Tetrodotoxin blocks a graded sensory response in the eye of Limulus.

The generalization that tetrodotoxin selectively blocks all-or none electrical activity of nerve and muscle but has negligible effect upon graded responses of sensory systeins does not appear to be valid for the Limulus eye. Tetrodotoxin reversibly blocks the graded transient component of this visual response, while the steady state component of the response is rela tively unaffected by the drug.

Membrane fatty acids associated with the electrical response in visual excitation.

The fatty acid composition of rat photoreceptor membranes was altered by dietary manipulation. A functional alteration was also observed in the component of the electroretinogram which is generated by the photoreceptors. A membrane fatty acid, docosahexaenoic acid, appears to be involved in the transduction process of visual excitation.

Visual membranes: specificity of fatty acid precursors for the electrical response to illumination.

Rat electroretinograms were measured as a function of dietary supplements of purified ethyl esters of linolenic acid, linoleic acid, and oleic acid. Polyunsaturated fatty acids derived from precursors of linolenic and linoleic acids appear to be important functional components of photoreceptor cell membranes, although in equal dietary concentrations, linolenic acid precursors affect electroretinogram amplitudes to a greater extent than linoleic acid precursors. The electrical response of photoreceptor cell membranes appears to be a function of the position of the double bonds as well as a function of the total number of double bonds in fatty acid supplements.

Lipid composition of Limulus photoreceptor membranes.

The lipid composition has been determined for rhabdomeric photoreceptor membranes of Limulus, and these data are compared with those from photoreceptor membranes of albino rats. The comparison is of interest because the membranes of these two photoreceptor cells regulate ionic transport differently during the response to illumination. 1. Phospholipid class composition of Limulus is similar, but not identical, to that of rats. The major differences are a greater percentage of sphingomyelin in Limulus and a greater percentage of phosphatidylethanolamine in the rat. 2. Ethanolamine plasmalogens, not observed in rat photoreceptor membranes, are present in Limulus photoreceptor fractions. 3. The level of cholesterol in Limulus is higher than that usually reported for vertebrate rod outer segments. 4. The predominant polyunsaturated fatty acids of Limulus photoreceptor membrane phospholipids are 20: 4(n-6) and 20: 5(n-3) with only traces of 22: 6(n-3). This is in sharp contrast with the large percentages of 22: 6(n-3) found in rat photoreceptors. 5. The fatty acid distributions of both membrane systems are highly unsaturated, but the ratio of (n-3) to (n-6) polyunsaturates is only 1.7 for Limulus as compared to 4.6 for rat.

Chemistry of photoreceptor membrane preparations from squid retinas.

Photoreceptor membrane preparations were made from retinas of the squid Loligo (Doryteuthis) plei for protein and lipid analysis. Lipid analysis was also completed on a single membrane preparation from Loligo pealei. (1) The membranes contain 75 wt. % protein and 25 wt. % lipid. Neutral lipids make up 26 mol % of the total lipid, the remaining 74% being phospholipid. No glycolipids were observed. (2) Free fatty acids and cholesterol comprise 8.6 and 17 mol %, respectively of the total lipid. No other neutral lipids were found. (3) Phosphatidylethanolamine and phosphatidylcholine are the major phospholipids. Lysophosphatidylcholine, lysophosphatidylethanolamine, sphingomyelin, and phosphatidylserine are present in small quantities. Phosphatidylinositol was not detected in the membranes. (4) The levels of polyunsaturated fatty acids, principally 20:4 omega 6, 20:5 omega 3, and 22:6 omega 3 are higher in the squid membranes than in any othr vertebrate or invertebrate retina that has been examined thus far. These acids account for 58 mol % of the fatty acids in phosphatidylcholine and phosphatidylserine, 75 mol % of the free fatty acids, and nearly 90% of the fatty acids of lyso- and phosphatidylethanolamine. The results from L. plei and L. pealei were indistinguishable. (5) Rhodopsin is the major protein of the membrane preparations and has a molecular weight of 50 500 +/- 850 determined by sodium dodecyl sulfate polyacrylamide gel disc electrophoresis.

Response properties of a sensory hair excised from Venus's flytrap.

Multicellular sensory hairs were excised from the leaf of Venus's flytrap, and the sensory cells were identified by a destructive dissection technique. The sensory layer includes a radially symmetrical rosette of 20-30 apparently identical cells, and the sensory cells are organized in a plane normal to the long axis of the sensory hair. The sensory cells were probed with intracellular glass electrodes. The resting membrane potential was about -80 mv, and the response to a mechanical stimulus consisted of a graded response and an "action potential." The action potential appears to be similar to the action potential which propagates over the surface of the leaf. In the absence of stimulation, the upper and lower membranes of a single sensory cell behave in an electrically symmetrical fashion. Upon stimulation, however, the upper and lower membranes become electrically asymmetrical. Limiting values for the response asymmetry were calculated on the hypothesis of an electrical model consistent with the histology of the sensory cells.