In this study, we have shown that the effective time window to start TH is sex-dependent. TH initiated earlier, i.e., 2 hours after injury, reduced extent of lesion in the cerebral hemisphere, hippocampus, and cerebral cortex for both sexes. However, the onset of hypothermia 6 hours after hypoxia worsened the results of morphological (percentage of degenerative cells in the hippocampus) and behavioral parameters, mainly in females. The best results of TH are achieved when treatment starts immediately or shortly after the hypoxic-ischemic event (Sabir et al. 2012; Cho et al. 2020).
Body weight is a developmental parameter which can provide important information about the protective effects of hypothermia. It is well known that HI can lead to motor deficits which, in turn, lead to feeding difficulties and affect animal’s body weight gain (Sanches et al. 2013; Fabres et al. 2018). The HI group showed a reduction in body weight as compared to SHAM and NAÏVE groups when evaluated 7 days after HI (P14). However, the TH-2h group did not show this decrease, indicating a beneficial effect of hypothermia when started 2h after HI. Such important improvement of this developmental parameter was not seen when hypothermia was started later, i.e., 4h and 6h after HI. Sabir and colleagues (2012 and 2014) did not observe any difference in weight gain between HI animals and animals treated with hypothermia (32ºC for 5h) started immediately or 3, 4, 6 or 12h after injury (Sabir et al. 2012, 2014). However, in study of Sabir et al. (2014) all animals that were submitted to neonatal HI, making it difficult to compare with our work.
The lack of weight loss observed in the TH-2h group may be associated with smaller volume of brain damage and, consequently, reduction in motor impairment. Here, animals belonging to the TH-2h group showed a decrease in lesion volume in every brain structure evaluated when compared to the HI group. However, when TH was started later (TH-4h and TH-6h groups), this neuroprotective effect of TH was blunted, suggesting TH started later may not be able to protect the brain properly. In agreement with Davson (2015) the cells can recover from an insult up to 6 hours after the event and this period is known as the latent period. Within this period, deleterious mechanisms can be initiated leading to spread of brain injury and progressive cell dysfunction (Davidson et al. 2015; Cho et al. 2020).
There are experimental and clinical evidences showing that the closer the onset of hypothermia is to the moment of HI, the greater the reduction of brain injury (Sabir et al. 2012, 2014; Thoresen et al. 2013). Sabir et al. (2012) observed that hypothermia started immediately or 3h after HI caused a reduction in brain injury, an effect that was not seen when hypothermia was started later (6h and 12h following HI) (Sabir et al. 2012). In addition, Park and colleagues (2015) did not observe neuroprotective effects when hypothermia was started 6 hours after injury (Park et al. 2015), corroborating with our findings.
In order to strengthen analysis of brain injury, we decided to calculate the pathological score(Thoresen et al. 1996) of brain structures from which the injury volume was assessed. The correlation between data of pathological score and lesion volume showed a very strong association proving both forms of analysis can be used. Although analysis of volume of brain injury and pathological score demonstrated a beneficial effect of hypothermia initiated 2h after HI, no sex-related differences were observed in such parameters. However, it is known that there is a difference in type of cell death depending on sex; in females, cell death is mainly dependent on caspase activation, whereas in males, cell death is mostly caspase-independent (Joly et al. 2004; Askalan et al. 2015; Netto et al. 2017). Therefore, an evaluation at the cellular level could help us find differences between sexes. The hippocampus was chosen for this analysis, since it is the most affected brain structure in the neonatal HI animal model (Dhikav and Anand 2012).
When percentage of degenerative cells in pyramidal layer of CA1 and hilus was evaluated, a sex-related effect was revealed. Animals from the HI group showed an 8-fold increase in percentage of degenerative cells for both sexes, which is in accordance to our previous study using only males (Fabres et al. 2020). TH reduced percentage of cells in degeneration when started up to 4h after injury, although the most remarkable neuroprotection was observed in the TH-2h group. In addition, in both hilus and CA1, the percentage of degenerative cells in females from the TH-2h group was around 15%, while in the TH-2h males this percentage was around 30% in hilus and 35% in CA1. These results corroborate with data showing that TH is able to reduce neuronal death in the CA1 area of the hippocampus three and seven days after injury (Xiong et al. 2009; Wood et al. 2016). The hilus of the dentate gyrus was chosen as being a region considered less vulnerable to HI than the CA1 area. Nevertheless, it was also shown that hypothermia started earlier was able to reduce cell degeneration in the hilus; on the other hand, when started 6h after HI (TH-6h group), the number of degenerating cells in females was even greater than that observed in the HI group.
Reduction in volume of brain injury and in percentage of degenerative cells are important factors indicative of the efficacy of TH; however, it is well known that brain injury caused by HI can extend for weeks to months (Davidson et al. 2015). Neuroinflammation is one of the main causes that extends the injury for longer periods. It is also known that male animals submitted to neonatal HI show increased microglial activation and upregulation of the inflammatory response (Netto et al. 2017). Astrocytes play a role in this process and reactive astrogliosis is also related to neuroinflammation (Davidson et al. 2015). In our study, TH started earlier (TH-2h) decreased GFAP fluorescence intensity in the CA1 area, suggesting a reduction in reactive astrogliosis for both, males and females.
