Homeostasis is a biological principle for regulation of essential physiological parameters within a set range. Behavioural responses due to deviation from homeostasis are critical for survival, but motivational processes engaged by physiological need states are incompletely understood. We examined motivational characteristics and dynamics of two separate neuron populations that regulate energy and fluid homeostasis by using cell type-specific activity manipulations in mice. We found that starvation-sensitive AGRP neurons exhibit properties consistent with a negative-valence teaching signal. Mice avoided activation of AGRP neurons, indicating that AGRP neuron activity has negative valence. AGRP neuron inhibition conditioned preference for flavours and places. Correspondingly, deep-brain calcium imaging revealed that AGRP neuron activity rapidly reduced in response to food-related cues. Complementary experiments activating thirst-promoting neurons also conditioned avoidance. Therefore, these need-sensing neurons condition preference for environmental cues associated with nutrient or water ingestion, which is learned through reduction of negative-valence signals during restoration of homeostasis.

The efficacy of fast synaptic inhibition is critically dependent on the accumulation of GABA A receptors at inhibitory synapses, a process that remains poorly understood. Here, we examined the dynamics of cell surface GABA A receptors using receptor subunits modified with N-terminal extracellular ecliptic pHluorin reporters. In hippocampal neurons, GABA A receptors incorporating pHluorin-tagged subunits were found to be clustered at synaptic sites and also expressed as diffuse extrasynaptic staining. By combining FRAP (fluorescence recovery after photobleaching) measurements with live imaging of FM4-64-labeled active presynaptic terminals, it was evident that clustered synaptic receptors exhibit significantly lower rates of mobility at the cell surface compared with their extrasynaptic counterparts. To examine the basis of this confinement, we used RNAi to inhibit the expression of gephyrin, a protein shown to regulate the accumulation of GABA A receptors at synaptic sites. However, whether gephyrin acts to control the actual formation of receptor clusters, their stability, or is simply a global regulator of receptor cell surface number remains unknown. Inhibiting gephyrin expression did not modify the total number of GABA A receptors expressed on the neuronal cell surface but significantly decreased the number of receptor clusters. Live imaging revealed that clusters that formed in the absence of gephyrin were significantly more mobile compared with those in control neurons. Together, our results demonstrate that synaptic GABA A receptors have lower levels of lateral mobility compared with their extrasynaptic counterparts, and suggest a specific role for gephyrin in reducing the diffusion of GABA A receptors, facilitating their accumulation at inhibitory synapses.

Ionic flux in defined cell populations mediates essential physiological and behavioral functions. Cell type-specific activators of diverse ionic conductances are needed for probing these relationships. We combined chemistry and protein engineering to enable systematic creation of a toolbox of ligand-gated ion channels (LGICs) with orthogonal pharmacologic selectivity and divergent functional properties. The LGICs and their small molecule effectors can activate a range of ionic conductances in genetically-specified cell types.LGICs constructed for neuronal perturbation can be used to selectively manipulate neuron activity in mammalian brains in vivo.The diversity of ion channel tools accessible from this approach will be useful for examining the relationship between neuronal activity and animal behavior, as well as for cell biological and physiological applications requiring chemical control of ion conductance.Ion channels are complex molecular machines with critical cell biological functions. Ligandgated ion channels (LGICs) provide rapid, remote control over conductances for different ions. In neurons, LGICs can be exploited for stimulation or silencing to examine causal relationships between electrical activity and animal behavior. Several neuron manipulation tools have been derived fromLGICs and G-protein coupled receptors (1-4) that can be genetically targeted and are reported to be orthogonal to endogenous systems. These tools are useful (5-7) but also face limitations such as ligand instability and lack of brain access (2), slow pharmacokinetics (6), the need to knockout endogenous alleles (3), or reliance on complex intracellular signaling pathways (4). Optogenetic tools (8-10) activate conductances with millisecond precision, but optimization of ion conductance properties has been limited and light targeting is invasive.To overcome these limitations, we have developed a strategy to create chimeric LGICs with distinct conductance properties derived from modular combinations of pharmacologicallyselective ligand binding domains (LBDs) and functionally diverse ion pore domains (IPDs). Within the Cys-loop receptor superfamily, the LBD of the α7 nicotinic acetylcholine receptor (nAChR) behaves as an independent actuator module that can be transplanted onto the IPDs of other Cys-loop receptors (11,12). These include at least 43 ion channel subunits in vertebrates (13), and many additional invertebrate (14) and prokaryotic (15) subunits. Distinct IPDs confer selectivity for chloride or calcium as well as nonspecific cations. For example, splicing the α7 nAChR LBD to the IPDs of the serotonin receptor 3a or the glycine receptor produces chimeric channels (α7-5HT3 or α7-GlyR) with α7 nAChR pharmacology and cation or chloride conductance properties, respectively (11,12). This modular property is a strong foundation for tailoring functional characteristics. However, the † To whom correspondence should be addressed. sternsons@janelia.hhmi.org (S.M.S.), HHMI Author ManuscriptHHMI Author Manuscript HHMI Auth...