Reproduction phase-related expression of beta-endorphin-like immunoreactivity in the nucleus lateralis tuberis of the female Indian major carp Cirrhinus mrigala: correlation with the luteinising hormone cells-ovary axis

The opioid peptide β-endorphin interferes with the pituitary-testis axis in the Mozambique tilapia Oreochromis mossambicus

Although the involvement of β-endorphin (β-ERP) in vertebrate reproduction has been suggested, its role in testicular activity is not clear in fish. We describe the influence of β-ERP on spermatogenesis in a cichlid fish in the present paper. In comparison to the control group, the administration of β-ERP (3 µg) caused a significant increase in the number of spermatogonia-A and spermatids. Following treatment with β-ERP (6 µg), a significant increase in the number of spermatogonia-A was observed, whereas the numbers of all the other germ cells, excluding spermatogonia-B, significantly decreased in comparison to those in the control group. In addition, treatment of fish with 6 µg β-ERP resulted in a significant reduction in the dimensions of the lumen and seminiferous lobules, the level of immunopositive androgen receptor (AR) expression in Sertoli cells, and the percentage of luteinizing hormone (LH) immunolabeled in the pituitary compared to those in the control group or the group treated with 3 µg β-ERP. In contrast, the intensity of AR immunoreactivity and the percentage of LH immunolabeling were substantially increased in fish treated with 3 µg β-ERP compared to those in the control group. These findings reveal for the first time that a low dose of β-ERP stimulates the recruitment of spermatogonia as well as spermateleosis, whereas a high concentration affects the recruitment of germ cells prior to meiotic division in tilapia. These results suggest that β-ERP exerts modulatory effects at the testicular and hypophysial levels through alterations in AR expression and LH secretory activity, respectively, in teleosts.

The stress - Reproductive axis in fish: The involvement of functional neuroanatomical systems in the brain

The neuroendocrine-stress axis of nonmammalian species is evolutionarily conserved, which makes them useful to serve as important model systems for elucidating the function of the vertebrate stress response. The involvement of hypothalamo-pituitary-adrenal (HPA) axis hormones in regulation of stress and reproduction is well described in different vertebrates. However, the stress response is a complex process, which appears to be controlled by a number of neurochemicals in association with hypothalamo-pituitary-interrenal (HPI) axis or independent of HPI axis in fish. In recent years, the participation of neurohormones other than HPI axis in regulation of stress and reproduction is gaining more attention. This review mainly focuses on the involvement of functional neuroanatomical systems such as the catecholaminergic neurotransmitter dopamine (DA) and opioid peptides in regulation of the stress-reproductive axis in fish. Occurrences of DA and opioid peptides like β-endorphin, enkephalins, dynorphin, and endomorphins have been demonstrated in fish brain, and diverse roles such as pain modulation, social behaviour and reproduction are implicated for these hormones. Neuroanatomical studies using retrograde tracing, immunohistochemical staining and lesion methods have demonstrated that the neurons originating in the preoptic region and the nucleus lateralis tuberis directly innervate the pituitary gland and, therefore, the hypophysiotrophic role of these hormones. In addition, heightened synthetic and secretory activity of the opioidergic and the dopaminergic neurons in hypothalamic areas of the brain during stress exposure suggest potentially intricate relationship with the stress-reproductive axis in fish. Current evidence in early vertebrates like fish provides a novel insight into the underlying neuroendocrine mechanisms as additional pathways along the stress-reproductive axis that seem to be conserved during the course of evolution.

Daily expression of a clock gene in the brain and pituitary of the Malabar grouper (Epinephelus malabaricus)

