Changes in the expression of calcium channels in the rat brain caused by imbalance of aromatic amino acids

Abstract

Ca ~ § channels expressed in Xenopus oocytes using mR.NA purified from the brain of the rats subjected to chronic treatment with L-phenylalanine (in order to model conditions typical of a congenital disease called phenylketonuria, PKU) were studied using a double microelectrode technique. The amplitude of Ca 2 § channel currents (lB,, 40 mM Ba 2. were used as a charge carrier), directed in the oocytes by mRNA from the brain of the animals with model PKU, was significantly lower compared with that in the control animals (145 __. 23 rlA vs 270 __. 38 nA, P < 0.025), even though the voltage dependence of both currents was similar and typical of that in high voltage-activated (HVA) Ca z § channels. In response to voltage-clamp pulses from the holding potential of -80 mV, the currents were activated around -30 mV, reached their maximum amplitude at +20 mV, and showed an apparent reversal potential between +60 and +70 inV. No currents that could be associated with the expression of low voltage-activated Ca 2 § channels were detected. A decrease in the overall HVA Ca z § current in the animals with model PKU occurred primarily because of decaying of the to-conotoxin-sensitive component (1 #M ta-conotoxin). This component accounted for about 64% of the total current amplitude in control animals and apparently was associated with the activity of the expressed N-type Ca z § channels. Based on the sensitivity to w-Aga-IVA (20/.tM), only about 10% of the net Ba z § current in control animals could be attributed to the activity of the expressed P/Q-type Ca z § channels. No significant difference in the expression of P/Q-type Ca z § channels between control and model PKU conditions were detected. In addition to the decreased amplitude of N-type component, HVA Ba 2. current in model PKU animals showed an accelerated run-down during prolonged recording (50%/h compared with 15%/h in control), suggesting some changes in the patterns of Ca z § channel regulation.Thus, our data suggest that hyperphenylalaninemic conditions affect genomic expression and/or stability of mRNA of preferentially N-type Ca 2 § channels, resulting in a decrease of the content of this mRNA in brain mRNA extract. These conditions also influence the type and manner of Ca 2 § channel regulation. In particular, underexpression of N-type Ca z § channels is consistent with a decrease in the overall number of synaptic contacts during PKU and may be one of the factors contributing to severe damage to the brain functions.