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PubMed1995

Block of cyclic nucleotide-gated channels in salamander olfactory receptor neurons by the guanylyl cyclase inhibitor LY83583.

1. Using whole cell voltage-clamp recordings, the guanylyl cyclase inhibitor LY83583 [6-(phenylamino)-5,8-quinolinedione] is shown to act as a potent blocker of cyclic nucleotide-gated (CNG) channels in isolated olfactory receptor neurons (ORNs) of the tiger salamander. 2. Under our experimental conditions, onset of the blockade by LY83583 occurs on the time scale of seconds and is completely reversed upon wash-out of the drug. Dose-response curves reveal a Kd of 1.4 microM (at -60 mV). Other data suggest that LY83583 acts within the CNG channel pore and that the channels must be in an activated state before the drug can exert its effect. 3. It appears that LY83583 can act on both CNG channels and soluble guanylyl cyclase (sGC) and that these two effects can be distinguished by their different recovery behaviors. The LY83583-induced blockade of CNG channels activated directly by guanosine 3',5' cyclic monophosphate (cGMP) is rapidly reversible (with a recovery time constant of approximately 3 s), whereas previous results have shown that no recovery is obtained during minute-long washing periods when the channels are activated indirectly through exogenous carbon monoxide application, which acts as a stimulator of sGC in ORNs. 4. LY83583 appears to be a novel and useful agent in examining neural functions due to CNG channel responses.

Ambystoma↗
PubMed1996

Receptive field of the retinal bipolar cell: a pharmacological study in the tiger salamander.

1. It is widely believed that signals contributing to the receptive field surrounds of retinal bipolar cells pass from horizontal cells to bipolar cells via GABAergic synapses. To test this notion, we applied gamma-aminobutyric acid (GABA) agonists and antagonists to isolated, perfused retinas of the salamander Ambystoma tigrinum while recording intracellularly from bipolar cells, horizontal cells, and photoreceptors. 2. As we previously reported, administration of the GABA analogue D-aminovaleric acid in concert with picrotoxin did not block horizontal cell responses or the center responses of bipolar cells but blocked the surround responses of both on-center and off-center bipolar cells. 3. Surround responses were not blocked by the GABA, antagonists picrotoxin or bicuculline, the GABAB agonist baclofen or the GABAB antagonist phaclofen, and the GABAC antagonists picrotoxin or cis-4-aminocrotonic acid. Combinations of these drugs were similarly ineffective. 4. GABA itself activated a powerful GABA uptake mechanism in horizontal cells for which nipecotic acid is a competitive agonist. It also activated, both in horizontal cells and bipolar cells, large GABAA conductances that shunted light responses but that could be blocked by picrotoxin or bicuculline. 5. GABA, administered together with picrotoxin to block the shunting effect of GABAA activation, did not eliminate bipolar cell surround responses at concentrations sufficient to saturate the known types of GABA receptors. 6. Surround responses were not blocked by glycine or its antagonist strychnine, or by combinations of drugs designed to eliminate GABAergic and glycinergic pathways simultaneously. 7. Although we cannot fully discount the involvement of a novel GABAergic synapse, the simplest explanation of our findings is that the primary pathway mediating the bipolar cell's surround is neither GABAergic nor glycinergic.

2-Amino-5-phosphonovalerate↗
PubMed1997

Impulse encoding mechanisms of ganglion cells in the tiger salamander retina.

