Differing immunoreactivities of somatostatin in the cortex and the hypothalamus of the rat

Summary Using an antibody against somatostatin (antiserum F), two somatostatin-immunoreactive systems, (i) a hypothalamic and (ii) an extrahypothalamic cortical system, are demonstrated in the rat. Another antiserum raised against somatostatin (antiserum BS 102) stains only the axons but not the perikarya of the hypothalamic system; the cortical somatostatin system does not react with this antiserum. The electron microscopic findings do not allow decision whether the above-mentioned hypothalamic and cortical neurons possess a common prohormonal form of somatostatin, immunoreactive only with antiserum F. They show, however, that the granules in both neuronal systems differ considerably; in the cortical neurons they measure approximately 65 nm in diameter, in the hypothalamic neurons 90–120 nm in diameter. Thus, both somatostatin systems are different and independent from one another.Supported by the Deutsche Forschungsgemeinschaft (Grant Nr. Kr 569/3) and the Stiftung Volkswagenwerk     

Melatonin microinjection into the medial preoptic area increases sleep in the rat

A wide variety of hypnotic compounds including triazolam, pentobarbital, ethanol and adenosine have been reported to enhance sleep when microinjected into the medial preoptic area (MPA) of the anterior hypothalamus of the rat. It is uncertain whether the pineal hormone melatonin, which may alter sleep/wake physiology in mammals, acts at this site. A previous report has indicated that a more widespread injection of melatonin into the hypothalamus of the cat induces sleep. In the present study we have examined the possibility that the MPA may mediate this effect. Nine adult rats were microinjected with melatonin 1 and 50 ug and vehicle into the MPA during the daytime in a repeated measures design study. It was found that melatonin increased total sleep time in a dose-dependent manner, primarily by increasing NREM sleep, and that wake time after sleep onset was significantly reduced. These data add melatonin to the growing list of compounds that increase total sleep after administration into the MPA, and suggest that the MPA may be a common site of action for such agents from a variety of pharmacologic classes. Based on previous studies, the possibility is raised that this sleep enhancement results from an alteration in function of the GABA(A)-benzodiazepine receptor complex.     

The preoptic area of the domestic fowl

Summary The preoptic area of the domestic fowl (Gallus gallus) was studied by means of the Golgi technique. At least two regions can be recognized: (i) a medial and (ii) a lateral area, clearly distinguishable laterally from the adjacent telencephalic regions. The dendritic organization of the preoptic area is quite uniform. The neurons can be classified as isodendritic elements. The magnocellular elements are few and irregularly scattered mostly in the periventricular grey of the medial preoptic area. Of relevant interest is also the observation of some bipolar and horizontal neurons in the dorsal part of the medial preoptic area, near the anterior commissure.     

Olfactory preference in the male rat depends on multiple chemosensory inputs converging on the preoptic area

Both volatile and nonvolatile molecules are involved in chemosensory communication in rodents. Volatile odors from physically inaccessible estrous females induced increased numbers of c-Fos-positive cells in the preoptic area (POA) and in the cortical nucleus of the amygdala (CoA) of male rats. The numbers of c-Fos-positive cells in the medial nucleus of the amygdala (MeA) increased in response to the nonvolatile odors of bedding soiled with the excreta of estrous females. In an alternate choice paradigm, male rats carrying ibotenic acid lesions in either the MeA or the CoA—or a combination of both—distinguished the odors of estrous females from those of males, although the time spent sniffing the stimuli was diminished. Males with POA lesions showed complete loss of this capability. Males carrying either of the lesions did not detect differences between estrous and anestrous females or between intact and orchidectomized males. Lesions in the POA or MeA severely impaired male sexual behavior, whereas a CoA lesion had no effects. Thus, c-Fos-positive cells in the CoA might be involved in chemosensory transmission relevant to certain social contexts, but not in the execution of male sexual behavior. The POA is indispensable for both olfactory preferences and sexual behavior. The residual olfactory preference in males with MeA or CoA lesions or the combination of both could reflect an additional route for chemosensory transmission from the main olfactory bulb to the POA.     

Ultrastructural features of the lateral preoptic area,median eminence and arcuate nucleus of the rat

Summary The arcuate nucleus, median eminence, and the lateral preoptic area from the brains of aldehyde-perfused male and female rats were examined by electron microscopy. In the lateral preoptic area, three neuronal types are described: a small light neuron, a larger light one, and a dark neuron resembling the larger light one in size and nuclear shape. Many myelinated axons are interposed among single neurons or neuronal pairs. The relationship of structures to each other is discussed. Several observations not previously reported are illustrated from tissue of the arcuate nucleus and median eminence.     

