Ultrastructural evidence for exocytosis in the median eminence of the rat

Ultrastructure of Hypothalamic Neurons and of the Median Eminence

Our light, and electron microscopic (EM) findings within the hypothalamic supraoptic (SO) and paraventricular (PV) nuclei of the normal female rabbit are in agreement with those reported earlier by other investigators for the same nuclei of the dog and rat. The neurons of these nuclei are the hypothalamic synthesis sites of the neurohypophyseal hormones.

With the exception of the arcuate nucleus, none of the hypothalamic nuclei associated with the control of adenohypohpyseal function have been studied extensively with the electron microscope. On the basis of our EM findings within the female rabbit hypothalamus, all neurons observed within the preoptic (PO) and suprachiasmatic (SCH) nuclei of the non-mated control animal were morphologically identical to the conventional neuron as described by Peters, Palay and Webster (1970). However, following coitus, castration and laparotomy, many neurons of these nuclei showed subcellular changes that have been repeatedly associated with enhanced protein synthesis. These large ‘neurosecretory’ neurons were usually located near capillaries and characterized by their well developed Rough endoplasmic reticulum (RER) and Golgi profiles, dense populations of mitochondria and lysosomes and by the presence of a homogeneous population of densecore vesicles (DCV) showing a peak distribution of 120-140 nm. Since similar neurons were not observed within the PO and SCH of the normal control rabbit it is suggested that we were observing functional states of the same type of neuron and that these ultrastructural changes occur in response to endocrine manipulation.

Two types of neurons described as ‘pale’ and ‘dark’ were observed within the arcuate nucleus of both the control and experimental female rabbit. Ultrastructurally, these neuron types were identical to those described by other investigators for the rat. It has been suggested that the ‘pale’ and ‘dark’ neurons of this hypothalamic nucleus represent functional states of the same type of cell. However, increases in the ratio of ‘dark’ to ‘pale’ neurons as observed within the arcuate nucleus of the rat following castration, were not seen in the rabbit. Similar findings were also not evident within the arcuate nucleus of the female rabbit following coitus.

As far as could be determined, all neurons of the ventromedial (VMN) nuclei of both the control and experimental rabbit were morphologically identical to the smaller, conventional type neuron. Certainly, ultrastructural changes similar to those observed within the PO and SCH nuclei of the female rabbit following coitus, castration or laparotomy, were never observed.

The basic zonation and subcellular organization of the female rabbit Median Eminence (ME) is similar to that described for other mammalian species. Our EM findings within the external layer of the rabbit ME, however, are not entirely in agreement with the earlier study of Duffy and Menefeef 1965). These investigators reported only one population of DCV within the axon terminals of the rabbit ME external layer. We feel that we have ultrastructural evidence for the presence of at least two distinct populations of DCV within this layer of the rabbit ME. Furthermore, since these vesicle populations occurred within separate axon profiles and terminals, differences in their content and origin are suggested.

Certainly, the relationship between releasing factors (RF) and the various populations of DCV observed within the external layer of the mammalian ME is not well established. The smaller (90 nm - 100 nm) DCV we have observed probably contain the catecholamines, while those of larger (120 nm - 140 nm) diameters may well represent the carriers of the RF associated with gonadotropic activity. The latter view is based primarily on our finding or numerous ‘vesicle ghosts’ within the axon terminals abutting the perivascular space (PVS) of portal capillaries of rabbits sacrificed at 10 minutes post-coitus. The mean diameters of 137±14 nm obtained for these ghosts strongly supports the suggested depletion of only the larger of the two DCV populations. Similar changes were not apparent within the axon terminals containing homogenous populations of only the smaller DCV.

Unquestionably, the precise hypothalamic synthesis sites for the RF associated with control of adenohypophyseal function, continues to provoke comment. From the results obtained from countless studies that have employed a variety of neuroendocrinilogical techniques, two main hypothalamic centers of RF synthesis have been suggested: a) the medial basal hypothalamus (MBH) or hypophysiotropic area (HTA) and b) the anterior hypothalamus. The ultrastructural studies carried out to date within this laboratoiy are in favour of the latter for the following reasons:

1) — the presence of large DCV and ‘vesicle ghosts’ within the external layer of the rabbit ME with diameters similar to those of the large (120-150 nm) DCV synthesized within the PO and SCH nuclei of the same animal in response to coitus, castration and laparotomy.

