Pigmented Rats Research Articles

Threshold shifts and enhancement of cortical evoked responses after noise exposure in rats

The effect of exposure to various types of noise (broadband, high-frequency or low-frequency) was studied in adult pigmented rats. Thresholds and amplitudes of middle latency responses (MLR) recorded from electrodes implanted on the surface of the auditory cortex were analyzed before and after noise exposure. Exposure to noise with intensities ranging from 105 to 120 dB for 1 h produced only temporary threshold shifts (TTS). Exposure to broadband noise produced TTS throughout the whole frequency range of the rat’s hearing, mostly expressed at frequencies of maximal hearing sensitivity (16–32 kHz). Hearing loss produced by high- or low-frequency noise exposure was related to the spectral characteristics of the noise. The exposure to high-intensity noise may also result in amplitude enhancement of the MLR. This phenomenon was seen mainly after broadband noise exposure and occurred in response to both low-frequency and high-frequency test stimuli. High-frequency and low-frequency noise produced amplitude enhancement mainly at frequencies which corresponded to the maximum exposure energy. In contrast to the relatively similar values of TTS obtained in different rats under the same conditions of noise exposure, great inter-individual variability was found in the MLR amplitude enhancement. In all rats the dynamics of recovery functions for amplitude enhancement were different from those for MLR thresholds. The data indicate that whereas post-exposure TTS are related to peripheral changes, the post-exposure MLR amplitude enhancement is most probably connected with a change in the processing of auditory information in the central nervous system.

Fluorescence Properties of a Hydrophilic Sensitizer in Pigmented Rats, Rabbits, and Monkeys

To evaluate fluorescence properties of mono-L-aspartyl chlorin e6 (NPe6; Meija Seika Kaisha, Ltd., Tokyo, Japan) photodynamic therapy, which allows real-time simultaneous imaging of choroidal and retinal vasculature during treatment without the addition of another dye. Four pigmented rabbits, 4 pigmented rats, and 2 African green monkeys were administered intravenous injections of the NPe6 dye. The animals were immediately placed in front of the scanning laser ophthalmoscope and the fundus was viewed with the helium-neon laser. The resulting fluorescence was viewed and recorded on super-VHS videotape. Fluorescence demonstrated clearly that NPe6 entered the retinal and choroidal circulation within seconds of intravenous injection. The concentration of NPe6 was diminished over a period of 1.5 hours in the monkey and 5 hours in the rat, as evidenced by considerable diminution of the intensity of fluorescence. NPe6 fluorescence allows evaluation of drug availability within the retinal and choroidal circulation and visualization of pathological lesions before commencement of photodynamic therapy.

Influence of UVA Light Stress on Photoreceptor Cell Metabolism: Decreased Rates of Rhodopsin Regeneration and Opsin Synthesis

There is considerable evidence indicating that rhodopsin is the chromophore mediating light damage to the rat retina caused by exposure to mid-visible wavelengths. Retinal damage is, however, more effectively produced by short-wavelength light, and little is known about the initiating events for this damage class. The present study sought to determine the involvement of rhodopsin bleaching in short-wavelength damage by examining rhodopsin levels and opsin synthesis at early time points following acute ultraviolet-A (UVA) exposures of the pigmented rat eye. A gradual decline in rhodopsin to 8% of the level in non-exposed control eyes occurred over a 1 hr exposure to 1500 μW cm-2of UVA light. When animals were placed in darkness following this exposure, rhodopsin had recovered to only 27% of control levels by 2 hr post-exposure indicating a very slow rate of regeneration. For later time points, animals were returned to dim cyclic light and by 2 days following exposure, rhodopsin levels had risen to 57% of control. In contrast, opsin levels at this same time point were unaffected by UVA exposure. Other observations indicating the UVA exposure affected photoreceptor cell metabolism included a 27% decrease in the rate of opsin synthesis between 1 and 2 days following exposure, and a 69% reduction in the rate of rod outer segment disk renewal during the initial 3 days following exposure.These data show that UVA light stress in the retina causes a gradual bleaching of rhodopsin followed by a slow rate of recovery and altered photoreceptor cell metabolism. These results are consistent with the concept that rhodopsin mediates UVA-induced retinal damage and the possible mechanisms by which this might occur are discussed in relation to alternative hypotheses currently in the literature.

