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