Trazodone and m-chlorophenylpiperazine. Concentration in brain and receptor activity in regions in the brain associated with anxiety.

Trazodone, its active metabolite 1-(m-chlorophenyl)piperazine (mCPP) and two isomers of mCPP were tested for affinity to the binding sites for [3H]flunitrazepam ([3H]FLU) and [3H]p-aminoclonidine ([3H]pAC) in the frontal cortex, amygdala and hippocampus of the rat. When tested at the binding site for [3H]flunitrazepam, trazodone showed an average IC50 of 1.7 mM in all three regions of the brain while mCPP yielded an average IC50 of 0.36 mM. These same two compounds, when tested at the binding site for [3H]p-aminoclonidine, resulted in an average IC50 of 4.5 microM for trazodone and 0.6 microM for mCPP. When plasma values of trazodone and mCPP in the rat were similar to those obtained in patients given therapeutic doses, the concentrations of trazodone were found to be between 6 and 7 microM and between 2 and 3 microM for mCPP in the same brain regions of the brain used in the binding assays. Thus, a sufficient concentration of trazodone and mCPP can accumulate in brain tissue to displace approx. 50% of the [3H]p-aminoclonidine from its binding site but very little [3H]flunitrazepam from its binding site. These results, combined with the reported anxiolytic effects of the alpha-2 agonist, clonidine and the noradrenergic hyperactivity theory of anxiety, indicate that the mechanism of the anxiolytic activity of trazodone, may be a direct action on the central alpha-2 binding site.