Hippocampal place cells support reward-related spatial memories by forming a cognitive map that over-represents reward locations. The strength of these memories is modulated by the extent of reward expectation during encoding. However, the circuit mechanisms underlying this modulation are unclear. Here we find that when reward expectation is extinguished in mice, they remain engaged with their environment, yet place cell over-representation of rewards vanishes, place field remapping throughout the environment increases, and place field trial-to-trial reliability decreases. Interestingly, Ventral Tegmental Area (VTA) dopaminergic axons in CA1 exhibit a ramping reward-proximity signal that depends on reward expectation and inhibiting VTA dopaminergic neurons largely replicates the effects of extinguishing reward expectation. We conclude that changing reward expectation restructures CA1 cognitive maps and determines map reliability by modulating the dopaminergic VTA-CA1 reward-proximity signal. Thus, internal states of high reward expectation enhance encoding of spatial memories by reinforcing hippocampal cognitive maps associated with reward.

Dopaminergic (DA) neurons exert profound influences on behavior including addiction. However, how DA axons communicate with target neurons and how those communications change with drug exposure remains poorly understood. We leverage cell type-specific labeling with large volume serial electron microscopy to detail DA connections in the nucleus accumbens (NAc) of the mouse (Mus musculus) before and after exposure to cocaine. We find that individual DA axons contain different varicosity types based on their vesicle contents. Spatially ordering along individual axons further suggests that varicosity types are non-randomly organized. DA axon varicosities rarely make specific synapses (<2%, 6/410), but instead are more likely to form spinule-like structures (15%, 61/410) with neighboring neurons. Days after a brief exposure to cocaine, DA axons were extensively branched relative to controls, formed blind-ended ‘bulbs’ filled with mitochondria, and were surrounded by elaborated glia. Finally, mitochondrial lengths increased by ~2.2 times relative to control only in DA axons and NAc spiny dendrites after cocaine exposure. We conclude that DA axonal transmission is unlikely to be mediated via classical synapses in the NAc and that the major locus of anatomical plasticity of DA circuits after exposure to cocaine are large-scale axonal re-arrangements with correlated changes in mitochondria.

Small alterations in extracellular acidity are potentially important modulators of neuronal signaling within the vertebrate retina. Here we report a novel extracellular acidification mechanism mediated by glial cells in the retina. Using self-referencing H+-selective microelectrodes to measure extracellular H+ fluxes, we show that activation of retinal Müller (glial) cells of the tiger salamander by micromolar concentrations of extracellular ATP induces a pronounced extracellular H+ flux independent of bicarbonate transport. ADP, UTP and the non-hydrolyzable analog ATPγs at micromolar concentrations were also potent stimulators of extracellular H+ fluxes, but adenosine was not. The extracellular H+ fluxes induced by ATP were mimicked by the P2Y1 agonist MRS 2365 and were significantly reduced by the P2 receptor blockers suramin and PPADS, suggesting activation of P2Y receptors. Bath-applied ATP induced an intracellular rise in calcium in Müller cells; both the calcium rise and the extracellular H+ fluxes were significantly attenuated when calcium re-loading into the endoplasmic reticulum was inhibited by thapsigargin and when the PLC-IP3 signaling pathway was disrupted with 2-APB and U73122. The anion transport inhibitor DIDS also markedly reduced the ATP-induced increase in H+ flux while SITS had no effect. ATP-induced H+ fluxes were also observed from Müller cells isolated from human, rat, monkey, skate and lamprey retinae, suggesting a highly evolutionarily conserved mechanism of potential general importance. Extracellular ATP also induced significant increases in extracellular H+ flux at the level of both the outer and inner plexiform layers in retinal slices of tiger salamander which was significantly reduced by suramin and PPADS. We suggest that the novel H+ flux mediated by ATP-activation of Müller cells and of other glia as well may be a key mechanism modulating neuronal signaling in the vertebrate retina and throughout the brain.

Self-referencing H-selective electrodes were used to measure extracellular H fluxes from Müller (glial) cells isolated from the tiger salamander retina. A novel chamber enabled stable recordings using H-selective microelectrodes in a self-referencing format using bicarbonate-based buffer solutions. A small basal H flux was observed from the end foot region of quiescent cells bathed in 24 mM bicarbonate-based solutions, and increasing extracellular potassium induced a dose-dependent increase in H flux. Barium at 6 mM also increased H flux. Potassium-induced extracellular acidifications were abolished when bicarbonate was replaced by 1 mM HEPES. The carbonic anhydrase antagonist benzolamide potentiated the potassium-induced extracellular acidification, while 300 μM DIDS, 300 μM SITS, and 30 μM S0859 significantly reduced the response. Potassium-induced extracellular acidifications persisted in solutions lacking extracellular calcium, although potassium-induced changes in intracellular calcium monitored with Oregon Green were abolished. Exchange of external sodium with choline also eliminated the potassium-induced extracellular acidification. Removal of extracellular sodium by itself induced a transient alkalinization, and replacement of sodium induced a transient acidification, both of which were blocked by 300 μM DIDS. Recordings at the apical portion of the cell showed smaller potassium-induced extracellular H fluxes, and removal of the end foot region further decreased the H flux, suggesting that the end foot was the major source of acidifications. These studies demonstrate that self-referencing H-selective electrodes can be used to monitor H fluxes from retinal Müller cells in bicarbonate-based solutions and confirm the presence of a sodium-coupled bicarbonate transporter, the activity of which is largely restricted to the end foot of the cell. The present study uses self-referencing H-selective electrodes for the first time to measure H fluxes from Müller (glial) cells isolated from tiger salamander retina. These studies demonstrate bicarbonate transport as a potent regulator of extracellular levels of acidity around Müller cells and point toward a need for further studies aimed at addressing how such glial cell pH regulatory mechanisms may shape neuronal signaling.

SummaryDetailing the ways drugs of abuse physically change dopaminergic circuits would provide new mechanisms for explaining addictive behaviors, future targets for therapeutic intervention, and insights into the nature of synaptic plasticity. We combine recent advances in genetic labeling with large volume serial electron microscopy to detail how normal dopaminergic (DA) axons interact with putative targets and how those interactions change in mice briefly exposed to cocaine. We find that while most DA boutons are devoid of obvious signs of synapses (i.e. synaptic vesicles or synaptic densities) many DA boutons physically interdigitate with both dendrites and excitatory and inhibitory axons. After a brief exposure to cocaine, we find evidence of large-scale structural remodeling: extensive axonal branching and frequent occurrences of axonal blind-ended “bulbs”, filled with mitochondria, reminiscent of axonal retraction in the developing and damaged brain. The number of physical interdigitations and vesicle filled boutons between DA axons and targets scales linearly with the length of axon whether in controls or cocaine exposed animals and the size or the type of interaction (i.e. axo-axonic or axo-dendritic) does not change. Finally, we find significant cell-type and sub-cellular specific increases in mitochondrial length in response to cocaine. Specifically, mitochondria in dopamine axons and local Nucleus Accumbens (NAc) dendrites are ~3.5 times and 2 times longer, respectively, in cocaine treated mice than controls. These results show for the first time the effects of cocaine on remodeling of dopamine circuitry and reveal new details on how dopamine neurons physical associate with downstream targets.