Cannabinoid CB(1) receptors colocalize with tyrosine hydroxylase in cultured fetal mesencephalic neurons and their activation increases the levels of this enzyme.

The incubation of cultured fetal mesencephalic neurons with Delta(9)-tetrahydrocannabinol (Delta(9)-THC) increased the activity of tyrosine hydroxylase (TH) and this increase was reversed by SR141716A, a specific antagonist for cannabinoid CB(1) receptors. In the present work, we extended these earlier observations by addressing two objectives. First, we characterized at a molecular level the presence of CB(1) receptors in cultured fetal mesencephalic neurons using two strategies: (i) analyzing the presence of CB(1) receptor gene transcripts by Northern blot, and (ii) measuring [3H]WIN-55,212-2 binding in membrane fractions obtained from these cells, as well as evaluating the potential increase in [35S]-guanylyl-5'-O-(gamma-thio)-triphosphate ([35S]GTPgammaS) binding caused by the activation of these receptors with WIN-55,212-2, a synthetic agonist. Northern blot analyses demonstrated the presence of small, but measurable levels of CB(1) receptor mRNA in cultured fetal mesencephalic neurons. The presence of these transcripts was accompanied by the presence of receptor binding protein, as revealed by a small, but specific, [3H]WIN-55, 212-2 binding in membrane fractions obtained from these cells. These CB(1) receptors are coupled to GTP-binding proteins, as the incubation of membrane fractions obtained from these cells with WIN-55,212-2 slightly, but significantly increased [35S]GTPgammaS binding. This fact indicated the existence, not only of receptor binding, but also of a functional receptor transduction pathway. As a second objective, we examined the potential colocalization of CB(1) receptors and TH in these cells by double-labelling immunocytochemistry. We also determined by Western blotting whether the previously observed Delta(9)-THC-induced increase in TH activity was accompanied by increased TH protein levels. Cultured fetal mesencephalic neurons exhibit diverse cell phenotypes, with CB(1) receptors localized only on TH-containing neurons. In addition, we found that the incubation of fetal mesencephalic neurons with medium containing Delta(9)-THC increased TH protein levels, in concordance with the previously reported increase in TH activity. Collectively, our results support the notion that CB(1) receptors are present in cultured fetal mesencephalic TH-containing neurons, despite their absence in the corresponding neurons in the adult brain. Thus, it is likely that the effects of cannabinoids on TH activity are direct. All this data strengthen the view that cannabinoid receptors are atypically located during brain development and that they might play an important role during this process, in particular on the phenotypical expression of TH-containing neurons.