Calcitonin gene-related peptide- and substance P-like-immunoreactive innervation of the anterior pituitary in the rat

Bioactive growth hormone in humans: Controversies, complexities and concepts

OBJECTIVE

To revisit a finding, first described in 1978, which documented existence of a pituitary growth factor that escaped detection by immunoassay, but which was active in the established rat tibia GH bioassay.

METHODS

We present a narrative review of the evolution of growth hormone complexity, and its bio-detectability, from a historical perspective.

RESULTS

In humans under the age of 60, physical training (i.e. aerobic endurance and resistance training) are stressors which preferentially stimulate release of bioactive GH (bGH) into the blood. Neuroanatomical studies indicate a) that nerve fibers directly innervate the human anterior pituitary and b) that hind limb muscle afferents, in both humans and rats, also modulate plasma bGH. In the pituitary gland itself, molecular variants of GH, somatotroph heterogeneity and cell plasticity all appear to play a role in regulation of this growth factor.

CONCLUSION

This review considers more recent findings on this often forgotten/neglected subject. Comparison testing of a) human plasma samples, b) sub-populations of separated rat pituitary somatotrophs or c) purified human pituitary peptides by GH bioassay vs immunoassay consistently yield conflicting results.

A prospective study of headache and neuropeptides in patients with pituitary adenomas

OBJECTIVE

To evaluate clinical criteria for headache associated with pituitary adenoma (HaPA) in the International Classification of Headache Disorders (ICHD) 3rd edition version criteria and further determine whether elevations of plasma calcitonin gene-related peptide and pituitary adenylate cyclase-activating peptide 1-38 (PACAP1-38) concentration contribute to HaPA.

METHODS

Demographic and clinical features of consecutive patients with pituitary adenoma were recorded. Plasma calcitonin gene-related peptide and PACAP1-38 concentrations in pituitary adenoma patients within 72 h pre- and post-operation were measured. Primary outcome for HaPA patients were 50% reduction of moderate-to-severe headache days at 3 months after discharge.

RESULTS

Sixty-three patients with pituitary adenoma were recruited, 33 (52.4%) of whom had headache. The patients who had HaPA presented with migraine-like (32.9%), tension-type-like (12.1%), and stabbing headache (9.1%). Non-functional adenoma was present in the majority of cases (82.5%). Surgical resection improved headache in 83.3% of cases at 3 month follow-up. Pre- and post-operative calcitonin gene-related peptide and PACAP1-38 levels were significantly higher in patients with headache than in those without headache (p < 0.05). Plasma calcitonin gene-related peptide and PACAP1-38 levels at 72 h post-operation were lower at 72 h after operation in patients who had greater improvement in headache compared with those who had little improvement, while plasma calcitonin gene-related peptide and PACAP1-38 levels were similar between these two groups preoperatively.

CONCLUSIONS

Most pituitary adenoma patients have non-functional adenoma, and half of this group have HaPA, indicating that the ICHD-3 criteria for HaPA with the emphasis on secretion status need further modifications. Lower plasma calcitonin gene-related peptide and PACAP1-38 concentrations at 72 h after operation may predict a better outcome in patients with HaPA.

Calcitonin gene-related peptide: neuroendocrine communication between the pancreas, gut, and brain in regulation of blood glucose

Background

Calcitonin gene-related peptide (CGRP), a ubiquitous neuropeptide, plays a diverse and intricate role in chronic low-grade inflammation, including conditions such as obesity, type 2 diabetes, and diabetes of the exocrine pancreas. Diabetes of exocrine pancreas is characterised by chronic hyperglycemia and is associated with persistent low-grade inflammation and altered secretion of certain pancreatic and gut hormones. While CGRP may regulate glucose homeostasis and the secretion of pancreatic and gut hormones, its role in chronic hyperglycemia after acute pancreatitis (CHAP) is not known. The aim of this study was to investigate the association between CGRP and CHAP.

Methods

Fasting blood samples were collected to measure insulin, HbA1c, CGRP, amylin, C-peptide, glucagon, pancreatic polypeptide (PP), somatostatin, gastric inhibitory peptide, glicentin, glucagon-like peptide-1 and 2, and oxyntomodulin. Modified Poisson regression analysis and linear regression analyses were conducted. Five statistical models were used to adjust for demographic, metabolic, and pancreatitis-related risk factors.

Results

A total of 83 patients were recruited. CGRP was significantly associated with CHAP in all five models (P-trend <0.005). Further, it was significantly associated with oxyntomodulin (P<0.005) and glucagon (P<0.030). Oxyntomodulin and glucagon independently contributed 9.7% and 7%, respectively, to circulating CGRP variance. Other pancreatic and gut hormones were not significantly associated with CGRP.

Conclusions

CGRP is involved in regulation of blood glucose in individuals after acute pancreatitis. This may have translational implications in prevention and treatment of diabetes of the exocrine pancreas.

Excitatory and inhibitory effects of prolactin release activated by nerve stimulation in rat anterior pituitary

BACKGROUND

A series of studies showed the presence of substantial amount of nerve fibers and their close relationship with the anterior pituitary gland cells. Our previous studies have suggested that aside from the classical theory of humoral regulation, the rat anterior pituitary has direct neural regulation on adrenocorticotropic hormone release. In rat anterior pituitary, typical synapses are found on every type of the hormone-secreting cells, many on lactotrophs. The present study was aimed at investigating the physiological significance of this synaptic relationship on prolactin release.