A study of nerve impulse generation in ganglion cells of the tiger salamander retina is carried out through a combination of experimental and analytic approaches, including computer simulations based on a single-compartment model. Whole cell recordings from ganglion cells were obtained using a superfused retina-eyecup preparation and studied with pharmacological and electrophysiological techniques, including phase plot analysis. Experimental efforts were guided by computer simulation studies of an excitability model consisting of five voltage- or ion-gated channels, which were identified from earlier voltage-clamp data. The ion channels include sodium, calcium, and three types of potassium channels, namely the A type (IK,A), Ca-activated potassium (IK,Ca), and the delayed rectifier (IK). A leakage channel was included to preserve input resistance continuity between model and experiment. Ion channel densities of Na and Ca currents (INa and ICa) for the single-compartment model were independently determined from phase plot analysis. The IK and IK,A current densities were determined from the measured width of impulses. The IK,Ca was modeled to respond to Ca influx, and a variable-rate Ca-sequestering mechanism was implemented to remove cytoplasmic calcium. Impulse frequency increases when either ICa or IK,Ca is eliminated from the model or blocked pharmacologically in whole cell recording experiments. Faithful simulations of experimental data show that the ionic currents may be grouped into small (IK,Ca, leakage, and stimulus), and large (INa, IK, IA, ICa) on the basis of their peak magnitudes throughout the impulse train. This division of the currents is reflected in their function of controlling the interspike interval (small currents) and impulse generation (large currents). Although the single-compartmental model is qualitatively successful in simulating impulse frequency behavior and its controlling mechanisms, limitations were found that specifically suggest the need to include morphological details. The spike train analysis points to a role for electrotonic currents in the control of the duration of the interspike intervals, which can be compensated by prolonged activation of gK,Ca in the single-compartment model. A detailed, multicompartmental model of the ganglion cell is presented in the companion paper.

Animals↗
PubMed1998

Synchronous period-doubling in flicker vision of salamander and man.

Periodic flashes of light have long served to probe the temporal properties of the visual system. Here we show that during rapid flicker of high contrast and intensity the eye reports to the brain only every other flash of light. In this regime, retinal ganglion cells of the salamander fire spikes on alternating flashes. Neurons across the entire retina are locked to the same flashes. The effect depends sharply on contrast and flash frequency. It results from a period-doubling bifurcation in retinal processing, and a simple model of nonlinear feedback reproduces the phenomenon. Pharmacological studies indicate that the critical feedback interactions require only cone photoreceptors and bipolar cells. Analogous period-doubling is observed in the human visual system. Under bright full-field flicker, the electroretinogram (ERG) shows a regime of period-doubling between 30 and 70 Hz. In visual evoked potentials from the occiput, the subharmonic component is even stronger. By analyzing the accompanying perceptual effects, we find that retinal period-doubling begins in the periphery of the visual field, and that it is the cause of a long mysterious illusory flicker pattern.

Animals↗
PubMed1998

Characterization of spontaneous inhibitory synaptic currents in salamander retinal ganglion cells.

Spontaneous and light-evoked postsynaptic currents (sPSCs and lePSCs, respectively) in retinal ganglion cells of the larval tiger salamander were recorded under voltage-clamp conditions from living retinal slices. The focus of this study is to characterize the spontaneous inhibitory PSCs (sIPSCs) and their contribution to the light-evoked inhibitory PSCs (leIPSCs) in ON-OFF ganglion cells. sIPSCs were isolated from spontaneous excitatory PSCs (sEPSCs) by application of 10 microM 6,7-dinitroquinoxaline-2,3-dione (DNQX) + 50 microM 2-amino-5-phosphonopentanoic acid (AP5). In approximately 70% of ON-OFF ganglion cells, bicuculline (or picrotoxin) completely blocks sIPSCs, suggesting all sIPSCs in these cells are mediated by GABAergic synaptic vesicles and gamma-aminobutyric acid-A (GABAA) receptors (GABAergic sIPSCs, or GABAsIPSCs). In the remaining 30% of - ganglion cells, bicuculline (or picrotoxin) blocks 70-98% of the sIPSCs, and the remaining 2-30% are blocked by strychnine (glycinergic sIPSCs, or GLYsIPSCs). GABAsIPSCs occur randomly with an exponentially distributed interval probability density function, and they persist without noticeable rundown over time. The GABAsIPSC frequency is greatly reduced by cobalt, consistent with the idea that they are largely mediated by calcium-dependent vesicular release. GABAsIPSCs in DNQX + AP5 are tetrodotoxin (TTX) insensitive, suggesting that amacrine cells that release GABA under these conditions do not generate spontaneous action potentials. The average GABAsIPSCs exhibited linear current-voltage relation with a reversal potential near the chloride equilibrium potential, and an average peak conductance of 319.67 +/- 252.83 (SD) pS. For GLYsIPSCs, the average peak conductance increase is 301.68 +/- 94.34 pS. These parameters are of the same order of magnitude as those measured in inhibitory miniature postsynaptic currents (mIPSCs) associated with single synaptic vesicles in the CNS. The amplitude histograms of GABAsIPSCs did not exhibit multiple peaks, suggesting that the larger events are not discrete multiples of elementary events (or quanta). We propose that each GABAsIPSC or GLYsIPSC in retinal ganglion cells is mediated by a single or synchronized multiple of synaptic vesicles with variable neurotransmitter contents. In a sample of 16 ON-OFF ganglion cells, the average peak leIPSC (held at 0 mV) at the light onset is 509.0 +/- 233.85 pA and that at the light offset is 529.0 +/- 339.88 pA. The approximate number of GABAsIPSCs and GLYsIPSCs required to generate the average light responses, calculated by the ratio of the charge (area under current traces) of the leIPSCs to that of the average single sIPSCs, is 118 +/- 52 for the light onset, and 132 +/- 76 for the light offset.

