Glutamatergic innervation of growth hormone-releasing hormone-containing neurons in the hypothalamic arcuate nucleus and somatostatin-containing neurons in the anterior periventricular nucleus of the rat

Somatostatin-immunoreactive neurons of the rat gut during the development

Somatostatin (SST) is a peptide expressed in the peripheral and central nervous systems, as well as in endocrine and immune cells. The aim of the current study is to determine the percentage of SST immunoreactive (IR) neurons and their colocalization with choline acetyltransferase (ChAT), neuronal nitric oxide synthase (nNOS), neuropeptide Y (NPY), and glial fibrillary acidic protein (GFAP) in the myenteric plexus (MP) and submucous plexus (SP) of the small intestine (SI) and large intestine (LI) of rats across different age groups from newborn to senescence using immunohistochemistry. In the MP of the SI and LI, the percentage of SST-IR neurons significantly increased during early postnatal development from 12 ± 2.4 (SI) and 13 ± 3.0 (LI) in newborn rats to 23 ± 1.5 (SI) and 18 ± 1.6 (LI) in 20-day-old animals, remaining stable until 60 days of age. The proportion of SST-IR cells then decreased in aged 2-year-old animals to 14 ± 2.0 (SI) and 10 ± 2.6 (LI). In the SP, the percentage of SST-IR neurons significantly rose from 22 ± 3.2 (SI) and 23 ± 1.7 (LI) in newborn rats to 42 ± 4.0 in 20-day-old animals (SI) and 32 ± 4.9 in 30-day-old animals (LI), before declining in aged 2-year-old animals to 21 ± 2.6 (SI) and 28 ± 7.4 (LI). Between birth and 60 days of age, 97-98% of SST-IR neurons in the MP and SP colocalized with ChAT in both plexuses of the SI and LI. The percentage of SST/ChAT neurons decreased in old rats to 85 ± 5.0 (SI) and 90 ± 3.8 (LI) in the MP and 89 ± 3.2 (SI) and 89 ± 1.6 (LI) in the SP. Conversely, in young rats, only a few SST-IR neurons colocalized with nNOS, but this percentage significantly increased in 2-year-old rats. The percentage of SST/NPY-IR neurons exhibited considerable variation throughout postnatal development, with no significant differences across different age groups in both the MP and SP of both intestines. No colocalization of SST with GFAP was observed in any of the studied animals. In conclusion, the expression of SST in enteric neurons increases in young rats and decreases in senescence, accompanied by changes in SST colocalization with ChAT and nNOS.

Hypothalamic neurons fully or partially expressing the dopaminergic phenotype: development, distribution, functioning and functional significance. A review

The hypothalamus is a key link in neuroendocrine regulations, which are provided by neuropeptides and dopamine. Until the late 1980 s, it was believed that, along with peptidergic neurons, hypothalamus contained dopaminergic neurons. Over time, it has been shown that besides dopaminergic neurons expressing the dopamine transporter and dopamine-synthesizing enzymes - tyrosine hydroxylase (TH) and aromatic L-amino acid decarboxylase (AADC) - the hypothalamus contains neurons expressing only TH, only AADC, both enzymes or only dopamine transporter. The end secretory product of TH neurons is L-3,4-dihydroxyphenylalanine, while that of AADC neurons and bienzymatic neurons lacking the dopamine transporter is dopamine. During ontogenesis, especially in the perinatal period, monoenzymatic neurons predominate in the hypothalamic neuroendocrine centers. It is assumed that L-3,4-dihydroxyphenylalanine and dopamine are released into the neuropil, cerebral ventricles, and blood vessels, participating in the regulation of target cell differentiation in the perinatal period and the functioning of target cells in adulthood.