Recent studies have revealed that, in addition to regulating the circadian system, clock genes such as cryptochrome (Cry) genes are involved in seasonal and lunar rhythmicity in fish. This study clarified the transcriptional characteristics of a Cry subtype (mgCry2) in the brain of the Malabar grouper, Epinephelus malabaricus, which is an important aquaculture species that spawns around the new moon. The cDNA sequence of mgCry2 showed high identity (97-99%) with fish Cry2 and had an open reading frame encoding a protein with 170 amino acids. Phylogenetic analyses revealed that mgCRY2 had high identity with CRY in other fish species. Real-time quantitative polymerase chain reaction (qPCR) showed the widespread distribution of mgCry2 in neural (brain, pituitary, and retina) and peripheral (heart, liver, kidney, spleen, gill, intestine, and ovary) tissues. When immature Malabar groupers were reared under a light-dark cycle (LD = 12:12) and the amounts of mgCry2 mRNA in the telencephalon and diencephalon were measured at 4-h intervals, the levels increased during photophase and decreased during scotophase. Day-night variation in mgCry2 mRNA abundance was also observed in the pituitary. These daily profiles suggest that mgCry2 is a light-responsive gene in neural tissues. In situ hybridization analyses showed that mgCry2 was strongly transcribed in the nucleus lateralis tuberis of the ventral hypothalamus, peripheral area of the proximal pars distalis, and the pars intermedia of the pituitary. We conclude that clock genes expressed in the pituitary and diencephalon play a role in entraining the endocrine network of the Malabar grouper to periodic changes in external cues.

Interaction between dopamine and neuropeptide Y in the telencephalon of the Indian major carp, Cirrhinus cirrhosus

In teleosts, while neuropeptide Y (NPY) has emerged as one of the potent regulators of GnRH-LH axis, entopeduncular nucleus (EN) in the ventral telencephalon serves as major site for NPY synthesis/storage. Neurons of the EN innervate preoptic area and pituitary, respond to gonadal steroids, undergo reproduction phase-related changes, and are believed to convey sex steroid-borne information to GnRH neurons. In spite of the importance of EN, the neural circuitry associated with the nucleus has not been defined. Aim of the present study is to examine the possibility of the dopaminergic regulation of EN. NPY-immunoreactive cells and fibers were extensively distributed in the forebrain and pituitary of Cirrhinus cirrhosus. NPY immunoreactivity was observed in the olfactory receptor neurons, ganglion cells of terminal nerve, and in neurons of area ventralis telencephali/pars lateralis, EN, nucleus preopticus periventricularis (NPP), and nucleus lateralis tuberis. NPY-fibers were observed in the dorsal telencephalon, tuberal area and pituitary. While the area ventralis telencephali/pars intermedialis (Vi) located just above the EN contained a distinct population of tyrosine hydroxylase neurons, their axons seem to innervate NPY neurons in EN. Superfused brain slices containing EN were treated with DA D1- and D2-like receptor agonists. NPY-immunoreactivity in the EN showed significant increase (P<0.001) following DA D1-like receptor agonist, SKF-38393 treatment, but DA D2-like receptor agonist, quinpirole was ineffective. DA may regulate NPY neurons in EN via D1-like receptors. DA-NPY interaction in the EN might be important in the central regulation of reproduction in teleosts.

Glucocorticoid synthesis inhibitor metyrapone blocks stress-induced suppression along luteinizing hormone secreting cells–ovary axis in the fish Oreochromis mossambicus

We showed previously that exposure to mild acute stressors leads to inhibition of follicular development and luteinizing hormone (LH) secretion in tilapia. In this study, we examined whether the hypothalamo–pituitary–interrenal axis was involved in such inhibition. Administration (i.p.) of corticotropin-releasing hormone (CRH) to stripped Oreochromis mossambicus (eggs manually removed from mouth brooder) during the ovarian cycle for 22 days resulted in a significant increase in the serum levels of cortisol, and significantly lower gonadosomatic and hepatosomatic indices concomitant with complete absence of stage V (vitellogenic) follicles in the ovary compared to controls. Furthermore, the LH secreting cells at the proximal pars distalis (PPD) in the pituitary gland showed weak immunostaining in contrast to the intensely stained immunoreactive cells in controls during prespawning phase. On the other hand, while exposure of fish to aquacultural stressors produced effects similar to that of CRH treatment, treatment of glucocorticoid synthesis inhibitor metyrapone to stressed fish during the ovarian cycle did not show significant serum cortisol response. The LH secreting cells in these fish showed intense immunostaining at the PPD in the pituitary gland, and the ovary contained stage V follicles similar to that of controls prior to spawning phase. These results suggest that the inhibitory effects of CRH treatment on LH secretion and recruitment of follicles for vitellogenic growth are mediated through the stress hormone cortisol in O. mossambicus.

