Diurnal rhythms in the responsiveness of hippocampal pyramidal neurons to serotonin, norepinephrine, gamma-aminobutyric acid and acetylcholine.

Radioligand binding studies have revealed the existence of endogenous circadian rhythms in the number of several receptors in the rat brain. The present microiontophoretic study was undertaken to assess diurnal rhythms in the responsiveness of rat hippocampal pyramidal neurons to serotonin (5-HT), norepinephrine (NE), gamma-aminobutyric acid (GABA), and acetylcholine (ACh). Between December and April, there was a significant diurnal variation in the responsiveness of hippocampal pyramidal neurons to 5-HT and ACh. Between May and August, the responsiveness to NE and ACh showed a diurnal variation. There was no diurnal variation in the responsiveness to GABA in either period of the year. Short-term exposure to constant light or darkness produced a phase-shift of the serotoninergic and cholinergic rhythms, suggesting their endogenous nature and their synchronization to clock-time by the light-dark cycle. The diurnal rhythms in responsiveness to 5-HT and NE underwent phase-shifts from the December-April to the May-August period in rats entrained to 12:12 light-dark cycle, suggesting the existence of seasonal modulation of these rhythms. These circadian rhythms in the postsynaptic responsiveness of hippocampal pyramidal cells and their seasonal fluctuation may be related to the diurnal variation of mood seen in major depression as well as to the seasonal incidence of this illness.