Striking Similarities between Clinical and Biological Properties of Ketamine and Ethanol: Linking Antidepressant-After Effect and Burgeoning Addiction
Ketamine is an old drug of abuse showing currently a new wave in its spread. Also, ketamine`s therapeutic quality is currently under strong observation, especially in terms of its value in the treatment of depression and suicidality. It`s a potential revolution in understanding the mechanisms of antidepressant treatment that single and repeated therapeutic administrations of sub-anesthetic ketamine doses are associated with a rapid and robust but transient antidepressant after-effect (ADE) in patients with treatment resistant major depression. There is increasing evidence that this ADE might result primarily from ketamine`s feature of being a non-competitive antagonist of glutamatergic N-methyl-D-aspartate (NMDA)-receptors embedded in synaptic membranes of neuronal cortico-limbic networks promoting an extracellular glutamate surge, thereby mediating changes in synaptic and cellular plasticity via local glutamate non-NMDA-receptors. Here, we focus on a couple of striking clinical and biological overlaps with ketamine and ethanol being a non-competitive antagonist of NMDA-receptors, too. Among them, a good portion is currently assumed to be specifically involved in both, the mechanisms of ADE (in the case of ketamine) and the development of addiction (in the case of ethanol). These overlaps are mainly addressed here in more detail, what may draw the reader in terms of the treatment of mood disorders to both, the possibility of a progressing transfer from ADE to addiction when repeatedly using therapeutic ketamine pulses, and on the other hand, a hypothesized therapeutic `antidepressant window´ of modest and cautious ethanol use in depressives, who are (still?) not addicted to ethanol. Of course, more frequent and intense use of ethanol or ketamine would prepare the brain to tolerance and dependence possibly using the same pathways.
v) Activation of mTOR-signaling pathways [12,13,50,51]. vi) Synaptic adaptations, such as increasing number and size of dendritic spines in rodent hippocampal and prefrontal neurons [13,52,53]. vii) Repeated administration induced tolerance in euphoriant and stimulating actions [21,54] as well as in ADE [55-57], own unpublished observations]. viii) Repeated administration induced gene expression of specific NMDA receptor subunits in cortico-limbic brain regions [20,32,5860]. ix) Down-regulation of cerebellum following repeated drug administration [59]. x) Chronic or regular intake was associated with the development of addiction and neurotoxic effects that could lead to brain atrophy [32,34,54,61]. xi) Bioavailability of oral ketamine is low (20%) [34] and that of ethanol decreased from about 85% in young people down to 40% in the elderly [62].