Mu opioid receptor in spermatozoa, eggs and larvae of gilthead sea bream (Sparus Aurata) and its involvement in stress related to aquaculture

In aquaculture, unfavourable conditions experienced during early development may have strong downstream effects on the adult phenotype and fitness. Sensitivity to stress, leading to disease, reduced growth and mortality, is higher in larvae than in adult fish. In this study, conducted on sea bream (Sparus aurata), we evidenced the presence of the mu opioid receptor in gametes and larvae at different developmental stages. Moreover, we evaluated the possibility of reducing the effects of artificially produced stress, altering temperature, salinity and pH, by naloxone (an opioid antagonist) and calcium. Results evidenced that mu opioid receptor is present in larvae and in gametes of both sexes and that, during larval growth, its expression level changes accordingly; furthermore, naloxone/calcium association is efficacious in increasing the survival period of treated larvae compared to controls. We conclude that in sea bream rearing, the use of naloxone/calcium against stress can improve fish farming techniques by reducing larval mortality and consequently increasing productivity.

Tyrosine hydroxylase in the olfactory system, forebrain and pituitary of the Indian major carp, Cirrhinus cirrhosus: organisation and interaction with neuropeptide Y in the preoptic area

Dopamine (DA) inhibits, whereas gonadotrophin-releasing hormone (GnRH) stimulates, luteinisiing (LH) cells in the pituitary of some but not all teleosts. A reduction in the hypophysiotropic dopaminergic tone is necessary for the stimulatory effect of GnRH on LH cells. Neuropeptide Y (NPY) has emerged as one of the potent, endogenous agent that modulates LH secretion directly or indirectly via GnRH. Involvement of NPY in the regulation of hypophysiotropic DA neurones, however, is not known, but there is good evidence suggesting an interaction in the mammalian hypothalamus. DA neurones, identified by tyrosine hydroxylase (TH)-immunoreactivity, were observed widely throughout the brain of the Indian major carp, Cirrhinus cirrhosus. The granule cells and ganglion cells of terminal nerve in the olfactory bulb, and cells in ventral telencephalon and preoptic area (POA) showed conspicuous TH immunoreactivity. In the POA, the nucleus preopticus periventricularis (NPP), divisible into anterior (NPPa) and posterior (NPPp) components, showed prominent TH-immunoreactivity. The majority of TH neurones in NPPa showed axonal extensions to the pituitary and were closely associated with LH cells. The NPPa also appeared to be the site for intense interaction between NPY and DA because it contains a rich network of NPY fibres and few immunoreactive cells. Approximately 89.7 ± 1.5% TH neurones in NPPa were contacted by NPY fibres. Superfused POA slices treated with a NPY Y2 -receptor agonist, NPY 13-36 resulted in a significant (P < 0.001) reduction in TH-immunoreactivity in NPPa. TH neurones in NPPa did not respond to NPY Y1 -receptor agonist, [Leu(31) , Pro(34) ] Neuropeptide Y treatment. We suggest that, by inhibiting DAergic neurones in NPPa via Y2 -receptors, NPY may contribute to the up-regulation of the GnRH-LH cells axis. The microcircuitry of DA and NPY and their interaction in NPPa might be a crucial component in the central regulation of LH secretion in the teleosts.

Naltrexone attenuates stress-induced suppression of LH secretion in the pituitary gland in the Cichlid fish Oreochromis mossambicus: evidence for the opioidergic mediation of reproductive stress response

Opioid peptide β-endorphin (β-EP) plays a modulatory role in vertebrate reproduction. However, the role of opioid peptides in reproductive stress response is least understood in fishes. The aim of the present study was to determine the effect of different doses of β-EP on luteinizing hormone (LH) secretion in normal and the opioid receptor antagonist naltrexone (NALT) in stressed female tilapia Oreochromis mossambicus. Administration of 4 μg β-EP, but not 0.5 or 1.5 μg β-EP, daily for 22 days caused suppression of LH-secreting cells at the proximal pars distalis of the pituitary gland, concomitant with a significant reduction in the mean GSI and HSI in 4 μg β-EP-treated fish compared to controls. On the other hand, exposure of the fish to mild acute stressors for 22 days caused changes in the LH-secreting cells similar to that of high dose of β-EP, whereas administration of NALT attenuated these effects. Taken together, the results indicate that increased concentration of β-EP as may occur during stressful conditions can cause suppression of LH secretion, leading to the inhibition of spawning, and that treatment of NALT attenuates the stress-induced inhibition of LH secretion in fish.

