Strain differences in freezing behavior of PVG hooded and Sprague-Dawley rats: differential cortical expression of cholecystokinin2 receptors.

PVG hooded, but not Sprague-Dawley (SD), rats showed remarkable freezing behavior on cat exposure. Experiments using cDNA microarray and reverse transcriptase-polymerase chain reaction (RT-PCR) demonstrated that cholecystokinin (CCK)2 receptors were expressed to a greater extent in the PVG hooded rats. A CCK2 antagonist, LY225910, inhibited freezing behavior in the PVG hooded rats while a CCK2 agonist, CCK-4, increased freezing behavior in the SD rats. These results strongly suggest that CCK2 receptors mediate the freezing behavior and the differential expression of these receptors underlie the strain difference in such behavior.