Maintenance of adenosine A1 receptor function during long-term anoxia in the turtle brain.
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It has been established that adenosine has a critical role in the extraordinary ability of the turtle brain to survive anoxia. To further investigate this phenomenon we compared rat and turtle brain adenosine A1 receptors using cyclopentyl-1,3-dipropylxanthine, 8-[dipropyl-2,3-3H(N)] ([3H]DPCPX) saturation binding analyses and determined the effects of prolonged anoxia (6, 12, and 24 h) on the adenosine A1 receptor of the turtle brain. The rat brain had a 10-fold greater density of A1 receptors compared with the turtle [rat cortex receptor density (Bmax) = 1,400 +/- 134.6 fmol/mg protein, turtle forebrain Bmax = 103.2 +/- 4.60 fmol/mg protein] and a higher affinity [dissociation constant (Kd) rat cortex = 0.328 +/- 0.035 nM, Kd turtle forebrain = 1.16 +/- 0.06 nM]. However, the turtle Kd is within the reported mammalian range, and the Bmax is similar to that reported for other poikilotherms. Unlike the mammal, in which A1 receptor function is rapidly compromised in anoxia, in the turtle forebrain no significant changes in the A1 receptor population were seen during 24-h anoxia. However, in the hindbrain, whereas the Bmax remained unchanged, the Kd significantly decreased from 2.1 to 0.5 nM after 6 h anoxia and this higher affinity was maintained at 12- and 24-h anoxia. These findings indicate that, unlike the GABAA receptor, the protective effectiveness of adenosine in the anoxic turtle brain is not related to an enhanced receptor number. Protection from a hypoxia-induced compromise in A1 receptor function and an increased A1 sensitivity in the hindbrain may be important factors for maintaining the adenosine-mediated downregulation of energy demand during long-term anoxia. (+info)
Properties of conditioned abducens nerve responses in a highly reduced in vitro brain stem preparation from the turtle.
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Previous work suggested that the cerebellum and red nucleus are not necessary for the acquisition, extinction, and reacquistion of the in vitro classically conditioned abducens nerve response in the turtle. These findings are extended in the present study by obtaining conditioned responses (CRs) in preparations that received a partial ablation of the brain stem circuitry. In addition to removing all tissue rostral to and including the midbrain and cerebellum, a transection was made just caudal to the emergence of the IXth nerve. Such ablations result in a 4-mm-thick section of brain stem tissue that functionally eliminates the sustained component of the unconditioned response (UR) while leaving only a phasic component. We refer to this region of brain stem tissue caudal to the IXth nerve as the "caudal premotor blink region." Neural discharge was recorded from the abducens nerve following a single shock unconditioned stimulus (US) applied to the ipsilateral trigeminal nerve. When the US was paired with a conditioned stimulus (CS) applied to the posterior eighth, or auditory, nerve using a delay conditioning paradigm, a positive slope of CR acquisition was recorded in the abducens nerve, and CR extinction was recorded when the stimuli were alternated. Resumption of paired stimuli resulted in reacquisition. Quantitative analysis of the CRs in preparations in which the caudal premotor blink region had been removed and those with cerebellar/red nucleus lesions showed that both types of preparations had abnormally short latency CR onsets compared with preparations in which these regions were intact. Preparations with brain stem transections had significantly earlier CR offsets as more CRs terminated as short bursts when compared with intact or cerebellar lesioned preparations. These data suggest that a highly reduced in vitro brain stem preparation from the turtle can be classically conditioned. Furthermore, the caudal brain stem is not a site of acquisition in this reduced preparation, but it contributes to the sustained activity of both the UR and CR. Finally, the unusually short CR onset latencies following lesions to the cerebellum are not further exacerbated by removal of the caudal brain stem. These studies suggest that convergence of CS and US synaptic inputs onto the abducens nerve reflex circuitry may underlie acquisition in this reduced preparation, but that mechanisms that control learned CR timing arise from the cerebellorubral system. (+info)
Spatial properties of horizontal cell responses in the turtle retina.
