Immunocytochemical ultrastructural study of hypothalamic neurons containing corticotropin-releasing factor in normal and adrenalectomized rats

Copeptin Levels Before and After Transsphenoidal Surgery for Cushing Disease: A Potential Early Marker of Remission

Context

Arginine-vasopressin and CRH act synergistically to stimulate secretion of ACTH. There is evidence that glucocorticoids act via negative feedback to suppress arginine-vasopressin secretion.

Objective

Our hypothesis was that a postoperative increase in plasma copeptin may serve as a marker of remission of Cushing disease (CD).

Design

Plasma copeptin was obtained in patients with CD before and daily on postoperative days 1 through 8 after transsphenoidal surgery. Peak postoperative copeptin levels and Δcopeptin values were compared among those in remission vs no remission.

Results

Forty-four patients (64% female, aged 7-55 years) were included, and 19 developed neither diabetes insipidus (DI) or syndrome of inappropriate anti-diuresis (SIADH). Thirty-three had follow-up at least 3 months postoperatively. There was no difference in peak postoperative copeptin in remission (6.1 pmol/L [4.3-12.1]) vs no remission (7.3 pmol/L [5.4-8.4], P = 0.88). Excluding those who developed DI or SIADH, there was no difference in peak postoperative copeptin in remission (10.2 pmol/L [6.9-21.0]) vs no remission (5.4 pmol/L [4.6-7.3], P = 0.20). However, a higher peak postoperative copeptin level was found in those in remission (14.6 pmol/L [±10.9] vs 5.8 (±1.4), P = 0.03]) with parametric testing. There was no difference in the Δcopeptin by remission status.

Conclusions

A difference in peak postoperative plasma copeptin as an early marker to predict remission of CD was not consistently present, although the data point to the need for a larger sample size to further evaluate this. However, the utility of this test may be limited to those who develop neither DI nor SIADH postoperatively.

Vasopressin and the regulation of hypothalamic-pituitary-adrenal axis function: implications for the pathophysiology of depression

The role of arginine vasopressin (AVPNP) in the control of adrenocorticotropic hormone (ACTH) secretion is explored, and in particular, its involvement in various stress response paradigms which may be of relevance in our understanding of the pathophysiology of depression. VP is released from two sites in the hypothalamus; the parvicellular division of the paraventricular nucleus (PVN), where corticotropin releasing hormone (CRH) is also formed, and from the magnocellular neurons of the supraoptic nucleus (SON) and the PVN. The intricate interaction with CRH, the other main ACTH secretagogue, and with glucocorticoids, the inhibitory feedback component of hypothalamic-pituitary-adrenal-axis (HPA) activity, is outlined. That VP plays an important role in the stress response is now beyond doubt. Examination of the impact of psychological stressors on the differential expression of VP and CRH at a hypothalamic and pituitary level has been facilitated by advances in molecular biological techniques. Of importance has been the cloning of the V1b receptor gene, the receptor at which AVP is active in the anterior pituitary. Chronic stress paradigms, associated with HPA hyperresponsiveness, and ACTH release following a novel superimposed stress, have been found with relative consistency to show a shift in the CRH:AVP ratio. This may relate to differing feedback sensitivity of AVP to glucocorticoid feedback restraint and the greater responsivity of AVP over CRH to chronic stimulatory stress input. Evidence for functionally distinct pools of ACTH releasing corticotropes, and the finding that AVP levels more closely correlate with ACTH levels than do CRH levels, suggest a more dynamic role for AVP in activity of the stress axis, and a primarily permissive function for CRH. The renewed interest in the role of VP in HPA axis activity may have important implications for furthering our understanding of psychiatric conditions such as depression, where significant dysregulation of this axis is seen. Elevated baseline cortisol, dexamethasone non-suppression and blunted CRH/ACTH release have been consistently documented. The possible contribution of VP to this hyperactivity, despite its known synergy with CRH, has been largely neglected. In animal models there is clear evidence that chronic psychological stressors increase the ratio of AVP to CRH production. Psychosocial stressors are intrinsically linked with depressive illness. The finding of elevated levels of AVP in postmortem studies of depressives and the lowering of CSF AVP levels by antidepressants, raises the question of the precise role of AVP in the overactivity of the HPA in depression, a finding that is currently attributed to overdrive of its HPA regulatory companion, CRH.