METHODS

The anterior pituitary of rat was sliced and stimulated with electrical field in a self-designed perfusion chamber. The perfusate was continuously collected in aliquots and measured by radioimmunoassay for prolactin levels. After statistic analysis, differences of prolactin concentrations within and between groups were outlined.

RESULTS

The results showed that stimulation at frequency of 2 Hz caused a quick enhancement of prolactin release, when stimulated at 10 Hz, prolactin release was found to be inhibited which came slower and lasted longer. The effect of nerve stimulation on prolactin release is diphasic and frequency dependent.

CONCLUSIONS

The present in vitro study offers the first physiological evidence that stimulation of nerve fibers can affect prolactin release in rat anterior pituitary. Low frequency stimulation enhances prolactin release and high frequency mainly inhibits it.

Intercellular communication within the rat anterior pituitary: XIV electron microscopic and immunohistochemical study on the relationship between the agranular cells and GnRH neurons in the dorsal pars tuberalis of the pituitary gland

Although numerous investigators in 1970s to 1980s have reported the distribution of LH-RH nerve fibers in the median eminence, a few LH-RH fibers have been shown to be present in the pars tuberalis. The significance of the finding remains to be elucidated, and there are few studies on the distribution of LH-RH neurons in the pars tuberalis, especially in the dorsal pars tuberalis (DPT). Adult male Wistar-Imamichi rats were separated into two groups: one for electron microscopy and the other for immunohistochemistry to observe LH-RH and neurofilaments. Pituitary glands attached to the brain were fixed by perfusion, and the sections were prepared parallel to the sagittal plane. The typical glandular structure of the pars tuberalis was evident beneath the bottom floor of the third ventricle, and the thick glandular structure was present in the foremost region. Closer to the anterior lobe, the glandular structure changed to be a thin layer, and it was again observed at the posterior portion. Then the pituitary stalk was surrounded with the dorsal, lateral, and ventral pars tuberalis. LH-RH and neurofilaments fibers were noted in the bottom floor, and some of them vertically descended to the gland. Adjacent to the glandular folliculostellate cells in the pars tuberalis, Herring bodies with numerous dense granules invading into the gland were present between the pituitary stalk and DPT. It was postulated that the "message" carried by LH-RH might have been transmitted to the cells in the DPT to aid in the modulation of LH release.

Administration of an antagonist of neurokinin receptors 1, 2, and 3 results in reproductive tract changes in beagle dogs, but not rats

SCH 206272, an antagonist of neurokinin receptors 1, 2, and 3, was administered orally by gavage for 1 month to 8- to 10-month-old dogs at doses of 0, 15, 30, or 60 mg/kg, and to 6-week-old rats at doses of 0, 30, 100, or 300 mg/kg. The most important changes occurred in the reproductive tract of the dogs at all doses. Absolute and relative group mean organ weights for the testes, prostate gland, epididymides, ovaries, and uterus were 33-86% lower than concurrent controls in groups receiving SCH 206272. Organ weight changes were not dose-related. Microscopic changes that correlated with the organ weight changes occurred in all groups receiving SCH 206272. For males, they included minimal to severe atrophy of the testes, epididymides, and prostate gland. In addition, the epididymides exhibited severe oligospermia or aspermia, minimal epithelial apoptosis and mild epithelial vacuolation. In female dogs, the ovaries and uteri appeared immature. Microscopic changes were similar in incidence and severity in dogs receiving 30 or 60 mg/kg, but were slightly less in dogs receiving 15 mg/kg. In contrast, similar findings were not observed in the reproductive tract of male or female rats, despite overlapping systemic, hypothalamic, and pituitary gland concentrations of SCH 206272.

Peptidergic innervation in pars distalis of the human anterior pituitary

It has been previously shown that the peptidergic nerve fibers are present in the anterior pituitary of monkeys, dogs and rats. In our study, which is reported here, thick nerve fiber bundles, large numbers of peptidergic nerve fibers and their varicosities, which are substance P (SP)-, calcitonin gene-related peptide (CGRP)- and galanin (GAL)-immunoreactive (ir), are found in the human pituitary stalk. All these peptidergic nerve fibers run along the pituitary stalk and enter the pars distalis, and some GAL-ir nerve fibers even reach the center of the human anterior pituitary as well as in parenchyma of adenohypophysis. The number of SP-ir nerve fibers is much more than that of other kinds of peptidergic nerve fibers. All these peptidergic nerve fibers are mainly located in the medial part of the gland and distributed in its dorsal-posterior region. A substantial amount of these peptidergic nerve fibers with numerous varicosities are found to be close to the glandular tissue in the pars distalis of the human anterior pituitary. Furthermore, same SP-ir and CGRP-ir cells have been demonstrated in the pars distalis of the anterior pituitary. More or less, these peptidergic nerve fibers came also from the meningeal shell and enter the parenchyma of the anterior pituitary. Whatever the function of peptidergic nerve fibers in the human anterior pituitary might be, the concept that the adenohypophysis is regulated only hormonally by way of the portal system could be challenged.

