Introduction: Using proton magnetic resonance spectroscopy imaging (1H-MRSI), the effects of early life stress (ELS) on nonhuman primate striatal neuronal integrity were examined as reflected by N-acetyl aspartate (NAA) concentrations. NAA measures were interrogated through examining their relationship to previously documented ELS markers -- cerebrospinal fluid (CSF) corticotropin-releasing factor (CRF) concentrations, hippocampal volume, body mass and behavioral timidity. Rodent models of depression exhibit increases in neurotrophic effects in the nucleus accumbens (NAcc). We hypothesized that rearing under conditions of ELS [Variable Foraging Demand: (VFD)] would produce persistent elevations of NAA concentrations (in absolute or ratio form) in ventral striatum/caudate nucleus (VS/CN) with altered correlation to ELS markers.
Methods: Eleven bonnet macaque males reared under VFD conditions and seven age-matched control subjects underwent 1H-MRSI during young adulthood. Voxels were placed over ventral striatum/caudate nucleus (VS/CN) to capture NAcc. Cisternal CSF CRF concentrations, hippocampal volume, body mass and response to a human intruder had been previously determined.
Results: VFD-reared monkeys exhibited significantly increased NAA/Cr concentrations in right VS/CN in comparison to normally-reared controls, controlling for multiple comparisons. In comparison to controls, VFD CSF CRF concentrations were directly associated with right VS/CN absolute NAA. Left hippocampal volume was inversely associated with left VS/CN N-acetyl aspartate/creatine (NAA/Cr) in VFD-reared but not in controls. Disruption of a normative inverse correlation between left VS/CN NAA and body mass was noted in VFD. Only non-VFD subjects exhibited a direct relationship between timidity response to an intruder and right VS/CN NAA.
Conclusion: ELS produced persistent increases in VS/CN NAA, which demonstrated specific patterns of association (or lack thereof) to ELS markers in comparison to non-VFD subjects. The data are broadly consistent with a stable nonhuman primate phenotype of anxiety and mood disorder vulnerability whereby in vivo indicators of neuronal integrity, although reduced in hippocampus, are increased in striatum. The findings may provide a catalyst for further studies in humans and other species regarding a reciprocal hippocampal/NAcc relationship in affective disorders.
Keywords: N-acetyl-aspartate (NAA); caudate nucleus; corticotropin-releasing factor (CRF); early life stress; hippocampal volume; nucleus accumbens; striatum.