Time-Dependent Effect of Melatonin on Glutamic Acid Decarboxylase Activity and CI Influx in Rat Hypothalamus.

Abstract The objective of the first series of experiments was to assess whether melatonin treatment modifies the activity of the y-aminobutyric acid synthesizing enzyme, glutamic acid decarboxylase, in the preoptic-medial basal hypotnalamic area, cerebral cortex and cerebellar cortex of rats receiving 25 to 300 mug of melatonin in the early morning and late evening in the diurnal cycle. A significant increase of apparent V(max) and K(m) of the enzyme was found in the hypothalamus of rats killed at the 12th h of the light phase (i.e. the time when lights were turned off) and receiving 25 to 300 mug/kg of melatonin 3 h earlier. In the early morning, only a 300 mug/kg dose of melatonin (injected in the 1st h of the light phase) was effective to increase V(max) and K(m) of hypothalamic glutamic acid decarboxylase 3 h later. In cerebral and cerebellar cortices, increases in V(max) and K(m) of enzyme activity were apparent only in the evening and with the highest melatonin dose employed (300 mug/kg). In a second series of experiments the activity of melatonin to modify in vitro(36)CI influx by 900 x g pellets of rat preoptic-medial basal hypothalamic area was studied at the 4th and 12th h of the light phase of daily photoperiod. Melatonin increased (36)CI(-) influx at a minimum concentration of 100 nM (in the morning) or 10 nM (in the evening). The effect of melatonin on (36)CI(-) influx was prevented by co-incubation with 100 muM picrotoxin. Addition of 10 to 100 muM of y-aminobutyric acid to the resuspended 900 x g pellets brought about a dose-dependent increase of (36)CI (-) influx. Preincubation with melatonin at threshold doses of 1 muM (in the morning) or 0.1 muM (in the evening) significantly augmented y-aminobutyric acid effect on (36)CI(-) uptake. These results indicate that melatonin facilitates pre- and postsynaptic activities of y-aminobutyric acid neurons, particularly in the hypothalamus, through an effect that displays a diurnal sensitivity compatible with the documented activity of the hormone on a number of physiological functions.