The role of substance P in respiratory control in the newborn : Effects of morphine and nicotine
Sammanfattning: We breathe in order to maintain oxygen, carbon dioxide and pH levels within the physiological range in response to the metabolic demands of the body. To achieve this, the respiratory control system is dependent on input from peripheral and/ central chemosensitive areas and on participation of different neuromodulator systems. This thesis focuses on the role of one of the neurotransmitters, substance P, involved in the complex and finely tuned control of respiration. It also explores how prenatal nicotine exposure affects the substance P-ergic system and the effects of morphine when this system is non-functional. To investigate if endogenously released substance P is necessary for the hypoxic ventilatory response (HVR) in the intact newborn rat pup (postnatal day five, P5) we used a neurokinin 1-receptor antagonist (RP67580) injected intracerebroventricularly. We could demonstrate that RP67580-treated animals displayed an altered HVR but normal respiration during normal conditions, indicating that endogenously released substance P is necessary for an adequate response to hypoxic stress. Furthermore, in situ hybridisation demonstrated that c-fos mRNA expression, used as a marker for neuronal activation, was decreased in respiration related areas in the brainstem in RP67580-treated animals, indicating structures involved in the perturbed HVR. We also used a transgenic mouse model (Tac1 -/-), lacking substance P and neurokinin A (NKA), to investigate the respiratory response to intermittent hypoxia and hypercapnic stress at P2-3 and at P8-10 to identify developmental changes. In vivo experiments, using flow-plethysmography, displayed an attenuated increase in tidal volume during intermittent hypoxia in transgenic mice, P8-10, whereas the younger animals did not differ from controls except from an altered breathing pattern with fewer apneas and more augmented inspiratory breaths with a pause during intermittent hypoxia. Brain-stem spinal cord preparations of P2-mice revealed that intermittent hypoxia did not induce an increase in burst frequency, reflecting long-term facilitation, in Tac1 -/- mice as displayed in controls. This was also manifested in vivo as an impaired augmentation of ventilation during posthypoxic periods. Furthermore, transgenic mice displayed a more prominent posthypoxic frequency decline in vivo and posthypoxic neuronal arrests appeared more often in vitro. In line with previous studies the hypercapnic response did not differ between strains, confirming that substance P is not involved. Thus, our results show that a functional substance P/NKA system is essential to generate an adequate respiratory response and that it is also involved in the plasticity of respiratory network during early development. Human sudden infant death victims have elevated levels of substance P-like immunoreactivity (-LI) in the brainstem and nicotine increases the risk for sudden infant death syndrome (SIDS) by up to four-fold. We could demonstrate elevated substance P-LI levels in the brainstem and alterations of the substance P-precursor, preprotachykinin A mRNA expression in carotid body and petrosal/jugular ganglia following prenatal nicotine exposure in newborn rat (P1). This may offer a biochemical link between nicotine exposure and SIDS. We also show an increase in morphine analgesia and reduced main (respiratory depression) and other side-effects in Tac1 -/- mice. Since morphine is a widely used analgetic drug, also in neonates, with a narrow therapeutic window, our result offers the possibility to decrease the activity of substance P/NK- receptor signalling and thereby improve the pharmacological potential of morphine. In conclusion, this thesis demonstrates the involvement of substance P in the HVR and plasticity of the respiratory network. Prenatal nicotine exposure severely affects the substance Pergic system, a possible underlying mechanism for SIDS. Furthermore, it offers a correlation between the functionality of the substance P-ergic system and the breathing disturbances seen in Rett syndrome.
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