Prolonged inhibitory potentials in layer III projection cells of the rat medial entorhinal cortex induced by synaptic stimulation in vitro.

The entorhinal cortex projects via layer III neurons directly to the hippocampal area CA1 and the subiculum. We studied the functional properties of the medial entorhinal cortex projection cells in horizontal hippocampal-entorhinal cortex combined slices. These cells displayed, upon single-shock synaptic stimulation, an excitatory postsynaptic potential followed by a fast and/or slow inhibitory postsynaptic potential. Short train repetitive stimulation subthreshold for generation of action potentials induced a slow hyperpolarization of up to 20 s. Pharmacological analysis shows that the slow hyperpolarization could be divided into three components: i) the first component, which lasted 1 s, was sensitive to GABA(B) receptor antagonists; ii) the second component lasting for about 6 s was sensitive to atropine, suggesting muscarinic acetylcholinergic nature of these responses; iii) a late component lasting for up to 20 s was sensitive to naloxone, suggesting a role for opioids in its generation. The finding that layer III projection neurons to the hippocampus proper develop long-lasting hyperpolarizations suggests possible control mechanisms for the output functions of the entorhinal cortex.