Dual excitatory and inhibitory effects of stimulation of intrinsic innervation of the anterior pituitary on adrenocorticotropic hormone release in the rat

The gland cells of the mammalian anterior pituitary are innervated by substantial amounts of nerve fibres, and there is evidence that the nerve fibres are functionally active. In the rat, the nerve fibres make typical excitatory synapses with corticotropes. The physiological significance of this synaptic relationship was investigated in the present study. The anterior pituitary of the rat was sliced and stimulated with electrical field in a chamber. The perfusate was continuously collected and immunoradioassayed for adrenocorticotropic hormone (ACTH). When the gland slices were stimulated at a high frequency of 10 Hz, there was a significant inhibition of ACTH secretion. Stimulation at a low frequency of 2 Hz resulted in a quick and transient excitation of ACTH release. The results indicate that stimulation of the nerve fibres in the anterior pituitary has dual excitatory and inhibitory effects on ACTH secretion.

Projections of the sexually dimorphic calcitonin gene-related peptide neurons of the preoptic area determined by retrograde tracing in the female rat

The medial preoptic area of the rat exhibits morphologic sex differences and is implicated in the control of sexually dimorphic behavior and function. Neurons expressing calcitonin gene-related peptide (CGRP) within the anteroventral periventricular (AVPV) and medial preoptic nucleus (MPN) of the medial preoptic area exhibit female-dominant sex differences in number through organizational and activational effects of gonadal steroids. The present study used retrograde tracing experiments to establish the projections of the AVPV and MPN CGRP neurons in the female rat. After the intraperitoneal administration of Fluoro-Gold to female rats (n = 5), we were unable to detect retrograde tracer in any CGRP-immunoreactive cells of the hypothalamus. Intracerebral injections of 50- to 100-nl volumes of Fluoro-Gold into the mediobasal hypothalamus resulted in up to 70% of CGRP neurons in the AVPV and MPN containing retrograde tracer. Similar large volume tracer depositions in the lateral septum, periaqueductal gray, two likely CGRP projection sites, resulted in no labeling of preoptic CGRP neurons. Experiments using small volume (30-nl) injections of Fluoro-Gold and green fluorescent microspheres at multiple sites in the mediobasal hypothalamus (n = 18) revealed that approximately 60% of AVPV and 30% of MPN neurons expressing CGRP were projecting to the region of the tuberal and ventral premammillary nuclei, with a minor projection to the dorsomedial nucleus. These findings demonstrate a major projection of the preoptic CGRP neurons to the posterior hypothalamus in the female rat and support further a functional role for these neurons in the sexually dimorphic regulation of reproductive functioning.

Evidence for nodose ganglion as the source of innervation to the anterior lobe of the pituitary gland

Recent studies have provided convincing evidence for the presence of peptidergic nerve fibers in the pituitary anterior lobe in several animal species. This study was aimed at elucidating the origin of this innervation by neuroanatomical tracing, denervation experiments, and immunohistochemistry. Immunohistochemistry against substance P and growth-associated protein 43 revealed a dense fiber plexus within the anterior lobe, and these markers were mostly colocalized. Retrograde tracing with Fluorogold from the pituitary gland stained neurons in the hypothalamus, superior cervical ganglia and the nodose ganglia. None of the Fluorogold-labelled neurons in the hypothalamus or superior cervical ganglion were substance P-immunoreactive, while many of the neuronal cell bodies in the nodose ganglion exhibited substance P immunoreactivity. There were no Fluorogold-labelled neurons in the trigeminal, otic or cervical dorsal root ganglia. Surgical transection of the pituitary stalk or bilateral removal of the superior cervical ganglion did not abolish the anterior lobe nerve fibers, and anterograde tracing with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindo-carbocyanine perchlorate from the pituitary stalk failed to stain any nerve fibers within the anterior lobe. Our findings suggest that the nodose ganglion neurons likely innervate the pituitary anterior lobe, while neither hypothalamus nor sympathetic superior cervical ganglion may be a source of this innervation. By showing a distinct neuronal system in the pituitary anterior lobe our findings (i) support the previous functional studies demonstrating a distinct regulation of the morphology of the anterior lobe innervation by hormonal changes, and (ii) suggest that the innervation of the pituitary anterior lobe is a part of the visceral innervation by the vagus nerve rather than a part of the other intracranial innervation. These findings provide a neuroanatomic basis for the reported observations about the neural regulation of the pituitary anterior lobe.

Galanin immunoreactive innervations of the anterior pituitary in the monkey and the dog

As we know, the anterior pituitary is regulated by hypothalamic hormones via the portal system. However, our recent studies have demonstrated the presence of a substantial amount of substance P (SP)- and calcitonin gene-related peptide (CGRP)-containing nerve fibers in the anterior pituitary. In the present paper, the existence of a considerable amount of galanin (GAL)like immunoreactive (ir) nerve fibers with numerous of varicosities in the anterior pituitaries of the Macaca mulatta monkey and the dog were reported. In the monkey, GAL-ir nerve fibers with a large amount of varicosities were mainly located in the medial part of the gland, dominantly in its dorso-posterior regions. A great majority of varicosities were found to be closely related to the glandular tissue although some were located along the walls of blood sinus. GAL-ir nerve fibers were traced to enter the pars distalis of the anterior pituitary from the stalk as well as from the meningeal sheath covering the upper part of the anterior pituitary. Numerous GAL-ir cells presented in the pars distalis and the pars intermedia of the adenohypophysis. In the dog pituitary, GAL-ir nerve fibers were mainly located in the tail part and some in oral region. A majority of GAL-ir nerve fibers were at the periphery of the gland, especially in the medial planes, although some could be found deep in the gland. They appeared in individual fibers or in patches. Many GAL-ir nerve fibers and fiber fascicles existed in the median eminence and the sheath covering the tail and the oral part of the pituitary.

