Seasonal Plasticity in the Peptide Neuronal Systems: Potential Roles of Gonadotrophin‐Releasing Hormone, Gonadotrophin‐Inhibiting Hormone, Neuropeptide Y and Vasoactive Intestinal Peptide in the Regulation of the Reproductive Axis in Subtropical Indian Weaver Birds

Abstract

Two experiments examined the expression of gonadotrophin-releasing and inhibiting hormones (GnRH-I, GnRH-II and GnIH), neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) in subtropical Indian weaver birds, which demonstrate relative photorefractoriness. Experiment 1 measured peptide expression levels in the form of immunoreactive (-IR) cells, percentage cell area and cell optical density in the preoptic area (GnRH-I), midbrain (GnRH-II), paraventricular nucleus (GnIH), mediobasal hypothalamus [dorsomedial hypothalamus (DMH), infundibular complex (INc), NPY and VIP] and lateral septal organ (VIP) during the progressive, breeding, regressive and nonbreeding phases of the annual reproductive cycle. GnRH-I was decreased in the nonbreeding and VIP was increased in INc in the breeding and regressive states. GnRH-II and NPY levels did not differ between the testicular phases. Double-labelled immunohistochemistry (IHC) revealed a close association between the GnRH/GnIH, GnRH/NPY, GnRH/VIP and GnIH/NPY peptide systems, implicating them interacting and playing roles in the reproductive regulation in weaver birds. Experiment 2 further measured these peptide levels in the middle of day and night in weaver birds that were maintained under short days (8 : 16 h light /dark cycle; photosensitive), exposed to ten long days (16 : 8 h light /dark cycle; photostimulated) or maintained for approximately 2 years on a 16 : 8 h light /dark cycle (photorefractory). Reproductively immature testes in these groups precluded the possible effect of an enhanced gonadal feedback on the hypothalamic peptide expression. There were group differences in the GnRH-I (not GnRH-II), GnIH, NPY and VIP immunoreactivity, albeit with variations in immunoreactivity measures in the present study. These results, which are consistent with those reported in birds with relative photorefractoriness, show the distribution and possibly a complex interaction of key neuropeptides in the regulation of the annual reproductive cycle in Indian weaver birds.

Changes in brain peptides associated with reproduction and energy homeostasis: Putative roles of gonadotrophin‐releasing hormone‐II and tyrosine hydroxylase in determining reproductive performance in response to daily food availability times in diurnal zebra finches

“…We performed immunohistochemistry (IHC) for ZENK (as a marker of neuronal activity) and GnRH-I, GnRH-II, GnIH, NPY, CART and TH (neuropeptides associated with reproduction and metabolism), using brain sections from five birds of each sex, as per standardised procedures in our laboratory. 18,20,21 Briefly, birds were deeply anaesthetised with ketamine-xylazine solution (0.003 mL g -1 body weight) and transcardially perfused successively with 50 mL saline The primary antibodies that we used are specific and have been used for IHC in passerine birds, including zebra finches. 18,20,21,48 Because GnRH antiserum recognises both GnRH-I and GnRH-II isoforms, we identified them based on immunoreactions with reference to their location in the brain (POA: GnRH-I; midbrain:…”

“…We quantified the expression of IHC-labelled peptide or protein by image analysis, as described previously. 18,20,21,46 Briefly, using The image analyses were conducted using nis-elements br, version 2.3 (Nikon, Tokyo, Japan). To avoid a staining-intensity bias, both, strongly (bright) and weakly (faint) stained-immunoreactive (IR) cells were counted in all images of the specified region on both sides.…”

“…18 Regardless of the differential cue sensitivity between photoperiodic and non-photoperiodic species, hypothalamic GnRH directly activates the HPG axis and regulates gonadal maturation and reproductive behaviour in breeding birds. [19][20][21][22] GnRH is recognised in two functional isoforms: hypothalamic (pre-optic area [POA]) GnRH-I controls gonadal maturation, whereas midbrain GnRH-II regulates reproductive behaviour. 23 At the same time, hypothalamic (paraventricular nucleus [PVN]) gonadotrophin-inhibiting hormone (GnIH) influences gonadal maturation and reproductive behaviour by its opposing effects on GnRH-I and GnRH-II neurones.…”

“…29,30 Besides the GnRH/GnIH system, brain peptides associated with metabolism and energy homeostasis could influence HPG axis activity. For example, orexigenic neuropeptide Y (NPY) affects reproduction via its direct effects on GnRH (NPY fibres lie in close proximity to POA GnRH-I neurones) 20 and/or indirect effects on feeding and energy homeostasis. 9,31,32 Hypothalamic NPY cells showed structural association with GnIH cells.…”

Scotostimulation of reproductive neural pathways and gonadal maturation are not correlated with hypothalamic expression of deiodinases in subtropical spotted munia

“…At the regulatory level, photoperiodic regulation of the hypothalamic‐pituitary‐gonadal ( HPG ) axis involves changes in the synthesis and/or release of hypothalamic peptides, particularly vasoactive intestinal peptide (VIP), neuropeptide Y (NPY) and gonadotrophin‐releasing and gonadotrophin‐inhibiting hormones (GnRH and GnIH) . In birds, hypothalamic VIP and NPY appear to be associated with sensing and translating photic and nonphotic messages, respectively, from the environment . Indeed, cerebrospinal fluid‐contacting VIP neurones of the lateral septal organ are suggested to act as the brain photoreceptor .…”