Buses run frequently along the length of Hwy 7 between Managua and the terminus at El Rama. If you're just passing through, make sure to hop on an expreso service, which are considerably faster than the ordinarios. Both options get packed to the rafters.
Old beat-up school buses serve the spur roads off the highway, but service is irregular at best. To truly explore the back roads of the region, a rental vehicle is the best way to go.