serotonin(out) + 2 H(+)(in) => serotonin(in) + 2 H(+)(out)
serotonin(out) + 2 h(+)(in) => serotonin(in) + 2 h(+)(out)
serotonin(out) + 2 H(+)(in) => serotonin(in) + 2 H(+)(out)
serotonin(out) + 2 h(+)(in) => serotonin(in) + 2 h(+)(out)