Electric membrane properties of adult mouse DRG neurons and the effect of culture duration

The electrical membrane properties (EMP) of adult mouse dorsal root ganglion (DRG) neurons were characterized by an extensive electrophysiological investigation of 450 cells. The neurons were divided into two types: an M-type having an action potential with monophasic falling phase and a B-type with a more complex biphasic or triphasic falling phase. Compared to M-type, B-type were “slow” neurons with a higher specific membrane resistance (R_m), and a longer time constant (τ), duration of action potential (Δt), and absolute refractory period (ARP). B-type also had a larger amplitude action potential, afterhyperpolarization and positive overshoot. The action potential of the M-type neuron had only a Na⁺ component while that of the B-type had both a Na⁺ and a Ca²⁺ component. After two days in culture, M-type neurons exhibited phase bright cytoplasmic granules, which were seldom observed for B-type neurons. Although neuron survival remained constant during the first six days in culture (DIV), the relative frequency of occurrence of the M-type decreased from 82 to 50%. Thereafter, it decreased more gradually to a final value of approximately 20% after 40 DIV. It was concluded that at least during the first 6 DIV and possibly through to 40 DIV, M-type neurons transformed into B-type. Both M- and B-type neurons showed significant and similar changes in their EMP with increasing DIV (up to 40 DIV). For M- and B-types combined, R_m increased approximately 142%, τ by 204%, and no significant change in specific membrane capacitance was observed. Rheobasic threshold depolarization decreased 58%, while the resting membrane potential decreased by only 19%. These changes in the EMP of adult neurons are strikingly similar to changes in EMP observed in adult denervated muscle and in cultures of either embryonic nerve or muscle. This similarity suggested that the adult DRG neurons in cell culture undergo progressive dedifferentiation because of isolation from their usual trophic interactions. Determination of neuronal membrane electrical characteristics provides a new method for evaluating the effects of various possible trophic agents, e.g., hormones and tissue extracts, on the state of differentiation of neurons in cell culture.     

Absence of actin genes in spirochetes

Probes of actin genes isolated fromSaccharomyces cerevisiae were hybridized with restriction endonuclease digests of the DNA of two spirochetes,Spirochaeta halophila andTreponema saccharophilum. Actin genes were not present in these spirochetes. Cytochalasins and colchicine had no effect on the growth or on the motility of these spirochetes. These data indicate that the spirochetes did not posses functional actin or tubulin proteins.     

Habitat availability,social attraction and nest distribution patterns in longear sunfish (Lepomis megalotis peltastes)

Synopsis Two forces, limited availability of preferred habitat and social attraction between conspecifics were hypothesized to explain nest aggregation in male longear sunfish. In the laboratory, males cluster their nests within a small segment of a much larger uniform area of nesting habitat. In the field, large areas of unused habitat suitable for nesting were found, suggesting that nesting aggregations are not formed as a result of limited preferred habitat. Males appear to be socially attracted during breeding, and social nesting presumably is a successful reproductive strategy for at least a portion of the population.     

Comparative anticholinergic potencies of R- and S- disopyramide in longitudinal muscle strips from guinea pig ileum

The anticholinergic potencies of R- and S- disopyramide were studied in isolated myenteric plexus longitudinal muscle strip preparations from guinea-pig ileum using two experimental procedures. The first procedure tested the relative potencies of the isomers in inhibiting electrically-stimulated contractions in 6 ileum preparations. A balanced crossover design was employed. The mean concentration of S-disopyramide required to inhibit electrically stimulated contractions by 50% was 4.6 × 10^?6 M and was about one-fourth of the concentration of R-disopyramide required to produce the same effect (p < 0.05). The second procedure tested the relative potencies of the isomers as direct antagonists of contractions induced by either histamine or acetylcholine in ileum preparations. Neither isomer antagonized the histamine-induced contractions. For the contractions induced by acetylcholine, the pA₂ value, obtained directly from Schild plots, was 6.25 for S- and 5.74 for R- disopyramide. However, the slope of the Schild plot for the S-isomer differed significantly from ?1, suggesting that other mechanisms in addition to direct antagonism of acetylcholine may be involved. Thus, the results of the experiments involving both the antagonism of electrically stimulated contractions and the direct antagonism of acetylcholine-induced contractions indicate (1) that both isomers of disopyramide have anticholinergic properties and (2) that S-disopyramide is about 3–4 fold more potent as an anticholinergic agent.