Current State of Understanding of the Role of PACAP in the Hypothalamo-Hypophyseal Gonadotropin Functions of Mammals

PACAP was discovered 30 years ago in Dr. Akira Arimura's laboratory. In the past three decades since then, it has become evident that this peptide plays numerous crucial roles in mammalian organisms. The most important functions of PACAP are the following: 1. neurotransmitter, 2. neuromodulator, 3. hypophysiotropic hormone, 4. neuroprotector. This paper reviews the accumulated data regarding the distribution of PACAP and its receptors in the mammalian hypothalamus and pituitary gland, the role of PACAP in the gonadotropin hormone secretion of females and males. The review also summarizes the interaction between PACAP, GnRH, and sex steroids as well as hypothalamic peptides including kisspeptin. The possible role of PACAP in reproductive functions through the biological clock is also discussed. Finally, the significance of PACAP in the hypothalamo-hypophysial system is considered and the facts missing, that would help better understand the function of PACAP in this system, are also highlighted.

Strategic Positioning of Connexin36 Gap Junctions Across Human Retinal Ganglion Cell Dendritic Arbors

Connexin36 (Cx36) subunits form gap junctions (GJ) between neurons throughout the central nervous system. Such GJs of the mammalian retina serve the transmission, averaging and correlation of signals prior to conveying visual information to the brain. Retinal GJs have been exhaustively studied in various animal species, however, there is still a perplexing paucity of information regarding the presence and function of human retinal GJs. Particularly little is known about GJ formation of human retinal ganglion cells (hRGCs) due to the limited number of suitable experimental approaches. Compared to the neuronal coupling studies in animal models, where GJ permeable tracer injection is the gold standard method, the post-mortem nature of scarcely available human retinal samples leaves immunohistochemistry as a sole approach to obtain information on hRGC GJs. In this study Lucifer Yellow (LY) dye injections and Cx36 immunohistochemistry were performed in fixed short-post-mortem samples to stain hRGCs with complete dendritic arbors and locate dendritic Cx36 GJs. Subsequent neuronal reconstructions and morphometric analyses revealed that Cx36 plaques had a clear tendency to form clusters and particularly favored terminal dendritic segments.

Connectivity of neuronal populations within and between areas of primate somatosensory cortex

Functions of the cerebral cortex emerge via interactions of horizontally distributed neuronal populations within and across areas. However, the connectional underpinning of these interactions is not well understood. The present study explores the circuitry of column-size cortical domains within the hierarchically organized somatosensory cortical areas 3b and 1 using tract tracing and optical intrinsic signal imaging (OIS). The anatomical findings reveal that feedforward connections exhibit high topographic specificity, while intrinsic and feedback connections have a more widespread distribution. Both intrinsic and inter-areal connections are topographically oriented across the finger representations. Compared to area 3b, the low clustering of connections and small cortical magnification factor supports that the circuitry of area 1 scaffolds a sparse functional representation that integrates peripheral information from a large area that is fed back to area 3b. Fast information exchange between areas is ensured by thick axons forming a topographically organized, reciprocal pathway. Moreover, the highest density of projecting neurons and groups of axon arborization patches corresponds well with the size and locations of the functional population response reported by OIS. The findings establish connectional motifs at the mesoscopic level that underpin the functional organization of the cerebral cortex.

