S14.2: A sexual dimorphic peptidergic system in an avian species: Development and possible function

1Institute for Animal Science and Animal Behaviour, Department of Physiology, Celle, Germany, fax 49 5141 381849, e-mail Grossmann@ktf.fal.de; ²Institute of Ecology, Vilnius, Lithuania; ³University of Maryland, College Park, USA

Arginine vasotocin (AVT) is one of the key hormones regulating hydromineral balance in birds. Its contribution to reproductive functions promoting oviduct contraction and oviposition was also unequivocally demonstrated (Arad and Skadhauge 1984; Simon-Oppermann et al. 1988; Shimada et al. 1986). In the domestic fowl, AVT immunoreactive perikarya have originally been described in the preoptic, supraoptic and paraventricular regions of the hypothalamus (Goossens et al. 1977; Tennyson et al. 1985). With minor variations, comparable plans of distribution were reported for AVT perikarya in other bird species (quail: Viglietti-Panzica 1986; canary: Kiss et al. 1987; zebra finch: Voorhuis & de Kloet 1992). Among all investigated avian species, AVT cells in extrahypothalamic locations were observed only in the brain of songbirds, canary and zebra finch. In these species clustered AVT perikarya were located in the bed nucleus of the stria terminalis (BNST; Kiss et al. 1987; Voorhuis & de Kloet 1992) on the border between the lateral septum and the dorsal diencephalon. This cell group and parvocellular AVT neurons in the dorsal region of the hypothalamic paraventricular nucleus exhibit a clear sexual dimorphism in the canary brain. AVT cell bodies and fibers are more intense in males than in females and steroid hormones appear to influence and regulate this dimorphism. Seasonal changes in AVT immunoreactivity with maximal intensity during breeding and the decline during moult coincide with the fluctions of plasma testosterone levels (Voorhuis et al. 1991). A number of findings have described that sex-related differences in vasotocinergic circuits show an earlier phylogenetic lineage. Sexually dimorphic AVT immunoreactive cells and fibers located in extrahypothalamic regions have been demonstrated in amphibians (Zoeller & Moore 1986) and reptiles (Stoll & Voorn 1985). This review is an attempt to summarize the growing body of data concerning neurochemical sex differences in the brain vasotocinergic system and its known sexually dimorphic behavioral and physiological effects in birds. Although the presence of a sexual dimorphism in vasotocinergic elements is a common feature of the BNST, interspecific differences do occur. While neither AVT immunoreactivity nor AVT gene expressing neurons were reported in the hen, AVT neurons are present in both sexes in the canary, but they are less abundant in females than in males (Voorhuis et al. 1988).

Among neuroendocrine circuitries in the brain the hypothalamo-neurohypophysial system (HNS) is, as far as its classical functions are concerned, one of the best understood structures. The endocrine AVT system is represented by the HNS and consist of magnocellular neurons, the vast majority of them are located within the hypothalamic nuclei supraopticus and paraventricularis. Axons travel via the hypothalamo-neurohypophysial tract to the posterior pituitary and from there the nonapeptide hormone is released into the blood stream. The morphological and functional organization is, in general, similar in most vertebrate species with some important exceptions: in most mammalian species arginine vasopressin is the predominant osmoregulatory neurohypophysial hormone, while its analogue oxytocin is responsible for reproductive functions like contraction of the myometrium during parturition and myoepithelial contraction during milk ejection. In birds, however, AVT takes over both osmoregulation and oviposition. Since the latter function of AVT is specific for females one should expect differences between sexes in the structure and physiological regulation of the AVT system. Despite the magnocellular AVT-system vasotocinergic neurons are widely distributed in the bird brain. Parvocellular AVT immunoreactive neurons have been described in the preoptic, peri- and paraventricular regions as well as in the tuberal region of the hypothalamus (for review see Korf et al. 1988)

