Chronic dietary treatment with antidepressants decrease brain Met-enkephalin-like immunoreactivity in the rat.

This report describes the effect of chronic dietary treatment with antidepressants or antimanic drugs on brain Met-enkephalin-like immunoreactivity in the rat. The chronic treatment was administered through food containing 250-1000 mg desipramine, imipramine or clomipramine, 200-800 mg amoxapine or mianserin, 750-1500 mg lithium chloride, or 50 mg haloperidol per 1 kg food for 40 days. In the striatum, each antidepressant decreased the enkephalin content, while lithium or haloperidol, having an antimanic effect, increased the enkephalin content. Following the chronic administration of each antidepressant tested, the concentration of the peptide was reduced in the nucleus accumbens, hypothalamus and thalamus. We further examined the acute effect of the antidepressant (10 or 20 mg/kg) on the striatal enkephalin content at 60 min after a single intraperitoneal injection. The striatal content was decreased after the acute treatment with each antidepressant tested. These results indicate that the antidepressants had an effect on the neuronal activity of Met-enkephalin not only after chronic treatment, but also after acute treatment. The reduction in Met-enkephalin-like immunoreactivity after prolonged treatment with the antidepressant may possible contribute to the mechanism of antidepressive action.