Evidence for a neuromuscular mechanism involved in the contractility of the ovarian follicular wall: fluorescence and electron microscopy and effects of tyramine on follicle strips

Effect of neostigmine on contractility of equine pre-ovulatory follicles: An in vitro study

In this study, the Authors investigated the modulatory effect of three single doses (10^-6, 10^-5, and 10^-4 M) of neostigmine on the spontaneous contractility of equine pre-ovulatory follicles in an isolated organ bath, to establish the relationship between this acetylcholinesterase inhibitor and ovulation, in the mare. The results indicate that neostigmine increases pre-ovulatory contractility in equine follicles at each dose, but in a different manner. Indeed, the rise in contractility induced by neostigmine at 10^-6 M and 10^-4 M was phasic, while at 10^-5 M it was tonic. The data obtained indicate possible implications of these drugs in the pharmacological modulation of equine ovulation.

alpha1-Adrenergic receptor in rat ovary: presence and localization

Noradrenaline modulates ovarian steroidogenesis, stimulates ovulation, and probably promotes follicular development in the ovary. It has been suggested that these effects of noradrenaline are mediated by alpha- and/or beta-adrenergic receptors (ARs) in the ovary. The purpose of the present study was to examine whether alpha(1)-AR is present in the rat ovary. In Western blotting, antibody against alpha(1)-ARs recognized a major protein in the ovary of adult (10-week-old) rats with a molecular weight of 80 kDa, which is similar to that of the alpha(1B)-AR subtype. Immunohistochemistry using this antibody showed that alpha(1)-AR was detected at various sites in the ovary, including large antral follicle, germinal epithelium at the circumference of large antral follicle, corpus luteum, and interstitial tissue. These results confirm that the ovary contains alpha(1)-AR (probably alpha(1B)-subtype), and suggest that this receptor mediates some of the activities of noradrenaline in the regulation of ovarian functions. Furthermore, we found that alpha(1)-AR is present in oocyte of large antral follicle, suggesting that noradrenaline acts on oocyte via this receptor.

Changes in intrafollicular pressure in the rat ovary by nitric oxide and by alteration of systemic blood pressure

BACKGROUND

ovulation is associated with degradation of the follicular apex vasodilatation and increased permeability of ovarian vessels. These changes may maintain or increase intrafollicular pressure (IFP) at ovulation to cause rupture of the follicular wall.

OBJECTIVE

to investigate the possible regulation of IFP during the ovulatory process.

STUDY DESIGN

immature Sprague-Dawley rats were primed with pregnant mare serum gonadotrophin (PMSG; 10IU) and given hCG (10IU) 48h later. The ovary was exposed 48-60h after PMSG, micropipette inserted into the Graafian follicle and the IFP measured at three time periods: preovulatory (PO) 48h after PMSG; midovulatory (MO) 4-7h after hCG; late ovulatory (LO) 9-12h after hCG. The offset of the nitric oxide synthase (NOS) inhibitor L-arginine methyl ester (L-NAME), the alpha(1)-adrenoceptor agonist phenylephrine and the beta-adrenoceptor agonist isoprenaline were tested.

RESULTS

phenylephrine given i.v. increased the systemic blood pressure, and significantly decreased the IFP in the LO phase (78% of pre-treatment value). Local administration of phenylephrine or isoprenaline (1ml of 1.5-15 microM) by superfusion over the ovary did not change the IFP. Local administration of L-NAME (1ml of 2 microM) significantly lowered (P<0.05) the IFP in the MO and LO phases, but was without effect in the PO phase.

CONCLUSION

this study reveals that IFP regulation may be related to changes of the systemic blood pressure and that NO may be one local ovarian mediator in IFP regulation.