Temporal sequence of changes in rat retina after UV-A and blue light exposure

Two spectral types of retinal light damage were induced in pigmented rats by irradiating small retinal patches at either 380 or 470 nm. The temporal sequence of changes in the retina was followed for up to 2 months by funduscopy and histology. For both damage types, fundus changes were best visible after 3 days. Histology showed that 380 nm specifically damaged photoreceptor cells, particularly the rods. All cell compartments of the rods, including the nucleus were affected already after 3 h. In the next days, damaged rods degenerated. At high doses (2.5×the funduscopic threshold dose) all rods in the irradiated area were lost, resulting in a local photoreceptor lesion, which was still present at 2 months after the irradiation. At 470 nm, damage occurred both in the photoreceptor layer and in the pigment epithelium. Acute changes, at 1 h after irradiation, consisted mainly of damaged mitochondria in these layers. Next, the pigment epithelium showed swelling, an altered melanin distribution and, at high doses (2.5×threshold), interruptions of the monolayer. Degeneration of photoreceptor cells was initially limited to a few scattered cells, but 3 days after high doses focal areas of massive degeneration were seen. At late stages, the cells of the pigment epithelium recovered and the photoreceptor layer showed a loss of cells. The results show that the spectral damage types are distinct in the early phases, indicating that different mechanisms are involved. Yet, the end effect of both damage types after exposure at doses up to 2.5×the funduscopic threshold is remarkably similar and consists of local photoreceptor lesions.

Fluoxetine-induced plasticity in the rodent visual system

We studied the effect of fluoxetine, a selective serotonin reuptake inhibitor, in the development and lesion-induced plasticity of retinotectal axons in pigmented rats. Neonatal rats received a daily injection of either fluoxetine or vehicle from postnatal day 1 (PND 1) to PND 10 or from PND 14 to PND 28 (fluoxetine, 7.5 and 10.0 mg/kg, respectively). In the latter group, some animals received a single lesion at the temporal periphery of the left retina at PND 21. Unoperated animals were use as the control. At the end of the treatment, the animals received an intraocular injection of horseradish peroxidase (HRP) in the right (intact) eye to trace the uncrossed retinotectal pathway. Chronic fluoxetine treatment, induced, in unoperated rats, an expansion of the retinal terminal fields along the rostro-caudal axis of the tectum both in the PND 10 and PND 28 groups. Following a retinal lesion in the left eye at PND 21, the vehicle-treated group showed a small reorganization of the intact uncrossed projection. In this group only a few terminals were labeled invading the denervated tectal surface one-week after the lesion. Fluoxetine-treated animals on the other hand, showed a great amplification of plasticity with a conspicuous sprouting of the uncrossed retinal axons into denervated areas. The data suggest that fluoxetine induces extensive axonal rearrangements in neonatal and juvenile central nervous system and amplifies neuroplasticity following retinal lesions late in development.

Processes of Blue Light–Induced Damage to Retinal Pigment Epithelial Cells Lacking Phagosomes

To experimentally clarify the processes of the changes induced by blue light directly on the retinal pigment epithelium (RPE) before the formation of phagosomes or the accumulation of lipofuscin. Methods: We developed a new experimental method in which primary cultured cells of very young pigmented rats were exposed to several intensities and durations of blue light (wavelength = 440 ± 10 nm). Results: At 1.0 mW/cm 2, the damage was limited to mitochondria. At 2.0 mW/cm 2, the cytoplasm exhibited large whorls of membrane or whorled inclusions, which were consistent with autophagic vacuoles. At 4.0 mW/cm 2, the RPE cells showed lysis of the cytoplasm and a nucleus that was consistent with necrosis. Conclusions: Our results suggested that damage induced by blue light to cultured RPE cells may originate in the mitochondria and end in necrosis. The type of cell death induced in the RPE by blue light seems to be determined mainly by the intensity of the light, but is also related to the duration of exposure.

