Reversed synaptic effects of hypocretin and NPY mediated by excitatory GABA-dependent synaptic activity in developing MCH neurons

Sevoflurane-induced learning deficits and spine loss via nectin-1/corticotrophin-releasing hormone receptor type 1 signaling

In recent years, the neurotoxicity of general anesthetics in the developing brain has been studied and raised great concern as a major health issue to the public and physicians. Sevoflurane inhalation may induce neurotoxicity expressed as memory and learning impairment in young animals. In the current study, we investigated the role of nectin-1 and corticotrophin-releasing hormone receptor type 1 (CRHR1) in sevoflurane-induced learning deficits and dendritic spines loss in neonatal mice. Neonatal mice (P7) were treated with 3% sevoflurane with 60% O₂ or 60% O₂ for 6 h. Cognitive function was evaluated by Y Maze, Object recognition test, and Morris Water Maze. Hippocampal nectin-1 and L-afadin expression assessed using western blot analysis. The dendritic spines morphology of the hippocampus was determined using Golgi impregnation on 7 d and 2 months old. Sevoflurane exposed to neonatal mice decreased hippocampal nectin-1 levels from 1 h to 2 months after sevoflurane inhalation and attenuated working and spatial memory and spinal number in adulthood, which could be reversed by nectin-1 overexpression and CRHR1 antagonist Antalarmin. Nectin-1 knockdown caused spatial learning deficits and dendritic spine loss and lower L-afadin protein expression. Sevoflurane-induced nectin-1 and L-afadin expression decrease was mediated by CRHR1 signaling in the hippocampus. This information can be used to develop targeted intervention aimed at decreasing the neurotoxicity of sevoflurane inhalation.

Orexin/Hypocretin Signaling

Orexin/hypocretin peptide (orexin-A and orexin-B) signaling is believed to take place via the two G-protein-coupled receptors (GPCRs), named OX1 and OX2 orexin receptors, as described in the previous chapters. Signaling of orexin peptides has been investigated in diverse endogenously orexin receptor-expressing cells - mainly neurons but also other types of cells - and in recombinant cells expressing the receptors in a heterologous manner. Findings in the different systems are partially convergent but also indicate cellular background-specific signaling. The general picture suggests an inherently high degree of diversity in orexin receptor signaling.In the current chapter, I present orexin signaling on the cellular and molecular levels. Discussion of the connection to (potential) physiological orexin responses is only brief since these are in focus of other chapters in this book. The same goes for the post-synaptic signaling mechanisms, which are dealt with in Burdakov: Postsynaptic actions of orexin. The current chapter is organized according to the tissue type, starting from the central nervous system. Finally, receptor signaling pathways are discussed across tissues, cell types, and even species.

Heightened stress response and cognitive impairment after repeated neonatal sevoflurane exposures might be linked to excessive GABAAR-mediated depolarization

OBJECTIVE

Children with repeated exposures to anesthesia at an early age are at an increased risk of cognitive impairment. Data in the literature link increased developmental depolarizing γ-aminobutyric acid (GABA) type A receptor (GABAAR) at younger age to neurodevelopmental disorders. Here we investigated the involvement of GABAergic signaling during development in mediating the adverse effects of repeated sevoflurane exposures.

METHODS

Sprague-Dawley male rats received repeated exposures to 3 % sevoflurane for 2 h daily for 3 consecutive days on postnatal days (P) 4, 5, and 6; maternally separated and unseparated rats served as controls. A subgroup of rats received three injections of the Na(+)-K(+)-2Cl(-) cotransporter inhibitor, bumetanide (1.82 mg/kg, intraperitoneally) 15 min prior to initiation of each sevoflurane exposure.

