A subsequent group noticed that this mutation is in a region important for modulation by G protein-coupled receptors (GPCRs). GPCR activation leads to inhibition of wild-type CaV2.1 currents. R192Q mutant channel currents are also decreased by GPCR activation, but by a smaller amount. A more recent group has confirmed some of these results by creating a R192Q knock-in mouse. They confirmed that the R192Q mutant activates at more negative potentials and that neurons producing these channels have much larger whole-cell current. This resulted in a much larger quantal content (the number of neurotransmitter packets released per action potential) and generally enhanced neurotransmitter release in R192Q-expressing neurons versus wild-type. Consequently, these mutant mice were more susceptible to cortical-spreading-depression than their wild-type counterparts. The most recent experiments on this mutant, however, have contradicted some of these results. In CaV2.1 knockout neurons transfected with human channels, P/Q-type currents from mutant channels are actually smaller than their wild-type counterpart. They also found a significant decrease in calcium influx during depolarization, leading to decreased quantal content, in mutant versus wild-type expressing neurons. Neurons expressing mutant channels were also less able to mediate inhibitory input and have smaller inhibitory postsynaptic currents through P/Q-type channels. Further testing with this and other mutants is required to determine their end effect on human physiology. FHM2 (ATP1A2)
The second subtype of familial hemiplegic migraine, FHM2, is caused by mutations in the gene ATP1A2 that encodes a Na+/K+-ATPase. This Na+/K+-ATPase is heavily expressed in astrocytes and helps to set and maintain their reversal potential.
Pathophysiology
FHM1 (CACNA1A)
The first discovered FHM locus was the CACNA1A gene (originally named CACNL1A4), which encodes the P/Q-type calcium channel CaV2.1. Currently, 17  mutations in this channel are known (table 1), and these mutations are distributed throughout the channel. Some of these mutations result in patients with notable cerebellar degeneration or other dysfunction, including one mutation (S218L), which may be related to severe responses to mild concussion, up to and including delayed cerebral edema, coma, and death. Fifteen of these mutants have received at least some further analysis at the electrophysiological level to attempt to determine how they might lead to the FHM1 phenotype. Contradiction in the literature is increasing as to the end result of these mutations on channel kinetics and neuronal excitability.A good example of this contradiction can be seen in the literature regarding the R192Q mutation. The first investigation of this mutation, using the rabbit isoform of the channel expressed in oocytes, found that it did not alter any measured channel properties. A subsequent report, using human channels expressed in HEK293 cells, found a small, hyperpolarizing shift in the midpoint for activation, a result common among FHM1 mutants. This shift results in channels that open at more negative potentials, thus have a higher open probability than wild-type channels at most potentials. This report also found that the R192Q mutant produced almost twice as much whole-cell current compared to wild-type channels. This is not due to a change in single channel conductance, but to an equivalent increase in channel density.