In a model of experimental focal cerebral ischemia, we have recently reported a strong correlation between the magnitude of ischemic depolarizations in the peri-infarct borderzone and the extent of histological injury. In the present study, we assessed the neurobehavioral consequences of spontaneously occurring and induced ischemic depolarizations in rats following middle cerebral artery (MCA) occlusion, as well as the effects of induced spreading depression (SD) in intact animals. Halothane-anesthetized, artificially ventilated Sprague-Dawley rats underwent photothrombotic MCA occlusion coupled with ipsilateral common carotid artery (CCA) occlusion. The electroencephalogram and direct current (DC) potential were recorded in the parietal infarct borderzone-corresponding to the cortical forelimb area-for 3 h following MCA occlusion. Group 1 rats (n = 9) received MCA/CCA occlusion, and the spontaneously occurring negative DC shifts were recorded in the ischemic borderzone. In Group 2 animals (n = 9), the (non-ischemic) frontal pole of the ipsilateral hemisphere was electrically stimulated in order to double the frequency of peri-infarct DC shifts occurring over the initial 3 h postocclusion. Group 3 consisted of intact rats (n = 3) in which SD was repeatedly evoked in the frontal pole. Four animals served as sham-operated controls. A battery of sensorimotor behavioral tests, consisting of beam balance, postural reflex and elicited forelimb placing, was applied in a blinded fashion. Sham controls and animals of Groups 1 and 2 were tested 24 h after surgery, and Group 3 rats were tested 2, 6 and 24 h after generation of SDs. A cumulative neurobehavioral index, ranging from 0 to 144, was calculated by adding the individual test results. Brains were perfusion-fixed 24 h following surgery for calculation of volumes of infarction and scattered neuronal injury. Functional outcome at 24 h was significantly worse in Group 2 animals (spontaneous plus induced ischemic depolarizations) (neurobehavior index 43 +/- 19, mean +/- S.D.) compared to Group 1 rats, in which only spontaneous depolarizations occurred (neurobehavior index 24 +/- 19, P < 0.05). The cumulative neurobehavioral index of Group 1 and 2 animals correlated positively with the volume of total ischemic injury (r = 0.765, P < 0.001) and with the frequency of ischemic depolarizations (r = 0.474, P < 0.05). Correlations between severe forelimb placing deficits and severe degrees of histological injury (necrosis or ischemic cell change) in the corresponding primary sensorimotor cortical region FR1 were significant in these rats. Group 3 rats showed severe neurobehavioral deficits at 2 and 6 h following SD stimulation (index 57 +/- 1 and 39 +/- 1, respectively) but returned to normal at 24 h (4 +/- 0). The findings indicate that cortical spreading depression is accompanied by transient neurobehavioral deterioration and that SD in the ischemic hemisphere of animals subjected to MCA occlusion worsened functional outcome 24 h after surgery.