Molecular Mechanisms of the Regulation of Liver Cytochrome P450 by Brain NMDA Receptors and via the Neuroendocrine Pathway-A Significance for New Psychotropic Therapies

Recent investigations have highlighted the potential utility of the selective antagonist of the NMDA receptor GluN2B subunit for addressing major depressive disorders. Our previous study showed that the systemic administration of the antagonist of the GluN2B subunit of the NMDA receptor, the compound CP-101,606, affected liver cytochrome P450 expression and activity. To discern between the central and peripheral mechanisms of enzyme regulation, our current study aimed to explore whether the intracerebral administration of CP-101,606 could impact cytochrome P450. The injection of CP-101,606 to brain lateral ventricles (6, 15, or 30 µg/brain) exerted dose-dependent effects on liver cytochrome P450 enzymes and hypothalamic or pituitary hormones. The lowest dose led to an increase in the activity, protein, and mRNA level of CYP2C11 compared to the control. The activities of CYP2A, CYP2B, CYP2C11, CYP2C6, CYP2D, and protein levels of CYP2B, CYP2C11 were enhanced compared to the highest dose. Moreover, CP-101,606 increased the CYP1A protein level coupled with elevated CYP1A1 and CYP1A2 mRNA levels, but not activity. The antagonist decreased the pituitary somatostatin level and increased the serum growth hormone concentration after the lowest dose, while independently decreasing the serum corticosterone concentration of the dose. The findings presented here unveil a novel physiological regulatory mechanism whereby the brain glutamatergic system, via the NMDA receptor, influences liver cytochrome P450. This regulatory process appears to involve the endocrine system. These results may have practical applications in predicting alterations in cytochrome P450 activity and endogenous metabolism, and potential metabolic drug-drug interactions elicited by drugs that cross the blood-brain barrier and affect NMDA receptors.

BASS: multi-scale and multi-sample analysis enables accurate cell type clustering and spatial domain detection in spatial transcriptomic studies

Spatial transcriptomic studies are reaching single-cell spatial resolution, with data often collected from multiple tissue sections. Here, we present a computational method, BASS, that enables multi-scale and multi-sample analysis for single-cell resolution spatial transcriptomics. BASS performs cell type clustering at the single-cell scale and spatial domain detection at the tissue regional scale, with the two tasks carried out simultaneously within a Bayesian hierarchical modeling framework. We illustrate the benefits of BASS through comprehensive simulations and applications to three datasets. The substantial power gain brought by BASS allows us to reveal accurate transcriptomic and cellular landscape in both cortex and hypothalamus.

Ablation of Growth Hormone Receptor in GABAergic Neurons Leads to Increased Pulsatile Growth Hormone Secretion

Growth hormone (GH) acts in several hypothalamic neuronal populations to modulate metabolism and the autoregulation of GH secretion via negative-feedback loops. However, few studies have investigated whether GH receptor (GHR) expression in specific neuronal populations is required for the homeostatic control of GH secretion and energy homeostasis. In the present study, we investigated the consequences of the specific GHR ablation in GABAergic (VGAT-expressing) or glutamatergic (VGLUT2-expressing) cells. GHR ablation in GABAergic neurons led to increased GH secretion, lean mass, and body growth in male and female mice. VGAT-specific GHR knockout (KO) male mice also showed increased serum insulin-like growth factor-1, hypothalamic Ghrh, and hepatic Igf1 messenger RNA levels. In contrast, normal GH secretion, but reduced lean body mass, was observed in mice carrying GHR ablation in glutamatergic neurons. GHR ablation in GABAergic cells increased weight loss and led to decreased blood glucose levels during food restriction, whereas VGLUT2-specific GHR KO mice showed blunted feeding response to 2-deoxy-D-glucose both in males and females, and increased relative food intake, oxygen consumption, and serum leptin levels in male mice. Of note, VGLUT2-cre female mice, independently of GHR ablation, exhibited a previously unreported phenotype of mild reduction in body weight without further metabolic alterations. The autoregulation of GH secretion via negative-feedback loops requires GHR expression in GABAergic cells. Furthermore, GHR ablation in GABAergic and glutamatergic neuronal populations leads to distinct metabolic alterations. These findings contribute to the understanding of the neuronal populations responsible for mediating the neuroendocrine and metabolic effects of GH.