Ambystoma↗
PubMed2000

Relationships between odor-elicited oscillations in the salamander olfactory epithelium and olfactory bulb.

Oscillations in neuronal population activity, or the synchronous neuronal spiking that underlies them, are thought to play a functional role in sensory processing in the CNS. In the olfactory system, stimulus-induced oscillations are observed both in central processing areas and in the peripheral receptor epithelium. To examine the relationship between these peripheral and central oscillations, we recorded local field potentials simultaneously from the olfactory epithelium and olfactory bulb in tiger salamanders (Ambystoma tigrinum). Stimulus-induced oscillations recorded at these two sites were matched in frequency and slowed concurrently over the time course of the response, suggesting that the oscillations share a common source or are modulated together. Both the power and duration of oscillations increased over a range of amyl acetate concentrations from 2.5 x 10(-2) to 1 x 10(-1) dilution of saturated vapor, but peak frequency was not affected. The frequency of the oscillation did vary with different odorant compounds in both olfactory epithelium and bulb (OE and OB): amyl acetate, ethyl fenchol and d-carvone elicited oscillations of significantly different frequencies, and there was no difference in OE and OB oscillation frequencies. No change in the power or frequency of OE oscillations was observed after sectioning the olfactory nerve, indicating that the OE oscillations have a peripheral source. Finally, application of 1.0 and 10 microM tetrodotoxin to the epithelium blocked OE oscillations in a dose-dependent and reversible manner, suggesting that peripheral olfactory oscillations are related to receptor neuron spiking.

Ambystoma↗
PubMed2000

I4AA-Sensitive chloride current contributes to the center light responses of bipolar cells in the tiger salamander retina.

