Glutamic acid decarboxylase in rat olfactory bulb: effect of ovarian steroids or male pheromones

The effect of ovarian steroids and pheromones on the activity of glutamic acid decarboxylase, the enzyme that synthesizes gamma-aminobutyric acid (GABA), was studied in the rat olfactory bulbs. The enzyme activity was measured in the main and accessory olfactory bulbs at 11:00 h and 17:00 h in ovariectomized rats, and in rats treated with ovarian steroids or exposed to male pheromones. The enzyme activity in both bulbs showed a diurnal fluctuation that was not affected in the accessory bulbs by the exposure to pheromones while the rhythm disappeared in the main bulbs. Estrogen and estrogen-progesterone treatments decreased the enzyme activity in both bulbs either in the morning or in the afternoon. The exposure of ovariectomized estrogen-primed rats to male pheromones reversed the effect of estrogen on the enzyme activity in the morning but not in the afternoon. Ovarian hormones plus pheromones prevented the steroid effect only in the morning. These results support the view that in olfactory bulbs, the GABAergic system can be modulated by endocrine and pheromonal factors.