Disruption of raphé serotonergic neural projections to the cortex: a potential pathway contributing to remote loss of brainstem neurons following neonatal hypoxic–ischemic brain injury

Abstract

Neuronal injury is a key feature of neonatal hypoxic-ischemic (HI) brain injury. However, the mechanisms underpinning neuronal losses, such as in the brainstem, are poorly understood. One possibility is that disrupted neural connections between the cortex and brainstem may compromise the survival of neuronal cell bodies in the brainstem. We investigated whether brainstem raphé serotonergic neurons that project to the cortex are lost after HI. We also tested if neuroinflammation has a role in disrupting brainstem raphé projections. Postnatal day 3 (P3) rats underwent unilateral carotid artery ligation followed by hypoxia (6% oxygen for 30 min). A retrograde tracer, choleratoxin b, was deposited in the motor cortex on P38. On P45 we found that retrogradely labelled neurons in the dorsal raphé dorsal, ventrolateral, interfascicular, caudal and ventral nuclei were lost after P3 HI. All retrogradely labelled neurons in the raphé nuclei were serotonergic. Numbers of retrogradely labelled neurons were also reduced in the ventromedial thalamus and basolateral amygdala. Minocycline treatment (45 mg/kg 2 h post-HI, 22.5 mg/kg daily P4-P9) attenuated losses of retrogradely labelled neurons in the dorsal raphé ventrolateral, interfascicular and ventral raphé nuclei, and the ventromedial thalamus. These results indicate that raphé neurons projecting to the cortex constitute a population of serotonergic neurons that are lost after P3 HI. Furthermore, neuroinflammation has a role in the disruption of raphé and thalamic neural projections. Future studies investigating the cellular mechanisms of axonal degeneration may reveal new targets for interventions to prevent neuronal losses after neonatal HI.

Deficits of brainstem and spinal cord functions after neonatal hypoxia–ischemia in mice

“…Furthermore, inhibition of neuroinflammation in the forebrain but not in the brainstem reduces the HI-induced raphé 5-HT losses (23). 5-HT raphé fibers projecting to the forebrain but not fibers that project to the medulla and the spinal cord (fibers from the raphé magnus, palidus, and obscurus) are disrupted after HI (6,24). We then proposed that the release of 5-HT in the medulla and the spinal cord might be affected since the 5-HT contents were modified in these regions after HI.…”

“…has been extensively used to investigate cognitive and sensorimotor functions; however, it should be emphasized that depending on the age of the insult, the results should be cautiously discussed. Previous studies interested in the 5-HT systems have used the rat model at P3 (6). Although still debated, P7 as corresponds to a brain development of 34-36 wk gestational age in the human fetus (10,33).…”

“…It is well established that there are volumetric reductions in certain brain areas of HI preterm infants including the thalamus, basal ganglia, and cerebral cortex. Only a few studies have examined neuronal injury in the brainstem in human neonates (5) or in animal models (6). Until now, no investigation has focused on the possible spinal cord defects following HI.…”

Long-Term Neuropathological Changes Associated with Cerebral Palsy in a Nonhuman Primate Model of Hypoxic-Ischemic Encephalopathy

“…In a neonatal rat model, Reinebrant et al demonstrated that disrupted serotonergic inputs to the motor cortex resulted in brainstem serotonergic neuronal loss after hypoxic-ischemic injury. [42] Disruption of the brainstem and spinal cord serotonergic system has also been proposed to account for the motor defects observed in neonatal mice using the Rice– Vanucci hypoxic-ischemic injury model. [43] Future studies in our UCO model investigating damaged neural pathways, such as serotonergic system pathway, after neonatal HIE may reveal cellular mechanisms of axonal degeneration relevant to the development of CP.…”

Chronic oral administration of minocycline to sheep with ovine CLN6 neuronal ceroid lipofuscinosis maintains pharmacological concentrations in the brain but does not suppress neuroinflammation or disease progression

“…Treatment trials in both animal models and human patients have been carried out for a number of neurodegenerative diseases, including multiple sclerosis, spinal cord injury, amyotrophic lateral sclerosis, Huntington disease, Parkinson disease and Alzheimer disease, with encouraging results in a number of cases. For example, recent rodent model trials have indicated efficacy against hypoxic-ischemic brain injury [23], blast-induced traumatic brain injury [24], Alzheimer disease-like amyloid pathology [25] and Huntington disease-like symptoms [26]. However, results from rodent model studies have not been very good indicators of human studies, indicating that larger animal models would be preferred, and there have also been some concerns about chronic treatment in some cases [20,21].…”