The Selective NMDA Receptor GluN2B Subunit Antagonist CP-101,606 with Antidepressant Properties Modulates Cytochrome P450 Expression in the Liver

Recent research indicates that selective NMDA receptor GluN2B subunit antagonists may become useful for the treatment of major depressive disorders. We aimed to examine in parallel the effect of the selective NMDA receptor GluN2B subunit antagonist CP-101,606 on the pituitary/serum hormone levels and on the regulation of cytochrome P450 in rat liver. CP-101,606 (20 mg/kg ip. for 5 days) decreased the activity of CYP1A, CYP2A, CYP2B, CYP2C11 and CYP3A, but not that of CYP2C6. The alterations in enzymatic activity were accompanied by changes in the CYP protein and mRNA levels. In parallel, a decrease in the pituitary growth hormone-releasing hormone, and in serum growth hormone and corticosterone (but not T₃ and T₄) concentration was observed. After a 3-week administration period of CP-101,606 less changes were found. A decrease in the CYP3A enzyme activity and protein level was still maintained, though no change in the mRNA level was found. A slight decrease in the serum concentration of corticosterone was also maintained, while GH level returned to the control value. The obtained results imply engagement of the glutamatergic system in the neuroendocrine regulation of cytochrome P450 and potential involvement of drugs acting on NMDA receptors in metabolic drug–drug interactions.

Defining the caudal hypothalamic arcuate nucleus with a focus on anorexic excitatory neurons

KEY POINTS

Excitatory glutamate neurons are sparse in the rostral hypothalamic arcuate nucleus (ARC), the subregion that has received the most attention in the past. In striking contrast, excitatory neurons are far more common (by a factor of 10) in the caudal ARC, an area which has received relatively little attention. These glutamate cells may play a negative role in energy balance and food intake. They can show an increase in phosphorylated Stat-3 in the presence of leptin, are electrically excited by the anorectic neuromodulator cholecystokinin, and inhibited by orexigenic neuromodulators neuropeptide Y, met-enkephalin, dynorphin and the catecholamine dopamine. The neurons project local axonal connections that excite other ARC neurons including proopiomelanocortin neurons that can play an important role in obesity. These data are consistent with models suggesting that the ARC glutamatergic neurons may play both a rapid and a slower role in acting as anorectic neurons in CNS control of food intake and energy homeostasis.

ABSTRACT

Here we interrogate a unique class of excitatory neurons in the hypothalamic arcuate nucleus (ARC) that utilizes glutamate as a fast neurotransmitter using mice expressing GFP under control of the vesicular glutamate transporter 2 (vGluT2) promoter. These neurons show a unique distribution, synaptic characterization, cellular physiology and response to neuropeptides involved in energy homeostasis. Although apparently not previously appreciated, the caudal ARC showed a far greater density of vGluT2 cells than the rostral ARC, as seen in transgenic vGluT2-GFP mice and mRNA analysis. After food deprivation, leptin induced an increase in phosphorylated Stat-3 in vGluT2-positive neurons, indicating a response to hormonal cues of energy state. Based on whole-cell recording electrophysiology in brain slices, vGluT2 neurons were spontaneously active with a spike frequency around 2 Hz. vGluT2 cells were responsive to a number of neuropeptides related to energy homeostasis; they were excited by the anorectic peptide cholecystokinin, but inhibited by orexigenic neuropeptide Y, dynorphin and met-enkephalin, consistent with an anorexic role in energy homeostasis. Dopamine, associated with the hedonic aspect of enhancing food intake, inhibited vGluT2 neurons. Optogenetic excitation of vGluT2 cells evoked EPSCs in neighbouring neurons, indicating local synaptic excitation of other ARC neurons. Microdrop excitation of ARC glutamate cells in brain slices rapidly increased excitatory synaptic activity in anorexigenic proopiomelanocortin neurons. Together these data support the perspective that vGluT2 cells may be more prevalent in the ARC than previously appreciated, and play predominantly an anorectic role in energy metabolism.

Enhanced NMDA receptor NR1 phosphorylation and neuronal activity in the arcuate nucleus of hypothalamus following peripheral inflammation

AbstractAim:To investigate the role of glutamate and N-methyl-D-aspartate (NMDA) receptors in central sensitization following peripheral inflammation in the arcuate nucleus (ARC) of the mediobasal hypothalamus.

METHODS

Mediobasal hypothalamic slices were prepared from rats undergoing peripheral inflammation, which was induced by a unilateral injection of complete Freund's adjuvant (CFA) into hind paw. Neuronal activation levels in the ARC were monitored by recording extracellular unit discharges. The NMDA receptor NR1 subunit (NR1) was measured using Western blot analysis.

