ABSTRACT
Sprouting of undamaged, adult neurones has been observed in a number of situations. For instance, in the peripheral nervous system of vertebrates, intact motoneurones exhibit terminal sprouting in response to axotomy of contralateral motoneurones (Rotshenker, 1979; Rotshenker & Tal, 1985) and in the central nervous system, neurones have been observed to sprout and occupy synapses vacated by lesioned fibres (Cotman, Nieto-Sampedro & Harris, 1981; Tsukahara, 1981). Furthermore, evidence also exists for dendritic sprouting and regression in the mature, human brain (Buell & Coleman, 1981). The plastic properties of the neurones of the freshwater pulmonate Helisoma have been examined in a number of regeneration and sprouting studies (e.g. Murphy & Kater, 1980; Bulloch & Kater, 1982; reviewed by Bulloch, 1985a). Recently, it was demonstrated that sprouting and retraction of a central neurite occurred from an undamaged adult neurone of Helisoma in response to stress (Bulloch, 1984). Specifically, sprouting occurred in a pair of buccal neurones, L5 and R5, and aestivation and body wall incision were the most effective treatments examined. Body wall incision results in haemolymph loss, and aestivation results in a loss of water and thus an increase in haemolymph osmolarity (Machin, 1975). These observations raise the possibility that the sprouting of neurones L5 and R5 may occur in response to changes in haemolymph osmoregulation. The current study directly tested the hypothesis that Helisoma neurones can sprout in response to a change in blood osmolarity. This was achieved by exposing snails to hyperosmotic and hyposmotic pond water which has known effects on blood osmolarity (Khan & Saleuddin, 1979).
