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Fos Expression by Naloxone in LHRH Neurons ofthe 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 are regulated by opioid neurons directly or indirectly. Further, we examined whether an ovulation‐blocking dosage of pentobarbital sodium (PB) would affect the NAL‐induced Fos expression. Female rats were infused 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 killed by i.v. injection with an overdose of PB at 11.30–12.00. Dual immunoperoxidase/immunofluorescence staining for both Fos and LHRH revealed that some LHRH immunoreactive (ir) neurons in the forebrain expressed Fos‐ir, associated with an increase in serum LH concentrations, but little co‐localization was found in rats in proestrus which were infused with saline as the control. The proportion of LHRH‐ir neurons which expressed Fos‐ir was about 35–62% in the caudal part of the forebrain including the mediobasal hypothalamus, and this was larger than that (10%) in the rostral part of the forebrain including the preoptic area. PB injection (32 mg/kg bw, i.p.) 15 min prior to the beginning of NAL 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 generator and our previous findings that NAL stimulates the LHRH pulse generator even in the PB‐blocked proestrous rat, these results strongly suggest that the LHRH pulse generator exists in the mediobasal hypothalamus which contains LHRH neurons that are responsive to NAL and express Fos protein.