FOS EXPRESSION BY NALOXONE IN LHRH NEURONS OF THE MEDIOBASAL HYPOTHALAMUS AND EFFECTS OF PENTOBARBITAL SODIUM IN THE PROESTROUS RAT

Because Fos is thought to be induced in neurons that are activated, we examined whether luteinizing hormone-releasing hormone (LHRH) neurons expressed Fos protein when they were stimulated by an opioid receptor antagonist naloxone (NAL), expecting to identify LHRH neurons which a re regulated by opioid neurons directly or indirectly. Further, we exa mined whether an ovulation-blocking dosage of pentobarbital sodium (PB ) would affect the NAL-induced Fos expression, Female rats were infuse d with naloxone (5 mg/kg/h) for 90 min (10.00-11.30) in the morning of proestrus, during which infusion blood sampling was done, and were ki lled by i.v. injection with an overdose of PB at 11.30-12.00. Dual imm unoperoxidase/immunofluorescence staining for both Fos and LHRH reveal ed that some LHRH immunoreactive (ir) neurons in the forebrain express ed Fos-ir, associated with an increase in serum LH concentrations, but little co-localization was found in rats in proestrus which were infu sed with saline as the control, The proportion of LHRH-ir neurons whic h expressed Fos-ir was about 35-62% in the caudal part of the forebrai n including the mediobasal hypothalamus, and this was larger than that (10%) in the rostral part of the forebrain including the preoptic are a. PB injection (32 mg/kg bw, i.p,) 15 min prior to the beginning of N AL infusion significantly enhanced the increase in LH secretion due to NAL, and also enhanced Fos-ir expression in LHRH-ir neurons, Together with the well-established fact that PB blocks the LHRH surge generato r and our previous findings that NAL stimulates the LHRH pulse generat or even in the PB-blocked proestrous rat, these results strongly sugge st that the LHRH pulse generator exists in the mediobasal hypothalamus which contains LHRH neurons that are responsive to NAL and express Fo s protein.