Efferent and afferent connections of the ventromedial hypothalamic nucleus determined by neural tracer analysis: implications for lordosis regulation in female rats.

Neural connections of the ventromedial hypothalamic nucleus (VMN) to and from forebrain and midbrain structures, which are involved in the neuroendocrine regulation of reproduction, were investigated. A retrograde (fluoro-gold [FG]) or an anterograde neural tracer (phaseolus vulgaris-leucoagglutinin [PHA-L]) was injected into the left side of the VMN in ovariectomized rats. Six days after injection with FG or 11 days after injection with PHA-L, brains were fixed and sectioned. After immunohistochemistry, digital images of FG-labeled neural cell bodies (FG-cells) or PHA-L-labeled fibers (PHA-L-fibers) were analyzed. Injection sites of FG and PHA-L were mainly in the ventrolateral VMN. Considerable numbers of FG-cells and PHA-L-fibers were present in the left side of the medial amygdala, ventral lateral septum, preoptic area, bed nucleus of stria terminalis, dorsomedial hypothalamic nucleus, arcuate nucleus, periventricular nucleus of thalamus, and midbrain central gray. The lateral dorsal raphe nuclei contained many PHA-L-fibers but few FG-cells. By contrast, both sides of the median raphe nucleus contained many FG-cells but few PHA-L-fibers. Reciprocal direct neural connection between the right and left side of the VMN were observed. The present results provide an anatomical basis for functional relationships between the VMN and these nuclei.

Inhibitory role of the serotonergic system on estrogen receptor α expression in the female rat hypothalamus.

The role of the serotonergic system in regulating the expression of estrogen receptor (ER) α in the hypothalamus was investigated in ovariectomized rats by injecting a serotonin synthesis inhibitor, parachlorophenylalanine (PCPA), or by destroying the dorsal raphe nucleus (DR). The number of ERα-immunoreactive (ir) cells was counted in the anteroventral periventricular nucleus in the preoptic area (AVPV), ventrolateral ventromedial hypothalamic nucleus (vlVMN), and arcuate nucleus (ARCN). Seven days after ovariectomy, 100mg/kg PCPA or saline was injected daily for 4 days. Alternatively, radiofrequency lesioning of the DR (DRL) or sham lesions were made on the same time of ovariectomy. One-day after the last injection of PCPA or 7 days after brain surgery, the brain was fixed for immunostaining of ERα and the number of ERα-ir cell were counted in the nuclei of interest. The mean number of ERα-ir cells/mm(3) (density) in the AVPV of the PCPA or DRL groups was statistically higher than that in the saline or sham group. In the vlVMN and ARCN of the PCPA or DRL groups, the mean density of ERα-ir cells was comparable to the saline or sham groups. These results suggest that the serotonergic system of the DR plays an inhibitory role on the expression of ERα in the AVPV, but not in the vlVMN and ARCN.

Postnatal changes in the number of serotonin-immunoreactive cells in midbrain raphe nuclei of male rats.

To clarify the developmental changes in serotonergic neurons in the subdivisions of the dorsal (DR) and median raphe (MR) nuclei before puberty, the extent of the nuclei and the number of serotonin (5-HT) immunoreactive (ir) cells were measured in 5-, 15-, and 30-day-old rats and 8-week-old (adult) castrated male rats. The brains were fixed and 50 µm frozen sections prepared. After immunostaining for 5-HT, the number of 5-HT-ir cells in a 0.2 × 0.2 mm frame in the dorsal, ventral and lateral subdivisions of the DR (dDR, vDR and lDR, respectively) and MR were counted. Total numbers of 5-HT-ir cells counted in the frame of three sections in each rat were expressed as the number of cells per cubic millimeter (density). The results indicated that the densities of 5-HT-ir cells in the MR were almost the same in all age groups. On the other hand, among the subdivisions of the DR, the mean density of 5-HT-ir cells in 15-day-old rats was higher than that in the 5-day-old group in the lDR only. The area of the three sections of the DR and of the MR was also measured. The area of the DR in 15-day-old rats was found to be twice that in the 5-day-old rats, and differed from the area in 30-day-old rats and adults. There were no differences among the age groups in the areas of the MR. The results indicate that the expression of 5-HT in the lDR and extent of the DR increased to adult levels from days 5 to 15 after birth. In the dDR, vDR and MR, expression of 5-HT at postnatal day 5 was at adult levels already.