The dark-adaptation of single units in the frog's retina and its relation to the regeneration of rhodopsin

Artificial light at night decreases the pupillary light response of dark-adapted toads to bright light

Artificial light at night (ALAN) is expanding worldwide. Many physiological effects have been reported in animals, but we still know little about the consequences for the visual system. The pupil contributes to control incoming light onto the retina. Sudden increases in light intensity evokes the pupil light reflex (PLR). Intrinsically photosensitive retinal ganglion cells (ipRGC) affect PLR and melatonin expression, which largely regulate circadian rhythms and PLR itself. IpRCG receive inputs from various photoreptors with different peak sensitivities implying that PLR could be altered by a broad range of light sources. We predicted ALAN to enhance PLR. Contrary to our prediction, dark-adapted cane toads Rhinella marina, exposed to ALAN (5 lx) for 12 days, exhibited a lower PLR than controls and individuals exposed to 0.04 lx, even after 1 h in bright light. We cannot conclude whether ALAN induced a larger pupil size in dark-adapted toads or a slower initial contraction. Nevertheless, the response was triggered by a light source with an emission peak (590 nm) well above the sensitivity peak of melanopsin, the main photoreceptor involved in PLR. Therefore, ALAN alters the capacity of toads to regulate the incoming light in the eye at night, which may reduce the performance of visually guided behaviors, and increase mortality by predators or road kills at night. This first study emphasizes the need to focus on the effect of ALAN on the vision of nocturnal organisms to better understand how this sensory system is altered and anticipate the consequences for organisms.

A frog's eye view: Foundational revelations and future promises

From the mid-19th century until the 1980's, frogs and toads provided important research models for many fundamental questions in visual neuroscience. In the present century, they have been largely neglected. Yet they are animals with highly developed vision, a complex retina built on the basic vertebrate plan, an accessible brain, and an experimentally useful behavioural repertoire. They also offer a rich diversity of species and life histories on a reasonably restricted physiological and evolutionary background. We suggest that important insights may be gained from revisiting classical questions in anurans with state-of-the-art methods. At the input to the system, this especially concerns the molecular evolution of visual pigments and photoreceptors, at the output, the relation between retinal signals, brain processing and behavioural decision-making.

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.

Modulation of synaptic transmission in the retina

Synaptic transmission between photoreceptors and horizontal cells can be modulated in at least two domains: amplitude and time. In teleost fish, synaptic transmission is modulated mainly in the amplitude domain. Cone-driven horizontal cells in this species require background illumination to maintain high light responsiveness, and they are strongly suppressed in prolonged darkness. Moreover, in light, cone horizontal cells are extensively coupled via gap junctions, and the coupling is reduced in strength after prolonged darkness. The dopaminergic interplexiform cells play a major role in the regulation of cone horizontal cell activity. They may release dopamine tonically in darkness, which suppresses the light responsiveness of horizontal cells and uncouples them. In amphibians, whose horizontal cells receive input from both rods and cones, the modulation appears to be in the time domain, i.e., the rise time of horizontal cell responses is slow in prolonged darkness and accelerated after background illumination. gamma-aminobutyric acid and glycine may mediate the changes in response rise time. Despite the differences of the neuromodulators involved, these species provide two complementary modes of modulation of synaptic transmission in the retina.

Ganglion cell performance at absolute threshold in toad retina: effects of dark events in rods

