TLR7 agonist modulation of postasphyxial neurophysiological and cardiovascular adaptations in preterm fetal sheep

Abstract

Activation of Toll-like receptors (TLRs) after hypoxic-ischemic brain injury can exacerbate injury but also alleviate cell loss, as recently demonstrated with the TLR7 agonist Gardiquimod (GDQ). However, TLR agonists also modulate vascular function and neuronal excitability. Thus, we examined the effects of TLR7 activation with GDQ on cardiovascular function and seizures after asphyxia in preterm fetal sheep at 0.7 gestation (104 days, term ∼147 days). Fetuses received sham asphyxia or asphyxia induced by umbilical cord occlusion for 25 min or asphyxia followed by a continuous intracerebroventricular infusion of 3.34 mg of GDQ from 1 to 4 h after asphyxia. Fetuses were monitored continuously for 72 h postasphyxia. GDQ treatment was associated with sustained, moderate hypertension for 72 h ( P < 0.05), with a transient increase in heart rate. Electroencephalographic (EEG) power was suppressed for the entire postasphyxial period in both groups, whereas EEG spectral edge transiently increased during the GDQ infusion compared with asphyxia alone ( P < 0.05), with higher β- and lower δ-EEG frequencies ( P < 0.05). This increase in EEG frequency was not related to epileptiform activity. After the GDQ infusion, there was earlier onset of high-amplitude stereotypic evolving seizures, with increased numbers of seizures and seizure burden ( P < 0.05). Hemodynamic function and seizure activity are important indices of preterm wellbeing. These data highlight the importance of physiological monitoring during preclinical testing of potential neuroprotective strategies.

Induction of Tertiary Phase Epileptiform Discharges after Postasphyxial Infusion of a Toll-Like Receptor 7 Agonist in Preterm Fetal Sheep

“…Consistent with our previous report [ 14 ], infusion of GDQ led to a mild, sustained hypertension, which resolved 72 h after asphyxia. This increase in fetal blood pressure was not attributable to increased FHR, supporting that it was mediated by increased peripheral vascular resistance.…”

“…GDQ administration was associated with transiently improved recovery of background EEG activity, similarly to our previous report [ 14 ], and reduced seizures in the secondary phase. These data are consistent with growing evidence that immunomodulators are associated with functional improvement and reduced seizures in animal models of HIE [ 28 , 29 ].…”

“…Greater numbers of EDs were strongly associated with more severe injury, as shown by reduced neuronal survival in the caudate nucleus and CA1/2 hippocampal region, whereas no association was found in the putamen, CA3 and CA4 hippocampal regions, dentate gyrus or the thalamic medial and geniculate nuclei. Finally, GDQ was associated with sustained postasphyxial hypertension, similar to our previous report [ 14 ], that resolved after 72 h. These findings highlight the potential for TLR agonists, such as GDQ, to modulate seizures following an asphyxia insult, but to induce delayed epileptiform activity, which may contribute to differential susceptibility of specific subcortical regions to neuronal injury.…”

“…However, there is growing evidence that cytokines can modulate neuronal excitability [ 30 , 31 , 32 ]. Supporting this hypothesis, the peak fetal plasma concentrations of interleukin (IL)-10 and tumor necrosis factor (TNF)-α in the present cohort [ 15 ] and in our previous study using the same model [ 14 ] correspond with the greatest suppression of seizures in the asphyxia + GDQ group. Both these cytokines are known to exert anticonvulsant effects.…”

“…For example, we previously reported that central infusion of the synthetic TLR7 agonist Gardiquimod (GDQ) from 1 to 4 h after asphyxia in preterm fetal sheep reduced white and grey matter damage after 3 days recovery [ 13 ]. Interestingly, in this cohort, central GDQ administration was associated with hypertension and transiently improved recovery of spectral edge frequency (SEF), and suppression of epileptiform transients [ 14 ]. In a subsequent study using the same protocol to assess whether neuroprotection was persistent, we found no significant improvement in total or immature/mature oligodendrocytes in the periventricular and intragyral white matter tracts after 7 days recovery [ 15 ].…”