The researchers from Vanderbilt University found that the presence of cannabinoid receptors in the central nucleus of the amygdala in a mouse model, and that nerve cells in this part of the brain make and release their own natural "endocannabinoids".
The researchers used high-affinity antibodies to "label" the cannabinoid receptors so they could be seen using various microscopy techniques, including electron microscopy, which allowed very detailed visualization at individual synapses, or gaps between nerve cells.
The paper's senior author and professor of Psychiatry and of Molecular Physiology and Biophysics, Sachin Patel, said that this discovery may help explain why marijuana users say they take the drug mainly to reduce anxiety.
He said that the study could be highly important for understanding how cannabis exerts its behavioral effects.
The researchers added that though marijuana's "exogenous" cannabinoids also can reduce anxiety, chronic use of the drug down-regulates the receptors, increasing anxiety, which can trigger "a vicious cycle" of increasing marijuana use.
The study is published in the journal Neuron.