Rod photoreceptor maturation does not vary with retinal eccentricity in mammalian retina

Purpose. Test the hypothesis that the development of mammalian rod outer segments (ROS) varies with retinal eccentricity. Methods. During the period of photoreceptor cell development, ROS lengths, opsin mRNA and (rhod)opsin were measured in central and peripheral retina of cows and pigmented rats. Published ROS length and/or rhodopsin data from albino rats, cows and monkeys were re-analyzed. Logistic growth curves were fitted to the newly obtained and published data. Within a species, growth in central and peripheral regions was compared. Results. The logistic growth curves fit all the data well and provide an excellent view of the developmental increases in ROS length, opsin mRNA and (rhod)opsin in each retinal region. Within a species, the growth curves for ROS length, opsin mRNA and (rhod)opsin concentration are superimposable. The age at which ROS length reaches 50% of its adult value is invariant with eccentricity. An exception to this pattern is the simian parafoveal ROS, which appears to have a delayed course of development. Conclusions. The hypothesis is disproved. Unlike rod photoreceptor cell genesis, ROS development is invariant with retinal eccentricity. Primate parafoveal ROS appear to have a different pattern of development.

Expression of vascular endothelial growth factor and its receptor (KDR/flk-1) mRNA in experimental choroidal neovascularization

Purpose. Vascular endothelial growth factor (VEGF) is an angiogenic peptide that has been suggested to be important in the pathogenesis of choroidal neovascularization. We investigated the transcription of VEGF and its receptor KDR/flk-1 genes during the development of experimentally induced choroidal neovascularization. Methods. Rat VEGF or KDR cDNA was inserted in PGEM or pBluescript to prepare antisense or sense riboprobes. Multiple krypton laser burns were applied to the posterior pole of pigmented rat eyes according to a previously described protocol which produces choroidal neovascularization. At intervals of up to 4 weeks after photocoagulation, the eyes were removed and cut into thin sections. The sections were subjected to in situ hybridization with digoxigenin (DIG)-labeled single-strand rat VEGF and KDR cDNA riboprobes. Results. In normal adult rat retinas, VEGF and KDR mRNA expression was mainly observed in the ganglion cell and the inner nuclear layers. During the development of neovascularization, VEGF and KDR mRNAs were detected in retinal pigment epithelial-like cells, fibroblast-like cells and endothelial cells in neovascular lesions. The level of expression was strongest at 1 week after photocoagulation in lasered lesions, and decreased by 4 weeks after photocoagulation. Conclusions. Our findings demonstrate that expression of VEGF and its receptor KDR may play a role in the formation of experimentally induced choroidal neovascularization. In this study, VEGF and its receptor were co-localized, suggesting that an autocrine and/or paracrine mechanism may be operative.

Phosphorothioate oligonucleotides induction into experimental choroidal neovascularization by HVJ-liposome system

