Possible change in noradrenergic receptor sensitivity following methylphenidate treatment: growth hormone and MHPG response to clonidine challenge in children with attention deficit disorder and hyperactivity.

Growth hormone (GH) and 3-methoxy-4-hydroxyphenelethylene glycol (MHPG) response was measured hourly for 4 hours in 8 children with Attention Deficit Disorder with Hyperactivity (ADD+H) following an acute single-dose of clonidine. The clonidine challenge was repeated before, during, and one day after 12 weeks of treatment with methylphenidate (MPH). Before MPH treatment, the plasma growth hormone (GH) rose to 31.3 +/- 4.6 (Mean +/- SE) ng/ml; during MPH treatment, the GH peak was only 14.8 +/- 3.2 ng/ml; one day after discontinuation of MPH, GH rose to only 20.8 +/- 3.9 ng/ml. MHPG release was inhibited by clonidine in all treatment conditions but tended to be more decreased during MPH treatment. Some children with ADD+H may have hypersensitivity of the post-synaptic alpha-1 noradrenergic receptor which is diminished by MPH treatment. The extent to which these effects are pharmacological or represent a change in receptor sensitivity requires further study.