Four series of experiments were performed to evaluate the effects of methamphetamine on the auditory system of guinea pigs. Methamphetamine was administered to guinea pigs via intraperitoneal injection, and the time course of latencies of waves I, II, III, and IV, interpeak latencies of waves I-III, III-IV, and I-IV of the ABR, and the near-field potential of the inferior colliculus and cochlear nerve were measured. These physiological data were analyzed in comparison with the distribution of methamphetamine in the inferior colliculus and cochlea following its immunohistochemical staining, as well as the serum methamphetamine concentration.
The following results were obtained;
1. The wave latencies and interpeak latencies of ABR decreased up to 120 minutes after the administration.
2. In the cochlear nerve recording, the amplitude of the response increased and the latency decreased up to 90 minutes after the administration. These changes in responses were, however more prominent in the inferior colliculus recording.
3. Immunohistochemical examination revealed that methamphetamine was present in the hair cells of the organ of Corti, spiral ligament, spiral ganglion cells and the inferior colliculus 30 minutes after the administration.
4. The serum concentration of methamphetamine rapidly reached its maximal level, and then dropped gradually, logarithmically, having become almost horizontal at a low level 6 hours later.
These results suggest that methamphetamine does exert an effect on the peripheral auditory system, as well as on the central nervous system, which is generally regarded as the effector organ. It is assumed that the pharmacological effect of methamphetamine on the cochlea involves excitation of sensory cells, afferent nerve fibers and spiral ganglion cells.