Purpose. The purpose of this study was to determine whether the inactivated hemagglutinating virus of Japan (HVJ)-liposome method can induce phosphorothioate oligonucleotides effectively into an experimentally-induced choroidal neovascularization of rats. We also examined whether anti-sense phosphorothioate oligonucleotides against VEGF could be induced into choroidal neovascularization as a therapeutic agent by the HVJ-liposome method. Methods. The experiments were conducted on a rat model of choroidal neovascularization. FITC-labeled phosphorothioate oligonucleotides were coencapsulated in liposomes. The liposomes were coated with the envelope of inactivated HVJ and injected into the vitreous cavity following photocoagulation of pigmented rat eyes. The eyes were removed following injection, fixed, frozen and cut into thin sections. Induction of oligonucleotides was observed under a laser confocal scanning microscope for fluorescence and the development of choroidal neovascularization was evaluated histopathologically. Results. Phosphorothioate oligonucleotides were effectively induced into ganglion cells and into the cells of the choroidal neovascularization induced by laser photocoagulation. Highly effective induction of oligos was observed 3 to 14 days after intravitreal injection of HVJ-liposomes after which the level decreased. Antisense oligonucleotides against VEGF were induced specifically into cells in the choroidal neovascularization, however neovascularization was still observed. Conclusions. Phosphorothioate oligonucleotides can be effectively induced into ganglion cells, and specifically into cells in choroidal neovascularization. Although antisense oligonucleotides against VEGF failed to prevent choroidal neovascularization, the HVJ-liposome method provided a highly effective means of inducing antisense oligos for in vivo antisense therapy.

Spatial Sound Detection and the Role of the Inferior Colliculus in the Long-Evans Rat

The ability of Long-Evans hooded rats (n=10) to detect sounds presented from sources in the horizontal plane at 0° elevation and the effects of bilateral lesions of the inferior colliculus on these abilities were examined. Rats were trained on a directional detection task which required animals to suppress licking responses in a conditioned avoidance paradigm when 100-ms noise bursts were presented at random from speakers at 45° intervals beginning at azimuth (0°). A task performance rate was determined by reducing the correct lick suppression rate for signal trials by the proportion of incorrect suppression responses on non-signal trials. Higher performance rates were observed for stimuli presented from 0-90° than for stimuli presented in the caudal hemifield prior to surgical procedures. Bilateral lesions restricted to the inferior colliculus reduced detection performance (p<0.05) and shifted the best performance rates from sounds presented at 0-45° to stimuli emitted from a 90° source (p<0.05). These results demonstrate that pigmented rats show differential detection levels for noise bursts presented from different locations throughout the horizontal interaural plane, and suggest that the inferior colliculus is involved in this aspect of directional hearing.

Expression of cell adhesion molecules and vascular endothelial growth factor in experimental choroidal neovascularisation in the rat

AIMSTo investigate the longevity and reproducibility of choroidal neovascularisation (CNV) induced by krypton laser photocoagulation in the rat. The presence of cell adhesion molecules (CAMs) and vascular endothelial growth factor...

Tacrolimus (FK506) Attenuates Leukocyte Accumulation After Transient Retinal Ischemia

Tacrolimus, an immunosuppressant agent, has been shown to reduce tissue injury and leukocyte accumulation after transient ischemia. This study was designed to evaluate quantitatively the inhibitory effects of tacrolimus on leukocyte rolling and on subsequent leukocyte accumulation in vivo after transient retinal ischemia and the protective effects of tacrolimus on ischemia-induced neural damage. Retinal ischemia was induced for 60 minutes in anesthetized pigmented rats by temporary ligation of the optic sheath. Tacrolimus was administered at 10 minutes after ischemic induction. At 4, 12, 24, and 48 hours after reperfusion, leukocyte behavior in the retinal microcirculation was evaluated in vivo with acridine orange digital fluorography. After 7 days of reperfusion, ischemia-induced retinal damage was evaluated histologically. Treatment with tacrolimus suppressed leukocyte rolling; the maximum number of rolling leukocytes was reduced by 60.1% at 12 hours after reperfusion (P<0.05). In tacrolimus-treated rats, the velocity of rolling leukocytes was significantly faster than in vehicle-treated rats (P<0.01). The subsequent leukocyte accumulation was reduced by 61.6% at 24 hours after reperfusion (P<0.01). Histological examination demonstrated the protective effect of tacrolimus on ischemia-induced retinal damage, which was more substantial in the inner retina (P<0.01). The present study demonstrated the inhibitory effect of tacrolimus on leukocyte rolling and on subsequent leukocyte accumulation and the therapeutic potency to neural injury after transient retinal ischemia.

Pharmacokinetic study in rats after single intravenous and oral administrations of [14C]-ITF-296.

