Neuroepithelial progenitors generate and propagate non-neuronal action potentials across the spinal cord

In the developing central nervous system, electrical signaling is thought to rely exclusively on differentiating neurons as they acquire the ability to generate and propagate action potentials. Accordingly, neuroepithelial progenitors (NEPs), which give rise to all neurons and glial cells during development, have been reported to remain electrically passive. Here, we investigated the physiological properties of NEPs at the onset of spontaneous neural activity (SNA) initiating motor behavior in mouse embryonic spinal cord. Using patch-clamp recordings, we discovered that spinal NEPs exhibit spontaneous membrane depolarizations during episodes of SNA. These rhythmic depolarizations exhibited a ventral-to-dorsal gradient with the highest amplitude located in the floor plate, the ventral-most part of the neuroepithelium. Paired recordings revealed that NEPs are coupled via gap junctions and form an electrical syncytium. Although other NEPs were electrically passive, we discovered that floor-plate NEPs generated large Na⁺/Ca²⁺ action potentials. Unlike in neurons, floor-plate action potentials relied primarily on the activation of voltage-gated T-type calcium channels (TTCCs). In situ hybridization showed that all 3 known subtypes of TTCCs are predominantly expressed in the floor plate. During SNA, we found that acetylcholine released by motoneurons rhythmically triggers floor-plate action potentials by acting through nicotinic acetylcholine receptors. Finally, by expressing the genetically encoded calcium indicator GCaMP6f in the floor plate, we demonstrated that neuroepithelial action potentials are associated with calcium waves and propagate along the entire length of the spinal cord. Our work reveals a novel physiological mechanism to generate and propagate electrical signals across a neural structure independently from neurons.

Axoglial synapses are formed onto pioneer oligodendrocyte precursor cells at the onset of spinal cord gliogenesis

Virtually all oligodendrocyte precursors cells (OPCs) receive glutamatergic and/or GABAergic synapses that are lost upon their differentiation into oligodendrocytes in the postnatal and adult brain. Although OPCs are generated at mid-embryonic stages, several weeks before the onset of myelination, it remains unknown when and where OPCs receive their first synapses and become susceptible to the influence of neuronal activity. In the embryonic spinal cord, neuro-epithelial precursors in the pMN domain cease generating cholinergic motor neurons (MNs) to produce OPCs when the first synapses are formed in the ventral-lateral marginal zone. We discovered that when the first synapses form onto MNs, axoglial synapses also form onto the processes of neuro-epithelial precursors located in the marginal zone as they differentiate into OPCs. After leaving the neuro-epithelium, these pioneer OPCs preferentially accumulate in the marginal zone where they are contacted by functional glutamatergic and GABAergic synapses. Spontaneous activity of these axoglial synapses was significantly potentiated by cholinergic signaling acting through presynaptic nicotinic acetylcholine receptors. Moreover, we discovered that chronic nicotine treatment significantly increases early OPC proliferation and density in the marginal zone. Our results demonstrate that OPCs are contacted by functional synapses as soon as they emerge from their precursor domain and that embryonic spinal cord colonization by OPCs can be regulated by cholinergic signaling acting onto these axoglial synapses.

Combining Cadherin Expression with Molecular Markers Discriminates Invasiveness in Growth Hormone and Prolactin Pituitary Adenomas

Although growth hormone (GH)- and prolactin (PRL)-secreting pituitary adenomas are considered benign, in many patients, tumour growth and/or invasion constitute a particular challenge. In other tumours, progression relies in part on dysfunction of intercellular adhesion mediated by the large family of cadherins. In the present study, we have explored the contribution of cadherins in GH and PRL adenoma pathogenesis, and evaluated whether this class of adherence molecules was related to tumour invasiveness. We have first established, by quantitative polymerase chain reaction and immunohistochemistry, the expression profile of classical cadherins in the normal human pituitary gland. We show that the cadherin repertoire is restricted and cell-type specific. Somatotrophs and lactotrophs express mainly E-cadherin and cadherin 18, whereas N-cadherin is present in the other endocrine cell types. This repertoire undergoes major differential modification in GH and PRL tumours: E-cadherin is significantly reduced in invasive GH adenomas, and this loss is associated with a cytoplasmic relocalisation of cadherin 18 and catenins. In invasive prolactinomas, E-cadherin distribution is altered and is accompanied by a mislocalisation of cadherin 18, β-catenin and p120 catenin. Strikingly, de novo expression of N-cadherin is present in a subset of adenomas and cells exhibit a mesenchymal phenotype exclusively in invasive tumours. Binary tree analysis, performed by combining the cadherin repertoire with the expression of a subset of known molecular markers, shows that cadherin/catenin complexes play a significant role in discrimination of tumour invasion.

