Prior cold water swim stress alters immobility in the forced swim test and associated activation of serotonergic neurons in the rat dorsal raphe nucleus

•Previous cold swim affects behavior in a second swim 24 h later.•Previous cold swim affects body temperature in a second swim 24 h later.•Previous cold swim affects 5-HT activity following a second swim 24 h later.

Prior adverse experience alters behavioral responses to subsequent stressors. For example, exposure to a brief swim increases immobility in a subsequent swim test 24 h later. In order to determine if qualitative differences (e.g. 19 °C versus 25 °C) in an initial stressor (15-min swim) impact behavioral, physiological, and associated neural responses in a 5-min, 25 °C swim test 24 h later, rats were surgically implanted with biotelemetry devices 1 week prior to experimentation then randomly assigned to one of six conditions (Day 1 (15 min)/Day 2 (5 min)): (1) home cage (HC)/HC, (2) HC/25 °C swim, (3) 19 °C swim/HC, (4) 19 °C swim/25 °C swim, (5) 25 °C swim/HC, (6) 25 °C swim/25 °C swim. Core body temperature (Tb) was measured on Days 1 and 2 using biotelemetry; behavior was measured on Day 2. Rats were transcardially perfused with fixative 2 h following the onset of the swim on Day 2 for analysis of c-Fos expression in midbrain serotonergic neurons. Cold water (19 °C) swim on Day 1 reduced Tb, compared to both 25 °C swim and HC groups on Day 1, and, relative to rats exposed to HC conditions on Day 1, reduced the hypothermic response to the 25 °C swim on Day 2. The 19 °C swim on Day 1, relative to HC exposure on Day 1, increased immobility during the 5-min swim on Day 2. Also, 19 °C swim, relative to HC conditions, on Day 1 reduced swim (25 °C)-induced increases in c-Fos expression in serotonergic neurons within the dorsal and interfascicular parts of the dorsal raphe nucleus. These results suggest that exposure to a 5-min 19 °C cold water swim, but not exposure to a 5-min 25 °C swim alters physiological, behavioral and serotonergic responses to a subsequent stressor.