Immune cells sport marijuana receptor.

Once considered the seat of the spirit and the source of emotions, the spleen lost that title when physiologists demonstrated that it does little more than filter, hold, and produce white blood cells.

But maybe ancient anatomists knew something that modern scientists do not.

Molecular biologists searching for molecules important for the migration of white blood cells have instead stumbled upon a new type of marijuana receptor, thus far found only in the spleen. This receptor molecule readily latches on to delta-9-tetrahydrocannabinol (THC), marijuana's active ingredient, says Sean Munro of the Medical Research Council Laboratory of Molecular Biology in Cambridge, England. It also accepts the body's natural version of THC, a substance called anandamide (SN: 2/6/93, p.88).

Munro and his colleagues had been using a genetic technique called polymerase chain reaction to search through lab-grown human lymphoid cells. "We never got what we were looking for," Munro recalls.

Instead, they discovered a receptor whose generic code has 44 percent of its sequence in common with the brain's known marijuana receptor. When the researchers inserted this genetic code into cells grown in the lab, the cells bound marijuana-like substances, though not to the same degree as the brain's cannabinoid receptor, Munro and his colleagues report in the Sept. 2 NATURE.

The researchers determined that the rat brain lacks this newly identified receptor, which is located on cells called macrophages that lie in the parts of the spleen where substances in the blood first encounter the immune system.

Another study, now accepted for publication, confirms that parts of rat spleen, as well as lymph nodes, contain marijuana receptors, says Miles Herkenham, who, with Allison B. Lynn, surveyed tissue using tagged molecules known to link with these receptors. They work at the National Institute of Mental Health in Bethesda, Md.

The fact that this receptor differs from the one in the brain offers hope that pharmaceutical companies may one day harness THC-like substances for medical uses without having to worry about patients getting high from the treatment, says Leslie L. Iverson of Merck, Sharp and Dohme Research Laboratories in Harlow, England.

In the spleen, THC-like compounds seem to affect the ability of cells to generate a messenger molecule that helps activate white blood cells, says Norbert E. Kaminski at Michigan State University in East Lansing. Last year, Kaminski discovered a marijuana receptor in mouse spleens.

Because the spleen and brain receptors are different and because other researchers have discovered marijuana receptors in fish and sea urchins, Munro suspects that this ancient psychoactive agent ties into an ancient -- and widespread -- internal signaling system for organisms.