What does the old man write to the papers? Why does Huck get especially nervous about the advertisement in the St. Louis paper?

From the text:

The old man had wrote a couple of times to the plantation below Orleans to come and get their run-away nigger, but hadn't got no answer, because there warn't no such plantation; so he allowed he would advertise Jim in the St. Louis and New Orleans papers; and when he mentioned the St. Louis ones it give me the cold shivers, and I see we hadn't no time to lose.