Constitutive excitation by Gly90Asp rhodopsin rescues rods from degeneration caused by elevated production of cGMP in the dark

Previous experiments indicate that congenital human retinal degeneration caused by genetic mutations that change the Ca(2+) sensitivity of retinal guanylyl cyclase (retGC) can result from an increase in concentration of free intracellular cGMP and Ca(2+) in the photoreceptors. To rescue degeneration in transgenic mouse models having either the Y99C or E155G mutations of the retGC modulator guanylyl cyclase-activating protein 1 (GCAP-1), which produce elevated cGMP synthesis in the dark, we used the G90D rhodopsin mutation, which produces constitutive stimulation of cGMP hydrolysis. The effects of the G90D transgene were evaluated by measuring retGC activity biochemically, by recording single rod and electroretinogram (ERG) responses, by intracellular free Ca(2+) measurement, and by retinal morphological analysis. Although the G90D rhodopsin did not alter the abnormal Ca(2+) sensitivity of retGC in the double-mutant animals, the intracellular free cGMP and Ca(2+) concentrations returned close to normal levels, consistent with constitutive activation of the phosphodiesterase PDE6 cascade in darkness. G90D decreased the light sensitivity of rods but spared them from severe retinal degeneration in Y99C and E155G GCAP-1 mice. More than half of the photoreceptors remained alive, appeared morphologically normal, and produced electrical responses, at the time when their siblings lacking the G90D rhodopsin transgene lost the entire retinal outer nuclear layer and no longer responded to illumination. These experiments indicate that mutations that lead to increases in cGMP and Ca(2+) can trigger photoreceptor degeneration but that constitutive activation of the transduction cascade in these animals can greatly enhance cell survival.

Transgenic mice carrying the H258N mutation in the gene encoding the beta-subunit of phosphodiesterase-6 (PDE6B) provide a model for human congenital stationary night blindness

Mutations in the beta-subunit of cGMP-phosphodiesterase (PDE6beta) can lead to either progressive retinal disease, such as human retinitis pigmentosa (RP), or stationary disease, such as congenital stationary night blindness (CSNB). Individuals with CSNB in the Rambusch pedigree were found to carry the H258N allele of PDE6B (MIM# 180072); a similar mutation was not found in RP patients. This report describes an individual carrying the H258N allele, who presented with generalized retinal dysfunction affecting the rod system and a locus of dysfunction at the rod-bipolar interface. Also described are preclinical studies in which transgenic mice with the H258N allele were generated to study the pathophysiological mechanisms of CSNB. While Pde6b(rd1)/Pde6b(rd1) mice have severe photoreceptor degeneration, as in human RP, the H258N transgene rescued these cells. The cGMP-PDE6 activity of dark-adapted H258N mice showed an approximate three-fold increase in the rate of retinal cGMP hydrolysis: from 130.1 nmol x min(-1) x nmol(-1) rhodopsin in wild-type controls to 319.2 nmol x min(-1) x nmol(-1) rhodopsin in mutants, consistent with the hypothesis that inhibition of the PDE6beta activity by the regulatory PDE6gamma subunit is blocked by this mutation. In the albino (B6CBA x FVB) F2 hybrid background, electroretinograms (ERG) from H258N mice were similar to those obtained from affected Rambusch family members, as well as humans with the most common form of CSNB (X-linked), demonstrating a selective loss of the b-wave with relatively normal a-waves. When the H258N allele was introduced into the DBA background, there was no evidence of selective reduction in b-wave amplitudes; rather a- and b-wave amplitudes were both reduced. Thus, factors other than the PDE6B mutation itself could contribute to the variance of an electrophysiological response. Therefore, caution is advisable when interpreting physiological phenotypes associated with the same allele on different genetic backgrounds. Nevertheless, such animals should be of considerable value in further studies of the molecular pathology of CSNB.

Removal of phosphorylation sites of gamma subunit of phosphodiesterase 6 alters rod light response