Extracellular and intracellular calcium sources mediating contractile responses of smooth muscle in bovine ovarian follicle and ovarian artery

The relative importance of extracellular and intracellular calcium sources mediating smooth muscle contraction in ovarian follicle and ovarian artery was assessed in experiments on the influence of nifedipine, D-600, amrinone, diethylstilbestrol (DES), lanthanum and/or calcium removal on contractions induced by K+ depolarization, by noradrenaline, histamine and acetylcholine. The K+-induced response was biphasic in the ovarian artery but not in the ovarian follicle. The K+-induced contraction in both preparations was greatly inhibited by nifedipine (1 microM), D-600 (10 microM) and lanthanum (2 mM). Although both phases of the responses in the ovarian artery appeared to be completely dependent on extracellular calcium, phase I was significantly more sensitive to nifedipine than phase II. Incubation in calcium-free medium for 15 min almost abolished the K+-induced contraction. Noradrenaline- and histamine-induced contractions of ovarian follicle were essentially unaffected by nifedipine (1 microM) and D-600 (10 microM) whereas the noradrenaline-induced contraction in ovarian artery was inhibited significantly by D-600 (1 and 10 microM) but not nifedipine (1 microM). In calcium-free medium containing EGTA (1 mM) the responses of ovarian follicle to noradrenaline and histamine were reduced by 26 and 22% respectively. When preparations were stimulated with noradrenaline more than one in calcium-free medium, the contraction decreased progressively compared to time-matched controls. The response was 34% of the control after 50 min in calcium-free medium containing EGTA. In the ovarian artery the response obtained (6% of control) was significantly smaller (P less than 0.05) than that in the follicle.(ABSTRACT TRUNCATED AT 250 WORDS)

GABA suppresses stimulation-induced release of [3H]-noradrenaline from sympathetic nerve fibres in bovine ovarian follicles

1 Strips from the bovine ovarian follicle wall were incubated in Krebs-Ringer solution containing [3H]-noradrenaline in order to saturate sympathetic nerve fibres with radiolabelled transmitter. This allowed the study of field stimulation-evoked transmitter release. 80.3 +/- 3.9% of the tritium released upon stimulation (10 Hz, pulse duration 1 ms, 10 V between the electrodes) was noradrenaline. 2 The stimulated release of tritium was totally blocked in calcium-free, EGTA (1 mM) containing medium or by 1 microM tetrodotoxin. Chemical sympathectomy (6-hydroxydopamine treatment) in vitro reduced the tritium content of the strip by 85%. The neuronal amine uptake blocker desipramine (0.6 microM) was almost equally effective in inhibiting the incorporation of tritium. The extraneuronal amine uptake blocker normetanephrine (10 microM) reduced the tritium content by 30%. Together, the results suggest that the electrically evoked release of tritium reflects the release of [3H]-noradrenaline from sympathetic nerve fibres. 3 gamma-Aminobutyric acid (GABA) concentration-dependently reduced the electrically evoked tritium release. Also the GABAB-receptor agonist baclofen (30 microM) reduced the stimulated tritium release whereas muscimol (100 microM), a GABAA-receptor agonist, failed to affect the release. 4 The selective GABAA-receptor antagonist bicuculline (3 and 100 microM) did not block the effect of GABA, while 3-amino-1-propanesulphonic acid (3-APA), a blocker of GABAB-receptors reversed the inhibitory effect of GABA. The results suggest that neuronal GABAB-receptors are involved in the GABA-evoked suppression of stimulated noradrenaline release.

Evidence for prejunctional GABAB receptors mediating inhibition of ovarian follicle contraction induced by nerve stimulation

The motor effects of gamma-aminobutyric acid (GABA) on the bovine ovarian follicle were studied in vitro using strips from follicle walls. Electrical field stimulation of nerves in the preparation, secured by tetrodotoxin blockade, caused a contraction that was almost totally abolished by phentolamine and only slightly affected by atropine. This mainly adrenergic neurogenic response was inhibited by GABA in a dose-dependent way. The GABAA-receptor antagonists, bicuculline and picrotoxin, did not affect the GABA action whereas the GABAB-receptor antagonist, homotaurine, significantly inhibited the GABA effect. The GABAA-receptor agonist, muscimol, did not affect the contractile response while the GABAB-receptor agonist, baclofen, imitated the action of GABA. On the other hand, GABA had no direct contractile or relaxing effect on the follicle strips nor did it interfere with the contractile response induced by noradrenaline or acetylcholine. The findings suggest that activation of prejunctional GABAB receptors inhibits transmitter release from mainly adrenergic nerves associated with the follicle, thereby affecting nerve-mediated tension in the follicle wall.

