Abstract

The red nucleus and the cerebellar interpositus nucleus were successively investigated in terms of multiple plastic structures mediating the classical conditioning of the rabbit`s nictitating membrane response (NMR). In experiment 1, monopolar tungsten stimulating electrodes and low-impedance monopolar tungsten recording microelectrodes were chronically implanted into the red nucleus and the interpositus nucleus respectively. All animals were trained with delay conditioning paradigm. In order to investigate any involvment of the red nucleus in the conditioning of NMR, animals of the experimental group were trained with the tone conditioned stimulus (CS) (400msec, 1kHz, 85dB, sine wave) and unconditioned stimulus (US) consisted of the electrical stimulation of the red nucleus (100msec, 400Hz, pulse width 0.1msec, 60~150㎂) and simultaneous corneal airpuff(100msec, 100g/cm²) in the five aquisition sessions. In two successive extinction sessions, the animals were trained with the tone CS and the electrical stimulation US of the red nucleus alone. Animals of the control group were trained with the tone CS and the corneal airpuff US in the same aquisition sessions and with the tone CS alone in the same extinction sessions. Results showed that the electrical stimulation US of the red nucleus neither facillitates the learning rate in the aquistion stage nor maintains the conditioned response(CR) in the extinction stage. It may be concluded that the red nucleus is a mere output structure and is not an essential one in the neural network mediating the conditioning of NMR. Multiple unit activity(MUA) recorded from the cerebellar interpositus nucleus was processed to 16msec-bin histogram and the NMR was averaged in each session. The averaged NMR was highly correlated with the histogram and the histogram of MUA preceded the averaged NMR. This supports the previous reports that the cerebellar interpositus nucleus may be the neural substrate of the conditioning of NMR. In experiment 2, stimulating electrodes and recording microelectrodes were chronically implanted into the cerebellar interpositus nucleus and the red nucleus respectively. All training methods were identical with the experiment 1, except for exchanging the stimulating target structure for the recording one. Results showed that the electrical stimulation US of the cerebellar interpositus nucleus severely retarded the aqusition of the CR. It is concluded that the cerebellar interpositus nucleus may be a structure of plasticity included in an essential neural network mediating the conditioning of NMR. Possible interpretations of the result, concerning the side effect of the electrical stimulation and the function of the inferior olive-cerebellar cortex-interpositus nucleus-inferior olive recurrent loop, are discussed.