The phosphodiesterase 6 gamma (PDE6 gamma) inhibitory subunit of the rod PDE6 effector enzyme plays a central role in the turning on and off of the visual transduction cascade, since binding of PDE6 gamma to the transducin alpha subunit (T alpha) initiates the hydrolysis of the second messenger cGMP, and PDE6 gamma in association with RGS9-1 and the other GAP complex proteins (G beta 5, R9AP) accelerates the conversion of T alpha GTP to T alpha GDP, the rate-limiting step in the decay of the rod light response. Several studies have shown that PDE6 gamma can be phosphorylated at two threonines, T22 and T35, and have proposed that phosphorylation plays some role in the physiology of the rod. We have examined this possibility by constructing mice in which T22 and/or T35 were replaced with alanines. Our results show that T35A rod responses rise and decay more slowly and are less sensitive to light than wild-type (WT). T22A responses show no significant difference in initial time course with WT but decay more rapidly, especially at dimmer intensities. When the T22A mutation is added to T35A, double mutant rods no longer showed the prolonged decay of T35A rods but remained slower than WT in initial time course. Our experiments suggest that the polycationic domain of PDE6 gamma containing these two phosphorylation sites can influence the rate of PDE6 activation and deactivation and raise the possibility that phosphorylation or dephosphorylation of PDE6 gamma could modify the time course of transduction, thereby influencing the wave form of the light response.

Effects of low AIPL1 expression on phototransduction in rods

PURPOSE

To investigate the impact of aryl hydrocarbon receptor-interacting protein-like (AIPL)-1 on photoreception in rods.

METHODS

Photoresponses of mouse rods expressing lowered amounts of AIPL1 were studied by single-cell and electroretinogram (ERG) recordings. Phototransduction protein levels and enzymatic activities were determined in biochemical assays. Ca2+ dynamics were probed with a fluorescent dye. Comparisons were made to rods expressing mutant Y99C guanylate cyclase activating protein (GCAP)-1, to understand which effects arose from elevated dark levels of cGMP and Ca2+.

RESULTS

Except for PDE, transduction protein levels were normal in low-AIPL1 retinas, as were guanylate cyclase (GC), rhodopsin kinase (RK), and normalized phosphodiesterase (PDE) activities. Y99C and low-AIPL1 rods were more sensitive to flashes than normal, but flash responses of low-AIPL1 rods showed an abnormal delay, reduced rate of increase, and longer recovery not present in Y99C rod responses. In addition, low-AIPL1 rods but not Y99C rods failed to reach the normal light-induced minimum in Ca2+ concentration.

CONCLUSIONS

Reduced AIPL1 delayed the photoresponse, decreased its amplification constant, slowed a rate-limiting step in its recovery, and limited the light-induced decrease in Ca2+. Not all changes were attributable to decreased PDE or to elevated cGMP and Ca2+ in darkness. Therefore, AIPL1 directly or indirectly affects more than one component of phototransduction.

GAP-independent termination of photoreceptor light response by excess gamma subunit of the cGMP-phosphodiesterase

We have generated a mouse with rod photoreceptors overexpressing the gamma inhibitory subunit (PDE6gamma) of the photoreceptor G-protein effector cGMP phosphodiesterase (PDE6). PDE6gamma overexpression decreases the rate of rise of the rod response at dim intensities, indicating a reduction in the gain of transduction that may be the result of cytoplasmic PDE6gamma binding to activated transducin alpha GTP (Talpha-GTP) before the Talpha-GTP binds to endogenous PDE6gamma. Excess PDE6gamma also produces a marked acceleration in the falling phase of the light response and more rapid recovery of sensitivity and circulating current after prolonged light exposure. These effects are not mediated by accelerating GTP hydrolysis through the GAP (GTPase activating protein) complex, because the decay of the light response is also accelerated in rods that overexpress PDE6gamma but lack RGS9. Our results show that the PDE6gamma binding sites of PDE6 alpha and beta are accessible to excess (presumably cytoplasmic) PDE6gamma in the light, once endogenous PDE6gamma has been displaced from its binding site by Talpha-GTP. They also suggest that in the presence of Talpha-GTP, the PDE6gamma remains attached to the rest of the PDE6 molecule, but after conversion of Talpha-GTP to Talpha-GDP, the PDE6gamma may dissociate from the PDE6 and exchange with a cytoplasmic pool. This pool may exist even in wild-type rods and may explain the decay of rod photoresponses in the presence of nonhydrolyzable analogs of GTP.

