Spatial Encoding Differences Between Horizontal and Vertical Planes

Post by Deborah Joye

What's the science?

The brain helps us navigate the world by creating a spatial map of our surroundings, combining sensory information about how our body is moving with inputs from our external environment. The brain has specialized cells, called hippocampal place cells, that track where we are in the environment by only firing when we are in the cell’s ‘preferred’ location, or ‘firing field’. Place cells work together with grid cells of the entorhinal cortex, which track distance travelled by creating a grid-like array of firing fields across the entire environment. When we move, place and grid cells use external landmarks to estimate location, and self-motion cues, like where the body is in relation to the surface and movement speed, to constantly update those estimations. However, the function of these cells has mostly been studied as animals move horizontally. Do these cells integrate sensory information the same way when we move vertically? This week in PNAS, Casali and colleagues record grid cells and place cells in freely behaving rats to demonstrate that the brain differently encodes movement in the vertical and horizontal planes.

How did they do it?

The authors recorded electrical activity from 148 grid cells in the medial entorhinal cortex of 11 rats and 72 place cells from the hippocampus of 3 rats as they explored an open field arena (horizontal surface only) or over a floor with an adjoining climbing wall (horizontal and vertical surfaces). To ensure that recorded brain activity was not due to the novelty of a new climbing environment, the authors recorded from rats that had extensive climbing experience. Since self-motion cues, such as running speed, and local field potentials are also important for grid cell and place cell function, the authors also recorded from separate cells that encode speed and measured local field potentials. If the grid plane is defined by gravity, then walking on a vertical surface instead of a horizontal plane should produce “stripes” (firing fields would be aligned in vertical stripes) in the recording of grid cell activity. Alternatively, if the grid plane is defined by the body plane, then firing fields should be grid-like (circular and evenly spaced) on the wall, just as they are on the floor. This is because even though the rat is moving vertically, the body is still parallel to the movement surface, as it would be if the rat was moving horizontally across the floor.

What did they find?

The authors found that grid cell firing patterns were different when rats climbed on the wall versus walking across the floor. During climbing, grid cells showed an overall reduction in firing activity with fewer, larger firing fields than those seen during horizontal movement. Grid cells also produced discrete firing fields during climbing, rather than the vertical “stripes,” that might be expected if grid cell firing while climbing on a vertical wall were due to gravity, suggesting that grid cell firing is adjusted by considering the rat’s body plane in relation to their movement surface. In contrast to grid cells, fewer hippocampal place cells were active during climbing but firing characteristics were otherwise not different between the horizontal and vertical planes. Lastly, recordings of firing rates and local field potentials from speed cells revealed that the brain’s encoding of movement speed was consistently underestimated during vertical climbing, which may contribute to the enlarged grid cell firing fields observed when rats were climbing.

What's the impact?

This study is the first to demonstrate that spatial representation in the brain is determined by an interaction between the body-plane alignment and the gravity axis; grid cells track distance differently when movement is over a vertical surface rather than a horizontal one. The speed-coding analysis suggests that this difference may result from underestimation of movement speed on the wall - the grid cells behave as though the animal is moving more slowly than it really is, thus producing larger, more widely spaced firing fields. Overall, this study suggests that the neural encoding of space is can distinguish horizontal from vertical movement, which may have adaptive consequences for animals that move over surface terrain.

Casali et al., Altered neural odometry in the vertical dimension, PNAS (2019). Access the original scientific publication here.

The Sleep-Wake Cycle Regulates Extracellular Tau

Post by: Amanda McFarlan

What's the science?

The aggregation of hyperphosphorylated tau protein is one of the primary markers of Alzheimer’s disease and is known to be highly correlated with neuronal and synaptic degeneration. Previous studies have shown that neurons release tau protein into the extracellular space–where it can spread from one synapse to another—and that this release is increased with elevated neuronal activity. Since the state of wakefulness, compared to sleep, is associated with increased activity, the authors hypothesized that tau may be regulated by the sleep-wake cycle. This week in Science, Holth and colleagues investigated the effect of the sleep-wake cycle on tau in rodents and humans.

