Reduction in extrasynaptic acetylcholine sensitivity of axotomized anterior pagoda neurones in the leech.

1. The effects of axotomy on the sensitivity of the leech anterior pagoda (AP) neurone to acetylcholine (ACh) and carbamylcholine (CCh) have been studied 1-5 days after axon interruption. 2. Hyperpolarizing responses to ionophoretically applied ACh and CCh have been recorded intracellularly from desheathed cell bodies of normal and axotomized neurones. The electrical properties of the membrane have also been measured in the same neurones. 3. Axotomy produced a progressive loss of sensitivity to both ACh and CCh with a similar percentage reduction. 4. No significant changes have been found in the time to peak and in the reversal potential of the responses to agonists, or in the number of drug molecules needed to combine with a single receptor to produce a response. 5. Interruption of nerve roots and connectives which do not contain the AP axon did not induce the alterations of ACh sensitivity observed after axotomy. 6. It is concluded that the loss of ACh sensitivity following axotomy is due to a reduction in density of functional ACh receptors (AChRs).