Opsin activation of transduction in the rods of dark-reared Rpe65 knockout mice

Rpe65 knockout mice (Rpe65-/-) are unable to synthesize the visual pigment chromophore 11-cis retinal; however, if these animals are reared in complete darkness, the rod photoreceptors accumulate a small amount of 9-cis retinal and its corresponding visual pigment isorhodopsin. Suction-electrode recording of single rods from dark-reared Rpe65-/- mice showed that the rods were about 400 times less sensitive than wild-type control rods and that the maximum responses were much smaller in amplitude. Spectral sensitivity measurements indicated that Rpe65-/- rod responses were generated by isorhodopsin rather than rhodopsin. Sensitivity and pigment concentration were compared in the same mice by measuring light responses from rods of one eye and pigment concentration from the retina of the other eye. Retinas had 11-35% of the normal pigment level, but the rods were of the order of 20-30 times less sensitive than could be accounted for by the loss in quantum catch. This extra desensitization must be caused by opsin-dependent activation of the visual cascade, which leads to a state equivalent to light adaptation in the dark-adapted rod. By comparing the sensitivity of dark-reared Rpe65-/- rods to that produced in normal rods by background light, we estimate that Rpe65-/- opsin is of the order of 2.5x10(-5) as efficient in activating transduction as photoactivated rhodopsin (Rh*) in WT mice. Dark-reared Rpe65-/- rods are less desensitized than rods from cyclic light-reared Rpe65-/- mice, have about 50% more photocurrent and degenerate at a slower rate. Retinas sectioned after 9 months in darkness show a larger number of photoreceptor nuclei in dark-reared animals than in cyclic light-reared animals, though both have fewer nuclei than in cyclic light-reared wild-type retinas. Both also have shorter outer segments and a lower free-Ca2+ concentration. These experiments provide the first quantitative measurement of opsin activation in physiologically responding mammalian rods.

The Y99C mutation in guanylyl cyclase-activating protein 1 increases intracellular Ca2+ and causes photoreceptor degeneration in transgenic mice

Guanylyl cyclase-activating proteins (GCAPs) are Ca2+-binding proteins that activate guanylyl cyclase when free Ca2+ concentrations in retinal rods and cones fall after illumination and inhibit the cyclase when free Ca2+ reaches its resting level in the dark. Several forms of retinal dystrophy are caused by mutations in GUCA1A, the gene coding for GCAP1. To investigate the cellular mechanisms affected by the diseased state, we created transgenic mice that express GCAP1 with a Tyr99Cys substitution (Y99C GCAP1) found in human patients with a late-onset retinal dystrophy (Payne et al., 1998). Y99C GCAP1 shifted the Ca2+ sensitivity of the guanylyl cyclase in photoreceptors, keeping it partially active at 250 nM free Ca2+, the normal resting Ca2+ concentration in darkness. The enhanced activity of the cyclase in the dark increased cyclic nucleotide-gated channel activity and elevated the rod outer segment Ca2+ concentration in darkness, measured by using fluo-5F and laser spot microscopy. In different lines of transgenic mice the magnitude of this effect rose with the Y99C GCAP1 expression. Surprisingly, there was little change in the rod photoresponse, indicating that dynamic Ca2+-dependent regulation of cGMP synthesis was preserved. However, the photoreceptors in these mice degenerated, and the rate of the cell loss increased with the level of the transgene expression, unlike in transgenic mice that overexpressed normal GCAP1. These results provide the first direct evidence that a mutation linked to congenital blindness increases Ca2+ in the outer segment, which may trigger the apoptotic process.

AIPL1, the protein that is defective in Leber congenital amaurosis, is essential for the biosynthesis of retinal rod cGMP phosphodiesterase

Aryl hydrocarbon receptor-interacting protein-like 1 (AIPL1) is a member of the FK-506-binding protein family expressed specifically in retinal photoreceptors. Mutations in AIPL1 cause Leber congenital amaurosis, a severe early-onset retinopathy that leads to visual impairment in infants. Here we show that knockdown of AIPL1 expression in mice also produces a retinopathy but over a more extended time course. Before any noticeable pathology, there was a reduction in the level of rod cGMP phosphodiesterase (PDE) proportional to the decrease in AIPL1 expression, whereas other photoreceptor proteins were unaffected. Consistent with less PDE in rods, flash responses had a delayed onset, a reduced gain, and a slower recovery of flash responses. We suggest that AIPL1 is a specialized chaperone required for rod PDE biosynthesis. Thus loss of AIPL1 would result in a condition that phenocopies retinal degenerations in the rd mouse and in a subgroup of human patients.

Early receptor current of wild-type and transducin knockout mice: photosensitivity and light-induced Ca2+ release