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1. Intracellular recordings were made from horizontal cells in the retina of the turtle Pseudemys scripta elegans. Spatial properties of the responses were determined using brief flashes of monochromatic light. 2. For a light stimulus in the form of a long narrow slit the peak response decayed approximately exponentially with displacement from the centred position. 3. With variation in the area of a centred circular patch, the peak response increased in a graded manner with stimulus area but was not proportional to area. 4. The model of electrical coupling in the horizontal cell layer proposed by Naka & Rushton (1976) was applied to the results. For the case of dim illumination a simplification is applicable, and the voltage distribution for circular and slit-shaped patches of light can be expressed in terms of two unknowns: the voltage resulting from diffuse illumination and a characteristic 'length constant'. 5. The measured variation of response amplitude was well described by the theory. Measured length constants were distributed from less than 100 mum to greater than 1 mm, and in a given cell the values determined by the slit displacement method and the area variation method were in reasonable agreement. 6. It is concluded that with dim illumination the model provides an accurate description of the voltage spread in the cells. Deviations were found to occur at higher intensities and possible reasons are discussed. 7. The implications of the model on the measurement of resistance changes during illunination are discussed. (+info)
The relation between intercellular coupling and electrical noise in turtle photoreceptors.
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1. Intracellular recordings from cones and rods in the retina of the turtle, Pseudemys scripta elegans, revealed that in darkness the cell voltage fluctuated spontaneously about its mean level. The fluctuations were reduced during bright steady illmination of the cell often to a level close to that obtained with the electrode outside the cell where the noise did not change significantly during illumination. 2. The magnitude of the intrinsic dark noise (voltage variance in darkness minus voltage variance in strong light) varied widely from cell to cell. In the noisiest cones it was about 0-4 mV2 while in quiet cones it was often as low as 0-01 mV2. The noise appeared radom and could be fitted by a Gaussian probability density function. 3. The spread of voltage in the network of coupled photoreceptors was estimated by measuring the spatial profile of the response to a brief flash of constant intensity moved across the retina. For a light stimulus in the form of a long narrow slit, the peak flash response usually decayed exponentially with displacement from the centred position. 4. For maximum responses less than about 5 mV in cones, the length constant of exponential decay, lambda, varied from less than 10 mum to greater than 35 mum, and the values obtained in opposite directions were often unequal. Background illumination did not significantly change lambda. In cells with extremely narrow spatial profiles, an exponential fit to the decay could not be made reliably. 5. Occasionally the spatial profiles had definite secondary peaks. In the most pronounced examples in a red-sensitive cone and in a rod the maxima were separated by about 20 and 50 mum respectively; for each, one peak was approximately as sharp as the optical stimulator while the second was broader. 6. Cones with short length constants displayed high dark noise while cones with long length constants were relatively quiet. 7. Three models of electrical coupling between cells were investigated: one based on a distributed network, one on a discrete square grid arrangement, and one on a discrete hexagonal array. Each model predicts a strong dependence of both noise and input resistance on length constant, and for tightly coupled cells each predicts that voltage variance is proportional to lambda-2. 8. The measured relationship between voltage variance and lambda in a large sample of cones was well described by both discrete models when the average cell spacing was taken to be approximately 15 mum. 9... (+info)
Extracellular recordings were obtained from the ganglion cell (GC) layer during correlated spontaneous bursting activity (SBA) in the immature turtle retina. Pharmacological agents were bath-applied, and their effects on burst and correlation parameters were determined. SBA requires synaptic transmission. It was blocked in the presence of curare and mecamylamine, two cholinergic nicotinic antagonists, and enhanced with neostigmine, a cholinesterase inhibitor. SBA was profoundly inhibited during blockade of glutamatergic receptors with the broad spectrum antagonist kynurenate and it vanished with 6,7-dinitroquinoxaline-2-3-dione (DNQX) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), two AMPA/kainate receptor antagonists. Blockade of NMDA receptors with D(-)-2-amino-5-phosphonopentanoic acid (D-AP-5) led only to a modest reduction in SBA. Blockade of GABAA receptors with bicuculline prolonged the duration of the bursts. Inhibition of GABA uptake with nipecotic acid led to a decrease in burst rate. Blockade of K+ channels with cesium (Cs+) and tetraethylammonium (TEA) led to a dramatic decrease in excitability. Burst propagation between neighboring GCs was reduced by K+ channel blockade. Gap junction blockade had no consistent effect on bursts or correlation parameters. None of these drugs had a strong effect on the refractory period between bursts. We conclude that correlated SBA in immature turtle GCs requires both cholinergic nicotinic and glutamatergic (mainly through AMPA/kainate receptors) synaptic transmission. GABAergic activity modulates the intensity and the duration of the bursts. Extracellular K+ is involved in lateral activity propagation and increases retinal excitability, which may be required for burst generation. (+info)
Modal behavior of cortical neural networks during visual processing.
