Blood pressure via telemetry during social confrontations in rats: effects of clonidine

Male Long-Evans rats were implanted with blood pressure transmitters and introduced as intruders for 60 min into the home cage of a reproductively active resident male rat. Physical interaction ended after 3-5 min when the intruder displayed clear submissive behaviors. A protective wire cage was placed over the intruder until the animal was returned to its home cage. Systolic (+29.3 +/- 3.6 mmHg) and diastolic (+25.7 +/- 3.7 mmHg) blood pressures, pulse pressure (+7.3 +/- 2.0 mmHg), and heart rate (+129.0 +/- 12.6 BPM) peaked in the intruder rats during the defeat and did not fully return to control levels until return to the home cage. These acute changes as well as the heart rate and blood pressure baselines did not change when the confrontations were repeated on alternating days for a maximum of three trials per week. Pretreatment with clonidine (0.01, 0.03, 0.06, and 0.1 mg/kg) led to a dose-dependent decrease in the heart rate response but blood pressure was reduced similarly for all doses. We conclude that acute "defeat" can lead to an abrupt, large increase in blood pressure and heart rate in normotensive, Long-Evans rats that is sustained even in the absence of physical contact with the threatening resident. This response is diminished but not prevented by administration of clonidine.