We have used suction-electrode recording to measure the early receptor current (ERC) from single, isolated mammalian photoreceptors. When a wild-type mouse rod was illuminated with light sufficient to close all the cGMP-gated channels, a succeeding bright laser flash bleaching a large proportion of the visual pigment produced an ERC, which at 37 degrees C consisted primarily of a single component of transient positive current. The amplitude of total charge movement of this component declined exponentially with successive flashes, consistent with the direct proportionality of the ERC to the quantity of pigment bleached. From the constant of exponential decline, it was possible to estimate the in vivo photosensitivity of mouse rhodopsin to be about 6 x 10(-9)microm(2) per molecule. We have also measured the ERC from rods of transducin-knockout mice, for which previous illumination to close the cGMP-gated channels was not required. The ERC of these rods was similar to that of wild-type rods but was followed by a slow component of outward current whose maximum amplitude in some cells approached that of the normal light response. This slow current was blocked by l-cis diltiazem, indicating that it was produced by ion flux through the cyclic nucleotide-gated channels of the outer segment; however, it could not have been produced by the normal transduction cascade, since it was recorded from rods lacking transducin. Since it was depressed by prior incorporation of the Ca(2+) buffer BAPTA, it was probably generated by light-activated Ca(2+) release earlier demonstrated in salamander and zebrafish. Recordings of the ERC from normal and mutant mice may provide a useful tool for the analysis of models of retinal disease, as well as exploration of the molecular origin of light-activated Ca(2+) release.

Spontaneous activity of opsin apoprotein is a cause of Leber congenital amaurosis

Mutations in Rpe65 disrupt synthesis of the opsin chromophore ligand 11-cis-retinal and cause Leber congenital amaurosis (LCA), a severe, early-onset retinal dystrophy. To test whether light-independent signaling by unliganded opsin causes the degeneration, we used Rpe65-null mice, a model of LCA. Dark-adapted Rpe65-/- mice behaved as if light adapted, exhibiting reduced circulating current, accelerated response turn-off, and diminished intracellular calcium. A genetic block of transducin signaling completely rescued degeneration irrespective of an elevated level of retinyl ester. These studies clearly show that activation of sensory transduction by unliganded opsin, and not the accumulation of retinyl esters, causes light-independent retinal degeneration in LCA. A similar mechanism may also be responsible for degeneration induced by vitamin A deprivation.

Dose-response effects of chronic lithium regimens on spatial memory in the black molly fish

Lithium is widely used in the management of bipolar disorder, yet memory impairment is a serious side effect. To assess the effects of lithium on spatial working and reference memories, we have employed a plus maze utilizing spontaneous alternation (SA) and place-learning paradigms in two experiments with the black molly fish. Four treatment groups were gavaged with 20 microl of a 10, 100, or 1000 mM lithium chloride (LiCl) solution or ddH(2)O vehicle every 12 h for 22 to 24 days. On Day 15, subjects began an 8-day SA task or a 10-day place-learning task. Results indicate that there is a significant difference in SA performance among the treatment groups for Days 1, 2, and 3. Results of the place-learning task indicate that the 1 M dose group needed significantly more trials to reach criterion and made significantly fewer correct first choices than the other dose groups. Capillary ion analysis determinations of plasma and brain lithium levels illustrate linear dose-response relationships to doses administered. Regression analyses indicate that there is a relationship between SA performance and plasma/brain lithium levels during the initial part of testing. Collectively, the results indicate that chronic lithium administration impairs spatial working and reference memories.

Measurement of cytoplasmic calcium concentration in the rods of wild-type and transducin knock-out mice

A 10 microm spot of argon laser light was focused onto the outer segments of intact mouse rods loaded with fluo-3, fluo-4 or fluo-5F, to estimate dark, resting free Ca(2+) concentration ([Ca(2+)](i)) and changes in [Ca(2+)](i) upon illumination. Dye concentration was adjusted to preserve the normal physiology of the rod, and the laser intensity was selected to minimise bleaching of the fluorescent dye. Wild-type mouse rods illuminated continuously with laser light showed a progressive decrease in fluorescence well fitted by two exponentials with mean time constants of 154 and 540 ms. Rods from transducin alpha-subunit knock-out (Tralpha-/-) animals showed no light-dependent decline in fluorescence but exhibited an initial rapid component of fluorescence increase which could be fitted with a single exponential (tau~1-4 ms). This fluorescence increase was triggered by rhodopsin bleaching, since its amplitude was reduced by pre-exposure to bright bleaching light and its time constant decreased with increasing laser intensity. The rapid component was however unaffected by incorporation of the calcium chelator BAPTA and seemed therefore not to reflect an actual increase in [Ca(2+)](i). A similar rapid increase in fluorescence was also seen in the rods of wild-type mice just preceding the fall in fluorescence produced by the light-dependent decrease in [Ca(2+)](i). Dissociation constants were measured in vitro for fluo-3, fluo-4 and fluo-5F with and without 1 mM Mg(2+) from 20 to 37 degrees C. All three dyes showed a strong temperature dependence, with the dissociation constant changing by a factor of 3-4 over this range. Values at 37 degrees C were used to estimate absolute levels of rod [Ca(2+)](i). All three dyes gave similar values for [Ca(2+)](i) in wild-type rods of 250 +/- 20 nM in darkness and 23 +/- 2 nM after exposure to saturating light. There was no significant difference in dark [Ca(2+)](i) between wild-type and Tralpha-/- animals.