Stress-induced inhibition of recruitment of ovarian follicles for vitellogenic growth and interruption of spawning cycle in the fish Oreochromis mossambicus

The tilapia, Oreochromis mossambicus, shows a short ovarian cycle of 24-26 days in nonmouthbrooding condition. In this study, the stripped female O. mossambicus were exposed to repeated mild acute stressors such as handling, chasing, frequent netting and low water levels daily for a period of 26 days. The follicular dynamics did not show significant difference during previtellogenic phase (day 12), whereas the mean number of stage IV (vitellogenic) follicles remained significantly lower compared with controls at the end of vitellogenic phase (day 18). The stage V (vitellogenic, preovulatory) follicles were completely absent in contrast to their presence in controls prior to spawning (day 23). The control fish spawned spontaneously after 24 days and entered mouthbrooding phase, whereas those exposed to stressors did not spawn. Furthermore, the serum levels of estradiol (E(2)) remained significantly lower concomitant with a significant increase in the serum cortisol concentration during vitellogenic and prespawning phase compared with those of the controls. The LH cells in the PPD of the pituitary gland showed weak immunoreactivity through vitellogenic and prespawning phase in fish exposed to stressors indicating the diminished secretory activity in contrast to the intensely stained ir-material in controls. The study reveals the disruptive effects of aquacultural stressors on the spawning cycle through suppression of LH and E(2) secretion along the pituitary-ovary axis. The results suggest that the ovarian stress response depends on the phase of the cycle and that the interruption of the spawning cycle is due to inhibition of recruitment of preovulatory follicles in O. mossambicus.

New insights into the neuroanatomical distribution and phylogeny of opioids and POMC-derived peptides in fish

This review re-evaluates the use of immunological probes to map enkephalinergic, dynorphinergic, and endorphinergic circuits in the CNS of lobe-finned fishes, ray-finned fishes, and cartilaginous fishes in light of the characterization of proenkephalin, prodynorphin, and POMC sequences from representatives of these groups of fish over the past 20 years. The use of α-MSH specific antisera is a reliable method for detecting POMC immunopositive cell bodies and fibers. Since α-MSH and β-endorphin are co-localized in the same neurons, these studies also reveal the distribution of endorphinergic networks. Met-enkephalin specific antisera can be used to detect enkephalinergic circuits in the CNS of gnathostomes because of the ubiquitous presence of this pentapeptide in the proenkephalin sequences of gnathostomes. However, the use of leu-enkephalin specific antisera to detect enkephalinergic networks is more problematic. While this immunological probe is appropriate for analyzing enkephalinergic networks in mammals and perhaps teleosts, for the lungfishes and cartilaginous fishes this probe is more likely able to detect dynorphinergic circuits. In this regard, there is a need to re-examine dynorphinergic networks in non-mammalian gnathostomes by using species specific antisera directed against dynorphin end-products.

Cocaine- and amphetamine-regulated transcript peptide (CART) in the telencephalon of the catfish, Clarias gariepinus: distribution and response to fasting, 2-deoxy-D-glucose, glucose, insulin, and leptin treatments

The cocaine- and amphetamine-regulated transcript peptide (CART)-containing system in the forebrain of Clarias gariepinus was studied with immunocytochemistry. While the immunoreactivity was prominently seen in the neurons of the entopeduncular nucleus (EN) located in the ventral telencephalon, CART-immunoreactive fibers were widely distributed in the dorsal and ventral telencephalon. In view of the established role of CART in energy metabolism, we investigated the response of the CART immunoreactive system to positive and negative nutritional conditions. Neurons of the EN and fibers in the different areas of the telencephalon showed significant reduction in CART immunoreactivity following 48 hours food deprivation, or 2 hours following intracranial administration of 2-deoxy-D-glucose (2DG, 100 ng/g body weight, a metabolic antagonist of glucose). However, intracranial injection of glucose (100 ng/g body weight) resulted in a distinct increase in CART immunoreactivity in these components. In mammals, insulin and leptin have been recognized as adiposity agents that convey peripheral energy status-related information to brain. Intracranial administration of insulin (3 mU/fish) and leptin (10 ng/g body weight) significantly increased CART immunoreactivity in the EN neurons and in the fiber network within 2 hours. Superfusion of the EN-containing tissue fragments in the medium enriched in glucose, insulin, or leptin evoked a significant increase in CART immunoreactivity in the EN neurons, but 2DG reduced the immunoreactivity. We suggest that CART-containing neurons of the EN, and fibers in the telencephalon, may process the energy status-related information and contribute to satiety.