Light-evoked currents in depolarizing and hyperpolarizing bipolar cells (DBCs and HBCs) were recorded under voltage-clamp conditions in living retinal slices of the larval tiger salamander. Responses to illumination at the center of the DBCs' and HBCs' receptive fields were mediated by two postsynaptic currents: DeltaI(C), a glutamate-gated cation current with a reversal potential near 0 mV, and DeltaI(Cl), a chloride current with a reversal potential near -60 mV. In DBCs DeltaI(C) was suppressed by L-2-amino-4-phosphonobutyric acid (L-AP4), and in HBCs it was suppressed by 6,7-dinitroquinoxaline-2,3-dione (DNQX). In both DBCs and HBCs DeltaI(Cl) was suppressed by imidazole-4-acetic acid (I4AA), a GABA receptor agonist and GABA(C) receptor antagonist. In all DBCs and HBCs examined, 10 microM I4AA eliminated DeltaI(Cl) and the light-evoked current became predominately mediated by DeltaI(C). The addition of 20 microM L-AP4 to the DBCs or 50 microM DNQX to HBCs completely abolished DeltaI(C). Focal application of glutamate at the inner plexiform layer elicited chloride currents in bipolar cells by depolarizing amacrine cells that release GABA at synapses on bipolar cell axon terminals, and such glutamate-induced chloride currents in DBCs and HBCs could be reversibly blocked by 10 microM I4AA. These experiments suggest that the light-evoked, I4AA-sensitive chloride currents (DeltaI(Cl)) in DBCs and HBCs are mediated by narrow field GABAergic amacrine cells that activate GABA(C) receptors on bipolar cell axon terminals. Picrotoxin (200 microM) or (1,2,5,6-tetrahydropyridine-4yl) methyphosphinic acid (TPMPA) (2 other GABA(C) receptor antagonists) did not block (but enhanced and broadened) the light-evoked DeltaI(Cl), although they decreased the chloride current induced by puff application of GABA or glutamate. The light response of narrow field amacrine cells were not affected by I4AA, but were substantially enhanced and broadened by picrotoxin. These results suggest that there are at least two types of GABA(C) receptors in bipolar cells: one exhibits stronger I4AA sensitivity than the other, but both can be partially blocked by picrotoxin. The GABA receptors in narrow field amacrine cells are I4AA insensitive and picrotoxin sensitive. The light-evoked DeltaI(Cl) in bipolar cells are mediated by the more strongly I4AA-sensitive GABA(C) receptors. Picrotoxin, although acting as a partial GABA(C) receptor antagonist in bipolar cells, does not suppress DeltaI(Cl) because its presynaptic effects on amacrine cell light responses override its antagonistic postsynaptic actions.

Ambystoma↗
PubMed2001

Effect of acute exposure to ammonia on glutamate transport in glial cells isolated from the salamander retina.

A rise of brain ammonia level, as occurs in liver failure, initially increases glutamate accumulation in neurons and glial cells. We investigated the effect of acute exposure to ammonia on glutamate transporter currents in whole cell clamped glial cells from the salamander retina. Ammonia potentiated the current evoked by a saturating concentration of L-glutamate, and decreased the apparent affinity of the transporter for glutamate. The potentiation had a Michaelis-Menten dependence on ammonia concentration, with a K(m) of 1.4 mM and a maximum potentiation of 31%. Ammonia also potentiated the transporter current produced by D-aspartate. Potentiation of the glutamate transport current was seen even with glutamine synthetase inhibited, so ammonia does not act by speeding glutamine synthesis, contrary to a suggestion in the literature. The potentiation was unchanged in the absence of Cl(-) ions, showing that it is not an effect on the anion current gated by the glutamate transporter. Ammonium ions were unable to substitute for Na+ in driving glutamate transport. Although they can partially substitute for K+ at the cation counter-transport site of the transporter, their occupancy of these sites would produce a potentiation of < 1%. Ammonium, and the weak bases methylamine and trimethylamine, increased the intracellular pH by similar amounts, and intracellular alkalinization is known to increase glutamate uptake. Methylamine and trimethylamine potentiated the uptake current by the amount expected from the known pH dependence of uptake, but ammonia gave a potentiation that was larger than could be explained by the pH change, and some potentiation of uptake by ammonia was still seen when the internal pH was 8.8, at which pH further alkalinization does not increase uptake. These data suggest that ammonia speeds glutamate uptake both by increasing cytoplasmic pH and by a separate effect on the glutamate transporter. Approximately two-thirds of the speeding is due to the pH change.

ATP-Binding Cassette Transporters↗
PubMed2002

Cytogenetic studies of Hynobiidae (Urodela)XVIII. A ZZ/ZW sex-determining mechanism in a hynobiid salamander species, Hynobius tokyoensis Tago.

