We have expanded these studies by doing

We have expanded these studies by doing EEG recording in the ET-1 model. Using this technique, we demonstrated that activation of either the ETA elvitegravir or the ETB receptor induces electrical seizures. ET-1 induced electrical seizures occurred in the presence of either the ETA receptor antagonist or the ETB receptor antagonist. These data strongly suggest that direct activation of ETB receptors is seizurogenic in immature rats. In addition to ETB activation being seizurogenic, our data indicate that this effect occurs in the absence of any significant effect on rCBF or pO2. Infusion of ET-1+ETA antagonist or infusion of the ETB agonist produced seizures but did not affect rCBF or pO2. Thus, there is dissociation between the presence of ischemia and the presence of seizures. Thus, the mechanism by which ETB activation induces seizures appears to be different from the mechanism by which ETA activation produces ischemia and seizures. This dissociation also extends to the induction of hippocampal damage in the experimental groups. Infusion of the ETB agonist did not produce any discernible hippocampal damage. Infusion of ET-1+ETA antagonist did lead to some hippocampal damage but this was much less than that induced by ET-1 alone or by ET-1+ETB antagonist. In conclusion, our data suggest that the induction of seizures does not obligatorily require ischemia or neuronal damage but can occur by other mechanisms related to ETB activation. Even though the induction of seizures did not require the presence of focal ischemia or hippocampal damage, when seizures did occur in our experimental animals, the severity of the seizures was correlated with the degree of hippocampal damage between experimental groups. This was particularly true following infusion of ET-1 alone where all measures of seizure severity were increased with increased damage, but the correlation was also present in the other experimental groups (ET-1+ETA antagonist or ET-1+ETB antagonist) where there was longer seizure duration with greater severity of damage. The reason for the differential action of ETA receptor activation (ischemia, tissue damage and seizures) versus ETB receptor activation (primarily seizures alone) is unknown and could be a fruitful field for further investigation. One area to explore might be the differential anatomical distribution of the two receptor types. ETA receptors are located predominantly in vascular smooth muscle cells, which is a logical distribution for inducing ischemia and tissue damage. On the other hand, it is less obvious how the distribution of ETB receptors on vascular endothelial cells, astrocytes and microglia might be related to the induction of seizures. Because of this conundrum, investigation of how ETB activation leads to seizures could provide new insights into epileptogenic mechanisms.
Conflict of interest
Acknowledgments We gratefully acknowledge the expert technical help by Mrs. Blanka Čejková, Mrs. Irinka Nešev and Mrs. Eva Lažková. We express gratitude to Dr. Rae Burchfiel, Dr. Kerry Thompson, Dr. Jaroslava Folbergrová, Dr. Přemysl Jiruška, Dr. Rastislav Druga, Ms. Thuy Hua, and Dr. Pavel Mareš for their helpful advice and comments. The present study was supported by grants P304/11/P383, P304/12/G069 and P304/14/20613S of the Grant Agency of the Czech Republic, and institutional support No. RVO: 67985823.
Introduction The endothelins have a variety of biological activities both in cardiovascular and non-cardiovascular tissues, including the urinary tract (Sullivan et al., 2000). The action of ETs is mediated through at least two distinct subtypes of receptors, the endothelin ETA and the endothelin ETB receptors. Saenz de Tejada et al. (1992) demonstrated the synthesis of endothelin-1 in epithelia, smooth muscle, and fibroblasts in human and rabbit urinary bladder. Radioligand binding experiments showed the presence of endothelin ETA and ETB receptors in the rabbit (Traish et al., 1992) and the rat urinary bladder (Saito et al., 2000). Functional studies indicated that endothelin-1 produced a contraction mediated by the endothelin ETA receptor but not by the endothelin ETB receptor in the urinary bladder of these animals Wada et al., 2000, Donoso et al., 1994. In the case of the guinea-pig urinary bladder, there are conflicting observations about the contractile effect of endothelin, with it causing weak contractile responses (Eglen et al., 1989) or no effect (Wiklund et al., 1989). The functional role of the endothelin ETB receptor in the urinary bladder is unclear.