Aims: 

PACAP-like molecules are expressed in a distinct neuron population of the central nervous system (CNS) and certain peripheral tissues (body wall, intestine) of the earthworms and they mediate renewing of neural tissues after brain extirpation.

Methods: 

Brain extirpation was carried out with two distinct methods, (1) simple cut off or (2) cauterization of circumpharyngeal connectives. Concentrations of PACAP-like molecules in various parts of the CNS and regenerating tissues of earthworms were determined by radioimmunoassay (RIA), expression of the specific PACAP receptor (PAC1-R) molecules in renewing tissues were detected with Western blot and immunocytochemistry, respectively.

Results: 

A strong upregulation of PACAP-like immunoreactivity was observed in the subesophageal ganglion after brain extirpation. A decreasing gradient of PACAP-like molecules from the subesophageal ganglion to circumpharyngeal connectives was found in regenerating earthworms with transected connectives. If circumpharyngeal connectives were cauterized PACAP-like molecules accumulated in the truncated ends. On the 7^th day of regeneration, detectable amount of PAC1-R molecules (molecular weight was about 50 KDa) appeared in the renewing tissues. Immunocytochemical observations showed that PAC1-R immunoreactive structures were morphologically heterogeneous; a part of labeled cells proved to be totipotent stem cells (named neoblasts in earthworms) while others represented various developmental stages of neural and muscle cells.

Conclusion: 

Our results prove that neural processes transport PACAP-like molecules to the site of regeneration where PAC1-R expressing cells are located suggesting that these molecules mediate both the migration of neoblasts to the site of the injury and their differentiation into neural, muscle and connective tissue cells resulting in the formation of a new brain.