Ultrastructural colocalization of vesicular cholecystokinin and corticoliberin in the periportal nerve terminals of the rat median eminence

Immunocytochemical detection of cholecystokinin and corticotrophin-releasing hormone neuropeptides in the hypothalamic paraventricular nucleus of the jerboa (Jaculus orientalis): modulation by immobilisation stress

The hypothalamic response to an environmental stress implicates the corticotrophin-releasing hormone (CRH) neuroendocrine system of the hypothalamic parvicellular paraventricular nucleus (PVN) in addition to other neuropeptides coexpressed within CRH neurones and controlling the hypothalamo-pituitary-adrenal (HPA) axis activity as well. Such neuropeptides are vasopressin, neurotensin and cholecystokinin (CCK). It has previously been demonstrated that the majority of the CRH neuronal population coexpresses CCK after a peripheral stress in rats. In the present study, we explored such neuroendocrine plasticity in the jerboa in captivity as another animal model. In particular, we studied CCK and CRH expression within the hypothalamic PVN by immunocytochemistry in control versus acute immobilisation stress-submitted jerboas. The results show that CCK- and CRH-immunoreactive neuronal systems are located in the hypothalamic parvicellular PVN. The number of CCK-immunoreactive neurones within the PVN was significantly increased (138% increase) in stressed animals compared to controls. Similarly, the number of CRH-containing neurones was higher in stressed jerboas (128%) compared to controls. These results suggest that the neurogenic stress caused by immobilisation stimulates CCK as well as CRH expression in jerboas, which correlates well with previous data obtained in rats using other stressors. The data obtained also suggest that, in addition to CRH, CCK is another neuropeptide involved in the response to stress in jerboa, acting by controlling HPA axis activity. Because CCK is involved in the phenotypical plasticity of CRH-containing neurones in response to an environmental stress, we also explored their coexpression by double immunocytochemistry within the PVN and the median eminence (i.e. the site of CRH and CCK corelease in the rat) following jerboa immobilisation. The results show that CCK is not coexpressed within CRH neurones in either control or stressed jerboa, suggesting differences between jerboas and rats in the neuroendocrine regulatory mechanisms of the stress response involving CRH and CCK. The adaptative physiological mechanisms to environmental conditions might vary from one mammal species to another.

Neuropeptides as synaptic transmitters

Neuropeptides are small protein molecules (composed of 3-100 amino-acid residues) that have been localized to discrete cell populations of central and peripheral neurons. In most instances, they coexist with low-molecular-weight neurotransmitters within the same neurons. At the subcellular level, neuropeptides are selectively stored, singularly or more frequently in combinations, within large granular vesicles. Release occurs through mechanisms different from classical calcium-dependent exocytosis at the synaptic cleft, and thus they account for slow synaptic and/or non-synaptic communication in neurons. Neuropeptide co-storage and coexistence can be observed throughout the central nervous system and are responsible for a series of functional interactions that occur at both pre- and post-synaptic levels. Thus, the subcellular site(s) of storage and sorting mechanisms into different neuronal compartments are crucial to the mode of release and the function of neuropeptides as neuronal messengers.

Glucocorticoids down-regulate lipopolysaccharide-induced de novo production of neurotensin mRNA in the rat hypothalamic, paraventricular, corticotrophin-releasing hormone neurons

OBJECTIVE

Intraperitoneal injection of the endotoxin lipopolysaccharide (LPS) produces inflammation accompanied by activation of the immune system and the secretion of cytokines. Cytokines stimulate the hypothalamo-pituitary-adrenal (HPA) axis to release the anti-inflammatory corticosterone which controls its own production by acting on the HPA axis. Upstream in the HPA axis are neuroendocrine corticotrophin-releasing hormone (CRH) neurons located in the paraventricular nucleus (PVN), whose multipeptidergic phenotype changes during inflammation: while CRH mRNA is up-regulated in these conditions, neurotensin (NT) mRNA expression is induced de novo. The negative feedback control of glucocorticoids on CRH production is well documented; however, their action on NT production in the PVN of the hypothalamus is poorly documented. The aim of this study was to determine if glucocorticoids modulate the de novo production of NT during inflammation.