Neurotropic effects of estrogen on the neonatal preoptic area grafted into the adult rat brain

Summary The preoptic area (POA) or cerebral cortex taken from newborn female rats were transplanted into the third ventricle of ovariectomized adult rats. From the day of transplantation, estradiol-17/ in a silastic capsule was implanted subcutaneously into host animals for 4 weeks. The POA or cerebral cortex transplants were examined at light- and electron-microscopic levels 4 weeks after transplantation. All of the POA or cortical grafts showed an appearance similar to normal neural tissue. Estrogen exposure for 4 weeks via the host induced a significant increase in the volume of the POA grafts. The neuronal population of the POA grafts exposed to estrogen was not significantly different from that of the POA grafts without estrogen treatment. However, the number of axodendritic shaft and spine synapses of the POA grafts exposed to estrogen was significantly greater than that of the POA grafts without estrogen treatment. In contrast, there was no significant difference in the volume of the cortical tissues transplanted into the brain between the control and estrogen-treated groups. These results suggest that estrogen has a stimulatory effect on the development of neuronal substrates in the intraventricular POA graft, increasing its volume and synaptic population.     

Participation of preoptic area TRPV4 ion channel in regulation of body temperature

Transient receptor potential vanilloid 4 (TRPV4) ion channel is a non-selective cation channel and its role in cutaneous thermosensation is emerging. It is expressed in many areas of the brain including the preoptic area (POA)/anterior hypothalamus which is the key neural site for thermoregulation. The present study was conducted to find out the role of TRPV4 ion channel in the POA in thermoregulation. Rats preimplanted with guide cannulae with indwelling styli 2.0 mm above the POA received TRPV4 agonist/antagonist/isotonic saline injections bilaterally in the POA using an injector cannula in three separate groups of six rats each. Body temperature (Tb) was recorded telemetrically by preimplanted radio transmitter in the peritoneal cavity. The injection of TRPV4 agonist (GSK1016790A) in the POA decreased Tb while its antagonist (RN1734) increased Tb. Immunohistochemical localization showed presence of TRPV4 ion channel in the POA. The results of the present study suggest that TRPV4 ion channels in the POA may play an important role in thermoregulation.     

Cardiovascular effects produced by injections of thyrotropin-releasing hormone in specific preoptic and hypothalamic nuclei in the rat

Microinjection of 1.4 pmol TRH (0.5 ng; 50–150 nl) into both the preoptic suprachiasmatic nucleus (pos) and the A6800–7000 region of the medial preoptic nucleus (pom) produced increases in blood pressure and heart rate of 7% and 19%, respectively; heart rate responses in these two areas were higher than those occurring in other areas tested. TRH induced a significant increase in blood pressure and heart rate in the posterior hypothalamic nucleus (nhp) and increased heart rate only in the anterior (nha) and dorsomedial (ndm) hypothalamic nuclei. A small decrease in both blood pressure and heart rate resulted with TRH injections in the A7050–7400 region of the pom. No changes in respiratory rate or rectal temperature were observed at any site with this dose of TRH. Preliminary studies into the mechanism of the cardiovascular actions of TRH suggested that inhibition of the parasympathetic nerves to the heart make a partial contribution to the TRH-induced heart rate increase in the pos and that adrenal catecholamine release mediates the TRH response in the nhp. Neither methylatropine pretreatment nor adrenalectomy prevented the response to TRH injected into the nha, suggesting that activation of the cardiac sympathetic nerves may mediate TRH actions in this region. In the ndm, neither methylatropine nor adrenalectomy prevented the response to TRH; however, there was a tendency for the response to be less after methylatropine. Therefore, both inhibition of the parasympathetic and activation of the sympathetic nervous systems may contribute to the response observed, but no adrenal involvement could be demonstrated. Discrete injections of 0.8 nmol TRH produced increases in heart rate and blood pressure in all preoptic and hypothalamic nuclei tested with accompanying changes in respiratory rate and rectal temperature in some areas. Lateral cerebral ventricle injections of as little as 2.8 pmol TRH produced increases in blood pressure and heart rate; cardiovascular responses to higher doses (0.8–22 nmol) in the ventricle were often accompanied by arousal, piloerection, “wet dog” shakes and changes in respiratory rate and rectal temperature. Previous immunohistochemical demonstration of nerve cells and fibers in the preoptic-hypothalamic area and the present finding of specific sites responsive to low dose TRH injections (1.4 pmol) both support a physiological role for this peptide in central control of the cardiovascular system.     

