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B Oakley

Publications and source records attributed to B Oakley.

At least 55 records · Page 3Linked to original sources
PubMed1987

Measurement of potassium turnover in rod photoreceptors in toad isolated retina using ion-selective microelectrodes.

Ion-selective microelectrodes (ISMs) were used to measure the turnover of intracellular K+ (Ki+) in rods in the isolated retina of the toad, Bufo marinus. The light-evoked hyperpolarization of rods decreases their passive K+ efflux, which in combination with active K+ uptake, decreases extracellular K+ concentration, Ko+.Rb+ substitutes for K+ in these processes. The turnover of Ki+ was measured as Rb+ and K+ were exchanged, using ISMs that were approximately five times more sensitive to Rb+ than to K+. When Ko+ was replaced by Rbo+, the light-evoked decrease in K+ efflux produced only a small change in ISM voltage, delta VISM, owing to the background of Rbo+. As Rbi+ replaced Ki+, the efflux shifted from K+ to Rb+ and delta VISM grew in amplitude. After loading the rods with Rbi+, Rbo+ was replaced by Ko+. The light-evoked decrease in Rb+ efflux lead transiently to a large delta VISM, since the change in Rbo+ was superimposed upon a background of Ko+. As Ki+ replaced Rbi+, the amplitude of delta VISM declined. When measured using this technique, the turnover of Ki+ was 95% complete in approximately 15 min. In low Ca2+ solutions, transmembrane fluxes of K+ (Rb+) increased and turnover of Ki+ occurred more rapidly. During background illumination, transmembrane fluxes of K+ (Rb+) decreased and turnover of Ki+ was slowed. These experiments have provided independent corroboration of earlier observations concerning rod K+ fluxes. This ISM-based technique also may be useful in measuring K+ turnover in other cell types.

Animals↗
PubMed1987

A sensitive period for the neural induction of taste buds.

Taste buds mature postnatally in the vallate papilla of the rat and reach a mean number of 610 by day 90. Although taste buds are neurotrophically dependent, the presence of widespread bilateral innervation permits more than 80% of the 610 vallate taste buds to survive after one IXth nerve is removed in adults. However, after a IXth nerve is removed at 0-3 d postpartum, about two-thirds of the vallate taste buds fail to develop. In the present investigation, the timing of the neural induction of taste buds was examined by unilaterally removing the IXth nerve at 12 different postnatal ages, from 0 to 75 d. Unilateral denervation revealed the existence of a sensitive period that is maximal from 0 to 10 d, when unilateral or bilateral interruption of the IXth nerve profoundly impairs the formation of taste buds. The number of taste buds that form is nonlinearly dependent upon the number of axons; at low levels of innervation, a doubling of the number of myelinated axons quintuples the number of taste buds. Thus, taste axons interact synergistically. In studying regeneration, we found that axons of both neonatal and adult IXth nerves elongate approximately 1.8 mm/d. Taste buds were re-formed more rapidly and a higher proportion were bilaterally innervated when regenerating axons and the sites of former taste buds were numerous. The proportion of bilaterally innervated taste buds could be approximated from the likelihood of random overlap of axons from the right and left IXth nerves. The greater ease with which taste buds are re-formed than developed suggests that taste bud regeneration does not recapitulate taste bud development.

Animals↗
PubMed1986

Decline of electrogenic Na+/K+ pump activity in rod photoreceptors during maintained illumination.

Light-evoked changes in membrane voltage were recorded intracellularly from rod photoreceptors in the isolated retina preparation of the toad, Bufo marinus, during superfusion with a solution containing pharmacological agents that blocked voltage-dependent conductances. Under these conditions, the amplitude of the hyperpolarizing photoresponse became much greater than under control conditions. The results of several experiments support the conclusion that this increase in photoresponse amplitude was due primarily to a voltage that was produced when the electrogenic current from the rods' Na+/K+ pump flowed across an increased membrane resistance (Torre, V. 1982. Journal of Physiology. 333:315). At the onset of a period of continuous illumination, the rod membrane first hyperpolarized and then began to repolarize, and after 180 s of illumination, the membrane voltage had recovered by 60-72% of its initial hyperpolarization. There did not appear to be any significant decrease in rod membrane resistance associated with this repolarization. Both the enhanced hyperpolarization at light onset and the slow repolarization during maintained illumination were blocked by superfusion with 10.0 microM strophanthidin. These data support the hypothesis that the activity of the rods' Na+/K+ pump declines progressively during maintained illumination. It is likely that the decline in pump activity produces significant changes in [K+]o in the subretinal space during maintained illumination.

