Andreas Zimmer at the University of Bonn, Germany, and his colleagues studied mice aged 2 months (young), 12 months (mature) and 18 months (old). They implanted mini-pumps under the mice’s skin, and for 28 days these released either low doses of marijuana’s main active compound, delta-9 tetrahydrocannabinol (THC), or a solution without the drug. After this period, the THC-treated mature and old mice performed as well as untreated young animals in memory and learning tests, whereas THC-treated young mice performed considerably worse.
In the brains of the treated mature and old mice, the structures of neuronal connections, or synapses, also reverted to those seen in untreated young animals, as did patterns of gene expression. All of these changes depended on the function of a brain receptor that binds certain neurotransmitters as well as THC.