Cholinergic modulation through biogenic amines during development of the chick spinal cord.

Our previous analyses of the functional maturation of cholinergic neurotransmission in the chick embryo spinal cord showed that 12 days of development represents a crucial stage in the maturation of cholinergic neurotransmission. Since supraspinal inputs are essential for the modulation of motoneuron activity in the embryo, we studied, using a pharmacological approach, the regulatory effects of biogenic amines on the developing cholinergic neurotransmission. We show that, in spite of the early occurrence of serotonin and norepinephrine in the developing spinal cord, the modulatory influences of these biogenic amines on motoneurons can only be effective from 12 days of incubation onwards. It also appears that the inputs of biogenic amines on cholinergic activity is a sequential event. Whereas serotonin (5-HT) begins to stimulate cholinergic activity from 12 days of incubation, norepinephrine inputs are only observed at later stages (17 days of development). These observations underline the specific turning point in the functional maturation of motoneurons, we noted in our previous studies. At the end of embryonic life a specific interrelationship between the biogenic amines was noted, its effect on the cholinergic system may lead to a more precise motor control, preparing the chick embryo for hatching.