Pharmacokinetics of [14C]-ITF-296 and its metabolites, ITF-1124 and ITF-1577, were studied in rats after a single intravenous (2.5 mg/kg) and oral (10 mg/kg) administration. Radioactivity was measured by LSC while unchanged drug and its metabolites in plasma were assayed by an HPLC-UV method. The absorption of [14C]-ITF-296 after oral administration is practically complete. Elimination of radioactivity occurs mainly in urine (higher than 80%) and the recovery of the dose (higher than 95%) takes place up to 96 h after both treatments. Both by i.v. and p.o. route the results show that the radioactivity is largely excreted in the bile and reabsorbed in the intestine. The tissue distribution study indicates that there is no accumulation or localization of radioactivity in the major organs or blood and no radioactivity levels are found 96 h after either treatment. In addition, whole body autoradiography confirms the tissue distribution pattern, showing no differences between albino and pigmented rats.

Effects of the GABA agonists baclofen and THIP on long-term compensation in hemilabyrinthectomised rats

Horizontal eye movements, elicited by sinusoidal rotation in darkness, were recorded with a magnetic search coil technique in pigmented rats, hemilabyrinthectomised 8–12 weeks before the investigation. Separate gains during rotation towards the lesioned side (LS) and the intact side (IS) were calculated by a computer program, demonstrating an asymmetry. Systemic single administration of the GABA B agonist baclofen caused a dose-related temporary rebalancing of the compensatory eye movements to the LS and the IS. At an optimal dose of 14 μmol/kg b.wt symmetry was achieved by excitation of eye movements during rotation to the LS and depression during rotation to the IS. Administration of the GABA A agonist THIP did not obviously reduce the asymmetry. It is suggested that stimulation of GABA B receptors modifies the tonic imbalance between the bilateral vestibular nuclei and/or the central processing of the input from the peripheral sensory organs.

Contrast Sensitivity Function of the Albino Rat Determined Electrophysiologically

Albinism alters the neural projections of the visual system. The authors wondered how this would affect visual function in rodents. They had previously shown that it doesn't alter the luminance threshold. They now explore visual acuity in the albino rat. In this work, they describe its contrast sensitivity function (CSF), as determined electro-physiologically. They recorded cortical visual evoked potentials (VEP) on six albino rats, stimulated by sinusoidal contrast reversal gratings. The curve showed the same characteristics that this function has in other mammals. Compared with the pigmented rat, the albino reached lower sensitivity values and showed a loss of sensitivity at high spatial frequencies. The estimated cut-off was 0.48 c/°, that is, 0.72 log units below the estimated cut-off for the pigmented rat under similar experimental conditions. VEP and behavioral cut-off were very close, the VEP estimation being slightly higher than the behavioral one.

Distribution of photon absorption rates across the rat retina

1. An investigation into the distribution of light intensity across the rat retina was carried out on excised, intact rat eyes exposed to Ganzfeld illumination from a helium-neon laser (543 nm). 2. Some of the light entering the eyes exits through the sclera where its intensity can be monitored with an optical 'pick-up' that samples the intensity coming from a small region of external sclera and underlying retina. The spatial resolution of the pick-up is such that it samples light that has passed through ca 2 % of the rods in the rat eye. 3. Some of the laser light is absorbed by the rod pigment, rhodopsin, which gradually bleaches. Bleaching in the retina, in turn, causes an exponential increase in intensity emanating from the sclera. By monitoring this intensity increase, we are able to measure two important parameters in a single bleaching run: the local rhodopsin concentration and the local intensity falling on the rods. 4. With an ocular transmission photometer, we have measured both the local intensity and the local rhodopsin concentration across wide regions of rat retina. Both pigmented and albino rats were studied. 5. The distributions of rhodopsin and intensity were both nearly uniform; consequently, the product, (rhodopsin concentration) x (intensity), was similarly nearly equal across the retina. This means that the initial rate of photon absorption is about the same at all retinal locations. 6. Interpreted in terms of photostasis (the regulation of daily photon catch), this means that the rate of photon absorption is about the same in each rod, viz. 14 400 photons absorbed per rod per second. Since this rate of absorption is sufficient to saturate the rod, one possible purpose of photostasis is to maintain the rod system in a saturated state during daylight hours.

