Motivational capacity to interact with the environment is fundamental for everyday healthy living. Motivated behaviors ultimately manifest through reward and aversion processes, where animals must approach positive rewarding stimuli and avoid negative aversive stimuli to survive. The neural mechanisms mediating these basic processes are not yet fully understood. However, technological advancements made in the past decade now allows us to investigate previously understudied brain regions involved with these processes. One such understudied region, the supramammillary nucleus (SuM), is a small posterior hypothalamic nucleus that provides dense projections throughout the cerebrum. Past research on SuM has focused on its role in arousal, learning, and memory. Our lab previously found that pharmacological stimulation of SuM neurons can reinforce behavior. In the current study, we designed experiments to further our understanding of the role the SuM and its related circuitry plays in reward and motivation. We first confirmed stimulation of SuM neurons is rewarding using a self-stimulation procedure with optogenetics involving channelrhodopsin-2 (ChR2) in wild-type (C57/BL7) mice. Mice with ChR2 and optic fibers in SuM quickly learned to respond on a lever reinforced by photostimulation and switch responding when lever assignments are reversed. Mice do not reliably self-stimulate when optic fibers are placed in areas adjacent to SuM, ie. in the mammillary bodies or ventral tegmental area. Next, using a Cre-dependent ChR2 and vGlut2-Cre, vGat-Cre or Th-Cre mice, we show this rewarding effect of SuM neuron stimulation is likely mediated by glutamatergic neurons, but not dopaminergic or GABAergic neurons. Then using optogenetic terminal-stimulation we dissect which glutamatergic projections from the SuM mediate self-stimulation behavior. Mice learned to respond for the stimulation of SuM glutamatergic neurons terminating in the septal area, but not terminals in the paraventricular thalamic nucleus (PVT), ventral subiculum, or diagonal band of Broca. In addition, mice show real-time place preference for activation of the SuM to septum circuit, and real-time place aversion for activation of the SuM to PVT circuit, indicating bivalent affective processes driven by SuM circuitry. To investigate the role of SuM neurons in food (sucrose) taking and seeking behavior, we conducted single-unit in-vivo electrophysiology experiments in mice seeking natural rewards. Most SuM neurons change their firing rates as a function of sucrose seeking, taking or both. Our results implicate the SuM and its downstream targets in motivational processes. As this circuitry is somewhat non-canonical in terms of classical reward circuits, we feel it warrants future research into its role in psychiatric disorders such as depression and addiction.