Quantitative radioautographic light and electron microscopic analysis of the localization of monoamines in the median eminence of the rat

Summary Serotonin containing structures in the median eminence of the rat have been studied by quantitative light and electron microscopic radioautography following intraventricular infusion of tritiated 5-hydroxytryptophan. One hour after injection of the tracer the highest density of silver grains was recorded in the ependymal and external zones, especially in the lateral palisade zone. The proportion of labelled neurosecretory terminals was also larger in the lateral palisade zone (29%) as compared with the medial palisade zone (13%), although the mean number of developed silver grains per one terminal was higher in the latter. On the average, 16% of neurosecretory terminals sequestered radiolabelled 5-hydroxytryptophan in the external zone of the rat median eminence. It is suggested that serotonin, like catecholamines, is discharged from neurosecretory terminals localized in the external zone and via the portal circulation affects the function of the anterior pituitary. The sites of origin of serotoninergic structures of the median eminence as well as the possible role of monoamine (catecholamine and indolamine) neurohormones in a dual peptidergic and monoaminergic control of anterior pituitary functions are discussed.This work was presented at the joint session of the Society of Anatomists, Histologists and Embryologists, the Society of Physiologists, and the Society of Neuroendocrinologists dedicated to the memory of Professor Wolfgang Bargmann, held in Leningrad, January 17, 1979     

Sexually dimorphic estrogen receptor α mRNA expression in the preoptic area and ventromedial hypothalamus of green anole lizards

Estradiol (E2) is important in activation of male reproductive behaviors, and masculinizes morphology of associated brain regions in a number of mammalian and avian species. In contrast, it is testosterone, rather than its metabolites, that is the most potent activator of male sexual behavior in green anole lizards. As in other vertebrate groups, however, E2 is critical for receptivity in females of this species. Aromatase, the enzyme which converts testosterone to E2, is more active in the male than female green anole brain, and appears to be actively regulated on a seasonal basis, suggesting some role for E2 in males. This study was designed to enhance our understanding of potential E2 actions by localizing and quantifying relative levels of estrogen receptor-alpha (ERα) mRNA in forebrain regions involved in masculine and feminine behaviors in anoles. These areas include the preoptic area (POA), ventromedial amygdala (AMY) and ventromedial hypothalamus (VMH). In situ hybridization was conducted in adult males and females collected during both breeding and non-breeding seasons. ERα mRNA was expressed in each brain region across sexes and seasons. However, expression was up to 3 times greater in the VMH compared to the POA and AMY. In the POA and VMH, expression was higher in females compared to males, independent of season. The increased receptor expression in females is consistent with E2 playing a larger role in female than male reproductive behaviors.     

Quantitative radioautographic light and electron microscopic analysis of the localization of monoamines in the median eminence of the rat

Summary By means of light and electron microscopic radioautography a quantitative study of the regional distribution of catecholamines in the median eminence of the rat was carried out. One hour after intraventricular injection of ³H-dopamine the highest radioautographic reaction was recorded in the external zone, especially in the lateral palisade zone where many neurosecretory terminals are separated from the basal lamina of the portal pericapillary space by a glial cuff. This area showed the highest percentage (52%) of labelled catecholamine containing neurosecretory terminals as well as the maximal silver grain density per one terminal. In the medial palisade zone where direct neurovascular contacts with the capillary loops prevail, only 27 per cent of neurosecretory terminals were found to harbour tritiated dopamine. On the average 35 per cent of neurosecretory terminals in the median eminence of the rat contain catecholamines (both dopamine and noradrenaline). Pretreatment of animals with reserpine strongly reduced the binding of the label. Per cent of labelled neurosecretory terminals as well as grain density over terminals were decreased in both the medial and lateral palisade zones, although to a lesser degree in the latter. The site of origin of catecholamine fibers as well as the mode of catecholamine action at the level of the median eminence are discussed.This work was presented at the joint session of the Society of Anatomists, Histologists and Embryologists, the Society of Physiologists, and the Society of Neuroendocrinologists dedicated to the memory of Professor Wolfgang Bargmann, held in Leningrad, January 17, 1979It is a pleasure to express our deep gratitude to Prof. P.P. Rumyantzev and Dr. N.V. Seina of the Institute of Cytology of the USSR Academy of Sciences, Leningrad, for their valuable advice on electron microscopic radioautography     