However, to obtain a more detailed assessment of astrocyte activation, we used Sholl’s concentric circles method to quantify astrocyte complexity (arbour length and number of processes) (Mestriner et al. 2011; Mari et al. 2019). These parameters were evaluated because increase in GFAP immunoreactivity as well as increase in number and compliance of processes are characteristics of reactive astrocytes as already demonstrated in other studies (Haim et al. 2015; Liddelow et al. 2017; de Fraga et al. 2021). Sholl analyses showed that the TH-2h female group had a reduced number of primary processes as well as length of processes compared to the HI group, which was not observed in males. Morphology of astrocytes may reflect the functional significance of neuroglial interactions (Sheikhbahaei et al. 2018). The glial scar formation triggered by neural lesions such as those produced after hypoxic-ischemic events, when uncontrolled, may impair neural communication and cell function (Sofroniew 2015; Sheikhbahaei et al. 2018). Therefore, although GFAP fluorescence intensity was used to infer increased astrocyte reactivity, females from the TH-2h group showed smaller number of primary processes and of shorter length, indicating more organized glial scar formation; this is conceivably beneficial for tissue repair (Sizonenko et al. 2008).
Astrocyte reactivity parameters (evaluated by GFAP immunofluorescence intensity and Sholl circles) also showed a strong association with percentage of degenerative cells in CA1. It was expected, since reactive astrogliosis is a phenomenon in which astrocytes branch off and extend their processes to form a glial scar at the locations where neurons have died. A study by Reddy and colleagues (2020) also observed a correlation between volume of hippocampus and the reactive astrogliosis marker GFAP+, corroborating the hypothesis of glial scar formation (Reddy et al. 2020).
In addition to the morphological parameters, an analysis of neurodevelopmental parameters is also important and can provide broader data on the neuroprotective effects of TH. Previous studies from our group(Tassinari et al. 2020) and from others(Lubics et al. 2005) have shown that HI causes an increase in latency to complete the negative geotaxis test, as seen here. However, together with the neuroprotective effects of TH on morphological and structural variables, we have also observed an improvement in the behavioral response in the test of negative geotaxis in animals from the TH-2h group, for both sexes. Jatana and colleagues (2006) showed that hypothermia was not able to reduce the latency in the negative geotaxis test 7 days after injury; however, the hypothermia protocol lasted for only 2h, which could explain differences in the results (Jatana et al. 2006). By starting hypothermia 3 hours after injury and using a prolonged period of hypothermia (48h), Ahn and colleagues (2018) observed a reduction in latency to perform the negative geotaxis test when animals reached 42 days of life (Ahn et al. 2018), but not at P14, P21, P28, and P35. It has already been observed in animal models that a protocol of hypothermia longer than 5h produces no additional neuroprotective effects (Sabir et al. 2012). Moreover, prolonged periods of maternal separation produce negative effects on behavior (Lehmann et al. 1999), which could explain the reason why longer periods of hypothermia do not lead to improvement in behavioral outcomes.
When the righting reflex was evaluated, only males from the TH-2h group showed a reduction in latency to complete the test as compared to the HI group. Yuan et al. (2015) also observed that hypothermia tended to reduce the latency to complete the righting reflex test compared to the HI group; however, sex-dependent effects were not assessed (Yuan et al. 2015). In the present study, TH was not able to improve the female response in the righting reflex test, regardless of the period in which it was started (2h, 4h or 6h after HI). Behavioral improvements observed in males and females in the negative geotaxis test, as well as in males in the righting reflex test, may be related to the reduction in volume of lesion and decreased percentage of degenerative cells observed in these groups. To test this hypothesis, a correlation between behavioral performance, morphological and structural parameters was run. We observed only a weak to moderate association beteween these variables, depending on the parameters evaluated.
The hippocampus is a structure that play important roles in spatial navigation and episodic memory (Soltesz and Losonczy 2018). Studies using gamma radiation showed alterations in hippocampal layers with pyramidal neurons exhibiting degeneration, i.e., showing characteristics of pyknosis, karyorrhexis and karyolysis (Li et al. 2014; Owoeye and Malomo 2015). Death of pyramidal neurons can disrupt the flow of information suggesting that memory and other functions of the hippocampus could potentially be affected (Owoeye and Malomo 2015).
Furthermore, among the structures evaluated in the present study, the hilus has a regulatory role for neuronal migration in neonates, which can continue into the adult stage (Saegusa et al. 2010, 2012). Lesions in this area may suggest alterations in development and plasticity of the hippocampus, which may reflect behavioral changes. However, behavioral tests used here are characterized by reflex movements and are more related to brain areas not evaluated in this study such as the brainstem (Heyser, 2003; Schneider and Przewłocki, 2005).
Our study has some limitations. First, we evaluated animals using only one time point (P14, i.e, 7 days after the HI event) and it is well known that the brain lesion can mature from days to months (Davidson et al. 2015). Therefore, further studies using animals from both sexes treated with TH and tracked for longer periods are needed, which would allow assessment of cognitive function using specific behavioral tests in adulthood. Second, we did not evaluate molecular mechanisms underlying the neuroprotective effects of TH. It is well described that TH has an effect in reducing brain metabolism leading to delayed cellular depolarization and, thus, reduction of intracellular calcium influx; however, understanding the molecular basis of TH effects can increase knowledge of its functioning and allow the combination of TH with other adjuvant therapies, in order to increase neuroprotective benefits (Davidson et al. 2015; Cho et al. 2020).