Bipolar cell gap junctions serve major signaling pathways in the human retina

Connexin36 (Cx36) constituent gap junctions (GJ) throughout the brain connect neurons into functional syncytia. In the retina they underlie the transmission, averaging and correlation of signals prior conveying visual information to the brain. This is the first study that describes retinal bipolar cell (BC) GJs in the human inner retina, whose function is enigmatic even in the examined animal models. Furthermore, a number of unique features (e.g. fovea, trichromacy, midget system) necessitate a reexamination of the animal model results in the human retina. Well-preserved postmortem human samples of this study are allowed to identify Cx36 expressing BCs neurochemically. Results reveal that both rod and cone pathway interneurons display strong Cx36 expression. Rod BC inputs to AII amacrine cells (AC) appear in juxtaposition to AII GJs, thus suggesting a strategic AII cell targeting by rod BCs. Cone BCs serving midget, parasol or koniocellular signaling pathways display a wealth of Cx36 expression to form homologously coupled arrays. In addition, they also establish heterologous GJ contacts to serve an exchange of information between parallel signaling streams. Interestingly, a prominent Cx36 expression was exhibited by midget system BCs that appear to maintain intimate contacts with bistratified BCs serving other pathways. These findings suggest that BC GJs in parallel signaling streams serve both an intra- and inter-pathway exchange of signals in the human retina.

Connexin36 Expression in the Mammalian Retina: A Multiple-Species Comparison

Much knowledge about interconnection of human retinal neurons is inferred from results on animal models. Likewise, there is a lack of information on human retinal electrical synapses/gap junctions (GJ). Connexin36 (Cx36) forms GJs in both the inner and outer plexiform layers (IPL and OPL) in most species including humans. However, a comparison of Cx36 GJ distribution in retinas of humans and popular animal models has not been presented. To this end a multiple-species comparison was performed in retinas of 12 mammals including humans to survey the Cx36 distribution. Areas of retinal specializations were avoided (e.g., fovea, visual streak, area centralis), thus observed Cx36 distribution differences were not attributed to these species-specific architecture of central retinal areas. Cx36 was expressed in both synaptic layers in all examined retinas. Cx36 plaques displayed an inhomogenous IPL distribution favoring the ON sublamina, however, this feature was more pronounced in the human, swine and guinea pig while it was less obvious in the rabbit, squirrel monkey, and ferret retinas. In contrast to the relative conservative Cx36 distribution in the IPL, the labels in the OPL varied considerably among mammals. In general, OPL plaques were rare and rather small in rod dominant carnivores and rodents, whereas the human and the cone rich guinea pig retinas displayed robust Cx36 labels. This survey presented that the human retina displayed two characteristic features, a pronounced ON dominance of Cx36 plaques in the IPL and prevalent Cx36 plaque conglomerates in the OPL. While many species showed either of these features, only the guinea pig retina shared both. The observed similarities and subtle differences in Cx36 plaque distribution across mammals do not correspond to evolutionary distances but may reflect accomodation to lifestyles of examined species.

Characterization of connexin36 gap junctions in the human outer retina

Retinal connexins (Cx) form gap junctions (GJ) in key circuits that transmit average or synchronize signals. Expression of Cx36, -45, -50 and -57 have been described in many species but there is still a disconcerting paucity of information regarding the Cx makeup of human retinal GJs. We used well-preserved human postmortem samples to characterize Cx36 GJ constituent circuits of the outer plexiform layer (OPL). Based on their location, morphometric characteristics and co-localizations with outer retinal neuronal markers, we distinguished four populations of Cx36 plaques in the human OPL. Three of these were comprised of loosely scattered Cx36 plaques; the distalmost population 1 formed cone-to-rod GJs, population 2 in the mid-OPL formed cone-to-cone GJs, whereas the proximalmost population 4 likely connected bipolar cell dendrites. The fourth population (population 3) of Cx36 plaques conglomerated beneath cone pedicles and connected dendritic tips of bipolar cells that shared a common presynaptic cone. Overall, we show that the human outer retina displays a diverse cohort of Cx36 GJ that follows the general mammalian scheme and display a great functional diversity.

