Possible modulation of auditory middle latency responses by nitric oxide in the inferior colliculus of anaesthetized rats

Abstract

Nitric oxide (NO) is a short-lived radical species endowed with intercellular signalling functions in the mammalian brain. In the present study we have investigated the effects of focal injection into one inferior colliculus of N ω-nitro- L-arginine methyl ester ( L-NAME), a nitric oxide synthase inhibitor, on the acoustic middle latency responses (MLRs) evoked by click stimuli and recorded from the auditory cortex in anaesthetized rats. Microinfusion of L-NAME (1.0 mM) did not alter the latency of MLRs nor did it affect the evoked brain stem responses (ABRs). By contrast, L-NAME reduced P 1a—N 1 amplitude of MLRs by 51.7 ± 6.6% (mean ± SEM; n = 5) and almost complete recovery to background amplitude was obtained 15–25 min after treatment. The less active isomer, D-NAME (1.0 mM; n = 5), failed to produce consistent effects on the evoked MLRs. A higher concentration of L-NAME (5.0 mM; n = 5) yielded a 69.0 ± 13.3% inhibition whereas maximum inhibition produced by 0.5 mM ( n = 3) L-NAME was ≌10% of control value. The inhibitory effect typically evoked by 1.0 mM L-NAME was prevented by treating rats with L-arginine (5.0 mM; n = 5), the endogenous precursor of NO synthesis. Reduction of MLR amplitude was also obtained in rats receiving intracollicular injection of dizocilpine (MK801; 1.0 μM) and LY274614 (1.0 mM), two selective [ N-methyl- D-aspartate (NMDA) receptor antagonists. In conclusion, the present data support a role for intracollicular NO in the processing and transmission of the acoustic input to the auditory cortex in the rat.