Effects of acute and sustained administration of the catecholamine reuptake inhibitor nomifensine on the firing activity of 2 J monoaminergic neurons

Nomifensine potently inhibits the reuptake of norepinephrine and dopamine in vitro. It is one of few antidepressants with marked potency to block dopamine reuptake that has ever been used clinically. Acute and sustained administration of nomifensine was investigated on the firing of monoaminergic neurons to understand its mechanism of action. In vivo extracellular recordings of locus coeruleus, ventral tegmental area and dorsal raphe nucleus neurons were obtained from male Sprague-Dawley rats. The intravenous injection of nomifensine in the locus coeruleus and ventral tegmental area yielded ED₅₀ values of 40 ± 1 and 450 ± 41 μg/kg, respectively, suggesting that nomifensine directly acted upon dopamine and norepinephrine neurons, since these values are proportional to its affinities for norepinephrine and dopamine transporters. There was no effect on 5-HT neurons. Nomifensine (5 mg/kg/day, subcutaneous, using minipumps) potently and significantly inhibited dopamine neuronal firing in the ventral tegmental area after 2 days, with recovery to normal after the 14-day treatment due to D₂ autoreceptor desensitization. Norepinephrine neuronal firing in the locus coeruleus was significantly decreased after 2 and 14 days. A significant increase in dorsal raphe nucleus 5-HT neuronal firing was seen after a two-day regimen, and remained elevated after 14 days. Desensitization of the 5-HT_1A receptor on 5-HT neurons of the dorsal raphe nucleus occurred after two days of nomifensine administration. Nomifensine likely treated depression by acting on dopamine, norepinephrine and 5-HT neurons, highlighting the importance of the functional connectivity between these three monoaminergic systems.