Neuronal shape and volume changes require accompanying cell surface adjustm
ents. In response to osmotic perturbations, neurons show evidence of surfac
e area regulation; shrinking neurons invaginate membrane at the substratum,
pinch off vacuoles, and lower their membrane capacitance. F-actin is impli
cated in reprocessing newly invaginated membrane because cytochalasin cause
s the transient shrinking-induced invaginations, vacuole-like dilations (VL
Ds), to persist indefinitely instead of undergoing recovery. To help determ
ine if cortical F-actin indeed contributes to cell surface area regulation,
we test, here, the following hypothesis: invaginating VLD membrane rapidly
establishes an association with F-actin and this association contributes t
o VLD recovery. Cultured molluscan (Lymnaea) neurons, whose large size faci
litates three-dimensional imaging, were used. In fixed neurons, fluorescent
F-actin stains were imaged. In live neurons, VLD membrane was monitored by
brightfield microscopies and actin was monitored via a fluorescent tag. VL
D formation (unlike VLD recovery) is cytochalasin insensitive and consisten
t with this, VLDs formed readily in cytochalasin-treated neurons but showed
no association with F-actin, Normally, however (i.e., no cytochalasin), VL
Ds were foci for rapid reorganization of F-actin. At earliest detection (1-
2 min), nascent VLDs were entirely coated with F-actin and by 5 min, VLD mo
uths (i.e., at the substratum) had become annuli of F-actin-rich motile lea
ding edge. Time lapse images from live neurons showed these rings to be mot
ile filopodia and lamellipodia. The retrieval of VLD membrane (vacuolizatio
n) occurred via actin-associated constriction of VLD mouths. The interplay
of surface membrane and cortical cytoskeleton in osmotically perturbed neur
ons suggests that cell surface area and volume adjustments are coordinated
in part via mechanosensitive F-actin dynamics.