RESULTS

Enhanced NR1 phosphorylation was observed in the ARC of CFA-inflamed rats. Compared with the control rats, the firing rate of spontaneous discharges in ARC neurons of inflamed rats was significantly higher, and it was significantly reduced both by an NMDA receptor antagonist (MK-801, 300 μmol/L) and by a non-NMDA receptor antagonist (CNQX, 30 μmol/L). Application of exogenous glutamate (200 μmol/L) or NMDA (25 μmol/L) resulted in increased neuronal discharges for ARC neurons, which was enhanced to a greater extent in inflamed rats than in control rats.

CONCLUSION

Glutamate receptor activation in the hypothalamic ARC plays a crucial role in central sensitization associated with peripheral inflammation.

Growth hormone/insulin-like growth factor I axis in neurodegenerative diseases

Neurodegenerative diseases (ND) are a group of heterogeneous disorders characterized by unknown etiology, subtle onset, and progressive involvement of neuronal systems leading to degeneration and dysfunction. They represent a challenge for basic science and clinical medicine because of increasing prevalence, social cost, complex biochemistry and pathology, and lack of mechanism-based treatments. Endocrine modifications may accompany the progression of ND, due to the intimate connections between central nervous and endocrine systems. Reported data on endocrine changes in different ND have often been non-conclusive or conflicting. GH/IGF-I axis is involved in the regulation of brain growth, development, and metabolism. Dysfunctions in GH/IGF-I axis in most of ND are therefore reviewed. Whether GH deficiency, when present, may act as a contributory factor in the pathogenesis of these diseases, or might represent a consequence of it is presently unknown. A thorough effort in investigating every possible involvement of GH/IGF-I axis is warranted, in the light of future possible therapeutic strategies.

Subcellular dynamics of somatostatin receptor subtype 1 in the rat arcuate nucleus: receptor localization and synaptic connectivity vary in parallel with the ultradian rhythm of growth hormone secretion

Growth hormone (GH) secretion in male rats exhibits a 3.3 h ultradian rhythm generated by the reciprocal interaction of GH-releasing hormone (GHRH) and somatostatin (SRIF). SRIF receptor subtypes sst(1) and sst(2) are highly expressed in GHRH neurons of the hypothalamic arcuate nucleus (ARC). We previously demonstrated an ultradian oscillation in binding of SRIF analogs to the ARC in relation to GH peaks and troughs. Here we tested the hypothesis that these ultradian changes in SRIF binding are due to differential plasma membrane targeting of sst(1) receptors in ARC neurons using immunocytochemistry and electron microscopy. We found that 87% of sst(1)-positive ARC neurons also synthesized GHRH. Subcellularly, 80% of sst(1) receptors were located intracellularly and 20% at the plasma membrane regardless of GH status. However, whereas 30% of the cell-surface sst(1) receptors were located perisynaptically or subsynaptically following exposure to high GH secretion, this fraction was increased to 42% following a GH trough period (p = 0.05). Furthermore, the relative abundance of symmetric and asymmetric synapses on sst(1)-positive dendrites also varied significantly, depending on the GH cycle, from approximately equal numbers following GH troughs to 70:30 in favor of symmetric, i.e., inhibitory, inputs after GH peaks (p < 0.02). These findings suggest that postsynaptic localization of sst(1) receptors and synaptic connectivity in the ARC undergo pronounced remodeling in parallel with the GH rhythm. Such synaptic plasticity may be an important mechanism by which sst(1) mediates SRIF's cyclical effects on ARC GHRH neurons to generate the ultradian rhythm of GH secretion.