1. The performance of ganglion cells in detecting flashes of light near the absolute threshold was studied in an isolated eye-cup preparation of toad retina. Retinal ganglion cells, through which all visual information from the rods must flow to the brain, are in a key position for evaluating the still unproven hypothesis that the absolute light sensitivity is limited by rod noise (Barlow, 1956). 2. The dark-adapted threshold intensity for these cells, which were selected on the basis of their high sensitivity, averaged 0.029 Rh* flash-1 (range 0.008-0.062), where Rh* signifies one photoisomerization per rod. On average, 46 photoisomerizations were needed per receptive field per flash to evoke a threshold response (range 16-84). 3. In the threshold region, frequency of responses versus mean flash intensity was determined. Threshold performance could be described by theoretical frequency of response curves, allowing intrinsic noise to be estimated in terms of an equivalent rate of photoisomerization-like (dark) events. In two completely characterized cells the rate of dark events corresponded to 0.03 and 0.06 Rh*DS-1, where Rh*D signifies one dark event per rod. 4. Threshold elevations produced by dim backgrounds were studied. The results of these experiments are consistent with a dark event rate equivalent to 0.046 Rh*DS-1, or 0.037 Rh*DS-1 after correcting for a probable decrease in summation time. 5. The rate of actual dark events (0.028 Rh*DS-1, 20 degrees C) measured in Bufo rods (Baylor, Lamb & Yau, 1980) is close to the equivalent rates determined here. Thus, for the ganglion cells signalling the dimmest lights, the dark events in rods appear to be the most significant intrinsic retinal noise source limiting detection.

The interplexiform-horizontal cell system of the fish retina: effects of dopamine, light stimulation and time in the dark

Interplexiform cells contact cone horizontal cells in the fish retina and probably release dopamine at synaptic sites. The effects of dopamine, certain related compounds, and light and dark régimes were tested on the intracellularly recorded activity of horizontal cells in the superfused carp retina to elucidate the functional role of the interplexiform cell. Dopamine application onto retinae kept in the dark for 30-40 min increased the size of the responses of cone horizontal cells to small-spot stimuli but decreased response size to large- and full-field stimuli. Dopamine also altered the response waveform of these cells; the transient at response onset increased in size and the depolarizing afterpotential decreased in size. Haloperidol, a dopamine antagonist, blocked these effects of dopamine application. Forskolin, an adenylate cyclase activator, increased the size of the responses of the cells to small-spot stimuli. Superfusion of vasoactive intestinal peptide did not produce any effects on horizontal cells. The results indicate that dopamine produces multiple physiological effects on cone horizontal cells by activation of an intracellular enzyme system. We propose that some of these effects are probably related to an uncoupling of the gap junctions between horizontal cells, but that other effects are most likely not explained on this basis and reflect additional changes induced in the cells by dopamine. After prolonged periods of darkness (100-110 min), compared with short periods (30-40 min), L-type cone horizontal cells exhibited responses similar to those obtained during dopamine application. Dim flickering or continuous light backgrounds did not mimic the effects of dopamine. Although dopamine application onto retinae after short-term darkness produced dramatic effects on L-type cone horizontal cells, little or no effect was observed when dopamine was applied while the effects of a previous dopamine application were still present or after prolonged darkness. These results suggest that interplexiform cells may release dopamine after prolonged darkness and that interplexiform cells may regulate lateral inhibitory effects mediated by L-type cone horizontal cells as a function of time in the dark.

Light and dark adaptation influences GABA receptor sites in the chick retina

The aim of the present study was to investigate the effect of environmental conditions such as light-and-dark-adaptation on the plasticity of GABA receptor sites in the chick retina. In chicks exposed to light for 5 hr (light-adapted), specific [3H]GABA binding was increased by 35% in comparison to the binding found in chicks maintained in darkness (dark-adapted). Conversely, in the retina of chicks exposed to darkness for 5 hr, specific [3H]GABA binding was decreased by 28% with respect to that found in chicks kept in the light. Scatchard analysis of the binding data revealed that the affinity of GABA for its receptor binding site was higher in the retinas of light-adapted chicks than in those of dark-adapted chicks (Kd values of 19.20 +/- 1.23 and 27.20 +/- 1.47 nM, respectively). On the contrary, the maximal number of binding sites (Bmax) remained unchanged in light- and dark-adapted chicks (5.2 +/- 0.10 and 5.3 +/- 0.15 pmol/mg protein, respectively). These results suggest the involvement of GABA receptors in the regulation of visual function.

