Pain sensitivity, thermal capability, and brain monoamine turnover in hypertensive rats

This study compared the pain sensitivity, thermal capability, and brain monoamine turnover in normotensive and hypertensive rats. Either the spontaneously hypertensive rats, the experimentally (renal or deoxycorticosterone acetate) hypertensive rats, or their normotensive controls were tested for pain sensitivity by the hot-plate procedure, with the plate maintained at 55 degrees C. The spontaneously hypertensive rats, but not the experimentally hypertensive rats, showed an elevated level of spontaneous pain threshold compared with their appropriate controls. However, both the spontaneously and the experimentally hypertensive rats displayed a reduced sensitivity of analgesic responses to morphine administration. In addition the thermal responses to external stresses in these groups of animals were assessed. It was found that, over an ambient temperature range of 8-30 degrees C, either spontaneously or experimentally hypertensive rats maintained their rectal temperature within a normal limit displayed by their appropriate controls. However, the metabolic rate levels of both spontaneously and experimentally hypertensive rats were elevated at ambient temperatures less than 22 degrees C. Furthermore, compared with their appropriate controls, these hypertensive rats also showed a greater degree of hypothermic responses to chlorpromazine injection. Biochemical examination revealed that either the spontaneously or the experimentally hypertensive rats had a lower serotonin, but a higher catecholamine, turnover in both the hypothalamus and the brain stem compared with their appropriate normotensive controls. The data indicate that, in a hypertensive state, changes in the monoamine turnover of different brain regions may be correlated with the above-mentioned alterations in pain sensitivity and in thermal capability.