Effects of neurotransmitters and vagus nerve stimulation on diencephalic and mesencephalic neuronal activity

The effects of microiontophoretically applied neurotransmitters and cervical vagus nerve stimulation on neuronal discharge frequency was determined for cells located in the lateral hypothalamus-lateral preoptic area (LH-LPA), medial hypothalamus, thalamus-zona incerta area, and midbrain of anesthetized rats. Seven barrel electrodes were utilized to record simultaneously from and apply gamma aminobutyric acid (GABA), dopamine, glutamate, acetylcholine and norepinephrine to single neurons. Microiontophoretically applied GABA resulted in similar decreases in neuronal discharge frequency in all central areas tested. Thalamus-zona incerta cells were the most sensitive to GABA and required the lowest ejection current for threshold responses. Dopamine application resulted in increases and decreases in hypothalamic and mesencephalic neuronal discharge frequency. Thalamus-zona incerta neurons were the most sensitive to dopamine and exhibited only decreased activity during its application. Glutamate application resulted in a non-specific excitation of neurons. Acetylcholine induced increases and decreases in neuronal discharge frequency. Thalamus-zona incerta cells were the most sensitive to acetylcholine application. Hypothalamic neurons were the most sensitive to norepinephrine induced decreases in neuronal activity and effects in this area persisted for a prolonged period after norepinephrine was applied. Neuronal discharge frequency was significantly increased during vagus nerve stimulation in all central areas studied. Only hypothalamic neurons also exhibited decreased activity during vagus nerve stimulation. Results are discussed in terms of previous neurochemical and neurophysiological data and in terms of the importance of vagal afferents in the control of central neuronal activity.