The Retinal TNAP

Accumulating evidence from recent literature underline the important roles of tissue non specific alkaline phosphatase (TNAP) in diverse functions as well as diseases of the nervous system. Exploration of TNAP in well characterized neural circuits such as the retina, might significantly advance our understanding regarding neural TNAP's roles. This chapter reviews the scarce literature as well as our findings on retinal TNAP. We found that retinal TNAP activity was preserved and followed diverse patterns throughout vertebrate evolution. We have consistently observed TNAP activity (1) in retinal vessels, (2) in photoreceptors and (3) in the majority of the studied species in the outer (OPL) and inner plexiform layers (IPL), where synaptic transmission occurs. Importantly, in some species the IPL exhibits several TNAP positive strata. These strata exactly corresponded those seen after quadruple immunohistochemistry with four canonical IPL markers (tyrosine hydroxylase, choline acetyltransferase, calretinin, protein kinase C α). Diabetes results in diminishing retinal TNAP activity before changes in canonical markers could be observed in a rat model. The presence of TNAP activity at critical sites of neurotransmission suggests its important and evolutionary conserved role in vision. In diabetes, the decreased TNAP activity indicates neurological alterations adding further evidence for the role of TNAP in brain diseases.

Connectivity of somatosensory cortical area 1 forms an anatomical substrate for the emergence of multifinger receptive fields and complex feature selectivity in the squirrel monkey (Saimiri sciureus)

Converging evidence shows that interaction of digit-specific input, which is required to form global tactile percepts, begins as early as area 3b in the primary somatosensory cortex with the involvement of intrinsic lateral connections. How tactile processing is further elaborated in area 1, the next stage of the somatosensory cortical hierarchy, is less understood. This question was investigated by studying the tangential distribution of intrinsic and interareal connections of finger representations of area 1. Retrogradely labeled cell densities and anterogradely labeled fibers and terminal patches were plotted and quantified with respect to the hand representation by combining tract tracing with electrophysiological mapping and intrinsic signal optical imaging in somatosensory areas. Intrinsic connections of distal finger pad representations of area 1 spanned the representation of multiple digits indicating strong cross-digit connectivity. Area 1 distal finger pad regions also established high-density connections with homotopic regions of areas 3b and 2. Although similar to area 3b, connections of area 1 distributed more widely and covered a larger somatotopic representation including more proximal parts of the finger representations. The lateral connectivity pattern of area 1 is a suitable anatomical substrate of the emergence of multifinger receptive fields, complex feature selectivity, and invariant stimulus properties of the neurons.

TNAP activity is localized at critical sites of retinal neurotransmission across various vertebrate species

Evidence is emerging with regard to the role of tissue non-specific alkaline phosphatase (TNAP) in neural functions. As an ectophosphatase, this enzyme might influence neural activity and synaptic transmission in diverse ways. The localization of the enzyme in known neural circuits, such as the retina, might significantly advance an understanding of its role in normal and pathological functioning. However, the presence of TNAP in the retina is scarcely investigated. Our multispecies comparative study (zebrafish, cichlid, frog, chicken, mouse, rat, golden hamster, guinea pig, rabbit, sheep, cat, dog, ferret, squirrel monkey, human) using enzyme histochemistry and Western blots has shown the presence of TNAP activity in the retina of several mammalian species, including humans. Although the TNAP activity pattern varies across species, we have observed the following trends: (1) in all investigated species (except golden hamster), retinal vessels display TNAP activity; (2) TNAP activity consistently occurs in the photoreceptor layer; (3) in majority of the investigated species, marked TNAP activity is present in the outer and inner plexiform layers. In zebrafish, frog, chicken, guinea pig, and rat, TNAP histochemistry has revealed several sublayers of the inner plexiform layer. Frog, golden hamster, guinea pig, mouse, and human retinas possess a subpopulation of amacrine cells positively staining for TNAP activity. The expression of TNAP in critical sites of retinal signal transmission across a wide range of species suggests its fundamental, evolutionally conserved role in vision.

Pathologic alterations of the outer retina in streptozotocin-induced diabetes

PURPOSE

Neurodegeneration as an early event of diabetic retinopathy preceding clinically detectable vascular alterations is a widely proven issue today. While there is evidence for the impairment of color vision and contrast sensitivity in early diabetes, suggesting deteriorated photoreceptor function, the underlying neuropathology of these functional alterations is still unknown. The aim of the present study was to investigate the effects of early diabetes on the outer retinal cells.

