A postsynaptic G-protein in hippocampal long-term potentiation

Lithium: a switch from LTD- to LTP-like plasticity in human cortex

Lithium, a simple cation, is the mainstay treatment of bipolar disorder. Deficient synaptic plasticity is considered one important mechanism of this disease. Lithium inhibits glycogen synthase kinase-3beta (GSK-3β), which is involved in the regulation of synaptic plasticity. In animal preparations, inhibition of GSK-3β by lithium up-regulated long-term potentiation (LTP) of excitatory synapses but down-regulated long-term depression (LTD). The effects of lithium on plasticity in the human brain are unexplored. We tested the effects of a single oral dose of 900 mg of lithium on LTP-/LTD-like plasticity in human motor cortex induced by established paired associative transcranial magnetic stimulation (PAS(LTP), PAS(LTD)) protocols. We studied 10 healthy adults in a placebo-controlled double-blind randomized crossover design. PAS-induced plasticity was indexed by change in motor evoked potential amplitude recorded in a hand muscle. In the placebo session, subjects were stratified, according to the known variability of the PAS(LTP) response, into PAS(LTP) 'LTP responders' and PAS(LTP) 'LTD responders' (n = 5 each). Lithium did not affect the PAS(LTP)-induced LTP-like plasticity in the 'LTP responders', but switched the PAS(LTP)-induced LTD-like plasticity in the 'LTD responders' to LTP-like plasticity. In contrast, lithium had no effect on the PAS(LTD)-induced LTD-like plasticity in the 'LTD responders'. We provide first-time evidence that lithium significantly modulates brain stimulation induced plasticity in human cortex. The switch from LTD- to LTP-like plasticity is best explained by the inhibitory action of lithium on GSK-3β. This conclusion is necessarily circumstantial because GSK-3β activity was not directly measured. We discuss that other important plasticity-related modes actions of lithium cannot explain our findings.

Influence of serotonin receptor 2A His452Tyr polymorphism on brain temporal structures: a volumetric MR study

Serotonin (5-HT) receptors 2A are expressed in brain regions involved in memory and learning processes. Recently, a functional single nucleotide polymorphism in the 5-HT2A receptor gene leading to an amino-acid substitution at residue 452 (His452Tyr) has been involved in memory performance, persons with the rare 452Tyr allele showing poorer memory performance compared to His452His subjects. To investigate a putative structural effect of this polymorphism on temporal areas typically involved in memory processes, we performed voxel-based morphometry (VBM) and region-of-interest (ROI) volumetric analysis on high-resolution magnetic resonance images in 15 carriers and 61 noncarriers of the 452Tyr allele. ROI volumetric analysis showed a significant reduction of the fractional volume of the temporal white matter in 452Tyr carriers (0.67+/-0.07 vs 0.73+/-0.08; P=0.007). VBM confirmed this finding and in addition showed reduced grey matter in the left hippocampus, left inferior temporal gyrus, and bilaterally in the middle and superior temporal gyrus. A possible effect on synaptic plasticity or neurodevelopment might explain the influence of the His452Tyr polymorphism on temporal brain structures, and this might be the basis for poorer memory performance in 452Tyr carriers. These findings should be considered preliminary and future replication is needed.