Apparent central neurotransmitter receptor changes induced by low-level lead exposure during different developmental phases in the rat

Three groups of male Wistar rats were exposed to low concentrations of lead during different phases of their development for periods of 21 days. Exposure was initiated at conception (group I), at parturition (group II), or after weaning had occurred (group III). Mean brain lead levels achieved immediately after exposure were 1.07 +/- 0.15, 0.84 +/- 0.06, and 0.44 +/- 0.06 microgram/g in groups I, II, and III, respectively. On Day 64 postpartum radioligand binding studies were performed to determine apparent receptor densities (Bmax values) and affinities (KD values) in brain membranes of the lead-exposed rats. Lead exposure during the neonatal phase (group II) resulted in significant increases of the apparent densities of forebrain alpha 1-adrenoceptors (by 92%), cortical beta-adrenoceptors (by 116%), and striatal D-2 dopamine receptors (by 133%). Lead exposure also resulted in a significant decrease of the apparent density of striatal muscarinic receptors in group I (by 43%) and group II (by 25%) and an increase in the apparent density of hippocampal S1 receptors in group II (by 78%). Lead exposure after weaning had occurred did not alter the apparent densities of any of the receptors investigated. Receptor affinity changes, which were not identical in the three groups of rats, were also induced by lead exposure. The developmental phase during which exposure occurs appears to be a determinant of the type of neurotransmitter receptor changes induced by lead.