The karyotype of Hynobius tokyoensis (2n = 56) was analyzed using three kinds of banding methods to determine the morphological differentiation of the sex chromosomes of this species. Salamanders and egg sacs were collected from seven localities around Tokyo, Japan. Of 28 chromosome pairs, microchromosome No. 21 was identified as a ZZ/ZW-type sex chromosome. The Z chromosome was acrocentric, whereas the W chromosome was submetacentric, with a heterochromatic, elongated short arm. Interestingly, the W chromosome is of three distinct types, W(A), W(B), and W(C), based on R-banding and Ag-NOR patterns. W(A) was detected in five populations from southern habitats, whereas W(B) and W(C) were detected in one population each from northern habitats. W(A), W(B), and W(C) were all found to carry Ag-NORs on their heterochromatic short arms. Considering the karyotypes of other species belonging to the same genus, we discuss the evolution of the sex chromosomes of H. tokyoensis.

Animals↗
PubMed1997

Tuning of electroreceptors in the blind cave salamander, Proteus anguinus L.

The blind cave salamander's (Proteus anguinus L.) sensitivity for electrical fields was determined by recordings from afferent fibers coming from ampullary electroreceptors. The animals were stimulated with single rectangular or continuous sinusoidal electrical signals passed through the cave water in the experimental tank. Best threshold sensitivities of the fibers for both kinds of signals were in the same range (0.1-0.5 mV/cm = 30-150 nA/cm2). The fibers showed phase-locked responses to rostrally cathodic phases of rectangular or sinusoidal stimuli and suppression to anodal phases. Units were tuned to 'best frequencies' (B.F., sine waves) between 20 and 40 Hz. Any functional, i.e. phase-related, firing or suppression of firing occurs in a total frequency range of less than 0.1 to around 100 Hz. This frequency range of tuning in the primary fibers was the same as found in the behavior, and this therefore reflects the peripheral sensory properties. Electrical sensitivity might be of special value for these cave dwelling animals, as they may detect and recognize prey animals by means of the prey's electrical field spreading into the surroundings, i.e. its electromyogram.

Afferent Pathways↗
PubMed1984

Quantitative analysis of salamander horizontal head nystagmus.

The horizontal optokinetic head nystagmus of salamanders (Salamandra salamandra) was investigated at various constant and sinusoidally modulated velocities. With constant velocities the best compensation (head velocity/stimulus velocity) measured was 0.6 at a stimulus velocity of 3 deg/s. With slow stimuli up to 5 deg/s, small oscillations in the slow phase of the nystagmus occurred. The nystagmus started with a delay of 1.1 s after onset of stimulus with an acceleration period of up to 6 s. After the stimulus terminated, the head was decelerated within (on average) 5 s, indicating a weak velocity storage element. The nystagmus frequency, the acceleration time, and the deceleration time were in correlation with the head velocity during the slow phase. With sinusoidal stimuli, nonlinearities were displayed: the gain was amplitude-dependent, and distortions of the sinusoidal head movements occurred with higher frequencies.

Animals↗
PubMed1984

A 'physiological' model for the salamander horizontal optokinetic reflex.

The salamander optokinetic head nystagmus can be explained by a comparatively simple negative feedback loop with few operational units within the loop. These units may easily be represented by properties of neural elements. In particular, the retinal slip velocity is transformed into an internal excitation by a velocity transfer function derived from pretectal recordings. A velocity storage element with input-output comparator is proposed as a recurrent inhibition network. A threshold element sums up the neuronal thresholds of the units involved. An internal gain factor may be interpreted as the transformation of the neural signal to a motor output. This output occurs with a stimulus-independent delay of 1 s, which is considered to be the sum of all conduction delays in the system. The model produces an output that is in excellent conformity with the optokinetic behavior. Additionally, the model predicts behavioral responses to optokinetic stimulation with extended stimulus ranges or with central lesions.

Animals↗
PubMed2004

Chloride equilibrium potential in salamander cones.

