PRIOR COLD WATER SWIM STRESS ALTERS IMMOBILITY IN THE FORCED SWIM TEST AND ASSOCIATED ACTIVATION OF SEROTONERGIC NEURONS IN THE RAT DORSAL RAPHE NUCLEUS

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 degrees C versus 25 degrees 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 I 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 degrees C swim, (3) 19 degrees C swim/HC, (4) 19 degrees C swim/25 degrees C swim, (5) 25 degrees C swim/HC, (6) 25 degrees C swim/25 degrees 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 degrees C) swim on Day 1 reduced Tb, compared to both 25 degrees 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 degrees C swim on Day 1, relative to HC exposure on Day 1, increased immobility during the 5-min swim on Day 2. Also, 19 degrees C swim, relative to HC conditions, on Day 1 reduced swim (25 degrees 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 degrees C cold water swim, but not exposure to a 5-min 25 degrees C swim alters physiological, behavioral and serotonergic responses to a subsequent stressor. Published by Elsevier Ltd. on behalf of IBRO.