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The network behavior of cortical cells during the processing of a light flash was characterized in an isolated, but functionally intact, turtle visual system. Rapid changes in intracellular membrane potential were monitored optically using a voltage-sensitive dye (VSD). Spatially coherent changes in membrane potential were determined by subjecting high-speed movies of the VSD signals to Karhunen-Loeve decomposition. In all experimental trials analyzed (n > 50), coherent activity was restricted to a small number of similar spatial patterns or modes. At least four modes (M(1,1), M(1,2), M(2,1), and M(2,2)) have an organizational structure similar to the normal modes of a vibrating membrane (drum). This empirical observation of modal activity provides a useful framework for analyzing the macroscopic behavior of cortical networks. (+info)
Upper respiratory tract disease in the gopher tortoise is caused by Mycoplasma agassizii.
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Upper respiratory tract disease (URTD) has been observed in a number of tortoise species, including the desert tortoise (Gopherus agassizii) and the gopher tortoise (Gopherus polyphemus). Clinical signs of URTD in gopher tortoises are similar to those in desert tortoises and include serous, mucoid, or purulent discharge from the nares, excessive tearing to purulent ocular discharge, conjunctivitis, and edema of the eyelids and ocular glands. The objectives of the present study were to determine if Mycoplasma agassizii was an etiologic agent of URTD in the gopher tortoise and to determine the clinical course of the experimental infection in a dose-response infection study. Tortoises were inoculated intranasally with 0.5 ml (0.25 ml/nostril) of either sterile SP4 broth (control group; n = 10) or 10(8) color-changing units (CCU) (total dose) of M. agassizii 723 (experimental infection group; n = 9). M. agassizii caused clinical signs compatible with those observed in tortoises with natural infections. Clinical signs of URTD were evident in seven of nine experimentally infected tortoises by 4 weeks postinfection (p.i.) and in eight of nine experimentally infected tortoises by 8 weeks p.i. In the dose-response experiments, tortoises were inoculated intranasally with a low (10(1) CCU; n = 6), medium (10(3) CCU; n = 6), or high (10(5) CCU; n = 5) dose of M. agassizii 723 or with sterile SP4 broth (n = 10). At all time points p.i. in both experiments, M. agassizii could be isolated from the nares of at least 50% of the tortoises. All of the experimentally infected tortoises seroconverted, and levels of antibody were statistically higher in infected animals than in control animals for all time points of >4 weeks p.i. (P < 0.0001). Control tortoises in both experiments did not show clinical signs, did not seroconvert, and did not have detectable M. agassizii by either culture or PCR at any point in the study. Histological lesions were compatible with those observed in tortoises with natural infections. The numbers of M. agassizii 723 did not influence the clinical expression of URTD or the antibody response, suggesting that the strain chosen for these studies was highly virulent. On the basis of the results of the transmission studies, we conclude that M. agassizii is an etiologic agent of URTD in the gopher tortoise. (+info)
Endothelial nitric oxide synthase (eNOS) is localized to Muller cells in all vertebrate retinas.
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The distribution of endothelial nitric oxide synthase immunoreactivity (eNOS-IR) was investigated in the retinas of all phylogenetic vertebrate classes by using a monoclonal eNOS antibody. Confocal light microscopy showed immunoreactive labeling in Muller cells of fish, frog, salamander, turtle, chicken, rat, ground squirrel, and monkey retina. In vascularized retinas (rat, monkey), astrocytes and some blood vessels were also stained. Furthermore, eNOS-IR was localized to axon terminals of turtle and fish horizontal cells. These observations are the first to show the presence of eNOS-IR in Muller glia and horizontal cell structures of the vertebrate retina. (+info)