Reticular formation and brainstem unitary activity: Effects of posterior hypothalamic and septal-limbic stimulation at reward loci

Stimulation at reward sites in septal-limbic regions had little influence on brainstem unitary activity, while, by comparison, stimulation at reward sites in the ventral regions of the posterior diencephalon had a marked influence. Reward effects in this latter region were obtained not only from areas typically associated with reward as the medial forebrain bundle (MFB), but somewhat surprisingly, were also obtained from regions dorsal to the MFB in H₂ field of Forel. Stimulation in these two ventral diencephalic regions had a similar strong influence on reticular formation (RF) units, but other brainstem units were significantly more influenced by H₂ field of Forel than by MFB. Distinctions between ventral posterior diencephalic rewarding and non-rewarding placements observed in behavioral testing procedures were also observed electrophysiologically. Stimulation at reward sites had a significantly greater influence on brainstem activity than did nonrewarding stimulation. There was no difference, however, between rewarding and non-rewarding placements when their effects on RF unitary activity alone were considered. It was concluded that RF may not represent a critical area of convergence for rewarding stimulation of the brain. Differences of effect of stimulation of reward loci on brainstem activity when either septal-limbic area and MFB or MFB and H₂ field of Forel were compared suggested the possibility that the view of a single brain mechanism subserving reward may be an oversimplification.