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Neural biology explains ejaculation

And sheds light on motivation and reward in male sexual behavior

Federation of American Societies for Experimental Biology

How does the body know it has had an ejaculation? And why does it care? Anatomically, it is more complex than it seems, says the University of Cincinnati scientist who last year identified the spinal cord cells that control ejaculation in rats and the neural pathway by which signals travel between the body's sexual organs to the brain.

At the Experimental Biology 2003 meeting in San Diego, Dr. Lique Coolen reviews work her laboratory has done in understanding ejaculation and then discusses her current work in how chemical signals on this pathway contribute to pleasure and reward, key elements in sexual behavior. Dr. Coolen is this year's recipient of the American Association of Anatomists' C. J. Herrick Award in Comparative Neuroanatomy.

Scientists had known for years that there must be a group of cells in the spinal center that control ejaculation. Following spinal cord injury that prevents sensation from reaching the brain, humans and other animals remain able to achieve erection and ejaculation upon stimulation. But the location of this spinal ejaculation generator remained a mystery until last August when Dr. Coolen and a postdoctoral fellow in her laboratory, Dr. William Truitt, reported their findings in Science. Dr. Coolen had targeted the lumbar spinothalamic neurons in the lower back because these neurons appeared active only after ejaculation and not during sexual arousal or mounting. When the researchers used a highly selective toxin to destroy the thalamic neurons in adult male rats, the rats appeared not to notice. They continued their sexual interest and behavior, including penetration of the female. But they no longer had ejaculations, confirming that these were the cells the researchers had been hunting.

With the ejaculation machinery identified as being part of the spinal cord, Dr. Coolen then turned her interest to the neural pathway that relayed ejaculation-related signs from the reproductive system to the brain. This turned out to be the same spinal cord neural population which in turn sends ejacultion-related signals to the thalamus. The lumbar spinothalamic neurons issue sensory signals related to ejaculation that also contribute to mating-induced activation within brain circuits involved in the regulation of motivation and reward, the mesolimbic and mesocortical system. Using neuroanatomical markers and measures of activation of receptors, the researchers were able to show that the brain released various neurochemicals during different stages of sexual behavior.

Rats are different than humans when it comes to sex in some ways, says Dr. Coolen. The male rat can have eight to ten ejaculations over a two hour period -- with five minute breaks in between --- before they lose interest in a receptive female. But, she says, most studies of sexual functioning have used rodents and the results have turned out to work well in humans. She hopes other researchers will be able to locate the same cells in spinal cells in humans and then develop treatments to make it easier for paraplegic men to ejaculate (important for those men who wish to have families) and to help the 30 percent of adult males who experience ejaculatory problems sometime in their lives.

And what about women? Dr. Coolen also is developing research plans to determine if the same cells that cause ejaculation in men exist in the lumbar spines of women and if so, what they do.

As for her new work in the pleasure-reward pathways, learning what the chemical signals are will mean learning ways to manipulate these signals and treat other sexual dysfunction as well.

(American Association fo Anatomists)

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