Procaine is used three times. The first time is in Part I, "The Hearth and the Salamander." My copy begins on page 3. The first time it's used in my copy is on page 23. When things "got dull," the firemen would alter the Hound's settings so it would chase cats, rats or chickens. They would bet on which animal would go first and the result would be quick, an injection of "massive jolts of morphine or procaine" (23).
The second time procaine is used is right after Montag kills Beatty. The Hound lunges for him, "the procaine needle snapping out of its single angry tooth" (106). This is about six pages into Part III.
The third time procaine is used is about 22 pages into Part III. From the window sill, Montag watches the Hound approach Faber's house as the Hound's "procaine needle flicked out and in" (122).