RESUMO
Neurons of the respiratory network in the lower brainstem express a variety of serotonin receptors (5-HTRs) that act primarily through adenylyl cyclase. However, there is one receptor family including 5-HT(2A), 5-HT(2B), and 5-HT(2C) receptors that are directed towards protein kinase C (PKC). In contrast to 5-HT(2A)Rs, expression and function of 5-HT(2B)Rs within the respiratory network are still unclear. 5-HT(2B)R utilizes a Gq-mediated signaling cascade involving calcium and leading to activation of phospholipase C and IP3/DAG pathways. Based on previous studies, this signal pathway appears to mediate excitatory actions on respiration. In the present study, we analyzed receptor expression in pontine and medullary regions of the respiratory network both at the transcriptional and translational level using quantitative RT-PCR and self-made as well as commercially available antibodies, respectively. In addition we measured effects of selective agonists and antagonists for 5-HT(2A)Rs and 5-HT(2B)Rs given intra-arterially on phrenic nerve discharges in juvenile rats using the perfused brainstem preparation. The drugs caused significant changes in discharge activity. Co-administration of both agonists revealed a dominance of the 5-HT(2B)R. Given the nature of the signaling pathways, we investigated whether intracellular calcium may explain effects observed in the respiratory network. Taken together, the results of this study suggest a significant role of both receptors in respiratory network modulation.
Assuntos
Mamíferos/metabolismo , Receptor 5-HT2A de Serotonina/metabolismo , Receptor 5-HT2B de Serotonina/metabolismo , Sistema Respiratório/metabolismo , Animais , Anticorpos/imunologia , Especificidade de Anticorpos/imunologia , Cálcio/metabolismo , Fluorescência , Regulação da Expressão Gênica/efeitos dos fármacos , Técnicas In Vitro , Masculino , Bulbo/citologia , Bulbo/efeitos dos fármacos , Bulbo/metabolismo , Camundongos , Ponte/citologia , Ponte/efeitos dos fármacos , Ponte/metabolismo , Ratos , Receptor 5-HT2A de Serotonina/genética , Receptor 5-HT2B de Serotonina/química , Receptor 5-HT2B de Serotonina/genética , Proteínas Recombinantes/metabolismo , Reprodutibilidade dos Testes , Respiração/efeitos dos fármacos , Sistema Respiratório/efeitos dos fármacos , Antagonistas do Receptor 5-HT2 de Serotonina/farmacologia , Transdução de Sinais/efeitos dos fármacosRESUMO
Rhythmic breathing movements originate from a dispersed neuronal network in the medulla and pons. Here, we demonstrate that rhythmic activity of this respiratory network is affected by the phosphorylation status of the inhibitory glycine receptor α3 subtype (GlyRα3), which controls glutamatergic and glycinergic neuronal discharges, subject to serotonergic modulation. Serotonin receptor type 1A-specific (5-HTR1A-specific) modulation directly induced dephosphorylation of GlyRα3 receptors, which augmented inhibitory glycine-activated chloride currents in HEK293 cells coexpressing 5-HTR1A and GlyRα3. The 5-HTR1A-GlyRα3 signaling pathway was distinct from opioid receptor signaling and efficiently counteracted opioid-induced depression of breathing and consequential apnea in mice. Paradoxically, this rescue of breathing originated from enhanced glycinergic synaptic inhibition of glutamatergic and glycinergic neurons and caused disinhibition of their target neurons. Together, these effects changed respiratory phase alternations and ensured rhythmic breathing in vivo. GlyRα3-deficient mice had an irregular respiratory rhythm under baseline conditions, and systemic 5-HTR1A activation failed to remedy opioid-induced respiratory depression in these mice. Delineation of this 5-HTR1A-GlyRα3 signaling pathway offers a mechanistic basis for pharmacological treatment of opioid-induced apnea and other breathing disturbances caused by disorders of inhibitory synaptic transmission, such as hyperekplexia, hypoxia/ischemia, and brainstem infarction.
Assuntos
Receptor 5-HT1A de Serotonina/metabolismo , Receptores de Glicina/metabolismo , Respiração , Animais , Linhagem Celular , Humanos , Masculino , Camundongos , Camundongos Endogâmicos C57BL , Camundongos Knockout , Camundongos Transgênicos , Periodicidade , Fosforilação , Receptor 5-HT1A de Serotonina/genética , Receptores de Glicina/genética , Proteínas Recombinantes de Fusão/genética , Proteínas Recombinantes de Fusão/metabolismo , Serotonina/metabolismoRESUMO
The cellular effects of serotonin (5-HT), a neuromodulator with widespread influences in the central nervous system, have been investigated. Despite detailed knowledge about the molecular biology of cellular signalling, it is not possible to anticipate the responses of neuronal networks to a global action of 5-HT. Heterogeneous expression of various subtypes of serotonin receptors (5-HTR) in a variety of neurons differently equipped with cell-specific transmitter receptors and ion channel assemblies can provoke diverse cellular reactions resulting in various forms of network adjustment and, hence, motor behaviour. Using the respiratory network as a model for reciprocal synaptic inhibition, we demonstrate that 5-HT(1A)R modulation primarily affects inhibition through glycinergic synapses. Potentiation of glycinergic inhibition of both excitatory and inhibitory neurons induces a functional reorganization of the network leading to a characteristic change of motor output. The changes in network operation are robust and help to overcome opiate-induced respiratory depression. Hence, 5-HT(1A)R activation stabilizes the rhythmicity of breathing during opiate medication of pain.
