Spatial learning in deer mice: sex differences and the effects of endogenous opioids and 60 Hz magnetic fields.

We examined the effects of brief exposure to weak 60 Hz extremely low frequency (ELF) magnetic fields and opioid systems on spatial behavior and learning in reproductive adult male and female deer mice, Peromyscus maniculatus. Sex differences were evident in spatial performance, with male deer mice displaying significantly better performance than female mice in the Morris water maze, whereby animals had to acquire and retain the location of a submerged hidden platform. Brief (maximum 5 min) exposure to weak (100 microT) 60 Hz magnetic fields during task acquisition significantly improved female performance, eliminating the sex differences in acquisition. The opiate antagonist, naltrexone, also improved female acquisition, though significantly less than the magnetic fields. These facilitatory effects involved alterations of "non-spatial" (task familiarization and reduction of related anxiety/aversive related behaviors) and possibly "spatial" aspects of the task. Enhancement of enkephalin activity with the enkephalinase inhibitor, SCH 34826, significantly reduced task performance by male deer mice. Both naltrexone and the 60 Hz magnetic fields attenuated the enkephalin mediated reductions of spatial performance. These findings indicate that brief exposure to 60 Hz magnetic fields can enhance water maze task acquisition by deer mice and suggest that these facilitatory effects on spatial performance involve alterations in opioid activity.

Male preference for the odors of estrous female mice is reduced by the neurosteroid pregnenolone sulfate.

The effects of the neurosteroid, pregnenolone sulfate (PS), on the responses of male mice to the odors of estrous female mice were examined in an odor preference test. Control untreated mice displayed a significant preference for the odors of an estrous female, spending more time in a Y-maze in the vicinity of the odors of an estrous than a non-estrous female. Administration of PS decreased male preference for the odors of estrous females, causing a significant dose-related (0.01-10 mg/kg) decrease in the amount of time spent in the proximity of the odors of the estrous female, while having significantly less of an effect on the responses to the non-estrous female odors. Neither pregnenolone nor sodium sulfate had any significant effects on the olfactory responses. The effects of PS were significantly reduced by peripheral administrations of the non-competitive N-methyl-D-aspartate (NMDA) receptor antagonist, MK-801, but were not significantly affected by either the GABAA antagonists, bicuculline and picrotoxin, or the benzodiazepine antagonist, Ro 15-1788. These results suggest that pregnenolone sulfate has inhibitory effects on olfactory mediated male sexual interest, preference, or 'motivation' that, in part, involve interactions with NMDA receptor mediated mechanisms.