Intercellular communication in the anterior pituitary

In addition to hypothalamic and feedback inputs, the secretory cells of the anterior pituitary are influenced by the activity of factors secreted within the gland. The list of putative intrapituitary factors has been expanding steadily over the past decade, although until recently much of the work was limited to descriptions of potential interactions. This took the form of evidence of production within the pituitary of factors already known to influence activity of secretory cells, or further descriptions of actions on pituitary cells by such factors when added exogenously. A new phase of discovery has been entered, with extensive efforts being made to delineate the control of the synthesis and secretion of the pituitary factors within the gland, regulation of the receptors and response mechanisms for the factors in pituitary cells, and measurements of the endogenous actions of the factors through the use of specific immunoneutralization, receptor blockade, tissue from transgenic animals, and other means. Taken together, these findings are producing blueprints of the intrapituitary interactions that influence each of the individual types of secretory cells, leading toward an understanding of the physiological significance of the interactions. The purpose of this article is to review the recent literature on many of the factors acting as intrapituitary signals and to present such finding in the context of the physiology of the secretory cells.

Suppression of adrenocorticotropic hormone release by stimulation of the nerve fibres in the anterior pituitary

With the recent revelation of considerable peptidergic innervation of the anterior pituitary in several mammalian species, including man, it becomes imperative to elucidate the physiological significance of such a morphological entity. We addressed this issue by employing an anterior pituitary slice in vitro superfusion system coupled with electrical field stimulation. Anterior pituitary slices of 0.8 mm were perfused with Krebs-Ringer bicarbonate-bovine serum albumin buffer in a superfusion chamber for 30 min before electrical field stimulation. A square current of 30 mA, 10 Hz and 0.5 ms was then applied for 10 min. The perfusate was collected every 10 min and measured for adrenocorticotropic hormone (ACTH) by radio-immunoassay. It was found that under the experimental condition the basal release of ACTH was suppressed by electrical field stimulation of the nerve fibres in the anterior pituitary. Furthermore, vasopressin was added as a secretagogue. The suppression of ACTH by electrical field stimulation became even more marked. This is the first physiological evidence of the effect of stimulation of the nerve fibres innervating the anterior pituitary on its secretory activity.

The role of amino acid neurotransmitters in the regulation of pituitary gonadotropin release in fish

Both glutamate and γ-aminobutyric acid (GABA) are involved in pituitary hormone release in fish. Glutamate serves 2 purposes, both as a neurotransmitter and as a precursor for GABA synthesis. Glutamate can be catabolized to GABA by the actions of 2 distinct but related enzymes, glutamate decarboxylase 65 (GAD65) and GAD67. They derive from 2 different genes that likely arose from an early gene duplication prior to the emergence of teleosts more than 400 million years ago. There is good evidence for the involvement of GABA in luteinizing hormone (LH) release in fish. The mechanism of GABA action to stimulate LH release appears to be a combination of effects on GnRH release, potentiation of gonadotropin hormone-releasing hormone (GnRH) action, and in some cases directly at the LH cell. These actions appear to be dependent on such factors as sex or sex steroid levels, and there may also be species differences. Nevertheless, the stimulatory effects of GABA on LH are present in at least 4 fish species. In contrast, convincing data for the inhibitory effects of GABA on LH release have only been observed in 1 fish species. The sites and mechanisms of action of amino acid neurotransmitters on LH release have yet to be fully characterized. Both N-methyl-D-aspartic acid (NMDA) and S-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors are likely to have important roles. We suggest that it is a receptor similar to the GABAA type which mediates the effects of GABA on LH release in fish, at least partially acting on the GnRH neuron, but likely directly acting at the gonadotroph as well. GABA may also be involved in regulating the release of other pituitary hormones in fish, namely follicle stimulating hormone (FSH = GTH-I), prolactin, and growth hormone. Based on the findings described in this review, a working model for the involvement of glutamate and GABA in the regulation of LH release in teleost fish is proposed.

Key words: glutamate, GABA, luteinizing hormone, muscimol, patch clamp electrophysiology, reproduction, fish.

Afferent input from rat slow skeletal muscle inhibits bioassayable growth hormone release

The release of a bioassayable form of growth hormone (BGH), distinct from growth hormone as measured by immunoassay (IGH), from the rat pituitary into the blood is differentially regulated by afferent input from fast and slow skeletal muscles. Specifically, activation of low-threshold fast muscle afferents for 15 min increased plasma BGH by 217 and 295% and decreased pituitary BGH by 68 and 45% in male and female rats, respectively. In contrast, activation of slow muscle afferents inhibited BGH release, decreasing plasma BGH by approximately 60% and increasing pituitary BGH by 30-50% in male rats. Female rats from which food had been withheld for approximately 12 h had elevated basal plasma BGH levels, which then were decreased by 81% after slow muscle nerve stimulation. Plasma IGH concentrations were unchanged after any nerve stimulation condition. These results demonstrate that regulation of BGH release can be differentially mediated through low-threshold afferent inputs from fast or slow skeletal muscle. Furthermore, the results indicate that BGH responses are independent of gender or feeding status.