Histochemistry and ultrastructure of adrenergic and acetylcholinesterase-containing nerves supplying follicles and endocrine cells in the guinea-pig ovary

The autonomic nerve supply of the guinea-pig ovary was investigated by a combination of light- and electron microscopy. At the light-microscopic level, adrenergic fibres were identified due to their formaldehyde-induced fluorescence. In addition, the ovary contained acetylcholinesterase-positive fibres. In all parts of the ovary, the adrenergic fibres were most numerous. At the ultrastructural level it was possible to identify the adrenergic nerve terminals with the aid of the false adrenergic transmitter, 5-hydroxy-dopamine. Thus, large numbers of adrenergic terminals, characterized by their content of 50-60 nm, electron-dense synaptic vesicles, were seen within the interstitial gland, where they formed close contacts with the endocrine cells (membrane-to-membrane distance, 20-100 nm). The follicular theca externa was also richly supplied by adrenergic nerves. At this location, close contacts (50-100 nm) were identified between the nerve terminals and the smooth muscle-like cells. Very few adrenergic nerve fibres were present in the theca interna of follicles or in the corpus luteum. Non-adrenergic nerve terminals, characterized by electron-lucent synaptic vesicles of 50-60 nm diameter, were observed together with the adrenergic fibres. They were always present in much lower numbers than the latter. No "p-type" nerves were identified by electron microscopy.

Contraction of the ovarian follicle induced by local stimulation of its sympathetic nerves

Strips of bovine ovarian follicle wall, known to contain smooth muscle cells innervated by adrenergic nerves, were dissected out and exposed to transmural electrical stimulation in an organ bath. A frequency-dependent contractile response was obtained with a maximum at 8-16 Hz. The response was abolished in the presence of tetrodotoxin and inhibited by bretylium, phentolamine and reserpine. Thus, stimulation of the sympathetic nerves in the ovarian follicle releases sufficient amounts of norepinephrine to produce a contraction of its wall, an effect mediated by alpha-adrenergic receptors.

The role of follicular smooth muscle cells in hamster ovulation

The role of contractile cells in mammalian ovulation is uncertain. In this study, we examined the morphology and distribution of cells within the theca externa of hamster follicles at various times during ovulation. Cells with all the ultrastructural features of smooth muscle (SMC) were found only in the basal hemisphere of the follicle. In contrast, the theca externa in the top half of the follicle was composed of fibroblasts. We next examined living hamster follicles during in vitro ovulation for morphological evidence of follicular contraction. The following changes in follicle shape were observed: (1) The base of the follicle moved apically; (2) follicles, which were initially spherical, became taller and thinner; and (3) after rupture, the apical follicle wall collapsed onto the surface of the ovary. To analyze the cause of these changes, sections of fixed follicles were examined by light and electron microscopy. During the final minutes before rupture, a V-shaped constriction formed in the base of the follicle. This constriction continued to narrow and by the time of rupture, it obliterated the basal part of the antrum. We concluded that the apical movement of the base of the follicle seen in living ovaries corresponds to the formation of the basal constriction seen in fixed sections. To determine if follicular SMC were involved in formation of the constriction, the ultrastructure of SMC was examined before, during, and after the constriction formed. The morphology of the SMC changed from the characteristic of relaxed or stretched SMC to that of contracted SMC when the constriction began to form. No other type of cell in the follicle wall showed these changes in morphology. We conclude that the formation of this constriction, and possibly the increase in height of preovulatory follicles and the collapse of the follicle wall after rupture, are due to contraction of SMC in the theca externa in the basal hemisphere of the follicle. This is the first morphological demonstration that follicular SMC contract prior to rupture of the follicle. The significance of these observations in mammalian ovulation is discussed.

The autonomic innervation of the interstitial gland of the rat ovary during pregnancy

The autonomic innervation of the interstitial gland of the rat ovary was studied on days 4, 6, 10, 14 and 18 of pregnancy with the acetylcholinesterase procedure, the Falck-Hillarp technique and electron microscopy after 5-hydroxydopamine treatment. Acetylcholinesterase-positive nerves were present as perivascular plexuses at all stages studied. Adrenergic nerves were present in the interstitial gland in all stages studied. The number and intensity of interstitial fluorescent adrenergic nerves increased as pregnancy progressed. Measurement of norepinephrine with the fluorometric procedure showed a highly significant (p less than 0.05) increase in the neurotransmitter in the ovary on days 14 and 18 as compared to day 4. Fine-structural studies after administration of the false transmitter, 5-hydroxydopamine, showed that the innervation of the steroidogenic cells of the interstitial gland is adrenergic.