Animals↗
PubMed1984

Transduction physiology of olfactory receptor cilia.

Electro-olfactograms (EOGs) evoked by 8 odorants from frog olfactory epithelia during ciliary regrowth and during epithelial regeneration were analyzed. During ciliary regrowth following detergent-induced ciliary removal, EOG amplitudes initially increase proportionately with ciliary length. EOGs reach maximal amplitudes after 2 days of growth, when cilia are 40 micron long. Therefore olfactory transduction sites are located primarily on cilia rather than on the dendrite terminal and most of the receptor current enters through the proximal portion of the cilium. Zinc sulfate lavage of the nasal cavity causes selective necrosis of the receptor epithelium. During epithelial regeneration, EOGs increase linearly with time from 13 days after zinc lavage, the time of first cilium emergence, through 30 days. The rate of increase is different for different odorants. At 30 days and within a period of a few days, EOG amplitudes increase abruptly, then asymptote. Thus the development of receptors for different substances occurs at different rates and occurs in two steps. The transition between the two developmental states is coincident with arrival of receptor axon terminals at the central nervous system and with the immobilization of the ciliary contractile apparatus. Since there is continual generation of new receptor neurons throughout life, EOGs recorded in a normal nose reflect a complex combination of the differing receptor processes of cells of differing developmental stages.

Animals↗
PubMed1984

Effects of low [Ca2+]0 upon [K+]0 during and after maintained illumination of the isolated retina of the toad.

Using K+-selective microelectrodes, [K+]0 was measured in the receptor layer of the isolated retina of the toad, Bufo marinus, during and after maintained adapting illumination. Lowering [Ca2+]0 to one-tenth of its control value produced larger, more rapid changes in [K+]0 at light onset and offset than under control conditions. Lowering [Ca+]0 also produced rod membrane depolarization in the dark and larger light-evoked changes in rod membrane voltage. The observed effects are consistent with a mechanism by which lowering [Ca+]0 increases sodium conductance in rods in the dark, which in turn increases [Na+]i and stimulates the rods' NA+/K+ pump. This putative mechanism may be used to explain several effects of low [Ca+]0 upon rod function observed previously.

Animals↗
PubMed1984

Trophic capacity of experimentally lengthened gustatory axons.

Long nerve stumps will maintain end-organs for a longer period of time than short nerve stumps. However, longer maintenance might result not from a longer nerve stump per se but from uncontrolled factors such as less interference with the target's blood supply when transecting a nerve far from its target. To control for the site of nerve transection and to evaluate the trophic capacity of lengthened axons, we extended the gerbil's (Meriones unguiculatus) IXth nerve (glossopharyngeal) by splicing in a 5.0 mm segment of the superior laryngeal nerve. Thus, when assessing nerve stump-length dependency 3-5 months later, both the control regenerated nerve and the lengthened experimental nerve could be transected at the same site on the IXth nerve trunk. The peak magnitude of the summated taste discharges of the experimentally lengthened IXth nerve declined at a rate exactly predicted by an earlier study of nerve stump-length dependency. We conclude that maintenance of synaptic transmission depends upon taste axon stump-length and not upon factors uncontrolled in conventional experimental designs of stump-length dependency which transect nerves at different locations. Control regenerated and experimentally lengthened nerves maintained similar numbers of taste buds. Thus, neither the added burden of maintaining longer axons nor the presence of Schwann cells of the superior laryngeal nerve splice degraded the IXth nerve's trophic performance.

Animals↗
PubMed1984

Reaccumulation of [K+]o in the toad retina during maintained illumination.

