Pineal modulation of the rat caudate-putamen spontaneous neuronal activity: roles of melatonin and vasotocin.

The effects of microiontophoretic application of melatonin and melatonin plus vasotocin on spontaneously active neurons of caudate-putamen in sham-operated and pinealectomized rats were studied. Extracellular unit recordings showed that in sham-pinealectomized rats, melatonin ejection primarily produced inhibition of the responsive neurons (74.1%), whereas only 24.9% of the neurons were excited. Iontophoretic ejection of vasotocin or melatonin+vasotocin produced, in both cases, an inhibition of 100% of the responsive neurons. In pinealectomized rats, iontophoretic melatonin ejection produced a similar percentage of inhibition (46.1%) and excitation (53.8%) of the responsive neurons. The simultaneous ejection of melatonin+vasotocin further increased the percentage of inhibition (88.8%) compared with the melatonin only treated group. Moreover, iontophoretic ejection of vasotocin inhibited 100% of the responsive neurons in pinealectomized rats. The actions of melatonin and vasotocin seem to be specific, because their effects are dependent on the amount of these compounds ejected, i.e., the intensity of the ejection current. These results indicate that the pineal compounds melatonin and vasotocin are neuromodulators of spontaneous neuronal activity of the rat caudate-putamen.