Differential response properties to amplitude modulated signals in the dorsal nucleus of the lateral lemniscus of the mustache bat and the roles of GABAergic inhibition.

We studied the phase-locking of 89 neurons in the dorsal nucleus of the lateral lemniscus (DNLL) of the mustache bat to sinusoidally amplitude modulated (SAM) signals and the influence that GABAergic inhibition had on their response properties. Response properties were determined with tone bursts at each neuron's best frequency and then with a series of SAM signals that had modulation frequencies ranging from 50-100 to 800 Hz in 100-Hz steps. DNLL neurons were divided into two principal types: sustained neurons (55%), which responded throughout the duration of the tone burst, and onset neurons (45%), which responded only at the beginning of the tone burst. Sustained and onset neurons responded differently to SAM signals. Sustained neurons responded with phase-locked discharges to modulation frequencies < or = 400-800 Hz. In contrast, 70% of the onset neurons phase-locked only to low modulation frequencies of 100-300 Hz, whereas 30% of the onset neurons did not phase-lock to any modulation frequency. Signal intensity differentially affected the phase-locking of sustained and onset neurons. Sustained neurons exhibited tight phase-locking only at low intensities, 10-30 dB above threshold. Onset neurons, in contrast, maintained strong phase-locking even at relatively high intensities. Blocking GABAergic inhibition with bicuculline had different effects on the phase-locking of sustained and onset neurons. In sustained neurons, there was an overall decline in phase-locking at all modulation frequencies. In contrast, 70% of the onset neurons phase-locked to much higher modulation frequencies than they did when inhibition was intact. The other 30% of onset neurons phase-locked to SAM signals, although they fired only with an onset response to the same signals before inhibition was blocked. In both cases, blocking GABAergic inhibition transformed their responses to SAM signals into patterns that were more like those of sustained neurons. We also propose mechanisms that could explain the differential effects of GABAergic inhibition on onset neurons that locked to low modulation frequencies and on onset neurons that did not lock to any SAM signals before inhibition was blocked. The key features of the proposed mechanisms are the absolute latencies and temporal synchrony of the excitatory and inhibitory inputs.