5-Hydroxytryptophan rescues serotonin response to stress in prefrontal cortex of hyperphenylalaninaemic mice

Abstract

Adult early treated hyperphenylalaninaemic patients can show specific deficits of prefrontal cortical functions. The development of additional therapeutic strategies for these patients requires the understanding of the mechanisms involved in phenylalanine-dependent impairment of fronto-cortical functions. We tested the hypothesis of phenylalanine interference with aminergic neurotransmission in the prefrontal cortex by evaluating, in vivo, amine release in adult Pah(enu2) mice, the genetic model of phenylketonuria. Mice of healthy background responded to a psychogenic stressor with the classic time-dependent increase of norepinephrine, dopamine and serotonin release from prefrontal cortical terminals. Neither the dopaminergic nor the serotoninergic responses were observable in the Pah(enu2) mice. Temporary reduction of circulating phenylalanine, by phenylalanine-free diet without amino- acid supplement, promoted recovery of the serotonin response only, demonstrating direct interference with serotonin synthesis in the mature brain. Evaluation of different steps of serotonin synthesis in the prefrontal cortex of hyperphenylalaninaemic mice demonstrated inhibition of cortical tryptophan hydroxylase activity. Finally, systemic administration of 5-hydroxytryptophan, the product of tryptophan hydroxylase activity, allowed frontal cortical serotonin response to stress in hyperphenylalaninaemic mice. Collectively, these results demonstrate that hyperphenylalaninaemia interferes with the ability of the mature prefrontal cortex to respond to psychological challenges, point to serotonin synthesis as the target of phenylalanine interference, and support the use of 5-hydroxytryptophan in lifelong treatment of hyperphenylalaninaemic subjects.

Prefrontal/Amygdalar System Determines Stress Coping Behavior Through 5-HT/GABA Connection

“…It was first demonstrated that, in mice, a stressful experience such as restraint induces a time-dependent increase of 5-HT output in the mpFC and of GABA in the BLA, in agreement with previous reports (Reznikov et al, 2009;Pascucci et al, 2009), and that selective depletion of cortical 5-HT canceled out these stress-induced responses, thus pointing to a controlling role of GABAergic transmission in amygdala by prefrontal 5-HT during stress. Electrophysiological studies indicated that previous stress and repeated administration of corticotrophin-releasing factor receptor agonist into the BLA reduces GABAergic neurotransmission in this structure, and this impairment is involved in the enhancement of conditioned fear behavior and anxiety (Rodríguez Manzanares et al, 2005;Rainnie et al, 2004).…”

“…Artificial CSF was pumped through the dialysis probe at a constant flow rate of 2 ml/min. Experiments were carried out 22-24 h after probe placement as previously described (Pascucci et al, 2009). For reverse microdialysis experiments, TTX (1 mm) was perfused in the BLA for 60 min before restraint stress, and L-allylglycine (1 mM) was perfused in the BLA for 30 min before FST.…”

“…Twenty microlitres of the dialysate samples were analyzed by HPLC. The HPLC analysis of 5-HT concentration in the dialysates was as previously described (Pascucci et al, 2009). GABA concentrations in the dialysates were determined as described by Rea et al (2005).…”

Tryptophan Hydroxylase as Novel Target for the Treatment of Depressive Disorders

“…Therefore, the behavioral phenotype observed in Pah -deficient mice can be partially attributed to lower TPH2 activity in the brain. Interestingly, administration of 5-HTP to Pah -deficient mice restored the cortical release of several monoamines in a model of restraint stress [157] .…”

“…Interestingly, it was recently shown that PAH ablation in the mouse causes 70% reduction in 5-HT production in the brain without a loss of the 5-HT precursor tryptophan [156] . Nonetheless, a dramatic reduction in brain 5-HTP levels as well as in the 5-HTP/tryptophan ratio was observed in Pah Enu2-/-mice, suggesting that accumulation of phenylalanine leads to the inhibition of Tph2 activity in vivo, corresponding to its effect on TPH2 activity in vitro [156,157] . Therefore, the behavioral phenotype observed in Pah -deficient mice can be partially attributed to lower TPH2 activity in the brain.…”