Kunio Hoshino

Hypothalamic GnRH pulse generator activity during the estradiol-induced LH surge in ovariectomized goats.

Electrophysiological manifestation of the hypothalamic GnRH pulse generator activity was examined during the LH surge induced by estradiol in ovariectomized goats. The characteristic increases in the frequency of multiple unit activity (MUA volley) associated with the pulsatile secretion of LH were recorded using electrodes implanted bilaterally in the medial basal hypothalamus. Estradiol was infused for 16 h at the rate of 3 micrograms/h, to induce an LH surge 10.0-11.5 h later. Regular recurrence of MUA volley was observed throughout the experimental period including the LH surge, but the interval between the MUA volleys became longer (p < 0.01) after the onset of the LH surge as compared with the pretreatment control period (32.9 +/- 2.1 vs. 60.0 +/- 5.1 min). These results suggest that an increased frequency of LH pulses is not a prerequisite for the LH surge in ovariectomized goats given estradiol, and imply that the positive feedback effects of estradiol on LH secretion appear to be exerted through a neuronal mechanism that is intrinsically different from the GnRH pulse generator.

Changes in the Gonadotropin- Releasing Hormone Pulse Generator Activity during the Estrous Cycle in the Goat

The hypothalamic GnRH pulse generator activity, which was assessed by the characteristic increases in the multiple unit activity (MUA) specifically associated with the pulsatile LH secretion (MUA volleys), was monitored throughout the estrous cycle of the female goat. Blood samples were collected daily for analysis of progesterone, estradiol and LH levels to monitor ovarian cyclicity. The frequency of the MUA volleys showed a reciprocal relationship with the plasma progesterone profile; being low during the luteal phase, whereas increased during the follicular phase until the onset of the subsequent luteal phase. On the other hand, the duration of the MUA volleys changed in parallel with plasma progesterone levels showing considerable prolongation in the luteal phase. More frequent blood sampling was carried out at 2-hour intervals for timing the LH surge during the follicular phase induced by prostaglandin F2alpha (PGF2alpha) administration. During the PGF2alpha-induced follicular phase, the MUA volley frequency was abruptly increased from 4 h after PGF2alpha injection, and this high frequency was maintained throughout the follicular phase except for the period around the preovulatory LH surge when the MUA volley frequency was lowered temporarily. These results were in accordance with our previous observation that the recurrence of MUA volleys continued with decreased frequency during the estradiol-induced LH surge in ovariectomized goats. The present study revealed that GnRH pulse generator activity changes dramatically during the estrous cycle in the goat, and it is suggested that GnRH pulse generator activity becomes fully potentiated following luteolysis to induce the final follicular development and to stimulate estradiol secretion, which eventually provokes another neural mechanism responsible for the induction of the LH surge.

Social environment affects synaptic structure in the glomerulus of the accessory olfactory bulb of the hamster

The accessory olfactory bulb (AOB) is the primary center of the vomeronasal chemosensory system. The effects of differential rearing on the numerical density and size of synapse as well as neuronal size were examined in the AOB of adult hamster. At 30 days from birth, male littermates were assigned to one of three experimental conditions. (1) the IC (isolated condition), where a male animal was housed alone, (2) the NC (neighbor condition), where one male was separated from two females by wire shields, and (3) the SC (social condition), where two males and two females were housed together. After 2 months of differential rearing, the AOBs of male littermates from each experimental set were prepared for morphological examination. The lengths of synaptic contact zones and the density of synapses in the glomeruli of the AOB as well as the area of somata of mitral/tufted cells were measured with an image analyzer. The synaptic contact zone was longer in the SC compared with both the IC and the NC while there was no difference in the synaptic density among the three groups. The somal area of mitral/tufted cells was larger in both the SC and the NC compared with the IC. These results indicate that the exposure to different rearing conditions induces differential morphological changes in both synapses and somata in the AOB of adult hamster.

Changes in plasma gonadotropins, ovarian steroids and inhibin concentrations in gilts following progesterone treatment with implantable osmotic pumps

Abstract Endocrine changes in cyclic gilts (10 months old, n=5) were examined during and following the measured administration of progesterone with implantable osmotic pumps, commencing during the late luteal phase of a cycle. Exogenous progesterone delivered from the pumps (115 mg/day) maintained high plasma progesterone levels (> 10 ng/ml) that effectively suppressed follicular growth and prevented the gilts from returning into estrus. Thus, 7 days progesterone implantation resulted in an extension of the estrous cycle by about a week. Progesterone withdrawal initiated the increase of plasma inhibin as well as of estradiol and the decrease of plasma FSH. In four out of five treated gilts, the exception being one gilt that developed follicular cysts, the LH surge was observed 6.3±0.3 (s.e.m.) days after the removal of progesterone implants. The overall plasma inhibin profiles were inversely correlated with those of FSH: during follicular development, inhibin levels were high while plasma FSH remained low, then, after the LH surge, a decline of plasma inhibin preceded the secondary surge of FSH around the time of ovulation. The method herein described appears to be a useful tool for synchronizing the preovulatory endocrine events in gilts.

Social Environment Affects Synapses in the Accessory Olfactory Bulb of Adult Hamster: Quantitative Electron Microscopic Study

There is increasing evidence that the complexity of the environment to which animals are exposed has an effect on fine synaptic structure, and that the effect varies among different regions of the central nervous system [1,2]. The vomeronasal system, consisting of the vomeronasal organ, the accessory olfactory bulb (AOB), and the higher vomeronasal centers receiving efferents from the AOB, is now thought to play a critical role in the perception and processing of conspecific chemical signals (pheromones) in mammals [3]. We have reported that differential rearing affects the synapses in the granule cell layer of the AOB and the molecular layer of the medial amygdaloid nucleus in adult rats [4,5]. The AOB is the primary nucleus in the vomeronasal system. Since synapses in the glomeruli of the AOB are formed between axons of vomeronasal sensory cells and dendrites of mitral/tufted (M/T) cells [6], they are the first synaptic contact in the vomeronasal pathway. The present study was therefore carried out to examine whether or not different pheromonal environments could affect synaptic structure in the vomeronasal system.