Novel aspects of glutamatergic signalling in the neuroendocrine system

L-glutamate, the main excitatory neurotransmitter, influences virtually all neurones of the neuroendocrine hypothalamus via synaptic mechanisms. Vesicular glutamate transporters (VGLUT1-3), which selectively accumulate L-glutamate into synaptic vesicles, provide markers with which to visualise glutamatergic neurones in histological preparations; excitatory neurones in the endocrine hypothalamus synthesise the VGLUT2 isoform. Results of recent dual-label in situ hybridisation studies indicate that glutamatergic neurones in the preoptic area and the hypothalamic paraventricular, supraoptic and periventricular nuclei include parvocellular and magnocellular neurosecretory neurones which secrete peptide neurohormones into the bloodstream to regulate endocrine functions. Neurosecretory terminals of GnRH, TRH, CRF-, somatostatin-, oxytocin- and vasopressin-secreting neurones contain VGLUT2 immunoreactivity, suggesting the co-release of glutamate with hypophysiotrophic peptides. The presence of VGLUT2 also indicates glutamate secretion from non-neuronal endocrine cells, including gonadotrophs and thyrotrophs of the anterior pituitary. Results of in vitro studies show that ionotropic glutamate receptor analogues can elicit hormone secretion at neuroendocrine/endocrine release sites. Structural constituents of the median eminence, adenohypophysis and neurohypophysis contain elements of glutamatergic transmission, including glutamate receptors and enzymes of the glutamate/glutamine cycle. The synthesis of VGLUT2 exhibits robust up-regulation in response to certain endocrine challenges, indicating that altered glutamatergic signalling may represent an important adaptive mechanism. This review article discusses the newly emerged non-synaptic role of glutamate in neuroendocrine and endocrine communication.

Impairment of GH secretion in amyotrophic lateral sclerosis is not affected by riluzole treatment

Amyotrophic lateral sclerosis (ALS), the most common motor neurone disorder in human adults, is characterized by selective and progressive degeneration of upper and lower motor neurones in the central nervous system. The main currently available drug for ALS treatment is riluzole, a compound that acts through inhibition of glutamate release, postsynaptic receptor activation, and voltage-sensitive channel inhibition. GH secretion, evaluated by GHRH+arginine (ARG) test, has recently been reported to be impaired in most untreated ALS patients. The aim of the present study was to evaluate whether riluzole administration could interfere with GH secretion and therefore with the diagnosis of adult GH deficiency. Ten patients (6 males, 4 females, mean age 59+/-11 yr) were studied performing GHRH+ARG test before and 3 months after starting riluzole treatment (100 mg/day). Blood samples for GH were collected at baseline, at 30 and 60 min. Both before and during riluzole treatment, 5 patients showed GH deficiency and 5 patients had a normal GH response according to body mass index (BMI). Mean peak GH levels were similar before and during riluzole treatment (13.4+/-10 vs 14.2+/-10.1 microg/l, p=ns). No significant correlation was observed between GH concentrations and age, BMI, disease duration, severity or clinical (bulbar/spinal) form. In conclusion, the present data confirm that GH secretion is impaired in a new series of ALS patients and indicate that riluzole treatment does not interfere with GH secretion. Thus, evaluation of GH secretion in ALS patients can also be performed without withdrawing riluzole treatment.

Vesicular glutamate transporter 2 protein and mRNA containing neurons in the hypothalamic suprachiasmatic nucleus of the rat

The hypothalamic suprachiasmatic nucleus is the key structure of the control of circadian rhythms and has a rich glutamatergic innervation. Besides the presence of glutamatergic afferents, several findings also suggest the existence of glutamatergic efferents from the suprachiasmatic nucleus to its target neurons in various prominent hypothalamic cell groups. However, there is no direct neuromorphological evidence for the presence of glutamatergic neurons in the suprachiasmatic nucleus. Therefore, the purpose of the present investigations was to try to clarify this question. Immunocytochemistry was used at the light and electron microscopy level to identify vesicular glutamate transporter type 2 (VGluT2) immunopositive (presumed glutamatergic) neurons in the rat suprachiasmatic nucleus. In addition VGluT2 mRNA expression in neurons of the nucleus was also addressed with radioisotopic in situ hybridization. Both at the light and electron microscopy level we detected VGluT2 positive neurons, which did not contain GABA, vasoactive intestinal polypeptide or vasopressin. Further, we demonstrated the expression of VGluT2 mRNA in a few cells within the suprachiasmatic nucleus; these glutamatergic cells were distinct from somatostatin mRNA expressing neurons. As VGluT2 is a selective marker of glutamatergic neuronal elements, the present observations provide direct neuromorphological evidence for the presence of glutamatergic neurons in the cell group.