DEVELOPMENT OF ADRENOCEPTIVE AND CHOLINOCEPTIVE RESPONSIVENESS IN THE NEONATAL RAT IRIS

The basic (pharmacologically inactivated) pupil diameter in neonatal Sprague-Dawley rats remains relatively stable at 2·2 mm for the first 12 days after birth, followed by a reduction to 1·7 mm by the 15th day and then an increase to 3·8 mm at 30 days and to 4·9 mm in the adult. The effects on pupil diameter of stimulating α-adrenergic, β-adrenergic and cholinergic receptors were studied separately and, in testing each receptor system, the other types were blocked with appropriate drugs. All three types of receptor become excitable about the 10th day after birth and they mature rapidly thereafter, differentiation of the cholinergic system being faster than that of the others. Comparison of the response of the α-adrenergic system to exogenous noradrenaline with that to endogenous noradrenaline, released indirectly by amphetamine, indicates that receptors probably mature in advance of the capacity of the associated nerve terminals to release maximal quantities of transmitter substance. The neonatal increase in iris noradrenaline levels was determined by the catechol O-methyl transferase (COMT) enzyme radiochemical assay technique. © 1979.