Penfluridol, chlorprothixene and haloperidol block fast axonal transport in an order of potency consistent with a mechanism related to inhibition of calmodulin.

The effects of the inhibitors of calmodulin penfluridol, chlorprothixene and haloperidol on fast axonal transport, the content of adenosine triphosphate and creatine phosphate and the density of axonal microtubules, were measured in spinal nerves of the bullfrog in vitro. These drugs inhibited the fast orthograde transport of [3H]leucine-labelled proteins: 35 microM penfluridol, 70 microM cis-chlorprothixene, and 200 microM haloperidol were needed to produce and approximately 50% inhibition of transport, and the order of potency was, therefore, penfluridol greater than cis-chlorprothixene greater than haloperidol; the trans isomer of chlorprothixene was as effective as the cis isomer-of chlorprothixene in inhibiting fast axonal transport. None of these drugs significantly reduced the density of microtubules in unmyelinated axons of nerves, incubated as for a transport experiment. Exposure to the concentration of these drugs which inhibited transport did not reduce significantly the content of adenosine triphosphate of the nerves, except for a 22% reduction by trans-chlorprothixene, and they had no significant effect on the content of creatine phosphate except for a 27% reduction by penfluridol and a 20% reduction by trans-chlorprothixene. The inhibition of axonal transport by these drugs can therefore not be explained either by an interference with oxidative metabolism or by disruption of microtubules. The order of potency of penfluridol, chlorprothixene and haloperidol as inhibitors of fast axonal transport parallels their known order of potency as antagonists of calmodulin; inhibition of axonal transport may therefore be related to inhibition of the function of calmodulin by these drugs.