Fig. 5.

Development of intertectal axons and their synapses. A: intertectal synaptic current is sensitive to picrotoxin (PTX). Tectal neurons were recorded from in whole cell, voltage-clamp configuration. Membrane potential was held at 0 mV, and 20 μM NBQX was added to the bath to isolate inhibitory components of the postsynaptic current (PSC). A bipolar electrode in the opposite tectal hemisphere was used to generate PSCs. A baseline of 50 traces was recorded (inset shows averaged responses, green trace). Then, 100 μM PTX was added to the bath. The cell was allowed to incubate in PTX for 10 min before recording was resumed. Inset black trace shows averaged response after PTX addition. The amplitude of the PSC was measured and is shown normalized to baseline (normalized baseline: 1.00 ± 0.08; +PTX: 0.48 ± 0.06; n = 12 cells, scale bar = 50 ms, 10 pA). B: intertectal synaptic current is sensitive to NBQX. A 2nd set of tectal neurons was recorded at −60 mV in the presence of 100 μM PTX. Baseline traces were collected as in A, and then 20 μM NBQX was added to the bath and allowed to circulate for 10 min. Inset: averaged traces before (green) and after (black) NBQX. Amplitudes of the PSCs were averaged and normalized to baseline (normalized baseline: 1.00 ± 0.11; +NBQX: 0.46 ± 0.04; n = 25 cells, scale bar = 50 ms, 20 pA). C: developmental increase in AMPA/NMDA postsynaptic currents (PSCs) in response to stimulating intertectal axons. PSCs were measured at −60 mV (AMPA, peak taken 10–30 ms following stimulus artifact) and +40 mV (NMDA, peak taken at >50 ms following stimulus artifact). AMPA/NMDA (absolute value of measured AMPA current amplitude divided by NMDA current amplitude) increased significantly from stage 46 (n = 9) to stage 48 (n = 33); *P < 0.05. Inset: example traces of currents recorded at −60 and +40 mV in stage 46 (gray) and stage 48 (black) neurons. Scale bar = 20 ms/50 pA. D: no developmental change in the GluN2B component of intertectal NMDAR-mediated currents. Pharmacologically isolated NMDAR-mediated responses were recorded at +40 mV before and after exposure to ifenprodil. Current amplitudes were normalized to baseline and are presented as %blockade (1 − remaining amplitude). The proportion of ifenprodil-sensitive NMDAR-mediated current was not different between stage 46 (n = 6) and stage 48 (n = 6). E: developmental decrease in paired-pulse ratio. Cells from stage 48 tadpoles (n = 13) showed significantly less facilitation (*P < 0.05) than at stage 46 (n = 17) in response to paired stimuli (50-ms interstimulus interval) to intertectal axons. Inset: example traces showing paired-pulse ratio responses in stage 46 (gray) and stage 48 (black) tadpoles. Scale bar = 20 ms/50 pA. Data are presented as means ± SE.