Sustained alterations of hypothalamic tanycytes during posttraumatic hypopituitarism in male mice

Traumatic brain injury is a leading cause of hypopituitarism, which compromises patients' recovery, quality of life, and life span. To date, there are no means other than standardized animal studies to provide insights into the mechanisms of posttraumatic hypopituitarism. We have found that GH levels were impaired after inducing a controlled cortical impact (CCI) in mice. Furthermore, GHRH stimulation enhanced GH to lower level in injured than in control or sham mice. Because many characteristics were unchanged in the pituitary glands of CCI mice, we looked for changes at the hypothalamic level. Hypertrophied astrocytes were seen both within the arcuate nucleus and the median eminence, two pivotal structures of the GH axis, spatially remote to the injury site. In the arcuate nucleus, GHRH neurons were unaltered. In the median eminence, injured mice exhibited unexpected alterations. First, the distributions of claudin-1 and zonula occludens-1 between tanycytes were disorganized, suggesting tight junction disruptions. Second, endogenous IgG was increased in the vicinity of the third ventricle, suggesting abnormal barrier properties after CCI. Third, intracerebroventricular injection of a fluorescent-dextran derivative highly stained the hypothalamic parenchyma only after CCI, demonstrating an increased permeability of the third ventricle edges. This alteration of the third ventricle might jeopardize the communication between the hypothalamus and the pituitary gland. In conclusion, the phenotype of CCI mice had similarities to the posttraumatic hypopituitarism seen in humans with intact pituitary gland and pituitary stalk. It is the first report of a pathological status in which tanycyte dysfunctions appear as a major acquired syndrome.

Data supporting a new physiological role for brain apelin in the regulation of hypothalamic oxytocin neurons in lactating rats

Apelin is a bioactive peptide identified as the endogenous ligand of the human orphan G protein-coupled receptor APJ in 1998. The present data show that apelin modulates the activity of magnocellular and parvocellular oxytocin (OXY) neurons in the lactating rat. A combination of in situ hybridization and immunohistochemistry demonstrated the presence of apelin receptor mRNA in hypothalamic OXY neurons. Double immunofluorescence labeling then revealed the colocalization of apelin with OXY in about 20% of the hypothalamic OXY-positive neurons. Intracerebroventricular apelin administration inhibited the activity of magnocellular and parvocellular OXY neurons, as shown by measuring the c-fos expression in OXY neurons or by direct electrophysiological measurements of the electrical activity of these neurons. This effect was correlated with a decrease in the amount of milk ejected. Thus, apelin inhibits the activity of OXY neurons through a direct action on apelin receptors expressed by these neurons in an autocrine and paracrine manner. In conclusion, these findings highlight the inhibitory role of apelin as an autocrine/paracrine peptide acting on OXY neurons during breastfeeding.

Ghrelin stimulation of growth hormone-releasing hormone neurons is direct in the arcuate nucleus

Background

Ghrelin targets the arcuate nucleus, from where growth hormone releasing hormone (GHRH) neurones trigger GH secretion. This hypothalamic nucleus also contains neuropeptide Y (NPY) neurons which play a master role in the effect of ghrelin on feeding. Interestingly, connections between NPY and GHRH neurons have been reported, leading to the hypothesis that the GH axis and the feeding circuits might be co-regulated by ghrelin.

Principal Findings

Here, we show that ghrelin stimulates the firing rate of identified GHRH neurons, in transgenic GHRH-GFP mice. This stimulation is prevented by growth hormone secretagogue receptor-1 antagonism as well as by U-73122, a phospholipase C inhibitor and by calcium channels blockers. The effect of ghrelin does not require synaptic transmission, as it is not antagonized by γ-aminobutyric acid, glutamate and NPY receptor antagonists. In addition, this hypothalamic effect of ghrelin is independent of somatostatin, the inhibitor of the GH axis, since it is also found in somatostatin knockout mice. Indeed, ghrelin does not modify synaptic currents of GHRH neurons. However, ghrelin exerts a strong and direct depolarizing effect on GHRH neurons, which supports their increased firing rate.

Conclusion

Thus, GHRH neurons are a specific target for ghrelin within the brain, and not activated secondary to altered activity in feeding circuits. These results support the view that ghrelin related therapeutic approaches could be directed separately towards GH deficiency or feeding disorders.