METHODS

The retinal pigment epithelium, photopigment expression, and density and morphology of photoreceptors were studied using immunocytochemistry in streptozotocin-induced diabetes in two rat strains. The fine structure of photoreceptors and pigment epithelium was also investigated with transmission electron microscopy.

RESULTS

Here we found that retinal thickness was unchanged in diabetic animals and that no significant increase in the number of apoptotic cells was present. Although the density of cones expressing middle (M)- and shortwave (S)-sensitive opsins was similar in diabetic and control retinas, we detected remarkable morphologic signs of degeneration in the outer segments of diabetic rods, most M-cones, and some S-cones. A decrease in thickness and RPE65 protein immunoreactivity of the pigment epithelium were evident. Furthermore, an increased number of dual cones, coexpressing both M- and S-opsins, was detected at the peripheral retina of diabetic rats.

CONCLUSIONS

Degenerative changes of photoreceptors and pigment epithelium shown here prior to apoptotic loss of photoreceptors may contribute to functional alterations reported in diabetic human patients and different animal models, thus may serve as a potential model for testing the efficacy of neuroprotective agents in diabetes.

Advent and recent advances in research on the role of pituitary adenylate cyclase-activating polypeptide (PACAP) in the regulation of gonadotropic hormone secretion of female rats

PACAP (ADCYAP1) was isolated from ovine hypothalami. PACAP activates three distinct receptor types: G-protein coupled PAC1, VPAC1, and VPAC2 with seven transmembrane domains. Eight splice variants of PAC1 receptor are described. A part of the hypothalamic PACAP is released into the hypophyseal portal circulation. Both hypothalamic and pituitary PACAP are involved in the dynamic control of gonadotropic hormone secretion. In female rats, PACAP in the paraventricular nucleus is upregulated in the morning and pituitary PACAP is upregulated in the late evening of the proestrus stage of the reproductive cycle. PACAP mRNA peak in the hypothalamic PVN precedes the LHRH release into the portal circulation. It is supposed that PACAP peak is evoked by the elevated estrogen on proestrous morning. At the beginning of the so-called critical period of the same day, PACAP level starts to decline allowing LHRH release into the portal circulation, resulting in the LH surge that evokes ovulation. Just before the critical period, icv-administered exogenous PACAP blocks the LH surge and ovulation. The blocking effect of PACAP is mediated through CRF and endogenous opioids. The effect of the pituitary-born PACAP depends on the intracellular cross-talk between PACAP and LHRH.

Layer-specific activity of tissue non-specific alkaline phosphatase in the human neocortex

The ectoenzyme tissue non-specific alkaline phosphatase (TNAP) is mostly known for its role in bone mineralization. However, in the severe form of hypophosphatasia, TNAP deficiency also results in epileptic seizures, suggesting a role of this enzyme in brain functions. Accordingly, TNAP activity was shown in the neuropil of the cerebral cortex in diverse mammalian species. However in spite of its clinical significance, the neuronal localization of TNAP has not been investigated in the human brain. By using enzyme histochemistry, we found an unprecedented pattern of TNAP activity appearing as an uninterrupted layer across diverse occipital-, frontal- and temporal lobe areas of the human cerebral cortex. This marked TNAP-active band was localized infragranulary in layer 5 as defined by quantitative comparisons on parallel sections stained by various techniques to reveal the laminar pattern. On the contrary, TNAP activity was localized in layer 4 of the primary visual and somatosensory cortices, which is consistent with earlier observations on other species. This result suggests that the expression of TNAP in the thalamo-recipient granular layer is an evolutionary conserved feature of the sensory cortex. The observations of the present study also suggest that diverse neurocognitive functions share a common cerebral cortical mechanism depending on TNAP activity in layer 5. In summary, the present data point on the distinctive role of layer 5 in cortical computation and neurological disorders caused by TNAP dysfunctions in the human brain.