The ontogenic appearance of tyrosine hydroxylase-, serotonin-, gamma-aminobutyric acid-, calcitonin gene-related peptide-, substance P-, and synaptophysin-immunoreactivity in rat pituitary gland

The initial appearance of tyrosine hydroxylase (TH)-, serotonin (5-HT)-, gamma-aminobutyric acid (GABA)-, calcitonin gene-related peptide- (CGRP), substance P-, and synaptophysin-immunoreactivity in the rat pituitary gland, and in the related brain regions was investigated. Several groups of TH-immunoreactive neurons were first detected in the brain stem on day E17, and in the hypothalamus on day E18, followed by TH-immunoreactivity in the median eminence and infundibulum on E19-E20. TH-positive fibers appeared in the posterior lobe on day E20 and in the intermediate lobe on day P0. 5-HT-immunoreactivity was first detected on day E17 in neurons and nerve fibers in the brain stem and in the median eminence, respectively. On day E18, a few 5-HT-immunoreactive fibers were detected in the posterior lobe of the pituitary, although they were consistently seen in the infundibulum from day E19. In newborn rats, some 5-HT-immunoreactive fibers, but no neurons, were seen in the hypothalamus. GABA immunoreactivity appeared on day E17 in several nerve fibers of the infundibulum and the posterior lobe. Some neurons in the cortex and ventral hypothalamus transiently expressed GABA-immunoreactivity on day E17. In newborn rats, a plexus of GABA-immunoreactive fibers was detected for the first time in the intermediate lobe. No CGRP-immunoreactive fibers could be detected in the prenatal pituitary. On day P10, CGRP-immunoreactive fibers were first observed in the anterior lobe. Later their number considerably increased, while only sporadic fibers could be found in the intermediate or posterior lobes. No substance P-immunoreactivity could be detected in any of the lobes in the embryonic or developing postnatal rat pituitary, instead the adult anterior lobe occasionally showed some substance P-immunoreactive fibers. Synaptophysin-immunoreactivity was first detected in the posterior lobe on day E20, followed shortly by its expression in the intermediate lobe in newborn rats. The time course of GABA and 5-HT expression revealed in the present study suggests that these transmitters, which are initially expressed in the developing pituitary clearly before synaptic maturation, may act as trophic molecules during the prenatal period.

Axons containing the growth associated protein GAP-43 specifically target rat corticotrophs following adrenalectomy

An extensive network of nerve fibers immunoreactive for the neuronal growth associated protein GAP-43 (GAP-43-IR) is present within the anterior pituitary (AP) of the rat, and the density of these fibers has been reported to increase 4 days after adrenalectomy (ADX). In the present study, we employed confocal dual-label immunofluorescence microscopy to determine whether GAP-43-IR fibers are specifically associated with corticotrophs at various intervals after ADX. A dramatic increase in the density of GAP-43-IR was apparent 4 days after ADX, and this increase was sustained at 7 and 14 days post-ADX. The percentage of corticotrophs in apparent contact with GAP-43-IR axons was 87% at 4 days after ADX and 92% at 14 days. In addition, fewer than 15% of GAP-43-IR terminals were associated with cells other than corticotrophs in either group. This highly specific targeting of corticotrophs during a period in which these cells are undergoing both hypertrophy and hyperplasia indicates that axonal sprouting is occurring in response to ADX. While the less intense GAP-43-IR in the AP of intact rats precluded precise quantitative analysis, the majority of corticotrophs also appeared to be selectively innervated in these animals. The observations that GAP-43-IR axons selectively contact corticotrophs, and that both the specificity and thoroughness of innervation are maintained by targeted growth of GAP-43-IR axons following ADX, strongly suggest that these fibers are of functional significance.

Calcitropic peptides: neural perspectives

In mammals and higher vertebrates, calcitropic peptides are produced by peripheral endocrine glands: the parathyroid gland (PTH), thyroid or ultimobranchial gland (calcitonin) and the anterior pituitary gland (growth hormone and prolactin). These hormones are, however, also found in the neural tissues of lower vertebrates and invertebrates that lack these endocrine organs, suggesting that neural tissue may be an ancestral site of calcitropic peptide synthesis. Indeed, the demonstration of CNS receptors for these calcitropic peptides and their induction of neurological actions suggest that these hormones arose as neuropeptides. Neural and neuroendocrine roles of some of these calcitropic hormones (calcitonin and parathyroid hormone) and related peptides (calcitonin gene related peptide, stanniocalcin and parathyroid hormone related peptide) are thus the focus of this review.

Synaptoid contacts between gland cells of the anterior pituitary of the rat

BACKGROUND

There is evidence suggesting cross-talk among gland cells of the anterior pituitary. We had reported a rare form of synaptoid contact between corticotrophs in the anterior pituitary of the dog. We then found similar synaptoid contacts with different characteristics in the rat, as described in the present article.

METHODS

Male Sprague-Dawley rats were used. The anterior pituitaries were prepared for ultrastructural study of substance P immunoreactivity of the anterior pituitary. Routine preembedding immunohistochemical staining was conducted, the sections were embedded in Epon 812 (Serva Feinbiochemica, Heidelberg, New York), and ultrathin sections were prepared.

RESULTS

In the anterior pituitary of the rat, synaptoid contacts were found between corticotrophs and lactotrophs. They appeared very close to typical synapses in the central nervous system, aside from evident weakness of presynaptic density.

CONCLUSIONS

The presence of synaptoid contacts suggests a form of cross-talk between the gland cells in the anterior pituitary of the rat.

