A study of the action, and the mechanism of action of serotonin on pyramidal neurones in the hippocampal slice preparation

1) Transverse hippocampal slices were prepared from rats and maintained in a recording chamber. Intracellular recording techniques were used to measure the amplitudes of the resting potentials (mean -63.4mV), the action potentials (mean 72.5mV), and the input resistance (mean 16.5M?) of neurones in the CA1 region of the hippocampus. Impalements of neurones could be maintained for up to 9.5hrs. 2) Impaled neurones in the CA1 region of the hippocampus were identified as pyramidal neurones after injection of the fluorescent dye Lucifer Yellow CH and subsequent microscopical examination of the slices. The projections of the apical and basal dendrites were complete and in some cases the axon could be identified and traced toward the subiculum. Injections of pyramidal neurones in the CA3 region and of granule cells in the area dentata demonstrated the different morphologies of the three types of neurone. 3) CA1 neurones were hyperpolarised by up to 10mV by serotonin applied locally by iontophoresis. This response was slow on onset and lasted between 30s and 3min. In contrast, glutamic acid and acetylcholine depolarised these neurones. 4) The amplitude of the hyperpolarisation induced by serotonin was dependent on the amplitude and duration of the iontophoresis current. A decreased input resistance of the neurones was associated with and followed the same time course as the potential change induced by serotonin. 5) The serotonin response was reversed at membrane potentials more negative than the reversal potential which was between -81mV and -104mV. This high reversal potential indicated that the response might be mediated by efflux of potassium ions from the neurones. 6) In ion substitution experiments, the serotonin response was shown to be dependent on the extracellular potassium ion concentration, but was independent of the chloride ion concentration. 7) The serotonin response was blocked by the putative serotonergic antagonist methysergide (100µM). However, the responses of the neurones to serotonin were not affected by cyproheptadine, methergoline, mianserin, and 1-propranolol which have also been proposed to be antagonists of serotonin in the central nervous system. 8) The significance of these results is discussed with reference to previous investigations by other workers