Moderate loss of cerebellar Purkinje cells after chronic bilateral common carotid artery occlusion in rats

Pathological effects of moderate ischemia (oligemia, hypoperfusion) are relevant in relation to vascular factors in dementia. Chronic bilateral common carotid artery occlusion (BCCAO) in adult Wistar rats induces oligemia and leads to acute changes in gene expression, subacute changes in cortical astrocytes and prolonged changes in white matter tracts, while largely sparing neurons in the forebrain areas. Dilation and remodeling of the basilar artery ensures blood flow to the forebrain. The present study examined the hypoxia-sensitive Purkinje cells in the cerebellum after 6 months of BCCAO using conventional neuropathological analysis, immunohistochemistry and high-precision design-based stereologic methods. Purkinje cells in the vermis region revealed abnormally shaped nuclei. A stereologic analysis showed that the mean total number of Purkinje cells within the vermis was statistically significantly smaller in the BCCAO animals than in the control animals (d = 11.8%; P < 0.0001). BCCAO had no significant effect on the mean volumes of the molecular layer, granule cell layer and white matter in the vermis or the entire cerebellum. Remodeling of the basilar artery indicated that secondary vascular perturbations might be responsible for the effects of BCCAO on the cerebellar Purkinje cells.

Behavioral abnormalities precede neuropathological markers in rats transgenic for Huntington's disease

Huntington's disease (HD) is caused by an expanded CAG repeat leading to the synthesis of an aberrant protein and to the formation of polyglutamine (polyQ)-containing inclusions and aggregates. Limited information is available concerning the association of neuropathological markers with the development of behavioral markers in HD. Using a previously generated transgenic rat model of HD (tgHD rat), we performed association studies on the time-course of behavioral symptoms (motor function, learning, anxiety) and the appearance of striatal atrophy, 1C2 immunopositive aggregates and polyQ recruitment sites, a precursor to these aggregates. At the age of 1 month, tgHD rats exhibited reduced anxiety and improved motor performance, while at 6 months motor impairments and at 9 months cognitive decline occurred. In contrast, polyQ recruitment sites appeared at around 6-9 months of age, indicating that HD-like behavioral markers preceded the appearance of currently detectable neuropathological markers. Interestingly, numerous punctate sites containing polyQ aggregates were also seen in areas receiving afferents from the densely recruiting regions suggesting either transport of recruitment-competent aggregates to terminal projections where initially 1C2 positive aggregates were formed or different internal properties of neurons in different regions. Furthermore, striatal atrophy was observed at the age of 12 months. Taken together, our findings support the hypothesis of a dynamic process leading to region- and age-specific polyQ recruitment and aggregation. The dissociation of onset between behavioral and neuropathological markers is suggestive of as yet undetected processes, which contribute to the early phenotype of these HD transgenic rats.

Selective striatal neuron loss and alterations in behavior correlate with impaired striatal function in Huntington's disease transgenic rats

Huntington's disease (HD) is an inherited neurodegenerative disorder characterized by selective striatal neuron loss and motor, cognitive and affective disturbances. The present study aimed to test the hypothesis of adult-onset neuron loss in striatum and frontal cortical layer V as well as alterations in behavior pointing to impaired striatal function in a recently developed transgenic rat model of HD (tgHD rats) exhibiting enlarged ventricles, striatal atrophy and pycnotic pyramidal cells in frontal cortical layer V. High-precision design-based stereological analysis revealed a reduced mean total number of neurons in the striatum but not in frontal cortical layer V of 12-month-old tgHD rats compared with age-matched wild-type controls. No alterations in mean total numbers of striatal neurons were found in 6-month-old animals. Testing 14-month-old animals in a choice reaction time task indicated impaired striatal function of tgHD rats compared with controls.

