Serine racemase deletion disrupts memory for order and alters cortical dendritic morphology

Abstract

There is substantial evidence implicating N-methyl-D-aspartate receptors (NMDARs) in memory and cognition. It has also been suggested that NMDAR hypofunction might underlie the cognitive deficits observed in schizophrenia as morphological changes, including alterations in the dendritic architecture of pyramidal neurons in the prefrontal cortex (PFC), have been reported in the schizophrenic brain post mortem. Here, we used a genetic model of NMDAR hypofunction, a serine racemase knockout (SR-/-) mouse in which the first coding exon of the mouse SR gene has been deleted, to explore the role of D-serine in regulating cognitive functions as well as dendritic architecture. SR-/- mice exhibited a significantly disrupted representation of the order of events in distinct experiences as showed by object recognition and odor sequence tests; however, SR-/- animals were unimpaired in the detection of novel objects and in spatial displacement, and showed intact relational memory in a test of transitive inference. In addition, SR-/- mice exhibited normal sociability and preference for social novelty. Neurons in the medial PFC of SR-/- mice displayed reductions in the complexity, total length and spine density of apical dendrites. These findings show that D-serine is important for specific aspects of cognition, as well as in regulating dendritic morphology of pyramidal neurons in the medial PFC (mPFC). Moreover, they suggest that NMDAR hypofunction might, in part, be responsible for the cognitive deficits and synaptic changes associated with schizophrenia, and highlight this signaling pathway as a potential target for therapeutic intervention.

Figure 1

Performance of the WT mice and SR −/− mice on object recognition, object displacement, and object order. A. Arrangement of stimuli in the object recognition test. A discrimination ratio was used to measure preference for a novel vs. a familiar object. B. Arrangement of stimuli in the object location test. A discrimination ratio was used to measure preference for a displaced vs. a stationary object. C. Arrangement of stimuli in the object order test. A discrimination ratio was used to measure preference for an older vs. a more recent object. *indicates p<.05 discrimination ratio less than zero; # indicates p<.05 difference in discrimination ratio between groups.

Figure 2. 56

Performance of the WT mice and SR −/− mice on odor sequence memory. A. Schematic of the apparatus and odors used to test the animals on sequence memory. B. A preference index was used to assess the amount of time spent digging in the item occurring earlier in the sequence compared to an item occurring later in the sequence. *indicates p<.05 discrimination ratio less than zero; # indicates p<.05 difference in discrimination ratio between groups.

Figure 3

Performance of WT mice and SR −/− mice on transitive inference. A. Schematic of the apparatus and odors used to train and test the animals on transitive inference. B. The number of days to reach criterion across the four stages of training was used to assess acquisition of the odor pairs. Percentage correct was used to assess performance on the training pairs during the transitive inference test. C. A preference index was used to assess the amount of time spent digging in B versus D (“transitive pair”) and A versus E (“non-transitive pair”) on the probe tests.

Figure 4

Performance of WT mice and SR −/− mice on sociability and social novelty. A. Apparatus used to assess sociability. A discrimination ratio was used to assess preference for a compartment containing a littermate as compared to an empty compartment. B. Apparatus used to assess social novelty. A discrimination ratio was used to compare the amount of time spent investigating a familiar versus novel animal.

Figure 5

Apical and basilar dendritic morphology of pyramidal neurons in the medial prefrontal cortex (mPFC). A. Golgi stained pyramidal neurons and computer-assisted reconstructions of representative neurons in WT (left) and SR−/− (right) mice. B., C. Sholl analysis was used to compare the apical dendrites of neurons from WT and SR−/− mice. D., E. Sholl analysis was also used to compare the basal dendrites of neurons from WT and SR−/− mice. *indicates p < 0.05 between groups.

Figure 6

Spine density analysis of pyramidal neurons in the medial prefrontal cortex (mPFC). A. Apical dendritic spines of a Golgi-stained pyramidal neuron in a WT (left) and a SR−/− mouse (right). B. Spine density analysis of both oblique branches and the distal end of apical dendrites in WT and SR−/− mice. C. Spine density analysis of branch order of basilar dendrites of pyramidal neurons in WT and SR−/− mice. Spine density was normalized to the number of spines per 10 µm of dendrite. *indicates p < 0.05 between groups.