2) — the absence of evidence for the storage of these large DCV within the somata of PO and SCH nuclei, suggesting their immediate transport toward the ME.

3) — the absence of any ultrastructural changes within neuron somata of the rabbit arcuate nuclei which might reflect enhanced neurosecretory activity in response to coitus and/or castration.

These ultrastructural findings within the rabbit hypothalamus may, therefore, provide the first evidence of a morphological nature for the actual release of RF from their ME storage sites, as well as their synthesis within certain neurons of the anterior hypothalamus.

The cellular localization of vasoactive intestinal peptide (VIP) in the mouse median eminence by immuno-electron microscopy

The aim of this study was to examine, by use of pre-embedding immunocytochemistry, the ultrastructural localization of vasoactive intestinal peptide (VIP) immunoreactivity in the mouse median eminence. VIP immunoreactivity was observed in axonal profiles. The VIP-immunoreactive axonal profiles were in close proximity to non-immunoreactive axonal profiles that contained dense granular vesicles and clear vesicles and also to processes of tanycytes. VIP-immunoreactive terminals were observed in the proximity of the perivascular space and in the neuropil. Our results suggest that VIP-immunoreactive axon terminals may possibly interact with other non-immunoreactive axon terminals containing peptide and/or other transmitters at the level of the median eminence or may be released to the portal vasculature thereby to effect anterior pituitary cells.

Ultrastructural demonstration of oxytocin and vasopressin release sites in the neural lobe and median eminence of the rat by tannic acid and immunogold methods

We have investigated the localization of oxytocin (OXT) and vasopressin (AVP) release sites in the neural lobe and median eminence (ME) of the rat. Ultrastructural and post-embedding immunocytochemical studies with protein A-gold as a marker were used for the identification of OXT and AVP neurosecretory granules (NSG). Release of the contents of these OXT and AVP NSG by exocytosis has been clearly demonstrated with the tannic acid-Ringer incubation method in axon terminals of the neural lobe and in varicose fibres with OXT and AVP NSG in the internal zone of the ME. As these fibers lack synaptic specializations, and are not located in the direct vicinity of capillaries of the primary portal plexus, these observations suggest that OXT and AVP are released in a paracrine way in the ME of the rat.

Structural changes in nerve endings of rat median eminence superfused with media rich in potassium ions

In vitro fragments of male rat mediobasal hypothalami were superfused with Krebs--Ringer solution in the presence or absence of CaCl2. Infusions containing up to 60 mM potassium chloride were applied, at the end of which tissues were fixed in osmium tetroxide and prepared for transmission electron microscopy. Control superfusions were run in parallel. Quantitative measurements performed on electron micrographs of the outermost palisade region showed significant (20-30%) increase in caliber of axon endings after intensive potassium ion stimulation. Ultrastructurally, widespread depletion of granular vesicles and microvesicles was found. Vesicle shift to the outer zone of the terminals, formation of membrane-bound tubules of the same diameter as microvesicles, and images of attachment and collapse of vesicles into the axolemma were found, particularly after 1 min stimulation. These findings were interpreted as consistent with exocytosis. Longer stimulations were followed by the appearance of large pleomorphic vacuoles that are probably the result of post-exocytotic membrane retrieval. Axon enlargement and vesicle depletion were absent in specimens superfused with calcium-free medium containing high potassium. The functional significance of these ultrastructural changes are interpreted as supporting the hypothesis that exocytosis of calcium-loaded microvesicles can contribute to extrude this ion from median eminence nerve endings during secretion.

