Reductions in synaptic proteins and selective alteration of prepulse inhibition in male C57BL/6 mice after postnatal administration of a VIP receptor (VIPR2) agonist

Abstract

Rationale: An abundance of genetic and epidemiologic evidence as well as longitudinal neuroimaging data point to developmental origins for schizophrenia and other mental health disorders. Recent clinical studies indicate that microduplications of VIPR2, encoding the vasoactive intestinal peptide (VIP) receptor VPAC2, confer significant risk for schizophrenia and autism spectrum disorder. Lymphocytes from patients with these mutations exhibited higher VIPR2 gene expression and VIP responsiveness (cAMP induction), but mechanisms by which overactive VPAC2 signaling may lead to these psychiatric disorders are unknown.

Objectives: We subcutaneously administered the highly selective VPAC2 receptor agonist Ro 25-1553 to C57BL/6 mice from postnatal day 1 (P1) to P14 to determine if overactivation of VPAC2 receptor signaling during postnatal brain maturation affects synaptogenesis and selected behaviors.

Results: Western blot analyses on P21 revealed significant reductions of synaptophysin and postsynaptic density protein 95 (PSD-95) in the prefrontal cortex, but not in the hippocampus in Ro 25-1553-treated mice. The same postnatally restricted treatment resulted in a disruption in prepulse inhibition of the acoustic startle measured in adult mice. No effects were observed in open-field locomotor activity, sociability in the three-chamber social interaction test, or fear conditioning or extinction.

Conclusion: Overactivation of the VPAC2 receptor in the postnatal mouse results in a reduction in synaptic proteins in the prefrontal cortex and selective alterations in prepulse inhibition. These findings suggest that the VIPR2-linkage to mental health disorders may be due in part to overactive VPAC2 receptor signaling during a critical time of synaptic maturation.

Fig. 1

Effects of early postnatal treatment with the VPAC2 receptor agonist Ro 25-1553 on PSD-95 (a) and synaptophysin (b) levels one week later in the hippocampus and prefrontal cortex. Mice were injected s.c. once daily with phosphate-buffered saline (PBS) or Ro 25-1553 (0.07 nmol/g) during P1-14. One week after the last injection, the hippocampus and prefrontal cortex were dissected, and then PSD-95 and synaptophysin (SYN) expression was analyzed by Western Blot. Expression levels were normalized to β-actin and are presented as fold change relative to control PBS results. The results are expressed as the mean ± SEM of 3 mice/group. *P < 0.05 compared with the PBS-treated group.

Fig. 2

Effects of early postnatal treatment with the VPAC2 receptor agonist Ro 25-1553 on open-field behavior determined at three to four months of age. Each mouse was placed individually into a clear Plexiglas chamber (27.5 cm × 27.5 cm × 20.5 cm) and the behavior was recorded for 20 min. The distance traveled in the peripheral and center area and the time spent in the center area are calculated. The results are expressed as the mean ± SEM of 8 mice/group.

Fig. 3

Effects of early postnatal treatment with the VPAC2 receptor agonist Ro 25-1553 on PPI of the acoustic startle response in mice. The percentage of PPI at each prepulse intensity is shown. The results are expressed as the mean ± SEM of 8 mice/group. *P < 0.05 compared with the PBS-treated group. ^†P < 0.05 compared with the Ro 25-1553 (P1-7)-treated group.

Fig. 4

Effects of early postnatal treatment with the VPAC2 receptor agonist Ro 25-1553 development on performance in the three-chamber social interaction test. After habituation, a mouse was placed in the central chamber of a clear Plexiglas box (41 cm × 60 cm × 23.5 cm) divided into three interconnected chambers and was given the choice to interact with either an empty wire cup (Empty) or a similar wire cup with an unfamiliar mouse (Stranger). The amount of time spent in each of the three chambers was recorded for the test mouse in a 10-min. The results are expressed as the mean ± SEM of 8 mice/group. **P < 0.01 compared with the middle chamber. ^††P < 0.01 compared with the empty side.

Fig. 5

Effects of early postnatal treatment with the VPAC2 receptor agonist Ro 25-1553 on memory formation, retention and extinction in the fear conditioning test. (a) Acquisition. BL; baseline, T; tone. (b) Contextual fear test. (c) Tone-cued fear test. **P < 0.01 compared with the baseline. (d) Cued fear extinction. BL; baseline. **P < 0.01 compared with the baseline in the PBS-treated group. ^†P < 0.05 compared with the baseline in the Ro 25-1553 (P1-7)-treated group. ^§P < 0.05 compared with the baseline in the Ro 25-1553 (P1-14)-treated group. The results are expressed as the mean ± SEM of 8 mice/group.