Using K+-selective microelectrodes, [K+]o was measured in the subretinal space of the isolated retina of the toad, Bufo marinus. During maintained illumination, [K+]o fell to a minimum and then recovered to a steady level that was approximately 0.1 mM below its dark level. Spatial buffering of [K+]o by Müller (glial) cells could contribute to this reaccumulation of K+. However, superfusion with substances that might be expected to block glial transport of K+ had no significant effect upon the reaccumulation of K+. These substances included blockers of gK (TEA+, Cs+, Rb+, 4-AP) and a gliotoxin (alpha AAA). Progressive slowing of the rods' Na+/K+ pump (perhaps caused by a light-evoked decrease in [Na+]i) also could contribute to this reaccumulation of K+ by reducing the uptake of K+ from the subretinal space. As evidence for a major contribution by this mechanism, treatments designed to prevent such slowing of the pump reversibly blocked reaccumulation. These treatments included superfusion with 2 microM ouabain, or lowering [K+]o, PO2, or temperature. It is likely that such treatments inhibit the pump, increase [Na+]i, and attenuate any light-evoked decrease in [Na+]i. The results are consistent with the following hypothesis. At light onset, the decrease in rod gNa will reduce the Na+ influx and the resulting rod hyperpolarization will reduce the K+ efflux. In combination with these reduced passive fluxes, the continuing active fluxes will lower both [K+]o and [Na+]i, which in turn will inhibit the pump. In support of this hypothesis, the solutions to a pair of coupled differential equations that model changes in both [K+]o and [Na+]i match quantitatively the time course of the observed changes in [K+]o during and after maintained illumination for all stimuli examined.

Animals↗
PubMed1983

Colchicine reduces myelin thickness and axoplasm volume.

A Silastic cuff containing either colchicine (1% w/v) or no colchicine was placed around the lingual disorder tympani nerve of the Mongolian gerbil. After 3 days of exposure to colchicine, the mean period of the myelin sheaths was 23% less than the period observed in nerves treated with cuffs lacking colchicine, while the average number of lamellae was unaltered. At the same time colchicine reduced the volume of axoplasm by an average of 19%, an effect which was independent of fiber diameter.

Animals↗
PubMed1983

Effects of maintained illumination upon [K+]0 in the subretinal space of the isolated retina of the toad.

Illumination of the vertebrate retina evokes a transient decrease in extracellular potassium concentration, [K+]0, in the subretinal space. During maintained illumination, [K+]0 recovers toward its dark-adapted level. The mechanisms most likely to contribute to this recovery process were examined by using K+-selective microelectrodes to measure [K+]0 in the isolated retina of the toad. Bufo marinus. Although both diffusion of K+ and changes in the rod membrane voltage contribute to the recovery of [K+]0 during maintained illumination, other factors are likely to be involved as well. It is suggested that this recovery process could be due in part to inhibition of the Na+/K+ pump in the rod photo-receptors during maintained illumination.

Animals↗
PubMed1983

Modulation of membrane conductance in rods of Bufo marinus by intracellular calcium ion.

Double-barrel micropipettes were used to pressure-inject EGTA into the outer segments of rods in the isolated retina of Bufo marinus. We used these pipettes to point voltage clamp the cell to its resting membrane voltage during the injection of EGTA in order to prevent changes in membrane voltage from occurring. The input conductance of the rod was assessed by measuring the incremental membrane current required to hyperpolarize the membrane by less than or equal to 10 mV. When the retina was bathed in normal Ringer solution, the injection of EGTA during point voltage clamp evoked an inward membrane current and in increase in input conductance. This observation is consistent with an EGTA-evoked increase in conductance for an ion with an equilibrium potential more depolarized than the resting membrane potential. Injections of control solutions that did not contain EGTA had no effect. The effects of injected EGTA were not altered by variations in the pH or buffering capacity of the injection solution, or by the addition of equimolar Mg2+. Furthermore, injections of a solution containing equimolar Ca2+ and EGTA were without effect. Thus, the observed effects of injected EGTA were due to the lowering of the [Ca2+]i. Replacement of extracellular Na+ with choline+ abolished both the response to light and the EGTA-evoked increase in input conductance. A low [Na+]o solution containing 10(-8) M-Ca2+ reduced the response to injected EGTA by approximately the same amount as it reduced the response to light. Replacement of extracellular Cl- by methanesulphonate was without significant effect on either the response to light or to injected EGTA. These results are consistent with the interpretation that a lowered [Ca2+]i increases primarily the sodium conductance, gNa, of the plasma membrane of the rod outer segment. The conductance that is affected by a lowered [Ca2+]i appears to have the same specificity as the light-dependent conductance. This conclusion is consistent with a hypothesis for visual transduction involving modulation of gNa by light-evoked changes in the [Ca2+]i.

