Role for neonatal D-serine signaling: prevention of physiological and behavioral deficits in adult Pick1 knockout mice

Abstract

NMDA glutamate receptors have key roles in brain development, function and dysfunction. Regulatory roles of D-serine in NMDA receptor-mediated synaptic plasticity have been reported. Nonetheless, it is unclear whether and how neonatal deficits in NMDA-receptor-mediated neurotransmission affect adult brain functions and behavior. Likewise, the role of D-serine during development remains elusive. Here we report behavioral and electrophysiological deficits associated with the frontal cortex in Pick1 knockout mice, which show D-serine deficits in a neonatal- and forebrain-specific manner. The pathological manifestations observed in adult Pick1 mice are rescued by transient neonatal supplementation of D-serine, but not by a similar treatment in adulthood. These results indicate a role for D-serine in neurodevelopment and provide novel insights on how we interpret data of psychiatric genetics, indicating the involvement of genes associated with D-serine synthesis and degradation, as well as how we consider animal models with neonatal application of NMDA receptor antagonists.

Figure 1. Behavioral changes in adult male Pick1 knockout mice in adulthood

A) Horizontal locomotor activity. There was a significant group by interval interaction [F(23, 383)=1.83, p=0.012]. Pairwise multiple comparison procedures (Holm-Sidak method) showed significantly more ambulatory activity in Pick1 knockout mice at the 60–90 min interval (*p<0.05), p=0.054 for the 90–120 interval, n=8 per group. B) Behavior in the elevated plus maze. No difference between wild-type and Pick1 knockout and mice was observed. n=7 per group. C) Time immobile in the forced swim test. No differences between wild-type and Pick1 knockout mice was observed. n=8 per group. D) Prepulse inhibition (PPI) of the acoustic startle. Pick1 knockout mice demonstrated significantly lower PPI at the prepulse intensities of 74, 78 and 82 dB. *p<0.05 vs. wild-type at the same prepulse intensity. n=11 (knockout), n=8 (wild-type). E) Spontaneous alternations in the Y maze. Pick1 knockout mice showed significantly fewer alternations in the Y maze compared to wild-type mice, *p<0.05, n=8 per group.

Figure 2. NMDA effects on prefrontal cortex pyramidal cell excitability in adult Pick1 knockout mice

Top: Examples of traces used to assess excitability 5 min before 4 µM NMDA application (left), after 5 min of NMDA perfusion, after 5 min of the D1 agonist SKF38393 (SKF, 2 µM), and during the combined perfusion of 4 µM NMDA and 2 µM SKF in a prefrontal cortical slice from a wild-type mouse. The rectangular area with the red line indicates the changes in the number of evoked action potentials over time with the three treatments (NMDA alone, SKF alone, NMDA+SKF, shaded areas). The vertical arrows indicate the time at which the traces on top were obtained. Right, population data indicating normalized responses to baseline after NMDA alone and NMDA + SKF. Bottom: Similar display for a representative prefrontal cortex prefrontal cortex pyramidal neuron in a Pick1 knockout mouse (left) and population data for NMDA and NMDA + SKF for all knockout mice recorded, showing a reduced NMDA effect in slices from Pick1 knockout mice.

Figure 3. Time line for D-serine treatment and summary of results

We have previously published that Pick1 knockout mice have reduced D-serine levels at P7 (neonatal stage), but not in adulthood (P56) (Hikida 2008). Based on those results, here we tested whether supplementation of D-serine during the neonatal period (P3–P17) would improve behavioral and electrophysiological deficits seen in adulthood and indeed it did. As a control experiment we also treated the mice with D-serine in adulthood (P56–P70), followed by a washout period of 1 week, and found no effect on behavior or electrophysiology.

Figure 4. Effect of neonatal D-serine treatment on PPI in adulthood

D-serine 500 (mg/kg) was injected daily for 2 weeks starting on postnatal day 3. The PPI deficit was significantly rescued in adult Pick1 knockout mice treated with D-serine in the neonatal stage. Two-way repeated measures ANOVA of the data for knockout mice revealed a significant effect of treatment, F(1,99)=4.8, p=0.041. n=8, wild-type with saline treatment; n=8, wild-type with D-serine treatment; n=5, Pick1 knockout mice with saline treatment; n=13, Pick1 knockout mice with D-serine treatment.

Figure 5. Rescue of electrophysiological deficits in adulthood by neonatal D-serine treatment, but not by adult D-serine treatment

Responses to NMDA application in prefrontal cortical brain slices from adult Pick1 knockout mice that had been treated with D-serine. Data are presented in the same format of those in Fig. 2. A) Neonatal D-serine treatment (from postnatal day 3 to day 16) rescued the electrophysiological changes in Pick1 knockout mice. B) Adult D-serine treatment did not rescue the electrophysiological changes in Pick1 knockout mice.