Stimulatory effect of calcitonin gene-related peptide on adrenocorticotropin release from rat anterior pituitary cells

In the present study, we examined the direct regulatory effect of rat calcitonin gene-related peptide (CGRP) on adrenocorticotropin (ACTH) release from rat cultured anterior pituitary cells. CGRP significantly increased ACTH release at concentrations of 10(-8)-10(-11) M. The ACTH release was gradually increased by CGRP concentrations lower than 10(-10) M, and was decreased at concentrations higher than 10(-9) M, presenting a bell-shaped dose-response curve. As well as having an additive effect on corticotropin-releasing factor-induced ACTH release, CGRP stimulated the accumulation of intracellular cAMP. The CGRP-induced ACTH release was inhibited by a protein kinase A inhibitor, suggesting that its stimulatory effect on the ACTH release was mediated via an adenylate-cyclase-protein kinase system. CGRP-like immunoreactive nerve fibers have been reported to innervate the anterior pituitary, so that the stimulatory effect of CGRP on the ACTH release suggests that this peptide may be involved in neural regulation of hormone secretion in the anterior pituitary.

Skeletal muscle afferent regulation of bioassayable growth hormone in the rat pituitary

There are forms of growth hormone (GH) in the plasma and pituitary of the rat and in the plasma of humans that are undetected by presently available immunoassays (iGH) but can be measured by bioassay (bGH). Although the regulation of iGH release is well documented, the mechanism(s) of bGH release is unclear. On the basis of changes in bGH and iGH secretion in rats that had been exposed to microgravity conditions, we hypothesized that neural afferents play a role in regulating the release of these hormones. To examine whether bGH secretion can be modulated by afferent input from skeletal muscle, the proximal or distal ends of severed hindlimb fast muscle nerves were stimulated ( approximately 2 times threshold) in anesthetized rats. Plasma bGH increased approximately 250%, and pituitary bGH decreased approximately 60% after proximal nerve trunk stimulation. The bGH response was independent of muscle mass or whether the muscles were flexors or extensors. Distal nerve stimulation had little or no effect on plasma or pituitary bGH. Plasma iGH concentrations were unchanged after proximal nerve stimulation. Although there may be multiple regulatory mechanisms of bGH, the present results demonstrate that the activation of low-threshold afferents from fast skeletal muscles can play a regulatory role in the release of bGH, but not iGH, from the pituitary in anesthetized rats.

Galanin-like immunoreactive nerve fibres in the anterior pituitary of the normal and adrenalectomized rat

Our previous studies have shown the presence of substantial amounts of substance P and calcitonin gene-related peptide-like immunoreactive nerve fibres in the anterior pituitary of the monkey, dog and rat. Furthermore, synaptic relationships have been demonstrated between these nerve fibres and the gland cells in the dog and rat. The substance P and calcitonin gene-related nerve fibres increase in number following adrenalectomy and ovariectomy, respectively. The present study was aimed to investigate the galanin-containing nerve fibres in the anterior pituitary of normal and adrenalectomized rats. The results showed only a small amount of galanin-like immunoreactive nerve fibres in the normal anterior pituitary, which were present among the gland cells as well as along the blood vessels. Following adrenalectomy, the number of galanin-like immunoreactive nerve fibres increased and ramification appeared more frequently. The results substantiate our hypothesis of a dual neural-humoral regulation of the mammalian anterior pituitary gland.

The relationship of galanin immunoreactive nerve fibers to glandular cells in the anterior pituitary in the monkey

The anterior pituitary is known to be regulated by hypothalamic hormones via the portal system. However, our recent studies have demonstrated the presence of a substantial amount of substance P (SP)-, calcitonin gene-related peptide (CGRP)- and galanin (GAL)- immunoreactive (ir) nerve fibers with numerous varicosities in the anterior pituitaries of the Macaca mulatta monkey. The present study investigated the relationship of the GAL-ir nerve fibers to the glandular cells. The M. mulatta monkeys were used and sections of the anterior pituitary were double immunostained. GAL-ir nerve fibers and/or varicosities were found in proximity to contact directly with corticotropes, somatotropes, lactotropes, gonadotropes and thyrotropes without any exception. These findings indicate that a direct neural factor may be involved in the regulation of adenohypophyseal secretion.

Innervation of the mammalian anterior pituitary: a mini review

The mammalian anterior pituitary was not known to be innervated other than by a few autonomic nerve fibers. Recent studies, however, have demonstrated otherwise. A hypothesis of neural-humoral dual regulation of the mammalian anterior pituitary has been postulated based on the following findings: (1) the presence of substantial amounts of nerve fibers in the anterior pituitary of a number of mammalian species; (2) close contact of the nerve fibers with the gland cells, even forming synapses; (3) the nerve fibers originate, as least partly, from the hypothalamus; (4) the nerve fibers respond actively to changes in hormonal levels of the organism; and (5) stimulation of the nerve fibers changes the secretory activities of the gland cells.