The role of PACAP in gonadotropic hormone secretion at hypothalamic and pituitary levels

The presence of pituitary adenylate cyclase-activating polypeptide (PACAP) and its mRNAin the three levels of the hypothalamo-hypophyseal-ovarian axis was previously demonstrated using immunohistochemistry, in situ hybridization, and reverse transcriptase polymerase chain reaction (RT-PCR). In the hypothalamus, PACAP is present in neuroendocrine effector cells and in the median eminence. In the anterior pituitary and ovary, PACAP is transiently present during the proestrous stage of the estrous cycle. In the pituitary, PACAP was observed in gonadotropes. In the ovary, PACAP was demonstrated in the granulosa cells of the preovulatory ovarian follicles. The effect of PACAP on luteinizing hormone (LH) secretion was demonstrated in in vivo and in vitro models. In our work we have studied the role of PACAP in gonadotropic hormone secretion at hypothalamic and pituitary levels. At the hypothalamic level, PACAP, administered intracerebroventricularly to female rats before the critical period of the proestrus stage, can inhibit LH release and ovulation. Its inhibiting effect is mediated through corticotropin-releasing factor (CRF) and endogenous opioids. PACAP administered to neonatal female rats delayed the onset of puberty by influencing the luteinizing hormone-releasing hormone (LHRH) neuronal system. In the pituitary gland, the release of PACAP depended on the stage of the estrous cycle and on the time of day the animals were sacrificed. On the day of proestrus, the number of PACAP-releasing cells showed a diurnal change with two peaks (in the morning and in the evening). The peak was much higher in the evening at the end of the LH surge than in the morning.

Distribution of PACAP and its mRNA in several nonneural tissues of rats demonstrated by sandwich enzyme immunoassay and RT-PCR technique

The presence of PACAP in various organs was previously demonstrated using immunohistochemistry and radioimmunoassay. The aim of our work was to get information whether the presence of immunoreactive PACAP in various organs, mainly in the gastric mucosa, also indicates the place of its synthesis. The immunoreactive PACAP and its mRNA were measured in parallel assays using sandwich enzyme immunoassay (S-EIA) and RT-PCR technique. PACAP and its mRNA were demonstrated in the pancreas, testes, adrenal glands, ovaries, and in the oxyntic mucosa of the stomach. These results support our previous observation that PACAP is present not only in the nervous system and endocrine glands, but might be synthetized in the oxyntic mucosa of the stomach as well.

PACAP and VIP in the photoneuroendocrine system (PNES)

The presence of PACAP and VIP was demonstrated in all the four levels of the photoneuroendocrine system (PNES) with the use of immunohistochemistry (IHC), radioimmunoassay (RIA), anterograde and retrograde tracing techniques, and cell immunoblot assay (CIBA). Both peptides play a physiological role in the PNES. According to our results both PACAP and VIP are involved in the regulation of the gonadotrop hormone secretion and their inhibitory role may be mediated through the neuronal chain of the PNES.

