serotonin(in) + n H(+)(out) = serotonin(out) + n H(+)(in)
serotonin(in) + n h(+)(out) = serotonin(out) + n h(+)(in)
serotonin(in) + n H(+)(out) = serotonin(out) + n H(+)(in)
serotonin(in) + n h(+)(out) = serotonin(out) + n h(+)(in)