Divergent Effects of Ketamine and the Serotoninergic Psychedelic 2,5-Dimethoxy-4-Iodoamphetamine on Hippocampal Plasticity and Metaplasticity.

In mice, the serotonergic psychedelic DOI, but not ketamine, enhanced neural plasticity and metaplasticity in the hippocampus 24 hours after a single dose. Brain slices from DOI-treated animals showed stronger synaptic responses and short-term potentiation compared to saline-treated controls, with evidence that these effects involve a presynaptic mechanism. Ketamine did not produce similar changes. These findings suggest that the therapeutic benefits of serotonergic psychedelics may be supported by a window of heightened neural plasticity, whereas ketamine's effects may rely on different mechanisms.

Study at a glance

Abstract

Serotonergic psychedelics and ketamine produce rapid and long-lasting symptomatic relief in multiple psychiatric disorders. Evidence suggests that despite having distinct molecular targets, both drugs may exert therapeutic benefit via their pro-neuroplastic effects. Following treatment with ketamine or serotonergic psychedelics, patients are reported to be more open to behavioral change, which is leveraged for psychotherapy-assisted reframing of narratives of the self. This period of enhanced behavioral change is postulated to be supported by a post-treatment window of enhanced neural plasticity, but evidence for such 'metaplastic' effects is limited. In this study, we tested for neural plasticity and metaplasticity in murine hippocampus. Brain slices were obtained from C57BL/6J mice 24 hours after treatment (intraperitoneal injection) with saline, ketamine, or the serotonergic psychedelic 2,5-Dimethoxy-4-iodoamphetamine (DOI). Extracellular fiber volleys (FVs) and field excitatory postsynaptic potentials (fEPSPs) were recorded in stratum radiatum of CA1 in response to stimulation of Schaffer collateral fibers before and after induction of short-term and long-term potentiation (STP, LTP). Before LTP induction, responses differed across treatment groups (F1,2 = 5.407, p = 0.00665), with fEPSPs enhanced in slices from DOI-treated animals (p = 0.0182), but not ketamine-treated animals (p = 0.9786), compared to saline. There were no treatment effects on LT (F1,2 = 0.6, p = 0.516), but there were on STP (F1,2 =, p = 0.0167), with enhanced STP in DOI-treated (p = 0.0352) but not ketamine-treated (p = 0.9999) animals compared to saline. A presynaptic component to the mechanism for the DOI effects was suggested by (1) significantly enhanced FV amplitudes (F1,2 = 3.17, p = 0.049) in DOI-treated (p = 0.0457) but not ketamine-treated animals compared to saline (p = 0.8677); and (2) enhanced paired pulse ratios (F1,2 = 3.581, p = 0.0339) in slices from DOI-treated (p= 0.0257) but not ketamine-treated animals (p = 0.4845) compared to saline. DOI, but not ketamine, induced significant neuroplastic and metaplastic effects at hippocampal CA1 synapses 24 hours after treatment, likely in part via a presynaptic mechanism.