The cellular localization of vasoactive intestinal peptide (VIP) in the mouse median eminence by immuno-electron microscopy

The aim of this study was to examine, by use of pre-embedding immunocytochemistry, the ultrastructural localization of vasoactive intestinal peptide (VIP) immunoreactivity in the mouse median eminence. VIP immunoreactivity was observed in axonal profiles. The VIP-immunoreactive axonal profiles were in close proximity to non-immunoreactive axonal profiles that contained dense granular vesicles and clear vesicles and also to processes of tanycytes. VIP-immunoreactive terminals were observed in the proximity of the perivascular space and in the neuropil. Our results suggest that VIP-immunoreactive axon terminals may possibly interact with other non-immunoreactive axon terminals containing peptide and/or other transmitters at the level of the median eminence or may be released to the portal vasculature thereby to effect anterior pituitary cells.

Immunolocalization of polysialic acid in the median eminence and neurointermediate hypophysial lobe of adult rats

Polysialic acid (PSA) is abundant on growing axons during brain development and down regulated on maturation. However, high amounts of this carbohydrate polymer have been found to persist in some regions of the adult rat brain including the mediobasal hypothalamus. In this study, confocal laser scanning microscopy combined with double fluorescence immunostaining was used to characterize the cellular localization of PSA throughout the median eminence and neurointermediate hypophysial lobe of adult rats. In these regions, polysialic acid-immunoreactivity (PSA-IR) generally appeared associated with fiber-like structures. Double immunostaining experiments demonstrated that, in addition to large axons of the neural lobe immunoreactive to vasopressin or oxytocin, PSA was constantly associated with fibers projecting into the intermediate hypophysial lobe immunoreactive to either gamma-aminobutyric acid (GABA) or tyrosine hydroxylase. Similarly, PSA-IR was detected on most, but not all the fibers immunoreactive to GABA or tyrosine hydroxylase dispersed throughout the neural lobe and the different layers of the median eminence. On the other hand, no PSA-IR was detected on axons immunoreactive to somatostatin or to corticotropin releasing hormone projecting throughout the median eminence, or on glial cell bodies and processes immunoreactive for glial fibrillary acidic protein (GFAP) or for vimentin dispersed throughout the median eminence and the neural lobe.

Short-term but not long-term adrenalectomy modulates amplitude and frequency of the CRH41 episodic release in push-pull cannulated median eminence of free-moving rats

CRH 41 release in push-pull cannulated median eminence (ME) was measured in unanesthetized male rats, 3 and 7 days after adrenalectomy (ADX) and in sham-lesioned controls. Perfusion started at 13.30 h and perfusate samples were collected at 5 min intervals for 3 h to estimate the mean release rate of CRH41. The major parameters of the neurohormone's episodic release pattern were analyzed using the Ultra algorithm. In a parallel study, 3 groups of similarly treated rats were used to measure plasma ACTH and hypothalamic CRH41. Three days after ADX, the plasma ACTH titers had risen 14-fold, the hypothalamic CRH41 content had decreased by 40%, while the CRH41 release in the ME had doubled as a result of a significant increase in most variables of the pulsatile release pattern: pulse frequency (+34%; P < 0.01), mean amplitude (+36%; P < 0.05), mean peak levels (+67%; P < 0.01) and mean pulse nadirs (x2.5; P < 0.01). Seven days after ADX, even though plasma ACTH had further increased to 30-times control levels, hypothalamic CRH41 content and CRH41 release in the ME had returned to almost control levels. The possible mechanisms of the discrepancy between the CRH and ACTH response time-courses following ADX are discussed.

Ultrastructural localization of galanin immunoreactivity in the rat median eminence

The aim of this study was to examine, by use of a pre-embedding immunoperoxidase technique, the ultrastructural localization of galanin immunoreactivity in the external layer of the rat median eminence. Galanin immunoreactivity was only observed in axonal profiles. Immunoreactive fibers were found in contact with the following non-immunoreactive structures: (1) axonal profiles that contain dense granular vesicles and clear vesicles, (2) axonal profiles that contain predominantly clear vesicles, (3) glial cell bodies, and (4) processes of tanycytes. Labeled terminals were also observed in the proximity of the perivascular space of the portal vessels. The results suggest possible interactions between galanin-immunoreactive terminals and other terminals containing peptide and/or other transmitters in the external layer of the median eminence.