Changes of body temperature and extracellular serotonin level in the preoptic area and anterior hypothalamus after thermal or serotonergic pharmacological stimulation of freely moving rats

Although many studies has been shown that serotonin (5-HT) in the preoptic area and anterior hypothalamus (PO/AH) is important for regulating body temperature (Tb), the exact role is not established yet due to conflicting results probably related to experimental techniques or conditions such as the use of anesthesia. The purpose of present study was to clarify the role of 5-HT in the PO/AH using the combined methods of telemetry, microdialysis and high performance liquid chromatography (HPLC), with a special emphasis on the regulation of Tb in freely moving rats. Firstly, we measured changes in Tb and levels of extracellular 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the PO/AH during cold (5 degrees C) and heat (35 degrees C) exposure. We also perfused fluoxetine (5-HT re-uptake inhibitor) and 8-hydroxy-2-(Di-n-propylamino)tetralin (8-OH-DPAT: 5-HT1A agonist) into the PO/AH. During both exposures, although Tb changed significantly, no significant changes were noted in extracellular levels of 5-HT and 5-HIAA in the PO/AH. In addition, although perfusion of fluoxetine or 8-OH-DPAT into the PO/AH increased or decreased extracellular 5-HT and 5-HIAA levels in the PO/AH respectively, but Tb did not change at all. Our results suggest that 5-HT in the PO/AH may not mediate acute changes in thermoregulation.     

Synaptology of luteinizing hormone-releasing hormone neurons in rat preoptic area

The ultrastructural appearance of luteinizing hormone-releasing hormone (LHRH) immunoreactive elements was studied in the medial preoptic area (MPOA) of adult male Fischer 344 rats. The purpose of the study was to determine the distribution and morphology of innervation of the LHRH neuron. Although not numerous, both axo-somatic and axo-dendritic synapses were present and generally of the asymmetric (Gray's II) category. Analyses of 56 profiles of 11 separate perikarya revealed only 7 axo-somatic terminals. The synaptic input to LHRH dendrites was a fraction of that to non-identified dendrites in the same electron micrographic fields; 0.4% of LHRH dendritic membrane was in synaptic contact compared to 6.6% of nonidentified dendritic membrane. In addition to receiving an input, LHRH processes were also seen to make synapses onto non-immunoreactive elements. Close examination of this material for evidence of contact between LHRH elements revealed two clear examples of synaptic interaction and several instances of close association in which no other elements intervened.     

Immunohistochemical study of the LHRH-synthesizing neuron system of aged female rats

Summary The LHRH-synthesizing neuron system was studied in young proestrous and old female rats, and in aged ovariectomized or reserpine-treated females. The medial preoptic area and septal region of old animals contains more LHRH positive perikarya compared to that of young proestrous rats. Reserpine treatment moderately increases the number of immunostainable LHRH cells, while ovariectomy is ineffective in this respect.The authors wish to thank Ms. Márta Szelier for her excellent technical assistance and Ms. Márta Soltész for the photographic work     

Oxytocin in the medial preoptic area facilitates male sexual behavior in the rat

Oxytocin (OT) is a versatile neuropeptide that is involved in a variety of mammalian behaviors, and its role in reproductive function and behavior has been well established. The majority of pharmacological studies of the effects of OT on male sexual behavior have focused on the paraventricular nucleus (PVN), ventral tegmental area (VTA), hippocampus, and amygdala. Less attention has been given to the medial preoptic area (MPOA), a major integrative site for male sexual behavior. The present study investigated the effects of intra-MPOA administration of OT and (d(CH2)51, Tyr(Me)2, Thr4, Orn8, Tyr-NH29)-vasotocin, an OT antagonist (OTA), on copulation in the male rat. The relationship between OT receptor (OTR) binding levels in the MPOA and sexual efficiency was also explored. Microinjection of OT into the MPOA facilitated copulation in sexually experienced male rats, whereas similar injections of an OTA inhibited certain aspects of copulation but had no significant effect on locomotor activity in an open field. Contrary to expectation, sexually efficient males had lower levels of OTR binding in the rostral MPOA compared to inefficient animals. The present data suggest that OT activity in the MPOA is not necessary for the expression of male sexual behavior but is sufficient to facilitate copulatory behaviors and improve sexual efficiency in sexually experienced male rats. These data also suggest that OTR activity in the MPOA stimulates anogenital investigation, facilitates the initiation of copulation, and plays a role in the sensitization effect of the first ejaculation on subsequent ejaculations.     

