Lipochondria and the light response ofAplysiagiant neurons

Abstract

The ultrastructure, absorbance, and elemental content of lipochondria present in the cytoplasm of Aplysia giant neurons have been investigated before and after 30-1,200 sec doses of white light at intensities which produce saturated light responses. The effects of exposure to the calcium ionophore A-23187 and to EGTA were also examined. The lipochondria of nonilluminated neurons are membrane-bound, and contain lipids, protein, Na, K, Mg Ca, Si, Cl, Br, P, and a pigment which is probably beta-carotene. The cytoplasm appeared to have little pigment. When neurons were illuminated for 20 min, 60-70% of the lipochondria showed marked ultrastructural alterations, the most notable being the appearance of membranous material. Earlier changes which occur after 30 sec of illumination include the appearance of paracrystalline arrays and mottling. Less than 10% of lipochondria in nonilluminated neurons have a similar appearance. These effects were greatly enhanced in illuminated neurons exposed to the calcium ionophore or EGTA. In nonilluminated neurons, the ionophore also produced ultrastructural changes. In frozen specimens, the calcium content of the most electron dense lipochondria of illuminated neurons was reduced. Other elements which were counted were also reduced. The lipochondria are the main intracellular site of photopigment. They may also act as an intracellular source for calcium which, as the accompanying paper indicated, may mediate phototransduction in Aplysia neurons.

The role of diffusion in the photoresponse of an extraretinal photoreceptor of Aplysia.

“…The neurone soma is spherical in shape and the nuclear radius is generally one-half the somal radius. The photopigment, probably a retinol derivative (Krauhs, Sordahl & Brown, 1977), is located in membranebound cytoplasmic granules called lipochondria (Arvanitaki & Chalazonitis, 1961;Henkart, 1975;Brown, Baur & Tuley, 1975), which also contain calcium (Baur et al 1977). The lipochondria are located in a shell extending from the plasma membrane inward toward the nucleus and are represented as small spheres in the diagram.…”

“…The lipochondria are located in a shell extending from the plasma membrane inward toward the nucleus and are represented as small spheres in the diagram. While the transmitter is not definitely established in photoresponsive Aply8ia neurones, there is a great deal of evidence that Ca2+ is involved Baur et al 1977;Brown et al 1975;Brown & Brown, 1973) and for convenience we refer to the transmitter substance as CaO+ in this paper. However, the possibility remains that some other substance activated by Ca2+ is the actual transmitter.…”

“…The nucleus generally occupies the cytoplasm up to r = j, and the lipochondria seem to be confined to the interval i < r < 1 (Baur et al 1977). Thus we may set f(r) = 0 for 0 < r < i.…”

The dual role of rhodopsin in vision: light‐driven charge translocation and formation of long‐lived photoproducts

“…For example, in light-sensitive neurons of marine mollusc Aplysia californica, Ca2+-loaded membrane vesicles were found [62][63][64]. The vesicles named lipochondria were shown to contain pigments with r-carotene and a 'retinol-like' compound as chromophores [65].…”

“…The vesicles named lipochondria were shown to contain pigments with r-carotene and a 'retinol-like' compound as chromophores [65]. Illumination induces release of Ca 2+ from lipochondria [63,64]. The same could be initiated by addition of a Ca 2+ ionophore [65].…”