We examined whether increases in blood-brain barrier (BBB) permeability occurring after
stroke can be exploited to apply protective substances selectively to ischemic tissue. To do this, the actions of the peripherally selective OP2 agonists,
EMD-61569 and
EMD-61747, have been compared with those of the centrally acting OP2 agonist,
GR-89696, in the rat permanent focal
ischemia model.
EMD-61569,
EMD-61747 and
GR-89696 all bound with high affinity to OP2 receptors and were potent agonists in the rabbit vas deferens functional assay. These substances also potently inhibited electrically-induced overflow of
dopamine from slices of rat nucleus accumbens.
EMD-61747 and
EMD-61569 penetrate poorly into the CNS under normal conditions and reverse
haloperidol-induced
L-DOPA accumulation in the nucleus accumbens of the rat only at high doses, in contrast to
GR-89696. Permanent unilateral occlusion of the middle cerebral artery (MCAO) was associated with a disruption of the BBB and an increase in the concentration of
EMD-61747 in the area of the
infarct.
GR-89696 at a dose of 0.1 mg/kg s.c. produced a reduction in
infarct volume by 38% after MCAO,
EMD-61569 and
EMD-61747 had no influence on swelling and ischemic damage. We conclude that
EMD-61747 and
EMD-61569 are potent OP2 agonists, which usually have a limited ability to penetrate the BBB. The change in the properties of the BBB in ischemic tissue was not sufficient to elicit neuroprotection, since both
EMD-61747 and
EMD-61569 were inactive in the focal
ischemia model. Conversely,
GR-89696 had a robust protective action, and probably powerful OP2-typical side effects as a consequence of its unrestricted central activity.