For appropriate function during posthatch life the brain must attain a sufficient degree of differentiation during embryonic and fetal development in order to cope with the change from a well protected prenatal life to the environmental conditions of postnatal life. The first visible products of AVT gene expression as well as detectable amounts of immunoreactive AVT peptide in the chicken brain were revealed at Day 6 of embryonic life (E6; Milewski et al. 1989; Mühlbauer et al. 1993). These data are in agreement with immunocytochemical findings showing AVT immunoreactive neurons in the chicken hypothalamus around E8 (Tennyson et al. 1986). It is unlikely, however, that AVT serves in systemic osmoregulation at this early stage because neither axon terminals nor the neurohypophysis have developed by then (Wingstrand 1954; Von Lawzewitsch 1969). Immunoreactive AVT has not been observed before E10 in neurohypophysial tissue (Tennyson et al. 1986). In the plasma AVT has been first detected at E14 by radioimmunoassay (Mühlbauer et al. 1993).

At least from E16 onwards the chick embryo is able to respond to osmotic challenge by increased AVT secretion into the blood, although an adult-like reaction is observed only after hatching (Klempt et al. 1992; Mühlbauer et al. 1992; Xu et al. 1992). Marked changes in the electrophysiological characteristics of the hypothalamo-neurohypophysial system of the chick embryo and the newly hatched chicken were observed during the perinatal period (Grossmann & Ellendorff 1986a; b). Identified magnocellular neurons of the hypothalamic nucleus paraventricularis in D1 chickens are clearly osmosensitive (Grossmann & Ellendorff 1990; Grossmann et al. 1994). Although in chick embryos at E18 and E19 as well as in newly hatched chickens, there are no differences in AVT plasma concentrations between males and females (Kisliuk & Grossmann 1994), both sexes do react differently to acute osmotic and voleamic stimuli at D1 (Kisliuk & Grossmann 1993). Females have a higher threshold in the reaction to haemorrhage: their AVT system does not respond to moderate haemorrhage while males respond by a significant elevation of AVT plasma concentration. To a more severe haemorrhage the AVT concentrations in females increase more slowly but reach at the end the same level as in males. The reaction to an acute hyperosmotic load is also much more pronounced in male than in female chickens at D1. One possible reason of the lower activity of the AVT system in female chicks might be the preadaptation to the specific water balance conditions which takes place in laying hens. Almost every day a normal laying hen looses, with its oviposited egg, approx. 30% of its blood volume attributed to water and one may expect that in females the hydromineral regulation is more tolerant to such kind of disturbances.

In adult chickens quantification of brain AVT mRNA content has revealed considerably more AVT transcripts in males compared to females (Barth et al. 1995). However, from these results it is unclear to what extent the magnocellular and parvocellular cell groups of the AVT system located in various brain areas contribute to this sex dimorphism. In a recent developmental study (Jurkevich et al. 1997) of the chicken parvocellular BnST we could demonstrate that AVT immunoreactive perikarya are first detectable already at E12, the earliest embryonic day of our study, in both sexes. At posthatch D1 and D2 there are clearly more AVT neurons in the male parvocellular BnST. During posthatch development the cell number and intensity of labelling in females decline rapidly, until by D70 the AVT immunoreactive elements have disappeared completely. The developmental time course of sex differences in brain AVT seems to be species-specific. While in gallinaceous birds sex dimorphism develops rather early, in the canary there is evidence that the sexually dimorphic AVT distribution in the BNST appears during the late phase of postnatal development (Voorhuis et al. 1991). In the latter species, sex differences in the vasotocinergic system are not as pronounced as in gallinaceous birds. AVT immunoreactive perikarya in the BnST could be demonstrated in the canary brain of both sexes, however, in females the intensity of staining is lower (Voorhuis et al. 1988). Studies in the quail have shown that the sexually dimorphic characteristics of the AVT system are comparable to those in the chicken. AVT immunoreactive perikarya could be observed exclusively in the parvocellular BnST of the male quail (Aste et al. 1996a) and this sex difference has been further confirmed at the level of AVT gene expression by in situ hybridization using a chicken AVT-specific cDNA probe (Aste et al. 1996b). It needs to be evaluated whether this sexual differentiation during development is accompanied by subsequent sex differences in behaviour and autonomic regulation during adulthood.

