Influence of age on stress responses to metabolic cage housing in rats

1. We studied the effect of isolation stress in 3- and 12-month-old rats in dividually housed in metabolic cages for 7 days. Urine (24 hr) was collecte d daily from one group of animals of each age. The other group was tested i n an open field and on a hot plate on days 1 and 7. 2. Total deambulation in the open-field test was lower in young than in old er rats both on day 1 (54.7 +/- 9.9 vs 80 +/- 8.9 crossings/session; P <0.0 4) and on day 7 (21 +/- 9 vs 48 +/- 7 crossings per session; P < 0.04) and decreased significantly in the two groups when tested on day 7 (P < 0.03). Latency to paw-licking in the hot-plate test was longer in young than in ol der animals on day 1 (14 +/- 2 vs 8 +/- 4 sec; P < 0.05) but was similar in the two groups on day 7. 3. Urinary excretions of norepinephrine (NE) and epinephrine (E) were deter mined by HPLC with electrochemical detection. Urinary NE in day 1 was simil ar in young and older animals (2627 +/- 828 vs 3069 +/- 598 ng/24 hr). In y oung animals NE excretion decreased along the study and was significantly ( P < 0.02) lower than on day 1 during the last 3 days of the study. Converse ly, in older animals urinary excretion of NE remained similar throughout th e study. On day 7 urinary excretion of NE in older animals was about two fo ld that in young rats. Urinary E was similar in young and older rats (341 /- 127 vs 532 +/- 256 ng/24 hr) on day 1 and showed a tendency to increase throughout the study. 4. Urinary monoamine oxidase inhibitory (IMAO) activity was determined by t esting the ability of urine extracts to inhibit rat liver MAO activity in v itro and was higher in young than in older animals throughout the study (da y 1, 54.8 +/- 4.2 vs 25.1 +/- 5.1%; P < 0.02). In young rats excretion of I MAO was significantly higher during the last 3 days of the study than on da y 1 (P < 0.05). In older animals urinary IMAO showed a tendency to increase at the end of the study. 5. Isolation stress caused by housing rats in metabolic cages results in di fferent behavioral and metabolic responses in young and older animals, Youn g animals exhibit a lower locomotor and analgesic response and excrete lowe r amounts of NE and higher IMAO activity in the urine than older rats. The metabolic and behavioral responses to isolation stress are highly dependent on the age of the animals tested. These results should be taken into consi deration when designing experiments requiring the use of metabolic cages.