Evidence that the serotonin transporter does not shift into the cytosol of remaining neurons after neonatal brain injury

Serotonergic mechanisms associated with experimental models of hypoxia: A systematic review

PURPOSE

The aim of this systematic review was to explore and discuss the literature concerning the effects of hypoxia or anoxia during the perinatal period on the serotoninergic network in rodents, through mechanisms that lead to changes in serotonergic neurons, levels, segments of central nervous system affected, 5-HT transporter, and 5-HT receptor.

METHODS

Literature searches were performed in Embase, Medline (PubMed), Web of Science, and SCOPUS, from April to July 2021, with a total of 1045 published studies found. Using a predefined protocol, as registered on the CAMARADES website, 10 articles were included in this review. The PRISMA statement was used for reporting this systematic review. The internal validity was assessed using the SYRCLE's risk of bias tool.

RESULTS

Our main findings show that hypoxia in the first days of postnatal life led to a disturbance in the serotonergic system with reduced in 5-HT fibers, reduced brain levels of 5-HT and 5-HIAA, reduced SERT protein expression, and reduced receptor 5-HT₇ . Putative mechanisms involving damage in the serotoninergic system include retrograde cell death resulting from primary damage mainly in forebrain areas, which impairs remote areas including serotonergic raphe nuclei. Other probable mechanisms associated with the serotoninergic network damage may be triggered by excitotoxic lesion and neuroinflammation.

CONCLUSION

Hypoxia at the beginning of an animal's life leads to modification of the serotonergic components associated with putative mechanisms that include cell damage and neuroinflammation.

Disruption to the 5-HT7 Receptor Following Hypoxia-Ischemia in the Immature Rodent Brain

It has become increasingly evident the serotonergic (5-hydroxytryptamine, 5-HT) system is an important central neuronal network disrupted following neonatal hypoxic-ischemic (HI) insults. Serotonin acts via a variety of receptor subtypes that are differentially associated with behavioural and cognitive mechanisms. The 5-HT₇ receptor is purported to play a key role in epilepsy, anxiety, learning and memory and neuropsychiatric disorders. Furthermore, the 5-HT₇ receptor is highly localized in brain regions damaged following neonatal HI insults. Utilising our well-established neonatal HI model in the postnatal day 3 (P3) rat pup we demonstrated a significant decrease in levels of the 5-HT₇ protein in the frontal cortex, thalamus and brainstem one week after insult. We also observed a relative decrease in both the cytosolic and membrane fractions of 5-HT₇. The 5-HT₇ receptor was detected on neurons throughout the cortex and thalamus, and 5-HT cell bodies in the brainstem. However we found no evidence of 5-HT₇ co-localisation on microglia or astrocytes. Moreover, minocycline treatment did not significantly prevent the HI-induced reductions in 5-HT₇. In conclusion, neonatal HI injury caused significant disruption to 5-HT₇ receptors in the forebrain and brainstem. Yet the use of minocycline to inhibit activated microglia, did not prevent the HI-induced changes in 5-HT₇ expression.

PACAP38 differentially effects genes and CRMP2 protein expression in ischemic core and penumbra regions of permanent middle cerebral artery occlusion model mice brain

Pituitary adenylate-cyclase activating polypeptide (PACAP) has neuroprotective and axonal guidance functions, but the mechanisms behind such actions remain unclear. Previously we examined effects of PACAP (PACAP38, 1 pmol) injection intracerebroventrically in a mouse model of permanent middle cerebral artery occlusion (PMCAO) along with control saline (0.9% NaCl) injection. Transcriptomic and proteomic approaches using ischemic (ipsilateral) brain hemisphere revealed differentially regulated genes and proteins by PACAP38 at 6 and 24 h post-treatment. However, as the ischemic hemisphere consisted of infarct core, penumbra, and non-ischemic regions, specificity of expression and localization of these identified molecular factors remained incomplete. This led us to devise a new experimental strategy wherein, ischemic core and penumbra were carefully sampled and compared to the corresponding contralateral (healthy) core and penumbra regions at 6 and 24 h post PACAP38 or saline injections. Both reverse transcription-polymerase chain reaction (RT-PCR) and Western blotting were used to examine targeted gene expressions and the collapsin response mediator protein 2 (CRMP2) protein profiles, respectively. Clear differences in expression of genes and CRMP2 protein abundance and degradation product/short isoform was observed between ischemic core and penumbra and also compared to the contralateral healthy tissues after PACAP38 or saline treatment. Results indicate the importance of region-specific analyses to further identify, localize and functionally analyse target molecular factors for clarifying the neuroprotective function of PACAP38.

Post-insult ibuprofen treatment attenuates damage to the serotonergic system after hypoxia-ischemia in the immature rat brain

There is currently no therapeutic intervention to stem neonatal brain injury after exposure to hypoxia-ischemia (HI). Potential neuroprotective treatments that can be delivered postinsult that target neuroinflammation and are safe to use in neonates are attractive. One candidate is ibuprofen. Ibuprofen is a nonsteroidal anti-inflammatory drug that inhibits cyclooxygenase enzymes and is used in neonates to treat patent ductus arteriosus. We investigated whether ibuprofen can inhibit neuroinflammation and attenuate neuronal damage manifested in a rodent model of preterm HI. Postnatal day 3 (P3) rat pups were subjected to HI (right carotid artery ligation, 30 minutes 6% O₂). Ibuprofen was then administered daily for 1 week (100 mg/kg P3 2 hours after HI, 50 mg/kg P4-P9; subcutaneously). Ibuprofen treatment prevented the P3 HI-induced reductions in brain serotonin levels, serotonin transporter expression, and numbers of serotonergic neurons in the dorsal raphé nuclei on P10. Ibuprofen also significantly attenuated P3 HI-induced increases in brain cyclooxygenase 2 protein expression, interleukin-1β, and tumor necrosis factor levels, as well as the increase in numbers of activated microglia. Thus, ibuprofen administered after an HI insult may be an effective pharmacologic intervention to reduce HI-induced neuronal brain injury in the preterm neonate by limiting the effects of neuroinflammatory mediators.