Selected contribution: PKC activation inhibits Ca(2+) signaling in tracheal epithelial cells kept in simulated microgravity

Microgravity has been shown to alter protein kinase C (PKC) activity; therefore, we investigated whether microgravity influences mechanically stimulated Ca(2+) signaling and ATP-induced Ca(2+) oscillations, both of which are modulated by PKC. Rabbit tracheal epithelial outgrowth cultures or suspended epithelial sheets were rotated in bioreactors to simulate microgravity. Mechanical stimulation of a single cell increased the cytosolic Ca(2+) concentration in 35-55 cells of both outgrowth cultures and epithelial sheets kept at unit gravity (G) or in simulated microgravity (smicroG). In outgrowth cultures, 12-O-tetradecanoylphorbol-13-acetate (TPA; 80 nM), a PKC activator, restricted Ca(2+) "waves" to about 10 cells in unit G and to significantly fewer cells in smicroG. TPA only slightly reduced the spread of Ca(2+) waves in epithelial sheets kept in smicroG but did not inhibit Ca(2+) waves of sheets kept in unit G. In both cell preparations from both conditions, TPA inhibited ATP-induced Ca(2+) oscillations; however, the effect was more pronounced in cells kept in smicroG. These results suggest that PKC activation is more robust in cells subjected to smicroG.

PKC role in mechanically induced Ca2+ waves and ATP-induced Ca2+ oscillations in airway epithelial cells

Mechanical stimulation of airway epithelial cells generates the Ca2+ mobilization messenger inositol 1,4,5-trisphosphate and the protein kinase (PK) C activator diacylglycerol. Inositol 1,4,5-trisphosphate diffuses through gap junctions to mediate intercellular communication of the mechanical stimulus (a "Ca2+ wave"); the role that diacylglycerol-activated PKC might play in the response is unknown. Using primary cultures of rabbit tracheal cells, we show that 12-O-tetradecanoylphorbol 13-acetate- or 1, 2-dioctanyl-sn-glycerol-induced activation of PKC slows the Ca2+ wave, decreases the amplitude of induced intracellular free Ca2+ concentration ([Ca2+]i) increases, and decreases the number of affected cells. The PKC inhibitors bisindolylmaleimide and Gö 6976 slowed the spread of the wave but did not change the number of affected cells. We show that ATP-induced [Ca2+]i increases and oscillations, responses independent of intercellular communication, were inhibited by PKC activators. Bisindolylmaleimide decreased the amplitude of ATP-induced [Ca2+]i increases and blocked oscillations, suggesting that PKC has an initial positive effect on Ca2+ mobilization and then mediates feedback inhibition. PKC activators also reduced the [Ca2+]i increase that followed thapsigargin treatment, indicating a PKC effect associated with the Ca2+ release mechanism.

Mechanical stimulation initiates intercellular Ca2+ signaling in intact tracheal epithelium maintained under normal gravity and simulated microgravity

We investigated mechanically induced cell-to-cell Ca2+ signaling in a preparation of rabbit tracheal epithelium close to its in vivo condition. We used confocal microscopy to analyze changes in intracellular free calcium concentration ([Ca2+]i) in intact ciliated tracheal mucosal explants loaded with the Ca2+-indicator dye, fluo-3. When a single cell in the epithelium was transiently stimulated with a microprobe, [Ca2+]i increased in the stimulated cell and then increased in surrounding cells. In the absence of extracellular Ca2+, the [Ca2+]i increases had a smaller amplitude and spread to fewer cells. Treatment of the cells with thapsigargin, in the presence of extracellular Ca2+, more markedly reduced the spread of elevated [Ca2+]i. These results suggest that the propagated [Ca2+]i increases are due to mobilization of Ca2+ from intracellular stores and, possibly, the influx of extracellular Ca2+. The mechanically stimulated [Ca2+]i increases were accompanied by propagated increases in ciliary beat frequency. Since microgravity has been shown to alter signal transduction, we investigated whether simulated microgravity affects the mechanically stimulated cell-to-cell Ca2+ signaling observed in tracheal epithelium. Tissues were maintained for 3-8 d in a rotating wall vessel which simulates microgravity conditions. Cells maintained in simulated microgravity exhibited mechanically induced [Ca2+]i increases not significantly different in magnitude, in speed of propagation, or in the number of cells involved, from tissue maintained at unit gravity. Our results suggest that intercellular Ca2+ signaling coordinates cellular activity, including ciliary beating, within the tracheal epithelium in vivo and that this function is not compromised in microgravity.

