SEX-RELATED EFFECTS ON BEHAVIOR AND BETA-ENDORPHIN OF DIFFERENT INTENSITIES OF FORMALIN PAIN IN RATS

The effects of two intensities of formalin pain on behaviour and beta- Endorphin (beta-EP) concentration in the brain and pituitary were stud ied in male and female rats. The animals were familiarized with the Ho le-Board apparatus for 3 days, and then, after a subcutaneous injectio n of formalin (50 mu l, 0.1 or 10%) or Sham-injection (Control) in the hindpaw, they were tested in the Hole-Board for 60 min. Licking, Flex ing and Paw-Jerk of the injected limb were recorded. beta-EP concentra tion was determined in the hypothalamus (HYP), the periaqueductal gray matter (PAG), the anterior pituitary (AP) and the neurointermediate l obe (NIL). Licking and Flexing durations were greater in females than males only with formalin 10%. Sex differences in beta-EP concentration between the Control groups were found in all tissues except the HYP; beta-EP levels were higher in females in the PAG and NIL, but greater in the AP in males. beta-EP concentration increased in males in the HY P and NIL with formalin 10%; in females, a decrease was found in the H YP with formalin 0.1%. The present results suggest that: (a) there are differences between males and females in the responses to formalin pa in, and the nature (pattern and duration) of the sex differences varie s according to the pain intensity; (b) there are differences in beta-E P concentration between the two sexes in control animals, and male and female rats also exhibit differences in the modifications of beta-EP in response to formalin-induced pain.