WASHINGTON (AP) _ Prostate trouble is an almost inevitable rite of passage for aging men. The walnut-sized gland can enlarge or become cancerous, but it's not something most men think they have to worry about until at least their 50s.

Less known is that prostates also can become inflamed, known as prostatitis _ an extremely painful, chronic disease that often hits younger men, in their 30s and 40s. Prostatitis accounts for 2 million visits to doctors a year, but they're largely in vain.

``We don't know what causes it, and we don't know how to effectively treat it,'' said Dr. Leroy Nyberg of the National Institutes of Health, which now is seeking sufferers for a $5.5 million study to better understand prostatitis. ``A lot of men are disabled by it.''

Enter the lowly rat. It seems that rats get inflamed prostates, too, especially when they're subjected to stress.

Why? Stress can spur excess production of a hormone called prolactin that seems to be involved in the inflammation, says North Carolina State University's Dr. C. Lee Robinette.

To Robinette, a veterinarian who has spent years studying rats and prostate disease, that's a red flag signaling that stress also may be behind much of the human misery.

Humans produce prolactin. And while no one has yet researched a stress connection, some men do report that their prostatitis seems to wax and wane with stress _ getting worse when they're under more pressure at work or there's an illness in the family, for example.

``It does appear that men in high-stress jobs, Type A personalities, are the ones that physicians see a lot of,'' agreed Nyberg.

Indeed, stress is one characteristic the NIH's new study will watch for this year as researchers try to pinpoint who is at risk for prostatitis, and then next year begin looking for treatments.

Prostatitis symptoms include pain in the genital area and lower back accompanied by frequent and urgent urination. It can cause burning or pain during urination or ejaculation, and is one cause of sexual impotence. Severe cases can be disabling.

No one knows just how many men have prostatitis, although experts estimate that half may experience it at some point. It can strike anytime, although of the first 100 men enrolled in the NIH's study, the average age is 41, and many sufferers are in their 30s, Nyberg said.

``Acute bacterial prostatitis'' is caused by an infection that quickly clears up, but it is rare. The vast majority of cases are ``chronic nonbacterial prostatitis,'' where symptoms go away and then return without warning. Doctors cannot find an infection, although for lack of anything else to help they often prescribe antibiotics _ a therapy that should never be used unless tests do uncover bacteria, Nyberg said.

It's the mystery that drove Robinette's rat research.

He discovered that the prostates of aging rats frequently would become inflamed, and that injecting younger rats with certain compounds could induce similar prostatitis _ creating an animal model to test what causes the disease.

When rats' prostates are inflamed, they have extra-high levels of prolactin. Stress causes the body _ rats' and people's _ to produce more prolactin. So Robinette subjected prostatitis-suffering rats to stress to see if they got worse than prostatitis-suffering rats left alone.

How do you stress a lab rat? Put it in a small plastic cone for 15 minutes, just small enough that it can't move. Restraint worries rats immensely, Robinette said.

He did this twice a day for four weeks. The stressed rats produced more prolactin and their prostatitis grew worse. The unstressed rats largely healed.

Rat studies don't always predict what happens in humans, and there's no way to know if rats felt more pain or other symptoms when they were stressed _ a key question for humans, Nyberg noted.

Still, the study suggests ``that maybe stress does exacerbate chronic prostatitis,'' something the NIH's study will examine, he said. To enroll, check http://www.niddk.nih.gov/health/kidney/kuru/win99/1.htm on the Internet.

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EDITOR'S NOTE _ Lauran Neergaard covers health and medical issues for The Associated Press in Washington.