Reward expectation extinction restructures and degrades CA1 spatial maps through loss of a dopaminergic reward proximity signal.

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.

A comparison of pectoral fin ray morphology and its impact on fin ray flexural stiffness in labriform swimmers.

The organization of tissues in appendages often affects their mechanical properties and function. In the fish family Labridae, swimming behavior is associated with pectoral fin flexural stiffness and morphology, where fins range on a continuum from stiff to relatively flexible fins. Across this diversity, pectoral fin flexural stiffness decreases exponentially along the length of any given fin ray, and ray stiffness decreases along the chord of the fin from the leading to trailing edge. In this study, we examine the morphological properties of fin rays, including the effective modulus in bending (E), second moment of area (I), segmentation, and branching patterns, and their impact on fin ray stiffness. We quantify intrinsic pectoral fin ray stiffness in similarly sized fins of two closely related species that employ fins of divergent mechanics, the flapping Gomphosus varius and the rowing Halichoeres bivittatus. While segmentation patterns and E were similar between species, measurements of I and the number of fin ray branch nodes were greater in G. varius than in H. bivittatus. A multiple regression model found that of these variables, I was always significantly correlated with fin ray flexural stiffness and that variation in I always explained the majority of the variation in flexural stiffness. Thus, while most of the morphological variables quantified in this study correlate with fin ray flexural stiffness, second moment of area is the greatest factor contributing to variation in flexural stiffness. Further, interspecific variation in fin ray branching pattern could be used as a means of tuning the effective stiffness of the fin webbing to differences in swimming behavior and hydrodynamics. The comparison of these results to other systems begins to unveil fundamental morphological features of biological beams and yields insight into the role of mechanical properties in fin deformation for aquatic locomotion.

The relationship between pectoral fin ray stiffness and swimming behavior in Labridae: insights into design, performance and ecology.

The functional capabilities of flexible, propulsive appendages are directly influenced by their mechanical properties. The fins of fishes have undergone extraordinary evolutionary diversification in structure and function, which raises questions of how fin mechanics relate to swimming behavior. In the fish family Labridae, pectoral fin swimming behavior ranges from rowing to flapping. Rowers are more maneuverable than flappers, but flappers generate greater thrust at high speeds and achieve greater mechanical efficiency at all speeds. Interspecific differences in hydrodynamic capability are largely dependent on fin kinematics and deformation, and are expected to correlate with fin stiffness. Here we examine fin ray stiffness in two closely related species that employ divergent swimming behaviors, the flapping Gomphosus varius and the rowing Halichoeres bivittatus To determine the spatial distribution of flexural stiffness across the fin, we performed three-point bending tests at the center of the proximal, middle and distal regions of four equally spaced fin rays. Pectoral fin ray flexural stiffness ranged from 0.0001 to 1.5109 µN m2, and the proximal regions of G. varius fin rays were nearly an order of magnitude stiffer than those of H. bivittatus In both species, fin ray flexural stiffness decreased exponentially along the proximodistal span of fin rays, and flexural stiffness decreased along the fin chord from the leading to the trailing edge. Furthermore, the proportion of fin area occupied by fin rays was significantly greater in G. varius than in H. bivittatus, suggesting that the proportion of fin ray to fin area contributes to differences in fin mechanics.