A biological mechanism explaining part of the mystery of acupuncture has been pinpointed by scientists studying rats.

Stimulation with electroacupuncture - a form of the therapy in which a small electric current is passed between a pair of needles - blunted activity in a key hormonal pathway linked to stress, chronic pain and mood, the researchers found.

The findings provide the strongest evidence yet that the ancient Chinese therapy has more than a placebo effect when used to treat chronic stress, it is claimed.

Rats had needles inserted at an especially powerful acupuncture point known as stomach meridian point 36 (St36).

Although linked to digestive problems and multiple other ailments, St36 is on the shin.

Traditionally, acupuncture involves unblocking energy paths known as meridians that flow around the body, keeping it in balance.

The new research showed that applying electroacupuncture to the St36 meridian point affected a complex interaction between hormones known as the hypothalamus pituitary adrenal (HPA) axis.

In stressed rats exposed to unpleasant cold stimulation, HPA activity was reduced.

Lead investigator Dr Ladan Eshkevari, from Georgetown University Medical Centre in Washington DC, said: "The benefits of acupuncture are well known by those who use it, but such proof is anecdotal.

"This research, the culmination of a number of studies, demonstrates how acupuncture might work in the human body to reduce stress and pain, and potentially depression. We have now found a potential mechanism, and at this point in our research, we need to test human participants in a blinded, placebo-controlled clinical study - the same technique we used to study the behavioural effects of acupuncture in rats.

"Some antidepressants and anti-anxiety drugs exert their therapeutic effects on these same mechanisms."

The study, reported in the journal Endocrinology, compared stressed rats given electroacupuncture, a "sham" therapy in which needles were not inserted in a meridian point, or no treatment. A fourth group of rats were not exposed to stress, and did not receive acupuncture.

Electroacupuncture delivered at St36 minutes after cold stimulation, or as a pre-treatment, was found to be effective at preventing rising levels of stress hormones.

Behaviour linked to depression and anxiety was reduced in the treated rats.

"This is the first report linking the effects of electroacupuncture at St36 to chronic stress-induced depressive and anxious behaviour in animals," Dr Eshkevari added.

"This work provides a framework for future clinical studies on the benefit of acupuncture, both before or during chronic stressful events."