Stress effects and neurosecretion in Earthworms: A Review

Earthworms, the poikilothermal first group of invertebrate, are subjected to stressful events, occasional (illumination), frequent (amputation due to predation) and seasonal (dehydration during summer), hydration (during monsoon), thermal (heat stress during summer, cold stress during winter) etc. These lowly evolved creatures often overcome the events of stress through eco-physiological adaptive changes in the central nervous system (CNS) neurosecretory system. In absence any endocrine gland and neurohaemal organ, their CNS is enriched with vascularisation and well defined neurosecretory cells (NSCs) (type A and type B). The type A NSCs - the source of peptide neurohormone and release their secretion through axonal transport, response mainly against osmotic stress (dehydration, hydration), light (when exposed to the sunlight), hypo and hyperthermia etc. The type B NSCs are source of amine neurohormone and release neurohormone mainly by perikarya and occasionally by axon. The type B cells show remarkable cytoplasmic changes in relation to metabolic compensation following food deprivation in earthworms. Both the type A and type B NSCs show adaptive response such axonal transport, cytoplasmic vacuoles followed normal secretory cycle (i.e. synthesis, storage, release) during amputation and subsequent regeneration in earthworm.