How did they do it?

The authors used in vivo microdialysis to examine the effect of the sleep-wake cycle on tau and lactate (regulated by neuronal activity) levels in the brain interstitial fluid in freely behaving mice. To do this, they surgically implanted a guide cannula into the left hippocampus of wild-type mice, and after recovery, inserted a microdialysis probe to collect brain interstitial fluid samples. The levels of tau and lactate in the brain interstitial fluid were then measured in mice that underwent one of three conditions: undisturbed sleep-wake cycle (control), manual sleep deprivation, and sleep deprivation with the infusion of tetrodotoxin (reduces neuronal activity). They further tested the effect of elevated wakefulness (sleep deprivation) on interstitial fluid tau and amyloid-β levels using chemogenetic activation (DREADDs) of the brain wake circuitry. Next, the authors investigated the effect of a longer period of sleep deprivation on the ability of injected tau fibrils to seed tau pathology as well as the ability of seeded tau pathology to proliferate in the brain and promote the misfolding of other tau proteins (tau spreading). They used a transgenic mouse model of tauopathy (P301S mice) and injected recombinant human tau fibrils into the hippocampus of young male mice (not yet expressing a tau pathology). Mice underwent either 28 days of sleep deprivation using the modified multiple platform technique or a control condition. Immunohistochemistry analyses were performed to determine whether tau aggregates had spread throughout the brain. Based on their results in mice, the authors also examined the effect of the sleep deprivation on tau levels in the cerebrospinal fluid of humans. A lumbar catheter was used to collect cerebrospinal fluid in adult participants as they underwent one night of normal rest and one night of sleep deprivation. The authors also measured levels of α-synuclein (a protein associated with increased neuronal activity) as well as other neuronal and glial proteins in the cerebrospinal fluid.

What did they find?

The authors determined that tau and lactate levels in the brain interstitial fluid were higher in control mice during the period of wakefulness compared to the period of sleep. They found that sleep deprivation caused an even greater increase in tau and lactate levels in the brain interstitial fluid. In mice that were both sleep deprived and infused with tetrodotoxin (reducing neuronal activity), there were no detectable changes in tau or lactate levels in the brain interstitial fluid. They also showed that increasing wakefulness using DREADDs significantly increased tau and amyloid-β in the interstitial fluid. Together, these findings suggest that higher tau levels during wakefulness and sleep deprivation might be a result of tau secretion due to increases in neuronal metabolism and synaptic strength. Next, the authors revealed that longer periods of sleep deprivation did not alter the ability of tau fibrils to seed the misfolding of other tau proteins at the injection site, but did increase the spreading of tau pathology throughout the brain compared to control conditions. They determined that in sleep deprived animals, tau spread from the hippocampus to the locus coeruleus, a brain region involved in wakefulness, was increased. In human studies, the authors found that levels of tau and synuclein in the cerebrospinal fluid were significantly increased with a night of sleep deprivation compared to a normal night of sleep. These findings suggests that tauopathies in humans might be sensitive to the sleep-wake cycle and in particular, sleep deprivation.

What's the impact?

This is the first study to show that levels of tau in the brain interstitial fluid are modulated by the sleep-wake cycle and increased with sleep deprivation in rodents. Furthermore, the authors showed that prolonged sleep deprivation in mice significantly increases tau spreading in the brain. Similarly, sleep deprivation was shown to increase tau levels in cerebrospinal fluid in humans. Altogether, this study provides evidence for a role of sleep and wake in regulating tau and tau pathology.

Holth et al. The sleep-wake cycle regulates brain interstitial fluid tau in mice and CSF tau in humans. Science (2019). Access the original scientific publication here.

Cholinergic Interneurons in the Nucleus Accumbens Inhibit Reward-Seeking Behavior

Post by Shireen Parimoo

What's the science?

