EFFECT OF KETAMINE ON THE EXCITABILITY OF DORSAL RAPHE NEURONS

The non-competitive NMDA antagonist ketamine is an old drug that traditionally has been used as an anesthetic with unique properties dissociative anesthesia. More recently, it has been repeatedly shown that a single dose of ketamine is able to quickly relieve major depressive symptoms (including suicidal thoughts) in severely affected patients. This effect appears to be transient (the effect lasts less than one week), robust and reproducible, although several questions on its clinical use remain to be addressed.
We therefore propose to test the hypothesis that ketamine and/or its metabolites acutely enhance serotoninergic (5HT) transmission by either modulating afferent synaptic transmission onto serotonin neurons of the dorsal raphe or changing their intrinsic excitability. We set out to directly test this hypothesis using whole-cell patch clamp recordings in brainstem slices from juvenile (P21-P30) rats. We used a conventional bicarbonate-based Ringer and a KCl-based intrapipette solution. This allowed us to discriminate presumed 5HT from non-5HT neurons. 5HT neurons were identified as cells generating an outward current > 30 pA in voltage clamp at -60 mV during superfusion of 100 nM of the 5HT1A agonist 8-OH-DPAT (calculated EK was = -93 mV).
We first tested Ketamine’s effect on sEPSCs by using 10µM gabazine and 1 µM CGP55845. We found that 10 µM racemic ketamine increase the AMPA EPSCs in terms of amplitude and frequency in the half of 5HT recorded neurons (N total = 18).This effect was not observed in the case of non-5HT recorded neurons (N = 6). The same effect was observed by the racemate of its metabolite 2,6 hydroxynorketamine (HNK). In current clamp recordings, both ketamine and HNK increased the firing of 5HT neurons in whole-cell recordings when AMPA, NMDA, GABAA and GABAB receptors were blocked.
In conclusion, ketamine and its metabolite HNK robustly enhance AMPA EPSCs onto a subgroup of pharmacologically identified 5HT neurons. It remains to be determined whether this subgroup projects to specific targets, given that subpopulations of DR 5HT neurons have been shown to have specific and non-overlapping projections (Ren et al., Cell 175, 472-487, 2018).