Two Spectral Types of Retinal Light Damage Occur in Albino as well as in Pigmented Rat: No Essential Role for Melanin

Earlier we showed that two spectral types of retinal damage occur in the pigmented rat. In the present study we investigated whether the same is true for albino rats. When investigating this issue we implicitly investigated the role of melanin in both damage types.An albinotic (Wistar) and a pigmented (Long Evans) strain of rats were used. Under anesthesia, a small part of the retina was irradiated at either 380 nm or at 470 nm. Three days later, the retina was analysed by funduscopy and prepared for light microscopy. Funduscopy showed no signs of damage in the albinotic retina. In the pigmented retina a decoloration of the fundus was noticed after irradiations starting from retinal doses of 0.6±0.1 J cm-2at 380 nm, and from 489±71 J cm-2at 470 nm. By light microscopy, retinal damage was found in the albino retina. The histologic manifestations at 380 nm differed from those at 470 nm. Irradiation at 380 nm at a dose of 0.5–0.9 J cm-2damaged a few scattered photoreceptor cells. At doses of 1.2–1.6 J cm-2all rods were damaged, while the other retinal layers showed no changes. These findings were similar to those found at 380 nm in the pigmented rat. At 470 nm, damage was found most prominently in the retinal pigment epithelium. These cells showed swelling and an increased number of dark inclusions. Threshold damage occurred at doses of 250–500 J cm-2. Again, the pathology in the pigmented rat was highly similar to that in the albino rat.The results show that both spectral damage types occur in albino as well as in pigmented retina. Therefore, melanin plays no crucial role in these light damage types.

Long-term deficits in otolith, canal and optokinetic ocular reflexes of pigmented rats after unilateral vestibular nerve section

Static and dynamic otolith, horizontal vestibular and optokinetic ocular reflexes were investigated in pigmented rats 1-6 and more months after unilateral vestibular nerve (UVN) section. Evoked responses were compared with published data from control rats studied under identical conditions. Static lateral tilt of UVN rats in the light evoked a vertical deviation in static eye position that was as large as in controls. In darkness, the evoked responses in UVN rats 6 months after the lesion were consistently smaller than in controls. Linear horizontal acceleration in darkness evoked vertical and torsional response components in UVN rats that were parallel-shifted towards lower gains and larger phase lags. Off-vertical axis rotation on a platform provoked responses that differed markedly from those recorded in intact rats with respect to the bias velocity component. These results suggest a permanent deficiency in the static and dynamic otolith-ocular reflex performance of UVN rats. Ocular responses to horizontal table velocity steps in darkness exhibited a direction-specific asymmetry in UVN rats. Step responses evoked by acceleration towards the intact side were larger in gain and longer in duration than responses evoked by acceleration towards the operated side. When compared with control data, responses to either side were reduced in UVN rats and the velocity store mechanism was barely activated by velocity steps towards the operated side. Responses evoked by horizontal optokinetic stimulation with constant pattern velocities were below control values in either direction. Slow-phase eye velocity saturated at much lower values than in intact rats, particularly during pattern motion towards the intact side. The duration of the optokinetic afternystagmus was asymmetrically reduced with respect to control data. Practically identical reductions in duration were found for vestibulo-ocular responses in the opposite directions. Behaving animals exhibited no obvious impairment in their spontaneous locomotory or exploratory activities. However, each UVN rat was impaired, even 2 years after the lesion, in its postural reaction to being lifted by the tail in the air. This observation suggests the presence of a permanent deficit in static and dynamic otolith-spinal reflexes that may be substituted on the ground by proprioceptive inputs.