BACKGROUND: GABAergic inhibition and effects of intracellular chloride ions on calcium channel activity have been proposed to regulate neurotransmission from photoreceptors. To assess the impact of these and other chloride-dependent mechanisms on release from cones, the chloride equilibrium potential (ECl) was determined in red-sensitive, large single cones from the tiger salamander retinal slice. RESULTS: Whole cell recordings were done using gramicidin perforated patch techniques to maintain endogenous Cl- levels. Membrane potentials were corrected for liquid junction potentials. Cone resting potentials were found to average -46 mV. To measure ECl, we applied long depolarizing steps to activate the calcium-activated chloride current (ICl(Ca)) and then determined the reversal potential for the current component that was inhibited by the Cl- channel blocker, niflumic acid. With this method, ECl was found to average -46 mV. In a complementary approach, we used a Cl-sensitive dye, MEQ, to measure the Cl- flux produced by depolarization with elevated concentrations of K+. The membrane potentials produced by the various high K+ solutions were measured in separate current clamp experiments. Consistent with electrophysiological experiments, MEQ fluorescence measurements indicated that ECl was below -36 mV. CONCLUSIONS: The results of this study indicate that ECl is close to the dark resting potential. This will minimize the impact of chloride-dependent presynaptic mechanisms in cone terminals involving GABAa receptors, glutamate transporters and ICl(Ca).

Ambystoma↗
PubMed2006

Male pheromone protein components activate female vomeronasal neurons in the salamander Plethodon shermani.

BACKGROUND: The mental gland pheromone of male Plethodon salamanders contains two main protein components: a 22 kDa protein named Plethodon Receptivity Factor (PRF) and a 7 kDa protein named Plethodon Modulating Factor (PMF), respectively. Each protein component individually has opposing effects on female courtship behavior, with PRF shortening and PMF lengthening courtship. In this study, we test the hypothesis that PRF or PMF individually activate vomeronasal neurons. The agmatine-uptake technique was used to visualize chemosensory neurons that were activated by each protein component individually. RESULTS: Vomeronasal neurons exposed to agmatine in saline did not demonstrate significant labeling. However, a population of vomeronasal neurons was labeled following exposure to either PRF or PMF. When expressed as a percent of control level labeled cells, PRF labeled more neurons than did PMF. These percentages for PRF and PMF, added together, parallel the percentage of labeled vomeronasal neurons when females are exposed to the whole pheromone. CONCLUSION: This study suggests that two specific populations of female vomeronasal neurons are responsible for responding to each of the two components of the male pheromone mixture. These two neural populations, therefore, could express different receptors which, in turn, transmit different information to the brain, thus accounting for the different female behavior elicited by each pheromone component.

Animals↗
PubMed2005

Temporal response of the tiger salamander (Ambystoma tigrinum) to 3,000 years of climatic variation.

BACKGROUND: Amphibians are sensitive indicators of environmental conditions and show measurable responses, such as changes in phenology, abundance and range limits to local changes in precipitation and temperature regimes. Amphibians offer unique opportunities to study the important ecological and evolutionary implications of responses in life history characteristics to climatic change. We analyzed a late-Holocene fossil record of the Tiger Salamander (Ambystoma tigrinum) for evidence of population-level changes in body size and paedomorphosis to climatic change over the last 3000 years. RESULTS: We found a significant difference in body size index between paedomorphic and metamorphic individuals during the time interval dominated by the Medieval Warm Period. There is a consistent ratio of paedomorphic to metamorphic specimens through the entire 3000 years, demonstrating that not all life history characteristics of the population were significantly altered by changes in climate on this timescale. CONCLUSION: The fossil record of Ambystoma tigrinum we used spans an ecologically relevant timescale appropriate for understanding population and community response to projected climatic change. The population-level responses we documented are concordant with expectations based on modern environmental studies, and yield insight into population-level patterns across hundreds of generations, especially the independence of different life history characteristics. These conclusions lead us to offer general predictions about the future response of this species based on likely scenarios of climatic warming in the Rocky Mountain region.

Animals↗
PubMed2006

Streambed microstructure predicts evolution of development and life history mode in the plethodontid salamander Eurycea tynerensis.

