Anticonvulsant Profiles of Three Hemorphin-4 Analogs with Rhodamine B in Mice

<b>Background/Objectives</b>: Hemorphins, considered to be bioactive atypical oligopeptides, are products of hemoglobin metabolism. Recently, our team reported the synthesis and characterization of three N-modified analogs of hemorphin-4 (H4) with rhodamine B (Rh). In the present study, the Rh-1, Rh-2, and Rh-3 compounds were intracerebroventricularly infused at doses of 1, 2.5, 5, and 10 µg/5 µL, respectively, and evaluated for their antiseizure activity in 6-Hz and maximal electroshock (MES) tests and in a pentylenetetrazol (PTZ)-induced kindling model in mice. Phenytoin and diazepam were used as the reference drugs. The role of opioid receptors (ORs) underlying their mechanism of action was also evaluated in silico and pharmacologically. <b>Results</b>: The three Rh-H4 compounds showed a good safety profile at a concentration of 100 µg/mL in the mouse embryonic fibroblasts. They suppressed psychomotor seizures and seizure spreading as follows: Rh-1 at doses of 5 and 10 µg/5 µL, Rh-2 at the highest dose, and Rh-3 at doses of 1-10 µg/5 µL, respectively. Administered at doses of 5 µg/5 µL (Rh-1 and Rh-3) and 10 µg/5 µL (Rh-2), the compounds suppressed clonic seizures in the kindled mice comparable to the reference drug diazepam. A combination of selective delta (DOR), kappa (KOR), and mu (MOR) OR antagonists with the highest doses of the Rh-1, Rh-2, and Rh-3 compounds was used to elucidate the possible role of ORs in the underlying mechanism related to their protective activity against seizure spread. Only the selective DOR antagonist, natrindole, suppressed the effect of the Rh-1 peptide analog on seizures. The OR antagonist naloxone prevented the antiseizure activity of Rh-1 in the kindled mice. The results of docking analysis also showed the model-specific interaction of the three Rh-H4 compounds with the OR. <b>Conclusions</b>: Our results suggest that the antiseizure activity of Rh-1 is mediated by the OR, and in particular by the DOR, while the mechanism underlying the antiseizure effect of Rh-3 is more complex and may involve other receptors.