Reduction of extracellular Na+ causes a release of Ca2+ from internal stores in airway epithelial cells

Exchange of physiological salt solution with Na(+)-free solution caused an increase in intracellular Ca2+ concentration ([Ca2+]i) in 86.3% of cultured airway epithelial cells within 75 s. [Ca2+]i returned to near baseline levels within 45 s and frequently showed oscillatory increases thereafter. When extracellular Na+ concentration ([Na+]o) was reduced to 10 and 60 mM, 59.0 and 8.0% of the cells increased [Ca2+]i, respectively. Low [Na+]o-induced increase in [Ca2+]i was not blocked by amiloride, benzamil, La3+, or the absence of extracellular Ca2+. Low [Na+]o-induced [Ca2+]i increase did not occur after thapsigargin treatment. These results indicated that low [Na+]o-induced [Ca2+]i increase is due to release of Ca2+ from intracellular stores. Because mechanical stimulation of a single cell causes a Ca2+ increase among many cells (Sanderson, M. J., A. C. Charles, and E. R. Dirksen. Mechanical stimulation and intercellular communication increases intracellular Ca2+ in epithelial cells. Cell Regul. 1: 585-596, 1990.) we assayed the effect of low [Na+]o on this mechanically induced response. In low [Na+]o, mechanically induced [Ca2+]i increase in the stimulated cell was reduced; however, [Ca2+]i increase in adjacent cells was normal. We suggest that a mechanically induced Na+ conductance in the stimulated cell contributes to [Ca2+]i changes. These signaling pathways may be involved in the maintenance of periciliary ion concentrations.

Stretch increases inositol 1,4,5-trisphosphate concentration in airway epithelial cells

Mechanical stimulation of airway epithelial cells with a microprobe leads to an increase in cytoplasmic [Ca2+] that appears to be due, in part, to release of Ca2+ from inositol 1,4,5-trisphosphate (IP3)-sensitive stores (Boitano et al., Science 258:292[1992]). To investigate whether intracellular IP3 concentration ([IP3]i) increases in response to mechanical stimulation, we grew confluent monolayers from rabbit tracheal mucosal explants on flexible substrates and measured [IP3]i after stretching the substrate. The effect of stretch on [IP3]i was measured in the presence of Li+, an inhibitor of IP3 degradation. In unstretched cells, IP3 measured approximately 5.1 pmol/10(6) cells, from which we estimated [IP3]i to be 1.8 microM. Addition of Li+ had no effect on resting [IP3]i. When the flexible cell support was stretched to increase its surface area by 13%, mean [IP3]i increased about 3-fold with a half-time of approximately 1 s. The increased [IP3]i was maintained in a plateau phase for approximately 8 s and then decayed to near the unstretched level over the next 10 s, despite the sustained application of stretch. A transient stretch (0.5 s) induced a similar rate of increase and peak [IP3]i; however, [IP3]i subsided without a plateau phase. The magnitude of the [IP3]i increase was proportional to stimulus intensity between 0 and 13% increase in substrate surface area. In addition, dissociated airway epithelial cells were exposed to hypotonic solution to induce cell swelling. [IP3]i increased about 4-fold above control levels after 10 s of exposure to hypotonic solution. Basal [IP3]i of dissociated cells in isotonic solution was estimated to be 0.7 microM. These results are consistent with mechanical stimulation leading to phospholipase C synthesis of IP3, which mediates intracellular and intercellular Ca2+ signaling.

Evidence for voltage-sensitive, calcium-conducting channels in airway epithelial cells

In airways epithelial cultures, mechanical stimulation induces intracellular Ca2+ concentration ([Ca2+]i) changes by causing Ca2+ entry and intracellular Ca2+ release. Mechanically induced Ca2+ fluxes across the plasma membrane are blocked by Ni2+ (Boitano, S., M. J. Sanderson, and E. R. Dirksen. J. Cell. Sci. 107: 3037-3044, 1994). In this report we use fluorescence imaging microscopy with fura 2 and intracellular recording of the transmembrane potential to further characterize Ca2+ flux in the plasma membrane of these cells. Mechanically induced Ca2+ influx is blocked by nifedipine. Addition of the dihydropyridine agonist BAY K8644 (2 microM) leads to a delayed increase of [Ca2+]i that is dependent on extracellular Ca2+. Switching to high extracellular K+ concentration ([K+]o) causes depolarization of the plasma membrane and a transient increase in [Ca2+]i. The number of cells that respond to high [K+]o is significantly decreased by Ni2+ (1 mM) or nifedipine (10 microM). Mechanical stimulation causes a rapid depolarization of the stimulated cell that can be suppressed by the K+ ionophore valinomycin. Valinomycin treatment also blocks mechanically induced Ca2+ dux. These results suggest that voltage-sensitive Ca(2+)-conducting channels exist in airway epithelial cells, and these channels contribute to the [Ca2+]i changes observed after mechanical stimulation or depolarization of the plasma membrane.