Expression of the mu opioid receptor and effects of the opioid antagonist naloxone on in vitro maturation of oocytes recovered from anoestrous bitches

The mu-opioid receptor (MOR) is expressed in bovine, human, equine and canine oocytes, and in seasonal breeders, it is expressed with higher intensity during the anoestrous phase. Supplementation of in vitro maturation (IVM) medium with opioid agents, agonists or antagonists, was shown to affect oocyte maturation in several species such as rat, bovine and equine. This study reports the effects of supplementing IVM medium with naloxone (Nx), an opioid antagonist, on nuclear and cytoplasmic maturation rate of oocytes recovered from anoestrous bitches. Cytoplasmic maturation was examined in terms of mitochondrial (mt) distribution. In order to confirm the receptor-mediated action of Nx, in oocytes of anoestrous bitches, MOR expression was analyzed by Western blot. Cumulus-oocyte complexes, recovered from the ovaries of bitches in anoestrous, were cultured in vitro and Nx was added at the concentrations of 1 x 10(-6), 1 x 10(-8) and 1 x 10(-10) M. The rate of oocytes resuming meiosis after culture in presence of 1 x 10(-6) M Nx (29%) was significantly higher than that of oocytes of control group (12%; p < 0.05). However, treatment with Nx did not affect mt distribution pattern. In denuded oocytes and in corresponding cumulus cells, a doublet of 65 and 50 kDa was observed. We conclude that, in oocytes of anoestrous bitches, MOR is expressed and Nx significantly improves nuclear maturation rate. Further studies should be performed to elucidate the expression of other opioid receptors, such as delta and kappa, and possible interactive effects of their antagonists on canine oocyte maturation.

Expression and localization of the mu-opioid receptor (MOR) in the equine cumulus-oocyte complex and its involvement in the seasonal regulation of oocyte meiotic competence

The micro-opioid receptor (MOR) was identified in equine oocytes, cumulus and granulosa cells. By RT-PCR, a 441bp fragment was observed. By immunoblotting, a 65 kDa band was detected in samples of winter anestrous whereas in cells recovered in breeding season, two bands, 65 and 50 kDa, were found. The 65 kDa band was significantly more intense in winter anestrous specimens. In samples recovered in the breeding season, this band significantly decreased with the raise of follicle size and was heavier in compact oocytes and cumulus cells. The protein was localized on the oolemma and within the cytoplasm of oocytes and cumulus cells. In vitro oocyte maturation rate (MR), analyzed by confocal microscopy for nuclear chromatin, microfilaments and microtubules, was reduced after the addition of 3 x 10(-8) M beta-endorphin in medium without additional hormones. Inhibitory effects of 10(-3) M Naloxone in oocytes collected in anestrous and spring transition were observed, both in presence and absence of hormones added to culture medium. Increased MRs were observed in oocytes collected in anestrous and cultured in presence of 10(-8) M Naloxone. The exposure to 10(-3) M Naloxone induced significant intracellular calcium increases in cumulus cells recovered all over the year. beta-Endorphin 3 x 10(-8) M induced significant calcium increases only in cumulus cells recovered in fall transition and anestrous. Naloxone 10(-8) M did not induce intracellular calcium modifications. We conclude that the MOR is differentially expressed in equine cumulus-oocyte complexes in the different seasons of the year and plays a role in the seasonal regulation of meiotic competence of equine oocytes.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Reproduction phase-related variations in neuropeptide Y immunoreactivity in the olfactory system, forebrain, and pituitary of the female catfish,Clarias batrachus (Linn.)

