Effect of psychotropic drugs on identified septohippocampal neurons

The effects of various psychotropic drugs (benzodiazepines, antidepressants, neuroleptics and nootropic drugs, a family of cognition activator agents) on firing rates of septohippocampal neurons, identified by electrical antidromic stimulation, were studied in the medial septum-nucleus of the diagonal band of Broca of rats anaesthetized with urethan. Extracellular potentials from single septohippocampal neurons were recorded using glass pipettes. Drugs were applied by either microiontophoresis or intravenous injections (i.v.). Benzodiazepines produced a marked depression of spontaneous firing rates of septohippocampal neurons whether applied i.v. (diazepam) or iontophoretically (flurazepam, midazolam). In addition, diazepam had a potent depressant effect on the rhythmically bursting activity of the septohippocampal neurons. Baclofen also had an inhibitory effect. Antidepressant drugs (applied by iontophoresis) as well as amphetamine, had a depressant effect on spontaneous firing rates. Neuroleptics (i.v.) had less significant or consistent effects on septohippocampal neurons, although the effects of haloperidol were usually inhibitory. Nootropic drugs were generally ineffective. These data indicate that most psychotropic drugs tested (with the exception of nootropic drugs) have an inhibitory effect on the spontaneous activity of septohippocampal neurons. However, benzodiazepines seem to be more active than antidepressants or neuroleptics. Oxotremorine (i.v.) had a potent excitatory effect on septohippocampal neurons. Atropine (i.v.) increased the septohippocampal neurons' firing rate in some cases. These results are discussed in view of the possible implication of the involvement of septohippocampal neurons in the mediation of the effects of psychotropic drugs on the central nervous system and, more specifically, on the cholinergic systems.