Fetal Ammon's horn transplants improve acquisition of a radial arm maze and a low-rate operant schedule in trimethyltin-treated rats

The results of previous studies indicated that block grafts of fetal hippocampal tissue made into cavities produced by aspiration lesions of the hippocampus in rats given the neurotoxin trimethyltin (TMT) significantly worsened the TMT-induced deficit in water maze acquisition. The purpose of the present study was to test the hypothesis that a procedure for transplantation that produced less destruction to the host brain and resulted in transplants with less mass might produce recovery in a spatial learning task in TMT-exposed rats. Acquisition of an externally cued (spatial) version of the radial arm maze (RAM), an internally cued version of the RAM, and of a differential reinforcement of low rate (DRL) operant schedule was assessed in normal rats, rats given TMT, and rats given TMT and stereotaxic implants of either fetal Ammon's horn or entorhinal cortex. The rats receiving Ammon's horn transplants made significantly fewer reentries into the baited arms in both maze configurations and fewer reentries into the nonbaited arms in the spatial RAM than rats in the TMT-only and TMT/entorhinal cortex transplant groups. The rats receiving transplants of Ammon's horn made significantly fewer responses and received more reinforcements during training on the DRL-20 schedule than rats receiving just TMT or rats receiving TMT and transplants of fetal entorhinal cortex. These results support the proposal that transplantation procedures that cause less damage to the host brain and result in transplants that do not occupy a large extent of the ventricular space increase the probability of behavioral recovery.

The effect of time following exposure to trimethyltin (TMT) on cholinergic muscarinic receptor binding in rat hippocampus

Adult male Long-Evans rats were given 6 mg/kg trimethyltin (TMT). Rats were killed 1, 3, 7, 14, 21, 35, or 60 d later. An untreated control group was included. Brain sections were processed using film autoradiography to visualize in the hippocampus either total muscarinic receptor binding ([3H]quinuclidinyl benzilate; [3H]QNB), or M1 receptors ([3H]pirenzepine; [3H]PZ), or M2 receptors ([3H]oxotremorine-M; [3H]OXO-M). A reduction in [3H]QNB binding was found in CA1 and CA3c 7 d after TMT, but not in CA3a, b, or the dentate gyrus. [3H]PZ binding was decreased throughout Ammon's horn by 14 d after treatment. [3H]OXO-M binding decreased 1 d after exposure in CA1 and in all subfields of Ammon's horn by d 3. Neither [3H]PZ or [3H]OXO-M binding decreased in the dentate gyrus of TMT-treated rat at any time point. The temporal patterns of receptor loss may be explicable by reference to timing of fiber and cell body degeneration reported in previous studies and the regional differences may account for discrepancies between reports of either substantial decreases or no loss in hippocampal muscarinic receptors after TMT exposure.

Effects of trimethyltin (TMT) on choline acetyltransferase activity in the rat hippocampus. Influence of dose and time following exposure

Trimethyltin (TMT) destroys specific subfields of the hippocampus in the rat. TMT also increases choline acetyltransferase (ChAT) activity in CA1 of Ammon's horn and the outer molecular layer of the dentate gyrus. This observation suggests that axonal sprouting occurs in the cholinergic septohippocampal system in response to TMT. However, neither does-response nor time course data are available for the effects of TMT on this enzyme. The effects of three dose levels of TMT on ChAT activity in CA1 and the dentate gyrus were determined in Experiment 1 and ChAT activity in these two areas was measured at six time points following exposure to TMT in Experiment 2. Only the highest dose of TMT (6 mg/kg) significantly increased ChAT activity. ChAT activity in the dentate gyrus increased significantly by 3 d after administration and continued to increase until 21 d after exposure. A significant increase was not observed in CA1 until 7 d after exposure to TMT. Asymptotic levels were still reached at d 21. These results indicate a steep dose-response curve for TMT-induced changes in ChAT activity in the hippocampal formation and that this marker of cholinergic activity is more sensitive to perturbation by TMT in the dentate gyrus than Ammon's horn.