METHODS

Using quantitative in situ hybridization histochemistry, we examined whether the absence (adrenalectomy) or excess (corticosterone implants) of glucocorticoids modulate de novo production of NT mRNA in the PVN during inflammation induced by LPS treatment.

RESULTS

A relatively low dose of LPS (50 microg/kg) that is not efficient to induce NT mRNA production in the PVN becomes efficient after adrenalectomy. Moreover, corticosterone excess reduces LPS-induced production of NT mRNA in the PVN.

CONCLUSION

Glucocorticoids exert a negative control on NT mRNA production in the PVN of the hypothalamus, and this effect requires that NT mRNA production be triggered, such as during inflammation.

Vasopressin-containing neurons of the hypothalamic parvocellular paraventricular nucleus of the jerboa: plasticity related to immobilization stress

The corticotropin-releasing hormone (CRH) neurons of the hypothalamic parvocellular paraventricular nucleus (PVN) have a high potential for phenotypical plasticity, allowing them to rapidly modify their neuroendocrine output, depending upon the type of stressors. Indeed, these neurons coexpress other neuropeptides, such as cholecystokinin (CCK), vasopressin (VP), and neurotensin, subserving an eventual complementary function to CRH in the regulation of the pituitary. Unlike in rats, our previous data showed that in jerboas, CCK is not coexpressed within CRH neurons in control as well as stressed animals. The present study explored an eventual VP participation in the phenotypic plasticity of CRH neurons in the jerboa. We analyzed the VP expression within the PVN by immunocytochemistry in male jerboas submitted to acute stress. Our results showed that, contrary to CRH and CCK, no significant change concerned the number of VP-immunoreactive neurons following a 30-min immobilization. The VP/CRH coexpression within PVN and median eminence was investigated by double immunocytochemistry. In control as well as stressed animals, the CRH-immunopositive neurons coexpressed VP within cell bodies and terminals. No significant difference in the number of VP/CRH double-labeled cells was found between both groups. However, such coexpression was quantitatively more important into the posterior PVN as compared with the anterior PVN. This suggests an eventual autocrine/paracrine or endocrine role for jerboa parvocellular VP which is not correlated with acute immobilization stress. VP-immunoreactive neurons also coexpressed CCK within PVN and median eminence of control and stressed jerboas. Such coexpression was more important into the anterior PVN as compared with the posterior PVN. These results showed the occurrence of at least two VP neuronal populations within the jerboa PVN. In addition, the VP expression did not depend upon acute immobilization stress. These data highlight differences in the neuroendocrine regulatory mechanisms of the stress response involving CRH/CCK or VP. They also underline that adaptative physiological mechanisms to stress might vary from one mammal species to another.

Costorage and coexistence of neuropeptides in the mammalian CNS

The term neuropeptides commonly refers to a relatively large number of biologically active molecules that have been localized to discrete cell populations of central and peripheral neurons. I review here the most important histological and functional findings on neuropeptide distribution in the central nervous system (CNS), in relation to their role in the exchange of information between the nerve cells. Under this perspective, peptide costorage (presence of two or more peptides within the same subcellular compartment) and coexistence (concurrent presence of peptides and other messenger molecules within single nerve cells) are discussed in detail. In particular, the subcellular site(s) of storage and sorting mechanisms within neurons are thoroughly examined in the view of the mode of release and action of neuropeptides as neuronal messengers. Moreover, the relationship of neuropeptides and other molecules implicated in neural transmission is discussed in functional terms, also referring to the interactions with novel unconventional transmitters and trophic factors. Finally, a brief account is given on the presence of neuropeptides in glial cells.

CCK mRNA expression in neuroendocrine CRH neurons is increased in rats subjected to an immune challenge

The expression of cholecystokinin (CCK) mRNA in neuroendocrine corticotropin-releasing hormone (CRH) neurons of the hypothalamic paraventricular nucleus (PVN) of male rats was examined 8 h following an acute immune challenge by intraperitoneal lipopolysaccharide (LPS, 250 microg/kg). Both quantitative, macroautoradiographic, single-label radioactive in situ hybridization histochemistry (ISHH) and qualitative dual-label ISHH were performed. Compared to controls, LPS-injected rats displayed increased (185%) parvicellular CCK mRNA expression levels, occurring in a majority (70%) of CRH neurons as revealed by dual-label ISHH.

