Microcirculatory patterns in adult rat cerebral hypophysis: a scanning electron microscope study of replicated specimens

Neurohypophyseal Neuregulin 1 Is Derived from the Hypothalamus as a Potential Prolactin Modulator

Although neuregulin 1 (Nrg1) has been identified in the rat hypothalamus, the localisation of Nrg1 in the hypothalamus-hypophyseal structure and its functions remain unclear and require further elucidation. In this study, we identified the existence of Nrg1β types I-III in the rat hypothalamus. We demonstrated that Nrg1 was partially localised in somatostatin-positive cells in the periventricular nucleus. It was also co-localised with arginine vasopressin in the supraoptic nucleus, median eminence and pituitary stalk. Nrg1 was also extensively distributed in the posterior pituitary (PP), including the projected neuronal fibres that surround the vascular structure and Herring bodies. Western blotting confirmed that these signals were primarily produced by soluble Nrg1 derived from a 45-kDa Nrg1 precursor mainly identified in the hypothalamus. Similar to Nrg1α, Nrg1β increased the prolactin (PRL) expression in rat pituitary RC-4B/C cells, which can be inhibited by an Akt inhibitor. In addition, Nrg1β had no apparent effect on growth hormone expression at the mRNA or protein levels. Collectively, we conclude that hypothalamic Nrg1 may be transported to the PP as the β form. We further hypothesise that Nrg1β may function via the regulation of PRL expression through a paracrine mechanism.

Pituitary Adenylate Cyclase Activating Polypeptide Messenger RNA in the Paraventricular Nucleus and Anterior Pituitary During the Rat Estrous Cycle1

The neuropeptide pituitary adenylate cyclase activating polypeptide (ADCYAP 1, or PACAP) has been demonstrated to enhance gonadotropin-releasing hormone (GnRH)-induced gonadotropin secretion and regulate gonadotropin subunit gene expression in cultures of anterior pituitary cells. In the present study, we used in situ hybridization and real-time polymerase chain reaction to examine the expression of Pacap mRNA within the paraventricular nucleus (PVN) and anterior pituitary throughout the estrous cycle of the rat. Levels of luteinizing hormone in serum and pituitary gonadotropin subunit mRNAs were evaluated and displayed cyclic fluctuations similar to those reported previously. Pacap mRNA expression in the PVN and pituitary varied significantly during the estrous cycle, with the greatest changes occurring on the day of proestrus. Pacap mRNA levels in the PVN declined significantly on the morning of diestrus. During proestrus, PVN Pacap mRNA levels significantly increased 3 h before the gonadotropin surge and then declined. Pituitary expression of Pacap mRNA also varied on the afternoon of proestrus with a moderate decline at the time of the gonadotropin surge and a significant increase later in the evening. Expression of the mRNA species encoding the 288 amino acid form of follistatin increased significantly following the rise in pituitary Pacap mRNA, at the termination of the secondary surge in follicle-stimulating hormone beta (Fshb) gene expression. These results suggest that PACAP is involved in events before and following the gonadotropin surge, perhaps through increased gonadotroph sensitivity to GnRH and suppression of Fshb subunit expression through increased follistatin, as previously observed in vitro.

Three-dimensional structural changes in cerebral microvessels after transient focal cerebral ischemia in rats: scanning electron microscopic study of corrosion casts

Pathological changes of cerebral microvessels in transient ischemia were investigated by scanning electron microscopy of vascular corrosion casts. Wistar rats were treated with middle cerebral artery (MCA) occlusion for 30 min, 1 h, 3 h, 4 h, 5 h or 7 h and subsequent reperfusion for 2 h. The ultrastructures of the cast were observed and computer-aided montage micrographs were obtained for visualization of the whole microvasculature in the ischemic brain hemisphere. Avascular areas representing ischemic areas were detected in the frontotemporal cortex and caudate putamen in the groups from 30 min to 5 h occlusion. Extravasation of the resin, which probably corresponded to the leakage of plasma or hemorrhage, was seen as spheroidal, conglomerative, large massive and worm-like types. The spheroidal type, which probably indicated a small leakage or minor hemorrhage, began to appear in the 30-min occlusion group. The conglomerative type, which probably indicated a larger leakage or moderate hemorrhage, appeared in the 3- to 5-h occlusion groups. The large massive and worm-like types, which probably indicated a significant hemorrhage, appeared in the 4- and 5-h occlusion groups. The number of these extravasations increased significantly in the 4-h occlusion group. Arterioles near the avascular area frequently showed vasospastic appearances, such as corrugations, fusiform indentations of endothelial nuclei, continuous circulatory constrictions and severe narrowing with interrupted branches. Arteriolar vasospasm possibly caused prolonged hypoperfusion even if reperfusion was achieved. The capillaries had a thin stringy appearance in the 4- and 5-h occlusion groups. These changes seemed to relate closely with increased intracranial pressure by brain edema or hemorrhage. The present study suggested that the risk of brain edema or hemorrhagic infarction increased beyond 3 h of MCA occlusion, and vasospasm of the arterioles might participate in stroke pathophysiology.