Caveolae and caveolin isoforms in rat peritoneal macrophages

Caveolea are special (highly hydrophobic) plasma membrane invaginations with a diameter of 50-100 nm. Their characteristic features are the flask- or omega-shape and the lack of basket-like coat composed of clathrin. Caveolin-an integral membrane protein-is the principal component of caveolae membranes in vivo. Multiple forms of caveolin have been identified: caveolin-1alpha, caveolin-1beta, caveolin-2 and caveolin-3. They differ in their specific properties and tissue distribution. In this paper we summarize the morphological and biochemical data providing strong evidence about the existence and function of caveolae in rat peritoneal macrophages. When studied electron microscopically, the surface of both resident and elicited macrophages exhibited omega- or flask-shaped plasma membrane invaginations. There was a significant difference, however, in the number of these profiles: whereas in resident cells only a small amount of them was found on the cell surface, in elicited cells they were abundantly present on the plasma membrane. Using an antibody against the VIP21/caveolin-1 isoform we showed that these plasma membrane pits were indeed caveolae. The number and the appearance of caveolae were found to be in close correlation with the functional activity of these phagocytotic cells, indicating that the formation of caveolae is a highly regulated process. Using Western blot analysis two different proteins ( approximately 29 and approximately 20 kDa)-both labelled with anti-caveolin antibodies-were identified in resident and elicited macrophages that have been isolated from rat peritoneal cavity. The approximately 20 kDa protein was labelled specifically only by anti-VIP21/caveolin-1, while the approximately 29 kDa protein was labelled by both anti-VIP21/caveolin-1 and anti-caveolin-2 antibodies. The presence of the approximately 29 kDa protein was highly characteristic of resident cells, and only a small amount of approximately 20 kDa protein was detected in these cells. Elicitation has resulted in a significant increase in the amount of approximately 20 kDa protein labeled only with anit-VIP21/caveolin-1. Our morphological (confocal and electron microscopical) studies have shown that in resident cells caveolin was present in the cytoplasm, in smaller vesicles and multivesicular bodies around the Golgi area. Only a very small amount of caveolae was found on the cell surface of these cells. In elicited macrophages, caveolae (labelled with anti-VIP21/caveolin-1 antibody) appeared in large numbers on the cell surface, but caveolin detected by anti-caveolin-2 was also found in small vesicles and multivesicular bodies. These data support the idea that the expression of the approximately 29 kDa (caveolin-related) protein is insufficient for caveolae formation in resident cells, it can function as a modified, macrophage-specific caveolin-2 isoform. Our results strongly suggest that caveolin-1 plays a crucial role in the formation of caveolae: it is the amount of caveolin-1 that regulates the appearance of caveolae on the plasma membrane. Studying the endocytotic processes of resident and elicited macrophages we have found that elicited macrophages bound and internalized significantly larger amounts of fluid phase marker (HRP) and immune complex (peroxidase-antiperoxidase-PAP) than resident cells. Serial section analysis, double labelled immunocytochemistry, and filipin treatment were used to demonstrate that caveolae can pinch off from the plasma membrane and can take part in endocytotic processes as alternative carriers in elicited macrophages.

Comparative study on the appearance of various bioactive peptides in foregut derivates during the ontogenesis

Bioactive peptides have an important multifunctional role in the gastrointestinal tract. In the present study we have investigated the dynamism of the appearance of PACAP (pituitary adenylate cyclase activating polypeptide), VIP (vasoactive intestinal polypeptide), gastrin, and secretin immunoreactivities in human foregut derivates during the ontogenesis using an immunohistochemical approach. None of these peptides were observed in the foregut derivates of an 8-week-old embryo. VIP immunoreactive nerve fibers appeared by the 11th week in the smooth muscle layers of the stomach. No other peptide immunoreactivities were observed of this stage. In 18- and 20-week old fetuses PACAP, secretin, and gastrin immunoreactive cells appeared in the developing glands of the stomach. In the duodenum gastrin immunoreactivity was present in the Lieberkühn's glands and secretin immunoreactive cells were seen between the surface epithelial cells. In the pancreas secretin immunoreactivity was found in the Langerhans islets; however, PACAP immunreactivity was observed in the exocrine portion. The distribution of VIP fibers did not change during the fetal life and it was similar to the adult pattern. According to our results the appearance of PACAP, secretin, and gastrin in the developing glands suggests their role in the proliferation and differentiation of the epithelial derivates.

Pituitary adenylate cyclase activating polypeptide (PACAP) is present in human and cat gastric glands

In the present work we have studied the occurrence of pituitary adenylate cyclase activating polypeptide (PACAP) in human and cat stomach mucosa using immunohistochemistry. As seen under a light microscope, there were many large rounded and ovoid cells that were PACAP immunopositive, mainly in the neck of the gastric glands of both species. The immunopositive material was predominant in the perinuclear area. The PACAP immunolabeling was specific because the preincubation of the antiserum with PACAP abolished the immunostaining. In human samples under electron microscope, the PACAP immunoreactive cells have shown the characteristics of parietal cells. In faintly stained cells, the localization of DAB reaction product was associated with the surface of the intracellular canaliculi. Cell labeling could not be observed besides parietal cells.