Effects of trimethyltin on acquisition and reversal of a light-dark discrimination by rats

The behavioral deficits produced by trimethyltin (TMT) are usually attributed to the hippocampal damage caused by this toxicant. The purpose of this experiment was to determine the effects of TMT administration on acquisition and reversal of a discrete trial light-dark discrimination. Acquisition of this task is impaired by hippocampal lesions but the effects of TMT on it are not known. Forty-five days after some of the rats were given one of three doses of TMT, adult, male Long-Evans rats were given 100 trials per day for 20 days to acquire a discrete trial lever press discrimination with lit cue lights located above the correct lever. At the end of this time the contingencies were reversed and the rats were given 30 more days of training. No significant group differences occurred during the first 20 days. A significant group effect was found for the 30 days of reversal training. The rats given the highest dose of TMT (6 mg/kg) obtained significantly more reinforcements during reversal training than the other groups. Because surgical hippocampal lesions generally impair both acquisition and reversal of visual discriminations, these data were unexpected and suggest that other factors than hippocampal damage enter into the behavioral effects of TMT.

Transplant-induced working memory deficits in hippocampectomized rats

This experiment determined the effects of transplantation of fetal hippocampus on the ability of male rats with hippocampal lesions to acquire versions of a radial arm maze that depended on either extramaze cues or intramaze cues for solution. Rats receiving transplants took significantly more trials than control rats to emit three consecutive errorless trials in the extramaze cue (spatial) variation of the maze. Rats with just hippocampal lesions never differed from any other group. No differences in this measure were found for the intramaze cue condition. Rats receiving transplants made more repeat entries into reinforced arms in both versions of the maze than control rats and more reentries into neverbaited arms in the spatial maze. Rats with hippocampal lesions failed to differ from any other group on this measure in the spatial maze, but were different from normal rats in the intramaze cue maze. These data suggest that in some tasks transplants of fetal tissue lead to greater behavioral impairment than lesions alone.

Angiotensin II binding sites in the hamster brain: localization and subtype distribution

This study was designed to characterize the distribution of angiotensin II (AII) binding sites in the hamster brain. Brain sections were incubated with [125I][sar1,ile8]-angiotensin II in the absence and presence of angiotensin II receptor subtype selective compounds, losartan (AT1 subtype) and PD123177 (AT2 subtype). Binding was quantified by densitometric analysis of autoradiograms and localized by comparison with adjacent thionein stained sections. The distribution of AII binding sites was similar to that found in the rat, with some exceptions. [125I][sar1,ile8]-angiotensin II binding was not evident in the subthalamic nucleus and thalamic regions, inferior olive, suprachiasmatic nucleus, and piriform cortex of the hamster, regions of prominent binding in the rat brain. However, intense binding was observed in the interpeduncular nucleus and the medial habenula of the hamster, nuclei void of binding in the rat brain. Competition with receptor subtype selective compounds revealed a similar AII receptor subtype profile in brain regions where binding is evident in both species. One notable exception is the medial geniculate nucleus, predominately AT1 binding sites in the hamster but AT2 in the rat. Generally, the AII binding site distribution in the hamster brain parallels that of the other species studied, particularly in brain regions associated with cardiovascular and dipsogenic functions. Functional correlates for AII binding sites have not been elucidated in the majority of brain regions and species mismatches might provide clues in this regard.

Surface morphology of fetal neural transplants into the lateral ventricles after hippocampal lesions

The surface morphology of transplants of rat fetal hippocampal tissue, and of cavities formed by aspiration lesion of the adult rat hippocampus and overlying neocortex into which the transplants were placed, was studied by scanning electron microscopy. The surface of lesion cavities was covered by a scar upon which occasional cellular profiles were found. The surface cells resembled supraependymal macrophages. Lesion cavities with a transplant showed similar scarring although the number of supraependymal structures was increased. Polymorphic cells and numerous fiber processes were observed both on the surface and embedded in the scar. Ependymal structures were seen on the non-damaged ventricular surfaces adjacent to the lesion site. These regions, however, also displayed increases in the number and types of supraependymal structures. Scanning electron microscopy demonstrated considerable variability in surface morphology of different transplants and over the surface of individual transplants. A transplant could show regions of scarring, areas covered by cells resembling ependymal cells, and regions covered by a dense matrix of fibers. In many regions the fibers coalesced to form a branching, web-like network over the transplant surface. Transmission electron microscopy indicated that the surface could be covered by ependymal cells or by the scar seen in scanning specimens. Some surface fibers were identified as axons. Cells on the surface of the transplants could be identified as neuronal, glial-like, and phagocytic. The cells and the possible effects of surface morphology on transplant function is discussed.