A comparison of the development of neuropeptide and MAP2 immunocytochemical labeling in the macaque visual cortex during pre- and postnatal development

The appearance of Substance P (SP) and Neuropeptide Y (NPY) has been studied using light microscopic immunocytochemical labeling throughout the complete developmental span of Macaca nemestrina monkey striate cortex. In the adult, 80% of the NPY + neurons occur in the white matter (WM) and most of the remainder are medium to large multipolar neurons in layer 2. Fibers occur in all layers except 4C and are very numerous, given the relatively small number of NPY+ cell bodies. NPY+ neurons first were seen at embryonic day (E) 75. Most neurons were in the intermediate zone (IZ), but a few were in the immature cortical plate (CP). An adultlike distribution was present by E125 for neurons and by birth for fibers, but fiber staining intensity and number increased to postnatal year 1 (P1yr). In adult cortex, numerous SP+ nonpyramidal neurons were present in layers 2–6 and WM, but SP+ fibers were surprisingly infrequent. During development, significant numbers of SP+ neurons were not seen in the CP until E113–125. Later prenatal ages had a prominent plexus of SP+ cell bodies and fibers at the layer 5/6 border. This plexus disappeared by P12wk due to either down-regulation of SP or cell death. SP+ neurons in IZ/WM were very sparse until birth after which they increased in number and staining intensity up to P1yr, suggesting a postnatal up-regulation of SP in a preexisting WM subpopulation. Cell densities were determined for SP, NPY, and the neuron-specific marker microtubule-associated protein 2 (MAP2) to clarify the developmental dynamics of IZ/WM neurons. MAP2+ cell densities in WM peaked around birth and then declined 20% in the outer half and 77% in the inner half of WM. SP+ cell density rose 57% from birth to P20wk and then declined 20% into adult hood. NPY+ cell density was fairly constant prenatally and then rose 300% by adulthood. Neuropeptide cell density changes took place predominatly in the outer WM. These data indicate that cell death does occur in the general population of monkey striate cortical WM neurons. In contrast, both SP+ and NPY+ cells are characterized by minimal cell death and a late expression of neuropeptides which causes an increase in neuropeptide+ cell density in postnatal WM. © 1993 John Wiley & Sons, Inc.     

Immunoreactive material resembling ovine prolactin in perikarya and nerve terminals of the rat hypothalamus

Summary The presence of prolactin (PRL)-like material is demonstrated in the brain of rats with the aid of anti-ovine PRL (oPRL) IgG as primary antibody in the unlabeled antibody-enzyme method. Immunoreactive deposits are visualized as an intraneuronal constituent with a widespread distribution in the hypothalamus and neural lobe of the pituitary. Dense networks of reactive nerve terminals derived from two prominent fibre tracts, a ventral (VHT) and a dorsal hypothalamo-neurohypophysial tract (DHT) are seen. The VHT is confined to the median eminence and pars oralis tuberis, the DHT to the pars caudalis tuberis. Both fibre tracts pass through the infundibular stalk into the neural lobe. The origin of the immunoreactive nerve terminals can be elucidated only to some extent. The VHT gives off beaded fibres entering the ependymal and glandular layer of the median eminence. Immunoreactive perikarya are observed in the supraoptic nucleus, the paraventricular nucleus, the anterior hypothalamic nucleus, the anterior commissural nucleus, the preoptic nucleus and the interstitial nucleus of the stria terminalis. A few of the immunoreactive perikarya are observed in close connection with brain vessels and the ependymal cells of the third ventricle. The results indicate that the anti-oPRL has a unique region specificity implying that only a segment of the mammalian PRL molecule is present in these nuclei of the brain. Fragments of PRL may function as neuromodulators or neurotransmitters in the rat brain.We are indebted to Dr. Mogens Hammer, Rigshospitalet, Copenhagen for the gift of Arg-VP and anti-VP, and to NIAMDD for the gift of ovine PRL, ratPRL, anti-rPRL, anti-hPRL and bovineSTH     