Effect of the lack of light impulses on the hypothalamic PACAP and C-fos immunoreactivities in rats

Pituitary adenylate cyclase activating polypeptide (PACAP) is a member of the secretin family. It is widely distributed in the central and peripheral nervous systems. The highest concentration of PACAP was found in the hypothalamus. In the present work it has been studied whether PACAP is involved in the mediation of photic stimuli to the anterior pituitary gland. We have examined the effect of the lack of light impulses on the hypothalamic PACAP and C-fos immunoreactivities. In adult rats 10 days after the removal of the eyes (surgical enucleation) and in those received monosodium glutamate treatment neonatally (chemical enucleation). The PACAP immunostaining enhanced in the hypothalamic magnocellular nuclei and in the extemal zone of the median eminence. C-fos immunoreactivity also enhanced in a few hypothalamic nuclei 2 hours after the surgical enucleation indicating that the lack of light impulses activated hypothalamic neurons which, in turn, might stimulate the release of PACAP into the portal circulation. It has been concluded that PACAP may be involved in photoendocrine regulations.

PACAP colocalizes with luteinizing and follicle-stimulating hormone immunoreactivities in the anterior lobe of the pituitary gland

Pituitary adenylate cyclase activating polypeptide (PACAP) and its close relative vasoactive intestinal polypeptide (VIP) were demonstrated in the anterior pituitary gland. The cells which exhibited PACAP immunoreactivity were oval or round shaped. Their distribution was similar to that of gonadotropes but the number of PACAP immunoreactive cells was less. Double labeling revealed that PACAP immunoreactivity partially colocalized with luteinizing and follicle-stimulating hormone; however, colocalization with other pituitary hormone immunoreactivities was not demonstrated. Our results suggest an autocrine or paracrine role of PACAP in the regulation of pituitary functions.

New aspects of the neuroendocrine role of PACAP

The presence of PACAP was revealed in the anterior pituitary with RIA, HPLC, and with the demonstration of its mRNA. The level of PACAP mRNA in the anterior pituitary is the highest during the proestrous LH surge. In our immunohistochemical studies we were able to demonstrate PACAP immunoreactive cells in the anterior pituitary. The shape and the distribution of PACAP immunoreactive cells were very similar to that of the gonadotropes; however, the number of PACAP cells was less than that of LH cells. Additionally, another PACAP-positive cell population with small diameter appeared in the proestrous stage, during pregnancy and lactation. Double labeling revealed that the major part of large PACAP cells exhibited LH immunoreactivity and those with a small diameter contained PRL. It is not clear whether the pituitary- or the hypothalamic-born PACAP, or both, influence pituitary LH and PRL secretion. I.c.v. administration of PACAP just prior to the critical period in the proestrous stage inhibited the expected ovulation and blocked the proestrus LH and PRL surge, although i.v. administration of PACAP had no effect. PACAP antiserum did not interfere with ovulation when i.c.v. or i.v. injection was used. Our results support the view that PACAP has a role in the control of LH and PRL secretion during the estrous cycle, pregnancy, and lactation. The inhibitory effect of PACAP on ovulation is mediated through the hypothalamus.

PACAP participates in the regulation of the hormonal events preceeding the ovulation

Pituitary adenylate cyclase activating polypeptide (PACAP) is a member of the secretin family. It was isolated and characterized in 1989. Its neuroendocrine role was demonstrated in vivo and in vitro systems. It seems that in vivo the effect of PACAP on the gonadotrop hormone secretion depends on the route of administration. It was reported that intravenous (i.v.) injection of PACAP elevated, while intra-cerebro-ventricular (i.c.v.) administration depressed plasma LH levels. In the present study it was demonstrated that PACAP, administered i.c.v. before the critical period of the proestrous stage, blocked the ovulation and prevented the proestrous LH surge in rats. The blocking effect of PACAP is not directly mediated by endogenous opioids because the antagonizing effect of Naloxone, an opioid receptor antagonist, was questionable. Under our experimental conditions we could not confirm the stimulating effect of i.v. administered PACAP.