Improper or delayed establishment of the mesocorticolimbic pathway may contribute to psychiatric disease, including ADHD (4
). Indeed the PFC is highly implicated in attentional and hyperactivity disorders, as therapeutic doses of MPH selectively modify neurotransmission in this area (15
). Here, we have shown that p35 KO compromised layer-specificity of mPFC afferents projecting to the NAc. Additionally, an increase in PFC innervation by TH-positive fibers, increased basal PKA activity, and decreased DA degradation likely contribute to potentiated DAergic neurotransmission in the PFC of p35−/−
mice. DA acts as a neuromodulator, but generally inhibits the firing of mPFC output neurons through the D2DR on pyramidal neurons and the activation of GABA interneurons (48
). Thus, perpetually high DAergic signaling may inhibit output firing of mPFC neurons in p35−/−
mice, and possibly ADHD patients.
mice were hyperactive and exhibited hypolocomotive responses to psychostimulants. The discrepancy between the locomotor responses of WT versus p35−/−
mice grew with each day of repeated dosing, as p35 KO mice were deficient in sensitization. The sensitizing effect of repeated exposure may depend upon both coordinated glutamate release in mPFC terminals of the NAc (51
) and proper DAergic modulation of mPFC output neurons (52
), which were disrupted by p35 KO. Inhibition of Cdk5 in adult rodents by pharmacological (34
) or transgenic means (35
) has generally potentiated psychostimulant responses. In contrast, here we report that constitutive Cdk5 dysregulation via p35 KO suppressed locomotor responses, further implicating the congenital defects in motor-reward circuitry in the ADHD-like phenotype.
While MPH is presently the most widely used drug treatment for ADHD, its long-term effects on development and catecholamine metabolism are poorly understood. Chronic exposure to MPH caused significant changes in catecholamine content and degradation in p35−/− mouse PFC, while controls were unaffected. This suggests a possible role for Cdk5 in regulating catecholamine metabolism. Cdk5 may normally protect PFC neurons from the effects of typical weekday MPH dosing so that DA levels are not overly depleted. However, these data also raise the possibility that high-dose chronic MPH may deplete DA and raise 5-HT levels in the PFC if mesocorticolimbic circuitry or Cdk5 activity is significantly altered. Future studies may be warranted to evaluate these parameters in ADHD patients.
Previously we reported that Cdk5 repressed D1DR/cAMP/PKA signaling through the phosphorylation of DARPP-32 (33
) and that p35 KO resulted in increased DA efficacy with regard to the phosphorylation of S845 GluR1 and T34 DARPP-32. Here we confirmed the potentiated PKA response to D1DR stimulation, but found that total levels of GluR1 were elevated and basal levels of P-T34 DARPP-32 were reduced in the striatum of p35−/−
mice, raising the possibility that other factors including genetic background fluctuation may now contribute to the increased efficacy of D1DR agonists in the absence of p35. Phosphorylation of GluR1 at S845 increases the peak open probability and potentiates AMPA receptor Ca2+
), while T34 phosphorylated DARPP-32 promotes phosphatase inhibition (47
). MPH potentiates PKA activity in acute striatal slices while antagonizing Cdk5-dependent phosphorylation of DARPP-32 (57
). Alterations in the basal phosphorylation of these sites in cortical and subcortical structures of p35−/−
mice suggest that the physiological properties of PFC and striatal neurons, which are dependent on the phosphorylation state of GluR1, DARPP-32 and possibly other downstream effectors of D1DR, may contribute to hyperactivity and paradoxical responses to stimulants observed in p35−/−
mice and ADHD patients. It is interesting to note that loss of another protein kinase that phosphorylates DARPP-32, casein kinase I, also results in a hyperactive phenotype (58
Reports of changes in ADHD patient glucose metabolism conflict (59
). As a noninvasive metabolic imaging technique with clinical acceptance, FDG-PET demonstrated clear perturbations in glucose metabolism, prompting our further study of the effects of p35 KO on mesocorticolimbic circuitry and function. The hypermetabolic brain activity of p35−/−
mice may result from reduced Cdk5/p35-dependent inhibition of neuronal excitability (62
). We also revealed that p35 KO caused increased mesolimbic connectivity and PFC neurotransmitter content possibly necessitating an increase in supportive energy expenditure. Increased glucose metabolism could also serve as an essential compensatory response, given the extent of anatomical defects resulting from p35 KO. Although the effects of anesthesia are controlled for in WT animals, these results must be interpreted carefully as isoflurane nonspecifically lowers brain metabolism (64
) and the glucose uptake of p35−/−
mice could be differently affected by this drug.
The pathoetiology of ADHD remains poorly understood and a number of independent genetic, epigenetic, and environmental factors may cumulatively contribute to the composite human disorder. Here we present evidence implicating Cdk5 dysregulation in ADHD. Interestingly, exposure to neurotoxins such as organophosphates, which can dysregulate Cdk5 (65
), has also been linked to a higher ADHD incidence in humans (66
). Combining the results of this study with the known role of Cdk5 in corticogenesis and regulation of the DA signal, we conclude that its dysfunction during development may contribute to ADHD etiology.