Hypothalamic reward mechanism: two first-stage fiber populations with a cholinergic component.

Refractory periods were estimated for fibers of the hypothalamic substrate of brain stimulation reward. Two nonoverlapping populations were evident: a homogeneous fast population and a more heterogeneous slow population. Cholinergic blockade eliminated the contribution of the fast but not the slow fibers, while dopaminergic blockade reduced responding without significantly influencing either directly activated fiber population. These data indicate that the hypothalamic reward substrate is more complex than has been widely appreciated; it contains two or more parallel subsystems, and one of these subsystems has a cholinergic link.