Effect of hypothalamic norepinephrine depletion on median eminence CRF-41 content and serum ACTH in control and adrenalectomized rats

In this study we examined the role of the noradrenergic innervation of the hypothalamus on the adrenalectomy-induced changes in median eminence (ME) CRF-41 and serum ACTH. 6-Hydroxydopamine (6-OHDA), the catecholaminergic neurotoxin, or vehicle was injected into the ventral noradrenergic bundle of male rats. One week later animals underwent adrenalectomy or sham operation and were sacrificed 18 or 120 h later. In sham-operated rats 6-OHDA did not affect ME CRF-41 content or serum ACTH. In vehicle-injected adrenalectomized rats ACTH was increased approximately 3-fold at 18 h and almost 6-fold at 120 h. At 18 h CRF-41 content was markedly depleted (reduced approximately 20-fold) but by 120 h CRF-41 content had partially recovered and was about 70% of control animals. In adrenalectomized animals, 6-OHDA lesions caused a complete inhibition of the increase in serum ACTH both at 18 h and at 120 h. Pretreatment with 6-OHDA partially attenuated the drastic reduction in ME CRF-41 content following adrenalectomy at 18 h. However, at 120 h, the neurotoxin prevented the recovery of CRF-41 following adrenalectomy. These results suggest that intact norepinephrine innervation to the hypothalamus is necessary for the increased production of ACTH following adrenalectomy and that its interruption interferes with both the adrenalectomy-induced ME CRF-41 reduction and subsequent recovery.

Hypophysiotrophic neurons are capable of altering the ratio of co-packaged neurohormones

Neuronal dense-core vesicles provide a mechanism whereby peptide messengers are secreted in discrete quanta. Here we report on the capacity of rat hypophysiotrophic corticotropin releasing factor-41 neurons to alter the peptide content as well as the size of dense-core vesicles after removal of glucocorticoid negative feedback by adrenalectomy. We demonstrate, using quantitative immunoelectron microscopy, that long-term adrenalectomy induces a progressive increase in the ratio of vasopressin to corticotropin releasing factor-41-immunoreactive sites in the dense-core vesicle compartment. The intravesicular concentration of vasopressin appeared to be the variable parameter while that of the corticotropin releasing factor-41 remained stable at all survival times after adrenalectomy. Moreover, observations for up to 5 weeks indicate that adrenalectomy results in a progressive increase in the mean volume of dense-core vesicles to about three times normal. These results suggest that the quantal size and the composition of dense-core vesicles are subject to long-term modulation. The capacity of corticotropin releasing factor-41 neurons to alter dense-core vesicles could enhance or diminish the efficacy of the hypothalamohypophyseal communication underlying physiological adaptation to stress, as well as pathological changes.

Effect of pharmacological adrenalectomy on corticotropin-releasing factor-like and arginine vasotocin immunoreactivities in the brain and pituitary of the eel: immunocytochemical study

Metopirone, a blocker of the 11 beta-hydroxylase, was used to suppress cortisol synthesis in the eel, producing a pharmacological adrenalectomy. After 1 and 2 days, corticotropin-releasing factor (CRF) immunoreactivity was reduced in the preoptic nucleus (PON) and was increased in the rostral neurohypophysis (NH) where CRF fibers ended in close proximity to ACTH cells. After 4 days, immunoreactive (ir) CRF increased in the PON or was similar to that of controls, and was reduced in the rostral NH. It was not affected in the caudal NH among intermediate lobe (IL) ramifications. Arginine vasotocin (AVT) immunostaining was often slightly increased in the PON, but changes were not apparent in the pituitary and AVT-ir fibers remained very scarce in the rostral NH. Metopirone significantly increased the cross-sectional area of CRF- and AVT-ir perikarya in the parvocellular region and the number of these perikarya as well as the ratio of CRF to AVT cell bodies in the totality of the PON. Colocalization of CRF and AVT, barely detected in control eels, occurred in 9.6% of CRF perikarya after 1-2 days, but in only 2.4% after 4 days. ACTH cells were rapidly and markedly stimulated. These immunocytochemical studies suggest that CRF synthesis, axonal transport, and release are increased in metopirone-treated eels, acting on the pituitary to stimulate ACTH release and interrenal cell activity. The participation of AVT in ACTH cell stimulation is less clear. These data are compared to those reported in mammals.

