Effects of novelty and pain on behavior and hippocampal extracellular ACh levels in male and female rats

In vivo microdialysis was used to assess the effects of novelty and pain on hippocampal ACh release in male and female rats. Experiments were carried out during the dark phase and consisted of 2 days of tests: on Day 1, after Baseline 1, animals were exposed to a new cage (Novelty) to which, 30 min later, a plastic cylinder (Object) was introduced. On Day 2, after Baseline 2, the Formalin test (50 mu l of formalin 10%, s.c. injected in the dorsal hindpaw) was carried out in the animal's home cage. All behaviors were rec orded. The extracellular levels of ACh in the dorsal hippocampus were estim ated, in IO-min samples, by assay of ACh in the dialysates by HPLC. On Day 1 the raw values of ACh were higher in females than in males, but no sex di fference was present when the percentage of change was considered. In both sexes the Novelty and Object tests induced an increase in ACh levels with r espect to Baseline. Higher levels of exploration were present in females th an males during the first IO min of Novelty. On Day 2, ACh release increase d in both sexes during the Formalin test. No sex difference in either ACh r aw values or the percentages of change were found. Females showed higher le vels of licking and lower levels of activity than males. The present study shows that novelty and pain induce similar hippocampal cholinergic activati on in male and female rats but different behaviors. The results are discuss ed in light of the several anatomical and functional sex differences presen t in the hippocampus. (C) 1999 Published by Elsevier Science B.V. All right s reserved.