The aim of this study was to determine whether neuropeptide Y (NPY) immunoreactivity in the cells and fibers in the forebrain and pituitary of Clarias batrachus is linked to the annual reproductive cycle. A steady rise in luteinizing hormone (LH) immunoreactivity was seen in the pituitary through preparatory (February-April) and prespawning (May-June) phases; it was greatly reduced during spawning (July-August; P < 0.001) and partially replenished during postspawning (September-November; P < 0.01) through resting (December-January) phases. Although NPY immunoreactivity in olfactory receptor neurons and olfactory nerve layer in olfactory bulb was gradually augmented during resting through prespawning phases (P < 0.001), attaining a peak in spawning phase (P < 0.001), a dramatic decline was encountered during postspawning phase (P < 0.001). A similar pattern was also observed in NPY-containing fibers of the medial olfactory tract (MOT) and pituitary. However, a different pattern of NPY immunoreactivity was observed in the neurons of nucleus entopeduncularis (NE) and nucleus preopticus periventricularis (NPP). Whereas these neurons and fibers in the forebrain showed significant augmentation during the resting through prespawning phases (P < 0.001), the immunoreactivity dramatically declined during spawning (P < 0.001) and was partially replenished in the postspawning phase. Testosterone injection of juveniles significantly augmented (P < 0.001) NPY immunoreactivity in NE neurons. We suggest that NPY cells of NE and NPP, and related fiber systems, might be involved in processing of sex steroid-borne information and regulation of the gonadotropin-releasing hormone-LH axis.

Immunohistochemical localization of cocaine- and amphetamine-regulated transcript peptide in the brain of the catfish,Clarias batrachus (Linn.)

The organization of cocaine- and amphetamine-regulated transcript peptide (CARTp, 54-102) immunoreactivity was investigated in the brain of the catfish, Clarias batrachus. CARTp-immunoreactivity was observed in several granule cells of the olfactory bulbs, in dot-like terminals around mitral cells, and in the fibers of the medial olfactory tracts. While several groups of discrete cells in the telencephalon showed CARTp-immunoreactivity, the immunostained fibers were widely distributed in the area dorsalis and ventralis telencephali. Immunoreactivity was seen in several periventricular and a few magnocellular neurons, and in a dense fiber network throughout the preoptic area. Varying degrees of immunoreactive fibers were seen in the periventricular region in the thalamus, hypothalamus, and pituitary. Some neurons in the nucleus preglomerulosus medialis and lateralis, central nucleus of the inferior lobes, nucleus lobobulbaris of the posterior tuberculum, and nucleus recessus posterioris showed distinct CARTp-immunoreactivity. Considerable immunoreactivity was seen in the optic tectum, rostral torus semicircularis, central pretectal area, and granule cells of the cerebellum. While only isolated immunoreactive cells were seen at three distinct sites in the metencephalon, a fiber network was seen in the facial and vagal lobes and periventricular and ventral regions of the medulla oblongata. The pattern of the CARTp distribution in the brain of C. batrachus suggests that it may play an important role in the processing of sensory information, the regulation of hormone secretion by hypophysial cell types, and motor and vegetative function. Finally, as in other animal species, CARTp seems to play a role in the processing of gustatory information.

Involvement of neuropeptide Y Y1 receptors in the regulation of LH and GH cells in the pituitary of the catfish, Clarias batrachus: an immunocytochemical study

Although neuropeptide Y (NPY) has been known to influence the release of luteinizing hormone (LH) and growth hormone (GH) from the pituitary gland of teleosts, the NPY receptor subtypes involved in the regulatory processes have not been fully defined. An attempt has been made to study the involvement of NPY Y1 receptors, if any, in mediating the NPY-triggered stimulation of the LH and GH secreting cells in the pituitary of the catfish, Clarias batrachus. NPY (10 ng/g of body wt) or NPY Y1 receptor agonist (Leu(31)-Pro(34)-NPY, 3 ng/g of body wt) were administered by the intracranial route and the responses by the LH and GH cells in the pituitary were investigated with the help of immunocytochemistry. Both the agents caused a highly significant decrease (P<0.001) in the immunoreactivity of LH cells. However, the treatment with NPY Y1 receptor antagonist (BIBP 3226, 1 ng/g of body wt), prior to NPY or NPY Y1 agonist, blocked the response by the LH cells; the profile of the cells was quite similar to that of the saline-injected control fish. GH cells also showed similar pattern of responses to these treatments. While NPY and NPY Y1 receptor agonist caused significant (P<0.001) decrease in the GH immunoreactivity, pretreatment with the NPY Y1 antagonist blocked the response. These results suggest that NPY may exercise a secretogogue-like action on the LH and GH cells in the pituitary of C. batrachus via NPY Y1 receptors.