Effects of Central Depressants on the Evoked Potentials in the Inferior Colliculus Caused by Acoustic Stimuli in the Cat

Abstract

It has been well-known that the electrical activity of the central nervous system is facilitated by relatively lower doses of central depressants.Many reports were presented to support the facilitatory mechanism in the brain of exprimental animals at lower dose levels of barbiturates : lowering of stimulation threshold for the recruiting response by King (1) and' Domino (2), increase in amplitude of the somatosensory evoked poten tial recorded from the internal capsule by King et al. (3), and increase in amplitude of the evoked potential recorded from the auditory cortex by Pradhan and Galambos (4).Nakai (5) and Nakai and Takaori (6) showed that pentobarbital sodium, chloralose and chlorpromazine increased the magnitude of the cortical potential evoked by click stimuli and accelerated the manifestation of the ensuing rhythmic after-discharges, while urethane and ethyl alcohol even in small doses reduced the amplitude of the evoked potential.The role of the inferior colliculus as a relay nucleus for the auditory pathway was investigated electrophysiologically by Ades and Brookhart (7) and Thurlow et al. (8).However, there is scarcely any systematic study concerning the effects of central depres sants on the evoked potential in the inferior colliculus. It is considered that the effect of central depressants on the collicular acoustic potential may differ from that on the cortical one. The difference may give some clue for the elucidation of the central audi tory mechanism. METHODSAll experiments were performed in a sound-proof room maintained at the tempera ture of 27°C using cats weighing 2.0 to 3.5 kg. Under ether anesthesia the exposed trachea was intubated and the head of animal was fixed on the stereotaxic instru ment of Todai-Noken type.The left occipital skull covering the cerebrum and cerebellum was , removed, and the left tentorium cerebelli was carefully resected. The left inferior colliculus was exposed for insertion of the recording electrode under direct inspection. The cerebral and cerebellar corticies were not removed. Thereafter, the spinal cord was sectioned at the level of C1 (encephale isole preparation) and the animal was maintained by artificial respiration. A monopolar insulated silver electrode, about

Effects of Volatile Anesthetics on the Evoked Potentials and Unitary Discharges in the Central Auditory System Caused by Click Stimuli in Cats

“…Brazier (23,24) has shown that the evoked potentials in the visual cortex and Nucleus centre median of thalamus to flash stimulation are inhibited by ether but not by pentobarbital or chloralose. Table 2 summarizes the effects of ether, chloroform, fluothane and nitrous oxide on the cortical and collicular evoked click potentials and unitary discharges in the present experiments comparing with those of pentobarbital sodium, urethane and chlorpromazine reported previously (14)(15)(16)(17). The evoked click potentials in the auditory cortex and in ferior colliculus were depressed by ether inhalation almost uniformly.…”

“…The facilitator), effects of small doses of pentobarbital sodium , chloralose and chlorpro mazine on the cortical evoked click potentials have been reported by Nakai et al (14)(15)(16) in cats. These effects were observed in the supra-thalamic level, and the evoked click potentials in the inferior colliculus were not augmented by these central depressants.…”

“…How ever, there are not enough informations about the mode of action of volatile anesthetics on the central auditory system. The present experiments are challenge to know the effects of inhalation of volatile anesthetics such as ether, chloroform, fluothane and nitrous oxide on the evoked potentials in the auditory cortex and inferior colliculus and the uni tary discharges in the inferior colliculus caused by click stimulation in the encephale isole preparation of cats and to compare the action of the volatile anesthetics with that of bar biturate, chloralose, urethane and chlorpromazine reported previously (14)(15)(16)(17).…”

“…The animal was kept in the sound-proof room maintained at 28°C. Details of the experimental pro cedures were followed the descriptions by Nakai et al (14)(15)(16)(17). Surgical procedures: Under ether anesthesia, the cat was inserted with tracheal cannula and the head was fixed on the stereotaxic instrument of Todai-Noken type.…”

“…Nakai et al (14)(15)(16) have demonstrated that small doses of pentobarbital sodium, chloralose and chlorpromazine facilitate the click responses in the auditory cortex of cats but urethane and ethyl alcohol depress them in proportion to the doses. These findings are noteworthy, because the sites of action of the central depressants are different each other in the auditory system which has considerably complicated fiber connection.…”