BACKGROUND: Habitat variation strongly influences the evolution of developmentally flexible traits, and may drive speciation and diversification. The plethodontid salamander Eurycea tynerensis is endemic to the geologically diverse Ozark Plateau of south-central North America, and comprises both strictly aquatic paedomorphic populations (achieving reproductive maturity while remaining in the larval form) and more terrestrial metamorphic populations. The switch between developmental modes has occurred many times, but populations typically exhibit a single life history mode. This unique system offers an opportunity to study the specific ecological circumstances under which alternate developmental and life history modes evolve. We use phylogenetic independent contrasts to test for relationships between a key microhabitat feature (streambed sediment) and this major life history polymorphism. RESULTS: We find streambed microstructure (sediment particle size, type and degree of sorting) to be highly correlated with life-history mode. Eurycea tynerensis is paedomorphic in streams containing large chert gravel, but metamorphoses in nearby streams containing poorly sorted, clastic material such as sandstone or siltstone. CONCLUSION: Deposits of large chert gravel create loosely associated streambeds, which provide access to subsurface water during dry summer months. Conversely, streambeds composed of more densely packed sandstone and siltstone sediments leave no subterranean refuge when surface water dries, presumably necessitating metamorphosis and use of terrestrial habitats. This represents a clear example of the relationship between microhabitat structure and evolution of a major developmental and life history trait, and has broad implications for the role of localized ecological conditions on larger-scale evolutionary processes.

Animals↗
PubMed2006

The role of thermal niche selection in maintenance of a colour polymorphism in redback salamanders (Plethodon cinereus).

BACKGROUND: In eastern North America two common colour morphs exist in most populations of redback salamanders (Plethodon cinereus). Previous studies have indicated that the different morphs may be adapted to different thermal niches and the morphological variation has been linked to standard metabolic rate at 15 degrees C in one population of P. cinereus. It has therefore been hypothesized that a correlated response to selection on metabolic rate across thermal niches maintains the colour polymorphism in P. cinereus. This study tests that hypothesis. RESULTS: We found that the two colour morphs do sometimes differ in their maintenance metabolic rate (MMR) profiles, but that the pattern is not consistent across populations or seasons. We also found that when MMR profiles differ between morphs those differences do not indicate that distinct niches exist. Field censuses showed that the two colour morphs are sometimes found at different substrate temperatures and that this difference is also dependent on census location and season. CONCLUSION: While these morphs sometimes differ in their maintenance energy expenditures, the differences in MMR profile in this study are not consistent with maintenance of the polymorphism via a simple correlated response to selection across multiple niches. When present, differences in MMR profile do not indicate the existence of multiple thermal niches that consistently mirror colour polymorphism. We suggest that while a relationship between colour morph and thermal niche selection appears to exist it is neither simple nor consistent.

Journal Article↗
PubMed2001

Apoptotic cell death of cultured salamander photoreceptors induced by cccp: CsA-insensitive mitochondrial permeability transition.

Photoreceptor degeneration is mediated by apoptosis in several animal models, although the underlying mechanisms are yet to be elucidated. We present here an apoptotic model based on a primary cell culture of tiger salamander photoreceptors, in which treatment with carbonyl cyanide m-chlorophenylhydrazone (cccp), a protonophore, induced apoptosis. Cells exposed to cccp showed condensed nuclei and displayed positive TdT-dUTP terminal nick-end labeling (TUNEL). In addition, 10-100 microM cccp rapidly induced a reduction of Delta psi(m) and > or = 30 microM cccp induced a significant leakage of calcein from mitochondria to cytosol and nucleus, indicating a change in mitochondrial inner membrane permeability. Cyclosporin A (CsA), a transition pore blocker, did not prevent the cccp-induced MPT or the cccp-evoked apoptotic cell death, suggesting that cccp-induced apoptotic process was mediated by a CsA-insensitive pathway. This cell model provides an in vitro tool for studying mechanisms of photoreceptor apoptosis in isolated photoreceptors and may provide clues to the etiology of retinal degeneration.

Animals↗