AJP - Regulatory, Integrative and Comparative Physiology, Vol 256, Issue 4 989-R996, Copyright © 1989 by American Physiological Society
Age sensitivity of osmoregulation and of its neural correlates in Aplysia
T. L. Skinner and B. Peretz
Department of Physiology and Biophysics, College of Medicine, University of Kentucky, Lexington.
Osmoregulation was studied in the marine mollusc Aplysia californica in
young, mature, and old adults. To monitor volume and osmoregulation, we
measured body weight, hemolymph osmolality, and chloride concentration.
These parameters were measured at regular intervals with animals in 90%
artificial seawater (90% ASW) for up to 36 h. They showed that the rates at
which Aplysia osmo- and volume regulate were significantly slowed with
increased age. However, no age effect was found in osmoregulation when the
hemolymph was diluted to 90% of control in animals without an external
stress, i.e., by injection of distilled H2O and keeping animals in 100%
ASW. Because the dilution bypassed the sensory receptors that detect
external changes of osmolality, this finding suggested that the slowed
osmoregulation involved age-impaired functioning of the neural pathway
mediating osmoregulation. Other evidence was from mature adults whose
osmoreceptive organ, the osphradium, was lesioned; they mimicked
osmoregulation measured in old adults. In preparations containing a portion
of the osmoregulatory pathway, the osphradium was stimulated by 90% ASW,
and the responsiveness of neuron R15, which putatively regulates
antidiuresis, was tested. The stimulus inhibited spiking in R15 from mature
adults but not in R15 from old adults or from osphradiallesioned mature
ones. In old Aplysia the refractoriness of R15 to osphradial stimulation
demonstrated that the effecacy of the pathway was impaired with increased
age; it helped explain the slower rate of osmoregulation. Possible changes
of osmoregulatory mechanisms and behavior compensating for the age
sensitivity of osmoregulation are discussed.(ABSTRACT TRUNCATED AT 250
WORDS)
