Effects of a single inescapable swim on long-term brain stimulation reward thresholds

Rats were subjected to a 15-min inescapable swim in a procedure which induces "behavioral despair" (immobility) 24 hr later. Their thresholds for brain stimulation reward were measured six times over the subsequent two weeks. Under one condition thresholds were first determined 3 hr postswim and, under a second condition, 24 hr postswim. Rats tested in the 3-hr condition showed elevated thresholds compared to preswim baselines. Elevations remained significant for six days. Thresholds for rats in the 24-hr condition did not change. Results are discussed in terms of a) the lack of correspondence between behavioral despair and brain stimulation reward models of depression, b) possible mechanisms for the 3-hr condition effect, and c) the significance of long-term changes in brain reward substrates.