RESULTS

The results showed that repeated neonatal sevoflurane exposures contribute to learning and memory impairment in the Morris water maze (MWM) at P60. The corticosterone level was significantly increased immediately after repeated neonatal sevoflurane exposures. Repeated neonatal sevoflurane exposures heightened the secretion of corticosterone in response to stress in P7 and P60 rats. Pretreatment of male rats prior to each sevoflurane exposure with bumetanide attenuated the corticosterone level immediately after repeated neonatal sevoflurane exposures, normalized endocrine response to stress at P7 and P60, and attenuated the sevoflurane-induced learning and memory impairment in the MWM.

CONCLUSION

These data suggested that the heightened stress response and cognitive impairment after repeated neonatal sevoflurane exposures might be linked to excessive GABAAR-mediated depolarization.

Anesthesia with sevoflurane in neonatal rats: Developmental neuroendocrine abnormalities and alleviating effects of the corticosteroid and Cl(-) importer antagonists

BACKGROUND

1.5 million children under 12 months of age are exposed to general anesthesia annually in the United States alone. Human and especially animal studies provide evidence that exposure to general anesthesia during the early postnatal period may lead to long-term neurocognitive abnormalities via poorly understood mechanisms. We investigated whether an immature stress response system and γ-aminobutyric acid (GABA) type A receptor activities are involved in mediating these abnormalities.

METHODS

Sprague-Dawley rats at postnatal days 4, 5 or 6 were anesthetized with 2.1% sevoflurane for 6h; maternally separated and house reared rats served as controls.

RESULTS

Sevoflurane anesthesia markedly increased corticosterone levels in rat pups of both genders. In adulthood, these rats responded to stress with heightened secretion of corticosterone and a greater increase in corticosterone levels in males versus females. Only male rats, previously exposed to neonatal sevoflurane, had a higher frequency of miniature inhibitory postsynaptic currents in CA1 neurons, spent a shorter time in open arms of the elevated plus maze (EPM) and exhibited impaired prepulse inhibition (PPI) of startle. Pretreatment of male rats prior to sevoflurane with the Na(+)-K(+)-2Cl(-) cotransporter inhibitor, bumetanide, or the mineralocorticoid receptor antagonist, RU28318, normalized endocrine responses to stress and the EPM behavior in adulthood, while only those pretreated with bumetanide exhibited normalized PPI of startle responses. Neither bumetanide nor RU28318 altered the effect of sevoflurane on synaptic activity.

CONCLUSIONS

Sevoflurane-enhanced neuronal excitation and elevated corticosteroid levels at the time of anesthesia contribute to the mechanisms initiating neonatal sevoflurane-induced long-term endocrine and neurobehavioral abnormalities.

Early expression of hypocretin/orexin in the chick embryo brain

Hypocretin/Orexin (H/O) neuropeptides are released by a discrete group of neurons in the vertebrate hypothalamus which play a pivotal role in the maintenance of waking behavior and brain state control. Previous studies have indicated that the H/O neuronal development differs between mammals and fish; H/O peptide-expressing cells are detectable during the earliest stages of brain morphogenesis in fish, but only towards the end of brain morphogenesis (by ∼85% of embryonic development) in rats. The developmental emergence of H/O neurons has never been previously described in birds. With the goal of determining whether the chick developmental pattern was more similar to that of mammals or of fish, we investigated the emergence of H/O-expressing cells in the brain of chick embryos of different ages using immunohistochemistry. Post-natal chick brains were included in order to compare the spatial distribution of H/O cells with that of other vertebrates. We found that H/O-expressing cells appear to originate from two separate places in the region of the diencephalic proliferative zone. These developing cells express the H/O neuropeptide at a comparatively early age relative to rodents (already visible at 14% of the way through fetal development), thus bearing a closer resemblance to fish. The H/O-expressing cell population proliferates to a large number of cells by a relatively early embryonic age. As previously suggested, the distribution of H/O neurons is intermediate between that of mammalian and non-mammalian vertebrates. This work suggests that, in addition to its roles in developed brains, the H/O peptide may play an important role in the early embryonic development of non-mammalian vertebrates.