The nucleus accumbens (NAc), located in the midbrain, regulates motivational or reward-seeking behavior primarily through dopaminergic signaling. For example, dopaminergic activity increases in response to cues that signal an upcoming reward, such as a tone signaling that food will be arriving soon. Conversely, activation of cholinergic interneurons in the NAc is associated with the inhibition of reward-seeking behavior when this behavior might not be beneficial, like seeking food when already full. However, the causal role of these interneurons in regulating cue-motivated behavior is not well-understood. This week in Biological Psychiatry, Collins and colleagues used behavioral conditioning and a combination of optogenetic and chemogenetic techniques to investigate the causal role of cholinergic NAc interneurons in cue-motivated behavior.

How did they do it?

Adult transgenic rats were were injected with adeno-associated virus carrying channelrhodopsin (ChR2) or a control yellow fluorescent protein, and the human M4 muscarinic receptor (hM4D(Gi)) or the control mCherry, which were selectively expressed in the NAc interneurons. Optical stimulation of ChR2 increases interneuron activity, whereas clozapine N-oxide (CNO) injection inactivates interneurons by binding to hM4D(Gi). Cue-motivated behavior was measured using the Pavlovian-to-instrumental transfer (PIT) test, which consisted of a training and a test phase. In the training phase, the rats underwent 8 days each of Pavlovian and instrumental conditioning. For Pavlovian conditioning, a predictive tone cue was repeatedly paired with a chocolate pellet reward; for instrumental conditioning, pressing a lever resulted in a chocolate pellet reward, but no tone was present. Rats were also exposed to a neutral tone in the absence of any rewards during a single session.

In the PIT test phase, the rats were presented with the neutral and reward-predictive cues. They had access to a lever but pressing it did not result in a reward. The authors tested the effect of cholinergic interneuron inactivation on cue-motivated behavior (i.e. lever-pressing) by injecting the rats with CNO or a vehicle and examining behavior before (baseline) and after cue presentation. They optically stimulated the interneurons at cue presentation to test the effect of interneuron activation on behavior. Finally, they injected DhbE into the NAc, which is an acetylcholine antagonist that binds to b2-containing nicotinic receptors. They then optogenetically stimulated the interneurons and recorded cue-motivated behavior to determine whether acetylcholine released from the interneurons acts on nicotinic receptors to regulate cue-motivated behavior.

What did they find?

When the reward predictive cue was presented, both the hM4D(Gi) and control rats had more lever presses compared to baseline or when the neutral cue was presented. However, the hM4D(Gi) rats showed much greater lever-pressing behavior compared to controls when CNO was administered (i.e. when interneuron activity was reduced). Optogenetic activation of cholinergic interneurons led to greater acetylcholine release in ChR2-expressing rats, which was accompanied by a reduction in reward-seeking behavior in ChR2-expressing rats but not in control rats. This means that inhibiting cholinergic interneurons increases cue-motivated behavior, whereas activating the interneurons reduces cue-motivated behavior. Lastly, in the absence of any stimulation, rats infused with an acetylcholine antagonist or a vehicle (control condition) showed greater lever-pressing behavior in response to the predictive cue, but not at baseline or in response to the neutral cue. However, when the cholinergic interneurons were optically activated, there was a reduction in cue-motivated behavior among rats infused with the vehicle, but no change in response to the cue among rats infused with the acetylcholine antagonist. Thus, even though the activation of interneurons normally decreases cue-motivated behavior, an acetylcholine antagonist disrupts the downstream effects of these interneurons by blocking the interaction between acetylcholine and b2-containing nicotinic receptors.

What's the impact?

This study demonstrates that nucleus accumbens cholinergic interneurons have an inhibitory effect on cue-motivated behavior and regulate this behavior by acting on downstream nicotinic receptors. These findings provide further insight into our current understanding of psychiatric conditions with dysfunctional motivational behavior, such as eating disorders and addiction, and have important implications for developing treatments for such disorders.

Collins et al. Nucleus accumbens cholinergic interneurons oppose cue-motivated behavior. Biological Psychiatry (2019). Access the original scientific publication here.