Repeated but not single administration of ketamine prolongs increases of the firing activity of norepinephrine and dopamine neurons.

BACKGROUND Clinical studies have shown that the rapid antidepressant effect of the glutamate N-Methyl-D-Aspartate receptor antagonist ketamine generally disappears within one week but can be maintained by repeated administration. Preclinical studies showed that a single ketamine injection immediately increases the firing and burst activity of norepinephrine (NE) neurons, but not that of serotonin (5-HT) neurons. It also enhances the population activity of dopamine (DA) neurons. In the present study, we investigated whether such alterations of monoamine neuronal firing are still present one day after a single injection, and whether they can be maintained by repeated injections. METHODS Rats received a single ketamine injection or 6 over 2 weeks and the firing activity of dorsal raphe nucleus 5-HT, locus coeruleus NE, and ventral tegmental area DA neurons was assessed. RESULTS One-day following a single injection of ketamine, there was no change in the firing activity of 5-HT, NE, or DA neurons. One day after repeated ketamine administration, however, there was a robust increase of the firing activity of NE neurons, an enhancement of burst and population activities of DA neurons, but still no change in firing parameters of 5-HT neurons. The increased activity of NE neurons was no longer present 3 days after the last injection, whereas that of DA neurons was still present. DA neurons were firing normally 7 days after repeated injections. CONCLUSION These results imply that the enhanced activity of NE and DA neurons may play a significant role in the maintenance of the antidepressant action of ketamine.