Neuroprotective Therapy for Argon-Laser Induced Retinal Injury

Laser photocoagulation treatment of the central retina is often complicated by an immediate side effect of visual impairment, caused by the unavoidable laser-induced destruction of the normal tissue lying adjacent to the lesion and not affected directly by the laser beam. Furthermore, accidental laser injuries are at present untreatable. A neuroprotective therapy for salvaging the normal tissue might enhance the benefit obtained from treatment and allow safe perifoveal photocoagulation. We have developed a rat model for studying the efficacy of putative neuroprotective compounds in ameliorating laser-induced retinal damage. Four compounds were evaluated: the corticosteroid methylprednisolone, the glutamate-receptor blocker MK-801, the anti-oxidant enzyme superoxide dismutase, and the calcium-overload antagonist flunarizine. The study was carried out in two steps: in the first, the histopathological development of retinal laser injuries was studied. Argon laser lesions were inflicted in the retinas of 18 pigmented rats. The animals were killed after 3, 20 or 60 days and their retinal lesions were evaluated under the light microscope. The laser injury mainly involved the outer layers of the retina, where it destroyed significant numbers of photoreceptor cells. Over time, evidence of two major histopathological processes was observed: traction of adjacent normal retinal cells into the central area of the lesion forming an internal retinal bulging, and a retinal pigmented epithelial proliferative reaction associated with subretinal neovascularization and invasion of the retinal lesion site by phagocytes. The neuroprotective effects of each of the four compounds were verified in a second step of the study. For each drug tested, 12 rats were irradiated with argon laser inflictions: six of them received the tested agent while the other six were treated with the corresponding vehicle. Twenty days after laser exposure, the rats were killed and their lesions were subjected to image-analysis morphometry. The extent of retinal destruction was assessed by measuring the lesion diameter and the amount of photoreceptor cell loss in the outer nuclear layer. Methylprednisolone and MK-801 were shown to ameliorate laser-induced retinal damage, whereas both superoxide dismutase and flunarizine were ineffective. Furthermore, MK-801 diminished the proliferative reaction of the retinal pigment epithelial cells. On the basis of our results we suggest that the pigmented rat model is suitable for studying and screening various compounds for their neuroprotective efficacy in treating retinal laser injury. We further suggest that glutamate might play a key role in mediating retinal injury induced by laser irradiation.

Effects of monocular enucleation on receptor binding and innervation pattern of the noradrenergic system in the superior colliculus of the pigmented rat

To examine visual activity-dependent regulation of the central noradrenergic (NAergic) system, we carried out in vitro autoradiography for α 2-, β-adrenergic and NMDA, non-NMDA glutamatergic receptors together with immunohistochemical analysis for dopamine β-hydroxylase in the major central visual structures, including the lateral geniculate nucleus, superior colliculus (SC), and visual cortex of pigmented rats, that had received monocular enucleation. β-Adrenergic receptor ( β-AR) binding in the SC contralateral to the enucleated eye was significantly decreased (82%, P<0.01) following monocular deprivation started at P12 and continued for 2 or 11 weeks. No significant change in β-AR binding was found in other structures examined. The number of varicosities in NAergic fibers was significantly increased following longer enucleation, i.e. for 11 weeks, in the contralateral SC (197%, P<0.001), whereas that in the ipsilateral SC was reduced (75%, P<0.001). Changes in α 2-adrenergic, NMDA, and non-NMDA glutamatergic receptor binding were small in these animals. Changes in neither β-AR binding nor innervation pattern of NAergic fibers were found in one-year-old rats that had received a comparable period of monocular enucleation. Furthermore, neither unilateral ablation of the visual cortex to reduce a different set of major afferents nor neonatal enucleation, which induced anatomical reorganization of the afferents, was found to be effective. These findings suggest that β-AR binding and innervation pattern of NAergic fibers in the SC are modified only when massive imbalance of retinal afferent activity is imposed during a limited period in early postnatal life (i.e. the sensitive period).