Neuroanatomical localization, pharmacological characterization and functions of CGRP, related peptides and their receptors

Calcitonin generelated peptide (CGRP) is a neuropeptide discovered by a molecular approach over 10 years ago. More recently, islet amyloid polypeptide or amylin, and adrenomedullin were isolated from human insulinoma and pheochromocytoma respectively, and revealed between 25 and 50% sequence homology with CGRP. This review discusses findings on the anatomical distributions of CGRP mRNA, CGRP-like immunoreactivity and receptors in the central nervous system, as well as the potential physiological roles for CGRP. The anatomical distribution and biological activities of amylin and adrenomedullin are also presented. Based upon the differential biological activity of various CGRP analogs, the CGRP receptors have been classified in two major classes, namely the CGRP1 and CGRP2 subtypes. A third subtype has also been proposed (e.g. in the nucleus accumbens) as it does not share the pharmacological properties of the other two classes. The anatomical distribution and the pharmacological characteristics of amylin binding sites in the rat brain are different from those reported for CGRP but share several similarities with the salmon calcitonin receptors. The receptors identified thus far for CGRP and related peptides belong to the G protein-coupled receptor superfamily. Indeed, modulation of adenylate cyclase activity following receptor activation has been reported for CGRP, amylin and adrenomedullin. Furthermore, the binding affinity of CGRP and related peptides is modulated by nucleotides such as GTP. The cloning of various calcitonin and most recently of CGRP1 and adrenomedullin receptors was reported and revealed structural similarities but also significant differences to other members of the G protein-coupled receptors. They may thus form a new subfamily. The cloning of the amylin receptor(s) as well as of the other putative CGRP receptor subtype(s) are still awaited. Finally, a broad variety of biological activities has been described for CGRP-like peptides. These include vasodilation, nociception, glucose uptake and the stimulation of glycolysis in skeletal muscles. These effects may thus suggest their potential role and therapeutic applications in migraine, subarachnoid haemorrhage, diabetes and pain-related mechanisms, among other disorders.

Synaptophysin immunoreactivity in the aging rat pituitary gland

The density of synaptophysin (SN)-immunoreactivity (IR) was examined in pituitary glands of aging male Sprague-Dawley rats. SN-IR was observed as dense dots among endocrine cells of the intermediate lobe, while the neural lobe contained numerous, highly dense immunopositive regions. Some anterior lobe secretory cells contained SN-IR within the cytoplasm, suggestive of the presence of the protein in secretory granules, but no dot-like staining was observed between endocrine cells of that region. A quantitative analysis of the dot-like SN-immunostaining within the intermediate lobe found that tissue from groups of rats aged 13 months, or 15-17 months, contained significantly fewer SN-immunopositive areas than did tissues from 8-month-old animals. Diminished SN immunostaining is suggestive of reduced numbers of synapses in the intermediate lobe, which may lead to alterations in regulation of pituitary hormone secretion from endocrine cells in the older animals.

Pituitary gland receives both central and peripheral neuropeptide Y innervation

Neuropeptide Y (NPY)-containing neural projections to the rat pituitary gland were studied by combining NPY immunohistochemistry with retrograde tracing with Fluorogold as well as central and peripheral denervations. Numerous pituitary-projecting, i.e. Fluorogold-labelled, neurons in the superior cervical ganglion, as well as in the hypothalamic magnocellular nuclei were NPY-immunoreactive (NPY-IR). In contrast, no other hypothalamic NPY-IR neurons, e.g. in the arcuate nucleus or the preoptic area, were observed to be projecting into the pituitary. Within the posterior lobe of the pituitary gland two morphologically distinct NPY-IR fiber populations were discovered, namely thinner parenchymal terminals, distinct from the neurosecretory terminals, and thicker, perivascular fibers. Neurosecretory nerve terminals, in contrast, were devoid of NPY-IR, being consistent with the previous reports on their sensitivity to osmotic stimulation. On the other hand, the anterior and intermediate lobes contained no NPY-IR fibers. Bilateral extirpation of the superior cervical ganglion resulted in disappearance of the perivascular NPY-IR fibers leaving the parenchymal NPY-IR fibers unaffected, while transection of the pituitary stalk abolished all of the parenchymal NPY-IR neurons, leaving the perivascular fibers unaffected. These findings together with the observed colocalization of tyrosine hydroxylase and NPY in the posterior lobe perivascular fibers indicated that they are sympathetic nerve endings. The thin parenchymal terminals, instead, are suggested to stem from central sources other than hypothalamus. Our findings indicate that the pituitary gland receives NPY-containing innervation from at least three distinct sources, and NPY may thus affect pituitary functions in various ways, such as blood flow and vasopressin release.

Alterations in GAP-43-immunoreactive innervation in the aging rat pituitary

The levels and distribution of the growth-associated protein, GAP-43, were examined in the pituitary glands of young and aging Sprague-Dawley rats, using immunohistochemical techniques on tissue sections and Western blot analyses. GAP-43-immunoreactive innervation was observed in sections in the intermediate and neural lobes of animals aged 8-15 months, while in the oldest rats studied (17 months), stained fibers were observed mainly in the neural, but not the intermediate lobe. Western blots revealed reduced levels of GAP-43 in samples from 15 month old animals, as compared to 12 month old rats, in the neurointermediate lobes. There was no immunoreactivity for GAP-43 in the anterior lobes in the tissue sections or in the blots in any of the glands examined. A diminished level of GAP-43 in pituitary innervation in aged animals suggests a reduced ability for nerve terminals to undergo 'plastic' changes in their relationship to target endocrine cells. Since GAP-43 has also been suggested to modulate neurotransmitter release, a reduction in the protein in aging nerve terminals may diminish availability of transmitters at presynaptic sites.