Phenotypical segregation among female rat hypothalamic gonadotropin-releasing hormone neurons as revealed by the sexually dimorphic coexpression of cholecystokinin and neurotensin

The neuroendocrine control of the gonad is exerted primarily by the gonadotropin-releasing hormone neurons located in the septum and the hypothalamus. Despite their sexually dimorphic activity, tonic in males and phasic in females, these neurons have not appeared qualitatively different between sexes in intrinsic organization or chemical phenotype. Here, by using multiple-label immunocytochemistry, it is demonstrated that the phenotype of gonadotropin-releasing hormone neurons is sex specific. In females only, 54.5% of them co-expressed cholecystokinin immunoreactivity and 29.4% additionally expressed neurotensin immunoreactivity. These multipeptidergic neurons were observed in the hypothalamus but not in the septum. During postnatal development, cholecystokinin and neurotensin immunoreactivities were first detected in gonadotropin-releasing hormone-containing axons of the median eminence at vaginal opening, suggesting an involvement of the neuropeptides in peri-ovulatory events. This peptidergic phenotype was not apparent in females ovariectomized as adults but was reinstated by estradiol treatment. In adult males, the testicle does not control this phenotype because orchidectomized adults did not display it, whatever the post-operative delay (one to five weeks) or substitutive chronic steroid treatment (testosterone or estradiol). The testicle may, however, masculinize the phenotype neonatally because estradiol or testosterone treatment in adulthood induced an expression of cholecystokinin immunoreactivity in gonadotropin-releasing hormone-containing axons of the median eminence in both males and females that were gonadectomized at birth. This procedure, however, failed to significantly induce an expression of neurotensin immunoreactivity, suggesting a role of the postnatal ovary on this element of the chemistry of gonadotropin-releasing hormone neurons.Thus, the gonad permanently organizes the gonadotropin-releasing hormone neuronal population, resulting, at least in females, in a mosaic of phenotypically distinct, functional subunits.

Prolactin: structure, function, and regulation of secretion

Prolactin is a protein hormone of the anterior pituitary gland that was originally named for its ability to promote lactation in response to the suckling stimulus of hungry young mammals. We now know that prolactin is not as simple as originally described. Indeed, chemically, prolactin appears in a multiplicity of posttranslational forms ranging from size variants to chemical modifications such as phosphorylation or glycosylation. It is not only synthesized in the pituitary gland, as originally described, but also within the central nervous system, the immune system, the uterus and its associated tissues of conception, and even the mammary gland itself. Moreover, its biological actions are not limited solely to reproduction because it has been shown to control a variety of behaviors and even play a role in homeostasis. Prolactin-releasing stimuli not only include the nursing stimulus, but light, audition, olfaction, and stress can serve a stimulatory role. Finally, although it is well known that dopamine of hypothalamic origin provides inhibitory control over the secretion of prolactin, other factors within the brain, pituitary gland, and peripheral organs have been shown to inhibit or stimulate prolactin secretion as well. It is the purpose of this review to provide a comprehensive survey of our current understanding of prolactin's function and its regulation and to expose some of the controversies still existing.

Glucocorticosteroids up-regulate the expression of cholecystokinin mRNA in the rat paraventricular nucleus