The existence of a clear sexual dimorphism has been demonstrated in the parvocellular BnST in a number of bird species (chicken: Jurkevich et al. 1996, 1997; quail: Viglietti-Panzica et al. 1992; 1994; canary: Voorhuis et al. 1988; 1991). This structure is known in the rat as an important integrative center involved in the control of sexual motivation and/or performance (Segovia & Guillamon 1993; van Furth et al. 1995) and central autonomic regulation (Moga et al. 1989). The presence of a sexual dimorphism in vasotocinergic elements is a common feature of the parvocellular BnST. While neither AVT immunoreactivity nor AVT gene expressing neurons were reported in the hen, AVT neurons are present in both sexes in the canary, but they are less abundant in females than in males (Voorhuis et al. 1988). In male chickens two densely packed parvocellular subgroups of AVT immunoreactive perikarya can be observed caudal of the anterior commissure. The dorsal cluster is located in the area of the BnST extending from the lateral ventricle to the dorsal aspect of the occipitomesencephalic tract. The ventral subgroup of AVT neurons is located ventrally to the caudal aspect of the anterior commissure in the dorsal position of the diencephalic paraventricular region (Fig. 1). Both cell clusters consist of numerous parvocellular bi- and multipolar cells surrounded by a dense plexus of beaded AVT containing fibers. The distribution pattern of AVT mRNA containing neurons in the parvocellular BnST of males is identical to the pattern of distribution of AVT immunoreactive cell bodies. In contrast, in hens immunoreactive perikarya and fiber tracts as well as AVT specific hybridization signals were completely absent in the parvocellular BnST.

It is reasonable to assume that the sex dimorphism in the AVT system is controlled and maintained by gonadal hormones. Although a number of findings support this assumption (Viglietti-Panzica et al. 1992; Voorhuis et al. 1988) there are some observations which cannot be explained solely on the level of androgens, e.g. the existence of both androgen and estrogen receptors in the quail parvocellular BnST (Balthazart et al. 1992) as well as a large population of aromatase-producing elements in the rat BnST (Roselli 1995). Since the BnST is a limbic structure it is reasonable to hypothesize that the observed sex difference in the AVT system is involved in specific behavioral and physiological regulation. Male social behavior may be one of the most plausible targets for such regulation. Administration of AVT stimulates mating behavior in chicken and pigeons (Kihlström & Dannige 1972), while in zebra finches aggressive behavior is enhanced (Goodson et al. 1996). An inhibitory effect of AVT on appetitive and consummatory aspects of the male sexual behavior has been described in the quail (Castagna et al. 1998). Putative effects of sexually dimorphic brain circuits on reproductive behavior may be mediated by other endocrine systems. It has been suggested that AVT may control the release and/or synthesis of gonadotropins. Recently, co-localization of AVT and GnRH-I in the chicken hypothalamus has been described (Dhondt et al. 1995). Thus, the sexual dimorphic AVT system may play a hey role in regulating either sexual behavior or gonadotropin release in birds by acting as neuromodulator and/or neurotransmitter. Further attempts are required to understand the specific contribution of AVT on these body functions.

This work was supported by grants from the Deutsche Forschungsgemeinschaft (DFG), the German Ministry of Agriculture and the Maryland Agricultual Experiment Station.

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Fig. 1. Microphotograph illustrating the distribution o AVT immunoreactive perikarya and fibers in the brain of male (A) and female (B) chicken. AVT immunoreactive parvocellular neurons and fibers are located in the cockerel but not in the hen BnST.