Sexually dimorphic expression of calcitonin gene-related peptide (CGRP) immunoreactivity by rat mediobasal hypothalamic neurons

Although the hypothalamic arcuate nucleus is a sexually dimorphic region of the rat brain, there are no reports of sex differences in the number of neurons containing specific neuropeptides within this structure. As cells synthesizing calcitonin gene-related peptide (CGRP) have been shown to exhibit sex differences in other steroid-receptive regions of the rat brain, we examined whether the CGRP-immunoreactive cells located in the mediobasal hypothalamus may also be sexually dimorphic. Immunostaining of sections from male and female colchicine-treated rats revealed a small population of CGRP-immunoreactive cells distributed throughout the arcuate nucleus. Immunoreactive cells were also detected in the lateral hypothalamic perifornical region, dorsomedial, posterior periventricular and ventral tuberomammillary nuclei, and zona incerta. Cell count analysis revealed approximately twice as many CGRP-immunoreactive cell profiles in the rostral (P < 0.01), middle (P < 0.001), and caudal (P < 0.01) thirds of the arcuate nucleus of male rats compared with females. A significant sex difference in immunoreactive cell numbers (male > female) was also detected within the caudal dorsomedial nucleus (P < 0.05) but not in the posterior periventricular nucleus, perifornical region and zona incerta. Although fibers immunoreactive for CGRP were identified in low density throughout the mediobasal hypothalamus, only female rats displayed prominent fiber staining in the periventricular region. Double-labelling immunofluorescence experiments revealed that the CGRP-immunoreactive cells within the zona incerta, but not the hypothalamus, were also immunoreactive for tyrosine hydroxylase; at least 60% of the A13 dopaminergic neurons co-express CGRP. These results provide evidence that sex differences exist in the number of specific neuropeptide-synthesizing cells within the hypothalamic arcuate nucleus and provide further examples of cell populations expressing CGRP immunoreactivity in a sexually dimorphic manner.

Expression and characterization of the substance P (NK1) receptor in the rat pituitary and AtT20 mouse pituitary tumor cells

Although substance P is known to take part in the regulation of the anterior pituitary, no conclusive evidence for the expression of the tachykinin NK1 receptor has been found yet in the pituitary or pituitary derived cells. With the reverse transcription-polymerase chain reaction (RT-PCR) method we could detect the low abundant transcripts of the NK1 receptor in the rat pituitary and in the AtT20 cell line (clone D16v). Furthermore, the functional expression of the NK1 receptor in AtT20 cells was confirmed by activation of the phosphatidylinositol-calcium second messenger system when the cells were treated with substance P. In addition, binding studies also indicated the functional expression of this receptor in AtT20 cells. Thus we provide the first evidence that the NK1 receptor is expressed in AtT20 cells and the rat pituitary.

Electron microscopic observation of substance P-like immunoreactive nerve fibres and innervation in the anterior pituitary of macaques

The anterior pituitary has recently been confirmed to be innervated by substance P-immunoreactive nerve fibres in the monkey and human. The present study investigated the morphology of the nerve fibres and their relationship with anterior pituitary gland cells in the macaque by means of immunoelectron microscopy. Many substance P-immunoreactive and unlabelled nerve fibres were distributed among the gland cells or formed nerve fibre bundles. These nerve fibres were unmyelinated and varicose, and contained a great many clear and/or large dense-cored vesicles. Substance P-immunoreactive cells were seen in the pars distalis of the anterior pituitary. Direct contacts between these nerve fibres and every cell type of the gland could be ascertained, including substance P-immunoreactive cells. Synaptoid contacts were seen among the substance P-immunoreactive varicosities and somatotropes, opiocortico-melanotropes and folliculostellate cells. Typical synapses were identified in somatotropes and opiocortico-melanotropes.

Pituitary adenylate cyclase activating peptide-38 (PACAP-38), PACAP-27, and PACAP related peptide (PRP) in the rat median eminence and pituitary

Pituitary adenylate cyclase activating peptide (PACAP) is a member of the vasoactive intestinal peptide-like peptide family. It is found in the hypothalamus, where the PACAP precursor is processed to form PACAP-38, the C-terminal truncated PACAP-27 and PACAP related peptide (PRP). Both PACAPs are potent stimulators of anterior pituitary adenylate cyclase activity, but the physiologically relevant anatomical sources of PACAP and possible importance of PRP in this regard are poorly understood. Using immunocytochemistry with epitope-specific antisera, we now show that PACAP38-, PACAP27- and PRP-positive nerve fibres are all present in the rat median eminence. The major immunoreactive species present was PACAP38. Numerous PACAP38-immunoreactive nerve fibres were observed in the internal layer and a few were present in the posterior pituitary lobe. The external layer of the median eminence contained a few PACAP-38-immunoreactive fibres and PACAP-38-positive nerve terminals were rarely seen in the perivascular portal spaces. Surprisingly, delicate PACAP-38-positive nerve fibres were identified in the anterior pituitary lobe intermingled between the pituitary cells although none of the secretory pituitary cells contained immunoreactive PACAP38, PACAP27 or PRP and preproPACAP mRNA was not detected in the gland by Northern blotting or in situ hybridization. PACAP-27- and PRP-immunoreactive nerve fibres and terminals were found in the same locations as PACAP-38 although generally in lower numbers. Specific radioimmunoassays and HPLC revealed that PACAP-38 accounts for the vast majority of the adenohypophyseal PACAP-immunoreactivity, whereas PACAP-27 and PRP were found in low to undetectable concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)