Animals↗
PubMed1983

Chronic impairment of axonal transport eliminates taste responses and taste buds.

A Silastic nerve cuff containing colchicine (1% w/v) was placed around the combined lingualchorda tympani nerve of the Mongolian gerbil (Meriones unguiculatus) to evaluate the role of axonal transport in the maintenance of taste buds. After 3 days the summated gustatory impulse discharges recorded from the chorda tympani nerve were reduced by 60%, while compound action potentials had not changed appreciably. The lingual-chorda tympani nerve underwent ultrastructural changes including a loss of microtubules, an increased prominence and disorientation of neurofilaments, and a significant shrinkage in the cross-sectional area of axoplasm. The shrinkage of axoplasm and the accumulation of mitochondria and cholinesterase at the nerve cuff provided evidence that the colchicine treatment acted to impair axonal transport. More substantial pathological changes were evident in nerve ultrastructure by 15 days when both the ipsilateral chorda tympani taste responses and fungiform taste buds were nearly absent. Control cuffs lacking colchicine had little effect on chorda tympani taste responses, taste buds, or nerve ultrastructure. Eight or 15 days of nerve exposure to lumicolchicine, an isomer of colchicine with low affinity for tubulin, had no significant effect on taste responses. [3H]Colchicine was used in the nerve cuff to demonstrate that colchicine must have acted directly upon the nerve trunk, rather than the taste buds, to cause the loss of taste responses and taste buds. [3H]Colchicine levels were equal in the two sides of the tongue, whereas both the functional and structural deterioration of the taste buds were restricted to the ipsilateral side. We conclude that the loss of taste responses and taste buds was caused by chronically impaired axonal transport in gustatory axons.

Animals↗
PubMed1982

Effects of maintained illumination upon [K+]0 in the subretinal space of the frog retina.

Changes in extracellular potassium ion concentration, [K+]0, were measured in the subretinal space of an in vitro preparation of bullfrog retina, pigment epithelium and choroid. In response to maintained illumination [K+]0 fell to a minimum value in 30-40 sec, and then began to recover, reaching a steady-state approximately 10 min after light onset. At light offset, [K+]0 overshot the dark-adapted baseline before recovering. Pigment epithelial membrane potentials followed [K+]0 during this entire timeperiod, causing parallel changes in the d.c. level of the vitreal electroretinogram (ERG). These changes in [K+]0 are very similar to those observed in the cat retina in vivo, except that they are slower in time-course by nearly an order of magnitude. The vitreal ERG, however, is much different in frog than in cat, since in frog, but not in cat, it is an indirect measure of [K+]0 in the subretinal space.

Animals↗
PubMed1982

Light-dependent effects of a hydrolysis-resistant analog of GTP on rod photoresponses in the toad retina.

Responses to 100-ms flashes were recorded intracellularly from dark- and light-adapted rod photoreceptors in the isolated retina of the toad, Bufo marinus. Properties of photoresponses were analyzed under each condition of adaptation when retinas were superfused with 1.0 mM guanosine 5'-[beta, gamma-methylene]triphosphate (p[CH2]ppG), a hydrolysis-resistant analog od GTP. When applied to retinas that previously had been subjected to intense light (approximately 30% bleach), p[CH2]ppG increased both the amplitude and duration of photoresponses. By contrast, treatment of dark-adapted retinas with p[CH2]ppG did not alter these response parameters. When similarly applied to either dark- or light-adapted retinas, GTP had no effects on amplitude or duration of photoresponses. These results are discussed in terms of GTP-dependent mechanisms for rod adaptation.

Animals↗
PubMed1981

Axonal transport maintains taste responses.