Neuropeptides regulating the activity of goldfish corticotropes and melanotropes

The goldfish (Carassius auratus) has proven an advantageous model for investigations of the neuroendocrine regulation of pituitary hormone secretion in teleost fishes. Investigations examining the secretion of adrenocorticotropin (ACTH) and melanocyte-stimulating hormone (MSH) from pituitary cellsin vitro have been used to identify neuropeptides influencing goldfish corticotrope and melanotrope activity. Ovine CRF, urotensin I (UI), arginine vasotocin (AVT), isotocin and angiotensins I and II stimulate the release of ACTH from corticotropesin vitro. Thyrotropin-releasing hormone (TRH), oCRF, UI and neuropeptide Y stimulate the release of MSH from melanotropesin vitro. Immunocytochemical studies have revealed the presence of separate CRF- and UI-immunoreactive perikarya in the hypothalamus suggesting the existence of two structurally similar, yet distinct, hypothalamic CRF-UI-like peptides. Interactions of AVT and CRF in the regulation of ACTH secretion is suggested from studies demonstrating the co-localization of AVT- and CRF-immunoreactivities in perikarya of the preoptic-hypophyseal system. These investigations demonstrate that the secretory activity of goldfish corticotropes and melanotropes is influenced by a diversity of neuropeptides of hypothalamic origin.

The effects of adrenalectomy, corticotropin releasing factor and vasopressin on the sympathetic firing rate of nerves to interscapular brown adipose tissue in the Zucker rat

The firing rate of the sympathetic efferent nerves to interscapular brown adipose tissue (IBAT) is lower in the obese rat compared with the lean rat. The present experiments show that adrenalectomy has no effect on nerve firing rate in the lean rat and a small but statistically nonsignificant effect in the obese rat. Injection of corticotropin releasing factor (CRF) into the IIIrd ventricle produced a dose dependent increase in the firing rate of the sympathetic nerves to interscapular brown adipose tissue (IBAT) in both lean and obese rats. The basal (unstimulated) level of firing was lower in the obese rat compared with the lean rat and remained significantly below lean values at each dose. The minimum dose of CRF to see an effect (125 ng) and the dose at which maximum effect on nerve firing rate was observed (500 ng) was similar in both genotypes. Injection of adrenocorticotropic hormone (ACTH) had no effect on nerve firing rate to IBAT. Central administration of vasopressin produced a significant increase in sympathetic firing rate to IBAT in both lean and obese rats. The temperature of IBAT was also significantly increased with vasopressin and the duration of the response was longer compared with CRF, but the minimum dose to see an effect was higher (2.5 micrograms). The response to vasopressin was greater in the obese rat compared with the lean rat but the maximum firing rate did not achieve levels observed in lean rats. Chronic infusion of CRF into the IIIrd ventricle of obese rats resulted in a reduction of food intake and body weight gain but IBAT mitochondrial GDP binding was unaltered by the treatment. These data are consistent with the hypothesis that the defect in the obese Zucker rat may be due to a glucocorticoid inhibition of CRF and/or vasopressin action in the CNS.

Studies on the sympathetic efferent nerves of brown adipose tissue of lean and obese Zucker rats

Previous studies have suggested that the sympathetic tone to brown adipose tissue (BAT) is reduced in the genetically obese (fa/fa) rat. The following experiments were designed to examine with electrophysiological techniques the activity of the sympathetic nerve innervating the interscapular BAT. The spontaneous activity of the efferent nerves was reduced in the obese (fa/fa) rat compared with the lean control. The activity of the nerve showed a linear relationship with changes in core temperature in both genotypes. Electrical stimulation of the ventromedial hypothalamus resulted in similar heat increments in BAT temperature for lean and obese, but this was associated with a smaller increase in nerve firing in the obese rat. Intracerebroventricular administration of glucose enhanced the nerve activity, whereas 2-deoxy-D-glucose reduced the nerve activity in both lean and obese rats. These data suggest that the sympathetic tone is suppressed in the genetically obese rat, but the response to temperature and central glucose metabolism is intact.

Fine structures of nerve fibers with corticotropin-releasing factor-like immunoreactivity in the rat lateral septum

The fine structures of nerve fibers with corticotropin-releasing factor (CRF)-like immunoreactivity in the rat lateral septum were investigated by means preembedding immunoelectron microscopy. A number of CRF axon terminals formed synapses with cell bodies of non-immunoreactive septal neurons. They occasionally had broad terminal bulges whose subregions showed little or no immunoreactivity for CRF. CRF axon terminals were also in synaptic contact with non-immunoreactive dendrites or dendritic spines. Some dendrites with CRF were postsynaptic to non-immunoreactive axon terminals.