ANALYSIS OF THE ROLE OF INHIBITION IN SHAPING RESPONSES TO SINUSOIDALLY AMPLITUDE-MODULATED SIGNALS IN THE INFERIOR COLLICULUS

Neurons in the central nucleus of the inferior colliculus (ICc) typica lly respond with phase-locked discharges to low rates of sinusoidal am plitude-modulated (SAM) signals and fail to phase-lock to higher SAM r ates. Previous Studies have shown that comparable phase-locking to SAM occurs in the dorsal nucleus of the lateral lemniscus (DNLL) and medi al superior olive (MSO) of the mustache bat. The studies of MSO and DN LL also showed that the restricted phase-locking to low SAM rates is c reated by the coincidence of phase-locked excitatory and inhibitory in puts that have slightly different latencies. Here we tested the hypoth esis that responses to SAM in the mustache bat IC are shaped by the sa me mechanism that shapes responses to SAM in the two lower nuclei. We recorded responses from ICc neurons evoked by SAM signals before and d uring the iontophoretic application of several pharmacological agents: bicuculline, a competitive antagonist for gamma-aminobutyric acid-A ( GABA(A)) receptors; strychnine, a competitive antagonist for glycine r eceptors; the GABAB receptor blocker, phaclofen, and the N-methyl-D-as partate (NMDA) receptor blocker, (-)-2-amino-5-phosphonopentanoic acid (APS). The hypothesis that inhibition shapes responses to SAM signals in the ICc was not confirmed. In >90% of the ICc neurons tested, the r ange of SAM rates to which they phase-locked was unchanged after block ing inhibition with bicuculline, strychnine or phaclofen, applied eith er individually or in combination. We also considered the possibility that faster lpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (A MPA) receptors follow high temporal rates of incoming excitation but t hat the slower NMDA receptors could follow only lower rates. Thus at h igher SAM rates, NMDA receptors might generate a sustained excitation that ''smears'' the phase-locked excitation generated by the AMPA rece ptors. The NMDA hypothesis, Like the inhibition hypothesis; was also n ot confirmed. In none of the cells that we tested did the application of AP5 by itself, or in combination with bicuculline, cause an increas e in the range of SAM rates that evoked phase-locking. These results i llustrate that the same response property, phase-locking restricted to low SAM rates, is formed in more than one way in the auditory brain s tem. In the MSO and DNLL, the mechanism is coincidence of phase-locked excitation and inhibition, whereas in ICc the same response feature i s formed by a different but unknown mechanism.