Stimulation of hippocampal dopaminergic receptors: behavioral and electrophysiological effects

Recent biochemical and pharmacological data have suggested a transmitting role for dopamine in the hippocampal formation. It was found that hippocampal dopaminergic afferents in the rat originate in the ventral tegmental area and the medial part of substantia nigra. In order to determine the influence of dopamine on hippocampal neurons, electrophysiological and behavioral changes after intrahippocampal injection were studied. The studies were carried out on nonanesthetized rabbits with chronically implanted intracerebral electrodes and intrahippocampal cannulas, and on rats: with intrahippocampal cannulas only. Additional studies were conducted on hippocampal slices maintained in vitro to determine the influence of dopamine on firing rate of hippocampal CA 1 neurons. An intrahippocampal injection of dopamine or apomorphine enhanced the persistence of EEG alert pattern and locomotor excitation. Injection of apomorphine to the rat hippocampus induced a dose-dependent increase in the locomotor activity. Similar results were obtained using the open field test: an increase in the frequency of line crossings and peeping episodes. Dopamine also influenced the firing rate of hippocampal neurons. All those effects were blocked by pretreatment with haloperidol or pimozide. These studies have proved an excitatory action of dopamine in the dorsal hippocampus of rats and rabbits. The excitation of electrical activity of the hippocampus induced synchronization of EEG in cerebral cortex, and behavioral arousal. The results obtained using open field test suggested that the increase of locomotor activity seems to express enhancement of exploratory type of activity and not simple stereotypic movements. On the basis of obtained data neuromodulatory role of dopamine in the hippocampus is proposed.