Behavioral, cellular and molecular changes were examined following axonal injury in the marine mollusc Aplysia californica. Unilateral nerve injury was performed by crushing the pleural-pedal connective and the peripheral pedal nerves innervating one side of the posterior body wall and the tail. The injury procedure severs the axons of the pleural sensory neurons resulting in the blockade of the tail-elicited siphon- withdrawal reflex. Partial reflex recovery is observed within 3 d and reaches 50% of the pretest value by six weeks postinjury. Retrograde staining of injured nerves combined with electrophysiological recordings from siphon motor neurons show that axons can regenerate through the crushed site and reconnect with the tail by three weeks postinjury. Moreover, the behavioral and electrophysiological measurements suggest that the contralateral sensory neurons may contribute to the early recovery of the siphon-withdrawal reflex. The levels of mRNAs coding for actin and calreticulin are elevated while the mRNAs coding for intermediate filament protein, sensorin A, FMRFamide are reduced in the ipsilateral pleural ganglia as detected by Northern blots. In the contralateral pleural ganglia, the levels of mRNAs coding for actin, sensorin A and FMRFamide are elevated. These molecular changes in both the ipsi- and contralateral sides are consistent with the hypothesis that both sides are participating in the behavioral recovery following unilateral axonal injury.

The identity of the types of different neurons in mammalian retinae is now close to being completely known for a few mammalian species; comparison reveals strong homologies for many neurons across the order. Still, there remain some cell types rarely encountered and inadequately described, despite not being rare in relative frequency. Here we describe in detail an additional ganglion cell type in rabbit that is bistratified with dendrites in both sublaminae, yet spikes only at light onset and has no response bias to the direction of moving bars. This ON bistratified ganglion cell type is most easily distinguished by the unusual behavior of its dendritic arbors. While dendrites that arborize in sublamina b terminate at that level, those that ascend to arborize in sublamina a do not normally terminate there. Instead, when they reach the approximate radius of the dendrites in sublamina b, they dive sharply back down to ramify in sublamina b. Here they continue to course even further away from the soma at the same level as the branches wholly contained in sublamina b, thereby forming an annulus of secondary ON dendrites in sublamina b. This pattern of branching creates a bistratified dendritic field of approximately equal area in the two sublaminae initially, to which is then added an external annulus of dendrites only in sublamina b whose origin is entirely from processes descending from sublamina a. It is coupled to a population of wide-field amacrine cells upon which the dendrites of the ganglion cell often terminate.

Axonal injury increases intracellular Ca 2؉ and cAMP and has been shown to induce gene expression, which is thought to be a key event for regeneration. Increases in intracellular Ca 2؉ and͞or cAMP can alter gene expression via activation of a family of transcription factors that bind to and modulate the expression of CRE (Ca 2؉ ͞cAMP response element) sequence-containing genes. We have used Aplysia motor neurons to examine the role of CRE-binding proteins in axonal regeneration after injury. We report that axonal injury increases the binding of proteins to a CRE sequencecontaining probe. In addition, Western blot analysis revealed that the level of ApCREB2, a CRE sequence-binding repressor, was enhanced as a result of axonal injury. The sequestration of CRE-binding proteins by microinjection of CRE sequence-containing plasmids enhanced axon collateral formation (both number and length) as compared with control plasmid injections. These findings show that Ca 2؉ ͞cAMP-mediated gene expression via CRE-binding transcription factors participates in the regeneration of motor neuron axons.