Two different types of dynorphin-A-immunoreactive terminals in rat substantia nigra

Summary The opioid peptide dynorphin A (1–17) is the third transmitter identified in the striatonigral projection, the other two being gamma-aminobutyric acid (GABA) and substance P. The ultrastructural features of the dynorphinergic terminals in substantia nigra/pars reticulata were studied using pre-embedding immunocytochemistry with the classical peroxidase-antiperoxidase-diaminobenzidine-method; these features were compared with GABAergic boutons visualized with an immunogold method. Two distinct types of dynorphin-A-immunoreactive boutons could be identified: (1) type A (81%) possessing characteristics similar to the GA-BAergic nerve endings in this region, i.e., large pleomorphic vesicles and symmetric synaptic contacts, (2) type B (19%) displaying asymmetric synaptic zones and small, mostly round vesicles. These results are in agreement with physiological studies suggesting a dual action of dynorphin A in substantia nigra.     

Afferent and efferent connections of the sexually dimorphic medial preoptic nucleus of the male quail revealed by in vitro transport of DiI

The medial preoptic nucleus of the Japanese quail is a testosterone-sensitive structure that is involved in the control of male copulatory behavior. The full understanding of the role played by this nucleus in the control of reproduction requires the identification of its afferent and efferent connections. In order to identify neural circuits involved in the control of the medial preoptic nucleus, we used the lipophilic fluorescent tracer DiI implanted in aldheyde-fixed tissue. Different strategies of brain dissection and different implantation sites were used to establish and confirm afferent and efferent connections of the nucleus. Anterograde projections reached the tuberal hypothalamus, the area ventralis of Tsai, and the substantia grisea centralis. Dense networks of fluorescent fibers were also seen in several hypothalamic nuclei, such as the anterior medialis hypothalami, the paraventricularis magnocellularis, and the ventromedialis hypothalami. A major projection in the dorsal direction was also observed from the medial preoptic nucleus toward the nucleus septalis lateralis and medialis. Afferents to the nucleus were seen from all these regions. Implantation of DiI into the substantia grisea centralis also revealed massive bidirectional connections with a large number of more caudal mesencephalic and pontine structures. The substantia grisea centralis therefore appears to be an important center connecting anterior levels of the brain to brain-stem nuclei that may be involved in the control of male copulatory behavior.     

Electronmicroscopic immunocytochemical study on the vasopressin-containing neurons of the thirsting rat

Summary Whereas in thirsting animals the perikarya of the nucleus supraopticus are nearly empty of neurosecretory granules as evidenced by electron microscopic observation, the perikarya are heavily stained by light microscopic immunohistochemical staining. In an attempt to discover the substrate responsible for the positive immunohistochemical staining in thirsting rats, the neurons of the supraoptic nucleus of normal and long-term thirsting animals were compared by electron microscopic immunocytochemistry (indirect PAP-method). In controls all parts of the vasopressin-synthesizing neuron are filled with elementary granules which render a positive and uniform reaction after immunostaining with the indirect PAP-method. The positively reacting fibers in the external zone of the median eminence contain smaller granules than those of the tractus supraoptico-hypophyseus. Within the nucleus suprachiasmaticus, no positive reaction after immunostaining was found. In long-term thirsting animals PAP-complexes as markers of vasopressin are located over the ergastoplasm and over the few small elementary granules. The processes within the nucleus supraopticus and the ballooned axons in the internal zone of the median eminence exhibit free, i.e. non granule-bound, PAP-complexes. Findings in the nucleus suprachiasmaticus and the median eminence of thirsting animals correspond to those in controls. The neurohypophysis is almost completely devoid of PAP-labeled elementary granules.From these results it can be concluded that during thirst vasopressin synthesis is increased in the ergastoplasm and that the hormone is transported partly in a non granule-bound form. Direct contacts between neurosecretory cells and the basal lamina are found more often in thirst-stressed animals and are typical of neurohemal regions. It is discussed whether these neurohemal regions may develop transitionally under stress.Supported by the Deutsche Forschungsgemeinschaft (Grant Nr. Kr 569/1) and Stiftung Volkswagenwerk. This work was presented in part at the 72nd meeting of the Anatomische Gesellschaft, Aachen 1977