The freeze-fractured median eminence. II. Developmental changes in the neurohemal contact zone of the median eminence of the rat

The neurohemal contact area of the median eminence was examined in adult, young, neonatal, and fetal rats in freeze-fracture preparations. While no specializations of the membranes of axonic terminals abutting on the perivascular space was observed, adjacent areas of the same membranes are rich in signs of hormone release. Signs of exocytosis are defined in the manner of Theodosis et al. (1978). Exocytotic pits with a single dense granule as a core are identified on P-faces along with mounds on corresponding E faces. These features appear near the time of birth, are especially numerous at 3 days, 3 weeks, and 5 weeks, and continue in lesser numbers into adulthood. Even more numerous and appearing even earlier, by the last day of fetal life, were P-face clusters of large particles usually in a pit, and pits without particles. These fall into 2 significantly different categories distinguished by the size of the pit. E face counterparts are large and small protuberances, respectively. Fenestrae of adult size and distribution are observed along the endothelium of portal vessels from the 20th day of fetal life on. Their frequency correlates well with other structural manifestations of a median eminence ready for the onset of functional activity at about the time of birth (Monroe and Paull 1974).

Ultrastructural dynamics of exocytosis in the ovulation-neurohormone producing caudo-dorsal cells of the freshwater snail Lymnaea stagnalis (L.)

The ultrastructural dynamics of exocytosis in the ovulation-stimulating neurosecretory Caudo-Dorsal Cells (CDC) of the freshwater snail L. stagnalis were studied after incubation of cerebral ganglia in Ringer's solutions with different concentrations of K+ and Ca2+. Detection of exocytosis was facilitated by the use of the tannic acid-glutaraldehyde fixation method (TAGO-method). In control Ringer (low K+) the frequency of exocytosis was rather low. Exocytosis mainly occurred as "terminal" exocytosis (TE); "intracellular" (ICE) and, particularly, "multiple" exocytosis (ME) took place infrequently. Incubation in high K+-containing Ringer strongly increased exocytotic activity. Compared to the controls the total frequency of exocytosis was 50 X as high, whereas TE, ICE and ME occurred 6 X, 47 X, and more than 300 X as frequently, respectively. In high K+/Ca2+-free Ringer the total frequency of exocytosis was only 2 X as high as in control Ringer. It is concluded that TE, ICE, and ME are normal, Ca2+-dependent exocytotic phenomena. The significance of their dynamics in response to K+-stimulation is discussed. The extremely high frequency of exocytosis, as well as the presence of "unaltered granule contents in transit", is explained by assuming that an exocytotic event in the CDC lasts rather long, viz. some minutes. The results may reflect the physiological mechanism by which the CDC release their ovulation hormone. The possible involvement of "clear" and "large" electron lucent vesicles in membrane reuptake after exocytosis is considered.

Secretory phenomenon of the adenohypophyseal cells viewed with the scanning electron microscope

Exocytosis from rat adenohypophyseal cells, probably somatotrophic, was studied in the scanning electron microscope after freeze fracturation of the glands. This technique permitted the distinguishing of a clearly delimited exoplasm interrupted by the passage of the secretory granules. It was postulated that the exoplasm could be involved in the control of the release of secretory granules in the endocrine cells studied. At the basal cell surface a simultaneous discharge of several secretory granules has been detected.

Ultrastructural localization of luteinizing hormone-releasing hormone in the median eminence of the rat

The nature of cells in the hypothalamus that produce hypothalamic hypophysiotropic hormones remains unsettled. To investigate this problem electron microscopic immunocytochemistry was performed on thin sections of the median eminence of proestrous rats using antibodies to synthetic luteinizing hormone-releasing hormone (LHRH) and the peroxidase-antiperoxidase (PAP) technique. PAP complexes indicating the presence of LHRH were found over small, homogeneously dense neurosecretory granules 75-90 nm in diameter that occurred in clusters within neurons. Positive staining was more prevalent in the palisade zone in the anterior median eminence than at more posterior levels. LHRH positive granules were not obsereved in neuronal processes in the neurohemal contact zone, but appeared closer to the pericapillary space in the posterior median eminence than at more anterior levels. Occasional groups of LHRH positive granules were also found in the internal layer and hypendymal zone. No staining was observed in tanycytes or glial elements. These results support the hypothesis that hypothalamic hypophysiotropic hormones are produced by neurons and are stored in granules of homogeneous size and density in nerve processes located in the median eminence in the proximity of the hypophysial portal plexus.