In vivo expression of the intermediate filament peripherin in rat motoneurons: modulation by inhibitory and stimulatory signals.

Peripherin is a type III intermediate filament which, in contrast to the neurofilaments, is strongly up-regulated after nerve injury. Although peripherin expression is stimulated in vitro by neurotrophins and cytokines, little is known about its in vivo regulation. In this report, we show that the in vivo down-regulation of peripherin expression to normal levels during regeneration closely correlates with target reconnection in rat facial motoneurons. Prevention of reconnection, by transection and suture, results in the persistence of strong peripherin expression for prolonged periods of up to 11months. This contrasts with the modulation of the p75 low-affinity neurotrophin receptor, whose expression returns to normal even in the absence of reconnection. We further demonstrate that blockade of the axonal transport in non-injured motoneurons increases the expression of peripherin. Blockade of the axonal transport simultaneously to, or after injury of, facial motoneurons does not abolish the axotomy-induced peripherin up-regulation. These data demonstrate that the in vivo expression of peripherin is normally restrained by a distal retrogradely transported inhibitory signal. Thus, peripherin up-regulation results primarily from a lack of supply in this factor. Our results show that stimulatory factors released at the injury site are not required for the initial up-regulation and maintenance of high peripherin expression. However, they appear to enhance this increase during the acute post-lesion phase. Peripherin expression is thus finely tuned by both glial cell-derived stimulatory and distal inhibitory signals that reflect neuron-target interactions.