Postpubertal feeding experience affects sex-specific spatial ability in rats

Developmental trajectory of contextual learning and 24-h acetylcholine release in the hippocampus

To determine the developmental trajectory of hippocampal function in rats, we examined 24-h changes in extracellular acetylcholine (ACh) levels and contextual learning performance. Extracellular ACh significantly correlated with spontaneous behavior, exhibiting a 24-h rhythm in juvenile (4-week-old), pubertal (6-week-old), and adult (9- to 12-week-old) rats. Although juveniles of both sexes exhibited low ACh levels, adult males had higher ACh levels than adult females. Moreover, juveniles exhibited much more spontaneous activity than adults when they showed equivalent ACh levels. Similarly, juveniles of both sexes exhibited relatively low contextual learning performance. Because contextual learning performance was significantly increased only in males, adult males exhibited better performance than adult females. We also observed a developmental relationship between contextual learning and ACh levels. Scopolamine pretreatment blocked contextual learning and interrupted the correlation. Since long-term scopolamine treatment after weaning impaired contextual learning in juveniles, the cholinergic input may participate in the development of hippocampus.

Activational and organisational effects of gonadal steroids on sex-specific acetylcholine release in the dorsal hippocampus

Acetylcholine (ACh) release in the dorsal hippocampus increases during stress, exploration or learning, exhibiting sex-specific 24-h release profile. We review the role of gonadal steroids on the ACh release in the dorsal hippocampus. In our studies, we found that male rats showed higher extracellular ACh levels than females, but gonadectomy decreased ACh levels in both sexes of rats and subsequently eliminated the sex difference. To examine the sex difference under comparable gonadal steroid levels, we implanted steroid capsules after gonadectomy. Oestradiol supplementation maintained circulating oestradiol to the levels in proestrous female rats, whereas testosterone capsules maintained circulating testosterone to the levels similar to intact male rats. Under comparable gonadal steroids levels, ACh levels were sex-specific. Testosterone replacement in orchidectomised rats clearly restored ACh levels, which were greater than ovariectomised testosterone-primed rats. Similarly, oestradiol replacement in ovariectomised rats successfully restored ACh levels, which were higher than orchidectomised oestradiol-primed rats. These results suggest sex-specific activational effects of gonadal steroids on ACh release. To further examine the organisational effect, female pups were neonatally treated with oil, testosterone, oestradiol, or dihydrotestosterone. These rats were bilaterally ovariectomised and a testosterone capsule was implanted at postnatal week 8. Neonatal treatment of either testosterone or oestradiol clearly increased ACh levels, whereas neonatal dihydrotestosterone treatment failed to change levels. These results suggest that: (i) gonadal steroids maintain the sex-specific ACh release in the dorsal hippocampus and (ii) neonatal activation of oestrogen receptors is sufficient to mediate masculinisation of the septo-hippocampal cholinergic system.