Temporally dissociable effects of ketamine on neuronal discharge and gamma oscillations in rat thalamo-cortical networks.

Sub-anesthetic doses of ketamine (1, 2, and 5 mg/kg intravenously) decreased the firing rate of neurons in the reticular thalamic nucleus, mediodorsal and centromedial thalamic nuclei, and layer VI of the medial prefrontal cortex in anesthetized male Wistar rats. Ketamine also reduced low-frequency oscillations across these areas while increasing gamma oscillations in the medial prefrontal cortex and mediodorsal/centromedial thalamic nuclei. Lower doses (0.25 and 0.5 mg/kg) had no effect. Unlike phencyclidine, ketamine's inhibition of reticular thalamic nucleus neurons did not disinhibit excitatory neurons in other areas, likely due to concurrent NMDA receptor blockade there. The early transient inhibition may relate to psychotomimetic effects, while prolonged gamma increases may underlie antidepressant action.

Study at a glance

Abstract

Sub-anesthetic doses of the non-competitive N-methyl-d-aspartate receptor (NMDA-R) antagonist ketamine evoke transient psychotomimetic effects, followed by persistent antidepressant effects in treatment-resistant depressed patients and rodents through still poorly understood mechanisms. Since phencyclidine (PCP) disinhibits thalamo-cortical networks by blocking NMDA-Rs on GABAergic neurons of the reticular thalamic nucleus (RtN), we examined ketamine's actions in the same areas. Single units and local field potentials were recorded in chloral hydrate anesthetized male Wistar rats. The effects of cumulative ketamine doses (0.25-5 mg/kg, i.v.) on neuronal discharge and oscillatory activity were examined in RtN, mediodorsal and centromedial (MD/CM) thalamic nuclei, and layer VI of the medial prefrontal cortex (mPFC). Ketamine (1, 2 and 5 mg/kg, i.v.) significantly decreased the discharge of MD/CM, RtN and layer VI mPFC pyramidal neurons. Simultaneously, ketamine decreased the power of low frequency oscillations in all areas examined and increased gamma oscillations in mPFC and MD/CM. Lower ketamine doses (0.25 and 0.5 mg/kg, i.v.) were ineffective. As observed for PCP, ketamine markedly inhibited the activity of RtN neurons. However, unlike PCP, this effect did not translate into a disinhibition of MD/CM and mPFC excitatory neurons, possibly due to a more potent and simultaneous blockade of NMDA-Rs by ketamine in MD/CM and mPFC neurons. Hence, the present in vivo results show that ketamine evokes an early transient inhibition of neuronal discharge in thalamo-cortical networks, following its rapid pharmacokinetics, which is likely associated to its psychotomimetic effects. The prolonged increase in gamma oscillations may underlie its antidepressant action.