Figure 1.

A tonic GABAergic current is present in middle/high- but not in low-CF NL neurons and is developmentally regulated. A, B, Schematic diagram of the avian auditory brainstem. NA, Nucleus angularis; NM, nucleus magnocellularis; SON, superior olivary nucleus; 8th n, cranial nerve VIII. The NL was divided into two regions according to the CF of the neurons (adapted from Rubel and Parks, 1975). C1, SR95531 (20 μm), a specific GABA_AR antagonist, blocked sIPSCs in both low- and middle/high-CF neurons, but only evoked an outward current in the middle/high-CF neuron. The dashed lines are artificial drawings to help visualize the effects of SR95531. C2, Recordings (2 s) under control conditions at an enlarged scale, with one sIPSC being shown at higher temporal resolution. C3, Distribution of the amplitudes of the sIPSCs recorded from the two sampled neurons under control conditions. C4, All-point histograms of the baseline currents fitted with a Gaussian function (control, black; SR95531, gray). The amplitude of the tonic current was defined as the difference in the means of the Gaussian fit between control conditions and in presence of SR95531. D, Amplitude of the tonic current, frequency, and amplitude of the sIPSCs in middle/high-CF neurons (MF/HF, n = 71) versus low-CF neurons (n = 10), from E17 to P1 animals. E, Development of the tonic current, the frequency, and amplitude of sIPSCs [E11, n = 6; E13, n = 9; E17–P1, n = 81; neurons sampled from all frequency regions were lumped (e.g., for E17–P1 animals, there were 71 middle/high-CF and 10 low-CF neurons)]. In this and subsequent figures, bars represent means ± SEM. *p < 0.05, **p < 0.01, and ***p < 0.001, t test or ANOVA post hoc Fisher's test. Cells were held at −60 mV for voltage-clamp experiments. LF, MF, HF, Low-, middle-, and high-CF neurons of the NL, respectively.