RGCs tested (82/93), 100 μM GLY induced an inward currentof 0.5~1 nA. Similar to the GABA responses, these responses showed significantdesensitization with a time course which could be fitted very well with amonoexponential equation, yielding a time constant (τ) of 1.65 ± 0.22 s (n = 36).Moreover, these responses were completely blocked by pre-incubation with 1 μMstrychnine (STR) (n = 7), indicating that they were mediated exclusively by ---8 ---英文摘要strychnine-sensitive and kinetically homologous GLY receptors. Using non-stationary variance analysis, the single channel conductance of this GLY receptor wasestimated to be ~3 pS. Dose-response curves were fitted to the averaged data obtainedby the application of different concentrations of GLY, yielding an EC50 of 44.5 ± 2.2μM with a Hill coefficiency of 1.41 ± 0.09 (n = 5). We further investigated interactions between GLY receptor and their antagonists.Current response to 100 μM GLY were recorded in the presence of differentconcentrations of STR, IC50 was found to be 37.2 ± 3.4 nM with a Hill coefficiencyof 0.88 ± 0.08. It was noted that STR at a concentration range between 10 and 200nM partially blocked the response and appeared to make it less transient and moresustained, so that the response in the presence of 200 nM st...