Abstract
The monovalent lithium cation (Li+) has been used as a first-line drug in the treatment of bipolar disorder for more than six decades. Despite extensive research the therapeutic mechanism of action of Li+ remains unknown. A large body of recent evidence from genetic linkage, genome-wide association, and pathways-based analytical studies have implicated neuronal membrane ion channels in bipolar disorder. Evidence from few in vitro biophysical studies using Li+ have also indicated possible membrane effects of Li+, but no specific ion channel targets have been identified. In addition, selected antiepileptic drugs with established ion channel blocking effects are used as alternatives to or in combination with Li+, to treat bipolar disorder. Substantial evidence therefore suggests that bipolar disorder might be a ‘channelopathy’, and that Li+ might mediate its therapeutic effect by acting on membrane ion channels.
This study sought to identify possible post-synaptic membrane ion channel target(s) of Li+ in projection neurons within the limbic system. For this purpose, an in vitro model consisting of the output neurons (mitral cells) in brain slice preparations of the mouse olfactory bulb was used, and the effect of Li+ was studied using patch-clamp electrophysiological techniques.
The results indicate that Li+ can modulate brain neuron membrane excitability in two ways. First, Li+ increased subthreshold membrane excitability, prior to the onset of action potential firing, as indicated by facilitated temporal integration of brief depolarizing inputs, and by shortening of the delay to the first action potential on removal of a hyperpolarising holding current. Second, Li+ altered neuronal membrane properties apparent at suprathreshold membrane potentials, indicated by decreased action potential rise slope, decay slope, and after-hyperpolarisation amplitude. These subthreshold and suprathreshold effects of Li+ were associated with increased spontaneous action potential firing frequency, and suggest that Li+ altered voltage-gated K+ currents.
The prominent effect of Li+ at subthreshold membrane potentials suggests that Li+ affects a subthreshold K+ current similar to ID, the delay current defined in several types of brain neuron. Specific pharmacological blockers of ID could reproduce the subthreshold effects of Li+, but did not occlude a further effect of Li+. The subthreshold membrane current sensitive to Li+ has functional biophysical characteristics as previously reported for ID, but the pharmacological results suggest a current carried by K+ channels with a subunit composition not previously reported for ID. Voltage-clamp recordings suggested that ion channels carrying this Li+-sensitive current are located away from the neuron cell body, possibly in the axon membrane as previous studies have suggested for ID. How Li+ acts on these K+ channels was not established, but blockade by elevated intracellular free Mg2+ (displaced by Li+ from intracellular Mg2+ binding sites) is proposed.
The effects of Li+ were compared to effects of antiepileptic drugs also used to treat bipolar disorder, carbamazepine (CBZ) and valproate (VPA). Unlike Li+ but as expected for these drugs, CBZ and VPA each suppressed spontaneous action potential firing, and while CBZ partially reproduced the subthreshold effects of Li+, VPA did not. These results suggest that relevant electrophysiological effects of mood-stabilising drugs might be shared at a higher functional level than the neuron membrane, at the neuronal network level.
Bipolar disorder involves deranged brain network oscillations and connectivity. The results of the thesis show for the first time that Li+ can influence post-synaptic integration and action potential timing in projection neurons of the brain, factors that can influence brain network oscillations and connectivity. Based on these new findings, the thesis proposes a model of bipolar disorder at the functional network level, offering to explain the effectiveness of mood-stabilising drugs in terms of their effects on specific neuron membrane and network properties.