[Show abstract][Hide abstract] ABSTRACT: Optogenetics is an extremely powerful tool for selective neuronal activation/inhibition and dissection of neural circuits. However, a limitation of in vivo optogenetics is that an animal must be tethered to an optical fiber for delivery of light. Here, we describe a new method for in vivo, optogenetic inhibition of neural activity using an internal, animal-generated light source based on firefly luciferase. Two adeno-associated viruses encoding luciferase were tested and both produced concentration-dependent light after administration of the substrate, luciferin. Mice were co-infected with halorhodopsin- and luciferase-expressing viruses in the striatum, and luciferin administration significantly reduced Fos activity compared to control animals infected with halorhodopsin only. Recordings of neuronal activity in behaving animals confirmed that firing was greatly reduced after luciferin administration. Finally, amphetamine-induced locomotor activity was reduced in halorhodopsin/luciferase mice pre-injected with luciferin compared to controls. This demonstrates that virally encoded luciferase is able to generate sufficient light to activate halorhodopsin and suppress neural activity and change behavior. This approach could be used to generate inhibition in response to activation of specific molecular pathways.
Frontiers in Behavioral Neuroscience 04/2014; 8:108. · 4.16 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Although the prefrontal cortex influences motivated behavior, its role in food intake remains unclear. Here, we demonstrate a role for D1-type dopamine receptor-expressing neurons in the medial prefrontal cortex (mPFC) in the regulation of feeding. Food intake increases activity in D1 neurons of the mPFC in mice, and optogenetic photostimulation of D1 neurons increases feeding. Conversely, inhibition of D1 neurons decreases intake. Stimulation-based mapping of prefrontal D1 neuron projections implicates the medial basolateral amygdala (mBLA) as a downstream target of these afferents. mBLA neurons activated by prefrontal D1 stimulation are CaMKII positive and closely juxtaposed to prefrontal D1 axon terminals. Finally, photostimulating these axons in the mBLA is sufficient to increase feeding, recapitulating the effects of mPFC D1 stimulation. These data describe a new circuit for top-down control of food intake.
[Show abstract][Hide abstract] ABSTRACT: Food restriction is known to enhance learning and motivation. The neural mechanisms underlying these responses likely involve alterations in gene expression in brain regions mediating the motivation to feed.
Analysis of gene expression profiles in male C57BL/6J mice using whole-genome microarrays was completed in the medial prefrontal cortex, nucleus accumbens, ventral tegmental area, and the hypothalamus following a 5-day food restriction. Quantitative polymerase chain reaction was used to validate these findings and determine the time course of expression changes. Plasma levels of the stress hormone corticosterone (CORT) were measured by enzyme-linked immunosorbent assay. Expression changes were measured in adrenalectomized animals that underwent food restriction, as well as in animals receiving daily injections of CORT. Progressive ratio responding for food, a measure of motivated behavior, was assessed after CORT treatment in restricted and fed animals.
Brief food restriction results in an upregulation of peripheral stress responsive genes in the mammalian brain. Time-course analysis demonstrated rapid and persistent expression changes in all four brain regions under study. Administration of CORT to nonrestricted animals was sufficient to induce a subset of the genes, and alterations in gene expression after food restriction were dependent on intact adrenal glands. CORT can increase the motivation to work for food only in the restricted state.
These data demonstrate a central role for CORT in mediating both molecular and behavioral responses to food restriction. The stress hormone-induced alterations in gene expression described here may be relevant for both adaptive and pathological responses to stress.
[Show abstract][Hide abstract] ABSTRACT: Orexin (hypocretin) signaling is implicated in drug addiction and reward, but its role in feeding and food-motivated behavior remains unclear.
We investigated orexin's contribution to food-reinforced instrumental responding using an orexin 1 receptor (Ox1r) antagonist, orexin -/- (OKO) and littermate wildtype (WT) mice, and RNAi-mediated knockdown of orexin. C57BL/6J (n = 76) and OKO (n = 39) mice were trained to nose poke for food under a variable ratio schedule of reinforcement. After responding stabilized, a progressive ratio schedule was initiated to evaluate motivation to obtain food reinforcement.
Blockade of Ox1r in C57BL/6J mice impaired performance under both the variable ratio and progressive ratio schedules of reinforcement, indicating impaired motivational processes. In contrast, OKO mice initially demonstrated a delay in acquisition but eventually achieved levels of responding similar to those observed in WT animals. Moreover, OKO mice did not differ from WT mice under a progressive ratio schedule, indicating delayed learning processes but no motivational impairments. Considering the differences between pharmacologic blockade of Ox1r and the OKO mice, animals with RNAi mediated knockdown of orexin were then generated and analyzed to eliminate possible developmental effects of missing orexin. Orexin gene knockdown in the lateral hypothalamus in C57BL/6J mice resulted in blunted performance under both the variable ratio and progressive ratio schedules, resembling data obtained following Ox1r antagonism.
The behavior seen in OKO mice likely reflects developmental compensation often seen in mutant animals. These data suggest that activation of the Ox1r is a necessary component of food-reinforced responding, motivation, or both in normal mice.