Transection of the gerbil's IXth nerve causes gustatory action potentials to decline in 1-6 h; the rate of decline is a linear function of the length of the nerve stump remaining attached to the tongue. To test the implication that taste discharge mechanisms depend upon axonal transport in the IXth nerve, we injected 40 nl of [3H]leucine into the petrosal ganglion of one IXth nerve of the gerbil, Meriones unguiculatus. Subsequent liquid scintillation counting of the petrosal ganglion, IXth nerve segments, and representative areas of the tongue indicated that labeled materials were transported down the IXth nerve primarily to the vallate and ipsilateral foliate taste papillae of the tongue. A significant impairment of axonal transport and a substantial decline in summated IXth nerve taste responses occurred within 2-3 h after colchicine was applied to the IXth nerve trunk between the tongue and the petrosal ganglion. Similarly, cooling the IXth nerve with a 3-10 degrees C thermocouple junction or metal probe impaired axonal transport and caused a taste response decline. Since impulse mechanisms of the nerve trunk continued to function distal to these sites of treatment, we concluded that the maintenance of taste discharges at the level of the taste bud depends upon unimpeded axonal transport.

Action Potentials↗
PubMed1980

The effect of nerve stump length upon mammalian taste responses.

A decline in the magnitude of the summated responses evoked by taste solutions began about 1 h following IXth nerve transection. In 41 IXth nerves transected at different lengths we found a linear relationship between the length of the IXth nerve stump remaining attached to the tongue and the time for the taste response amplitude to fall to 50% of its control magnitude. The existence of a nerve stump length dependency suggests that IXth nerve taste responses in the gerbil are maintained by the axonal transport of trophic substances to the tongue.

Animals↗
PubMed1979

Intracellular potassium ion activity in resting and stimulated mouse pancreas and submandibular gland.

Intracellular potassium ion activities (aiK+) and membrane potentials were measured with double-barrelled, potassium-specific microelectrodes in superfused mouse pancreas and submandibular gland. Stimulation with the cholinergic agonist bethanechol caused a marked decrease in aiK+ in the submandibular gland, whereas no change in aiK+ could be detected in the pancreas. This indicates that bethanechol increases the permeability of the cell membranes to potassium ions in the submandibular gland but not in the pancreas. Pancreatic acinar cells hyperpolarized promptly when the extracellular potassium ion activity was restored after a prolonged period of potassium deprivation. In comparison, the recovery of aiK+ was a slow process. This finding gives support to the view that the hyperpolarization is due to electrogenic sodium pumping.

Animals↗
PubMed1979

Effects of the rod receptor potential upon retinal extracellular potassium concentration.

It has been hypothesized that the light-evoked rod hyperpolarization (the receptor potential) initiates the light-evoked decrease in extracellular potassium ion concentration, [K+]o, in the distal retina. The hypothesis was tested using the isolated, superfused retina of the toad, Bufo marinus; the receptor potential was recorded intracellularly from red rods, and [K+]o was measured in the photoreceptor layer with K+-specific microelectrodes. In support of the hypothesis, variations in stimulus irradiance or duration, or in retinal temperature, produced qualitatively similar effects on both the receptor potential and the decrease in [K+]o. A mechanism for the relationship between the receptor potential and the decrease in [K+]o was suggested by Matsuura et al. (1978. Vision Res. 18:767-775). In the dark, the passive efflux of K+ out of the rod is balanced by an equal influx of K+ fromthe Na+/K+ pump. The light-evoked rod hyperpolarization is assumed to reduce the passive efflux, with little effect on the pump. Thus, the influx will exceed the efflux, and [K+]o will decrease. Consistent with this mechanism, the largest and most rapid decrease in [K+]o was measured adjacent to the rod inner segments, where the Na+/K+ pump is most likely located; in addition, inhibition of the pump with ouabain abolished the decrease in [K]o more rapidly than the rod hyperpolarization. Based upon this mechanism, Matsuura et al. (1978) developed a mathematical model: over a wide range of stimulus irradiance, this model successfully predicts the time-course of the decrease in [K+]o, given only the time-course of the rod hyperpolarization.

Animals↗