Adrenalectomy abolishes corticosteroid feedback onto the hypothalamic-pituitary-adrenal axis. This results in an increased biosynthetic and secretory activity of corticotropin-releasing hormone (CRH) neurons of the hypothalamic paraventricular nucleus (PVN), sustained in the absence of hormone replacement. In the PVN, cholecystokinin (CCK) is present both in parvicellular CRH-containing and in magnocellular oxytocin (OXY)-containing neurons. We presently studied the glucocorticoid feedback regulation of the expression of cholecystokinin (CCK) mRNA in rats after: (i) adrenalectomy, (ii) sham surgery or (iii) adrenalectomy with corticosterone replacement. Using 35S-labeled CRH and p-CCK cRNA probes and in situ hybridization, CRH and CCK mRNAs were radiolabeled. The total amount of hybridization labeling (integrated density), was quantified in adjacent series of cryosections regularly spaced throughout the PVN. The OXY mRNA detection served to identify PVN magnocellular areas. Adrenalectomy was shown to induce: (i) a 75% increase in CRH mRNA labeling in the PVN, (ii) a concomitant 43% decrease in CCK mRNA labeling but only in the anterior part of the PVN and occurring both in CCK/CRH area (two thirds of it) and CCK/OXY area (one third of it) and (iii) that they were fully reversed by corticosterone replacement. Thus, glucocorticoids that are well known to negatively feedback on CRH expression in parvicellular PVN neurons are also capable of positively regulating CCK expression in anterior PVN neurons, both in parvicellular and magnocellular areas.

Decrease in phorbol ester-induced potentiation of noradrenaline release in synapsin I-deficient mice

Synapsin I is involved in regulating amino acid neurotransmitter release, but has a less clear role in noradrenergic nerve terminals. To better understand the role of synapsin I in the function of noradrenergic nerve terminals, we compared noradrenaline release in wild-type and synapsin I-deficient mice. No difference was found in the accumulation or in the Ca(2+)-independent release of [(3)H]noradrenaline in cerebrocortical synaptosomes from wild-type and synapsin I-deficient mice. Synaptosomes lacking synapsin I also displayed no gross alterations in either the time course or the Ca(2+)-dependency of [(3)H]noradrenaline release when stimulated by depolarizing secretagogues or ionophore treatment. In wild-type synaptosomes, activation of protein kinase C by phorbol ester treatment resulted in a Ca(2+)-dependent increase in [(3)H]noradrenaline release evoked by depolarizing secretagogues and ionophore treatment. The phorbol ester-mediated enhancement of [(3)H]noradrenaline release evoked by depolarizing secretagogues, but not by ionophore treatment, was greatly reduced in synapsin I-deficient synaptosomes. These results indicate that synapsin I plays a role in regulating noradrenaline release.

Increased adrenocorticotropin responses to acute stress in Otsuka Long-Evans Tokushima Fatty (type 2 diabetic) rats

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat is a new diabetic strain of rats whose disease closely resembles human type 2 diabetes. We measured plasma adrenocorticotropic hormone (ACTH) and corticostrone levels, and iodine-125-labeled ovine corticotropin-releasing factor ([125I]oCRF) binding in the anterior pituitary after ether-laparotomy stress in OLETF rats to examine the alteration of the hypothalamic-pituitary-adrenal (HPA) axis. In addition, we examined ACTH secretion following CRF administration in vivo and in vitro to characterize the mechanisms regulating the HPA axis in OLETF rats. Body weight, plasma glucose and insulin levels in OLETF rats were significantly higher than that in Long-Evans Tokushima Otsuka (LETO) rats. Basal plasma ACTH levels tended to be higher in OLETF rats than in LETO but it did not reach statistical significance. Ether-laparotomy stress dramatically increased plasma ACTH levels at 2 h after the stress both in either OLETF and LETO rats; the peak plasma ACTH level in OLETF rats following the stress was significantly greater than in LETO rats. Plasma ACTH levels following CRF (2 microg/kg, i.v.) in OLETF and LETO rats showed statistically significant increases at 10 and 30 min after CRF administration compared to ACTH levels at 0 min, however, the peak plasma ACTH level in OLETF rats at 10 min after CRF administration was significantly greater than in LETO rats. In contrast to ACTH levels, no significant differences in corticosterone levels between OLETF and LETO were observed at any of the time points. CRF (10 ng/ml) significantly increased ACTH secretion in pituitary cultures from OLETF compared to LETO rats. These data reveal a complex regulation of the endocrine system in this diabetic condition and suggest that HPA axis may be more stimulated during acute stress in diabetes mellitus than in unaffected subjects.