Derivatization of progesterone to a neurally active steroid by pituitary neurointermediate lobe

The melanotrophs of the neurointermediate lobe and peptidergic terminals of the neural lobe are regulated by gamma-aminobutyric acid (GABA) via GABA-A receptors and therefore, may be important sites for the modulatory actions of neurally active steroids. These steroid compounds might be produced peripherally, synthesized de novo in the pituitary, or derivatized from circulating steroids, each pathway having different physiological implications. In the present study, we show that neurointermediate lobe tissue can derivatize progesterone to the neurally active steroid 3 alpha-hydroxy-5 alpha-pregnan-20-one. The neurointermediate lobe was found to be four times as active as anterior pituitary and mediobasal hypothalamus in conversion of progesterone to 3 alpha-hydroxy-5 alpha-pregnan-20-one; mediobasal hypothalamus was relatively more active in the production of the intermediate 5 alpha-pregnan-3,20-dione. The identity of the compounds was confirmed by the method of serial isotopic dilution. We observed rates of synthesis in the neurointermediate lobe consistent with the production of physiologically relevant quantities of 3 alpha-hydroxy-5 alpha-pregnan-20-one from concentrations of progesterone which can occur naturally. In support of these findings, we demonstrate the presence of 3 alpha-hydroxysteroid oxidoreductase in neurointermediate lobe by immunocytochemistry.

Evidence for diverse pathways of hypophysiotropic hormone transport in mammals

Comparative studies of mammalian hypothalamic-pituitary relationships have revealed striking variations in hypophysiotropic systems and in portal vascular architecture. Immunocytochemical studies indicate that mammalian GnRH, GHRH and somatostatin systems can project to all portions of the neurohypophysis (median eminence, infundibular stem and pituitary neural lobe). In rats, primary secretion sites are located within the median eminence and upper infundibular stem, whereas in bats, most projections extend into the lower infundibular stem and pituitary neural lobe. In ferrets and monkeys, sites of secretion appear to extend throughout the neurohypophysis, from median eminence to proximal neural lobe. In this review, these interspecific differences are examined in light of observed structural variations in portal vascular systems. Correlations suggest that hypophysiotropic hormones can be delivered to target cells in the pars distalis by diverse routes, with some species relying more heavily on long and others on short portal transport. These patterns may have important functional implications with respect to regulatory mechanisms operating within the hypothalamic-pituitary complex.

Three-dimensional analysis of vasospastic major cerebral arteries in rats with the corrosion cast technique

BACKGROUND AND PURPOSE

Although mice, rats, and other small animals are commonly used for molecular biology research, their use in the evaluation of cerebral vasospasm after subarachnoid hemorrhage is somewhat problematic because of the correspondingly small size of their cerebral vessels. We have already reported that the corrosion cast technique was useful for evaluating newly formed cerebral vessels in neural grafts in these small animals. In the present study we applied the corrosion cast technique to the evaluation of hemolysate-induced cerebral vasospasm in rats and performed three-dimensional analysis for comparison. The casting was done 10 minutes after the hemolysate injection, so that only acute "vasospasm" was assessed.

METHODS

After withdrawal of 0.1 mL cerebrospinal fluid, 0.2 mL hemolysate (n = 9) or saline (n = 10) was injected into the cisterna magna of male Sprague-Dawley rats weighing between 300 and 350 g. Ten minutes later, perfusion of a semipolymerized casting medium was performed at an injection pressure of 100 to 120 mm Hg. The brains were immersed and corroded in 10% NaOH solution. After these procedures, the basilar artery as well as peripheral vessels was analyzed morphologically with scanning electron microscopy. Conventional histological analysis with the use of paraffin-embedded section with hematoxylin-eosin staining was also performed, and the results were compared with those for the corrosion cast methods.

RESULTS

In the saline-injected group, SEM showed that the inner surface of the basilar artery was smooth and the form of the endothelial cell was printed on the surface of the cast. In the hemolysate-injected group, the basilar artery showed an apparent vasospasm over its entire length, and corrugation was observed on the inner surface of the basilar artery in a three-dimensional fashion. Higher magnification revealed that the nuclei of the endothelial cells were distorted. Local narrowing of the basilar artery and vasospasm in the arteries of the anterior circulation and in peripheral arteries were also observed. Measurement of the inner diameter of the basilar artery showed 37.8% contraction in the hemolysate-injected group compared with the saline-injected group by the corrosion cast method. This degree of vasospasm was similar to that observed by the conventional histological method.

CONCLUSIONS

In this report we show that detailed three-dimensional observation in the rat can be performed qualitatively and quantitatively with the corrosion cast technique. We conclude that this method derives an accurate measurement of the diameter of rat major cerebral arteries and is more reliable for analyzing vasospasm in rats than angiography and other conventional procedures.

Blood flow patterns in the rat pancreas: a simulative demonstration by injection replication and scanning electron microscopy

Scanning electron microscopy of vascular casts prepared by arterial injections of intentionally reduced amounts of resin showed that in the rat pancreas, the casting medium fills blood capillaries in the endocrine islets more promptly than those in the exocrine lobules and secretory ducts. Furthermore, the exocrine lobules containing endocrine islets allowed a more rapid resin flow through the insulo-acinar portal route than those lobules lacking an islet. The capillaries of secretory ducts were the last portions to be filled with resin. Since the resin used in this study was as viscous as blood and injected under a physiological pressure, the microcirculatory patterns demonstrated by the present method reflect the physiological flow pattern of blood in the pancreas.