Responsiveness and receptive field size of carp horizontal cells are reduced by prolonged darkness and dopamine

In the fish retina the interplexiform cells contain dopamine and provide a centrifugal pathway from the inner plexiform layer to horizontal cells of the outer plexiform layer. Dopamine application reduced the responsiveness and receptive field size of cone horizontal cells, as did a prolonged period of complete darkness. Other results suggest that the interplexiform cells may release dopamine after a prolonged period in the dark. The interplexiform-horizontal cell system may modify the strength of the antagonistic surrounds of retinal neurons as a function of time in the dark.

Center and surround excitation in the receptive fields of frog retinal ganglion cells

We have reexamined the receptive fields of frog retinal ganglion cells focussing on their surround properties. Carefully excluding artifacts due to stimulation of the (Gaussian) RF center, we found that spiking responses can be elicited by step stimulation of any receptor type in the surrounds of all the classes 1-4 Maturana et al. (1960) (J. gen. Physiol. 43, 129-175). The surround responses are antagonized by the responsive center and suppressed by the inhibitory surround, but are seen because of their slower dynamics. The responsive surround differs spectrally from the center: in the latter, cones and green rods compete, in the former, their signals sum.

Rhodopsin and visual adaptation: analysis of photoreceptor thresholds in the isolated skate retina

Photoreceptor thresholds in the isolated retina of the skate, determined by extracellular measurement of the photoreceptor potential during periods of light and dark adaptation, were analyzed in relationship to prevailing states of the visual pigment. The starting assumption of the analysis is that relative levels of three forms of the pigment molecule [native rhodopsin (R), a photoactivated intermediate (R*), and bleached pigment (B)] govern (quasi-) stable levels of threshold measured (a) during exposure of the retina to background light of fixed incident intensity (Ib), and (b) after irradiation that bleaches a defined fraction (B) of the rhodopsin. It is shown that experimental data are described well by the equation It/ It0 = (1 - B)-1 X F X (1 + 0(3)B), where F = [1 + 0(1)Ib(1 - B) + 0(2)B]. In this equation, It/ It0 is the relative threshold for detection of a test flash; (1 - B) approximates the relative efficiency of quantum capture; and 0(1) - 0(3) are constants. For values of 0(1) - 0(3) yielding an optimal fit to experimental data, log (It/ It0 ) approximately log F over a broad range of values of Ib and B. It is further shown that the algebraic form of the term F in the above equation is consistent with the predictions of a (steady-state) model for the role of the pigment molecule in photoreceptor adaptation. The model proposes that R* and B desensitize the photoreceptor by acting (in qualitatively similar fashion) to reduce the availability of E, an intracellular substance whose activation supports generation of the flash response. Results of the analysis are discussed in relation to the Dowling- Rushton equation (Dowling, 1960, 1963; Rushton , 1961), and to the results of more recent studies examining light and dark adaptation.

Remote pattern reversal reduces the proximal negative response of the goldfish retina

1. Using the eyecup preparation, proximal negative responses (PNR) to small test spots of different irradiance were recorded with (a) a stationary peripheral black and white grating surrounding the test spot, and (b) with contrast reversal of the same grating. In the latter case, the PNR-amplitude was reduced by a magnitude that was dependent on the frequency of contrast reversal. The reduction was maximum (approximately 50%) for a frequency of 8-10 Hz. 2. The attenuation was constant for PNR-amplitudes greater than half the maximum value, but increased for smaller responses. The fact that the intensity-response curve was not merely shifted towards higher values on the log intensity axis, indicates that the suppression was an effect neither of stray light nor of adaptive processes in the distal retina. 3. The effect of a single shift of the grating (by half a cycle) on the PNR was studied at different delays between grating shift and test spot presentation. Strong suppression of the PNR was found for delays between 100 ms (shift preceding test spot) and -50 ms (test spot preceding grating shift), with a maximum at about 30 ms. 4. This long-range effect of peripheral transient stimulation is of inhibitory nature, and probably related to Werblin's windmill effect.

Rhodopsin kinetics in the cat retina

The bleaching and regeneration of rhodopsin in the living cat retina was studied by means of fundus reflectometry. Bleaching was effected by continuous light exposures of 1 min or 20 min, and the changes in retinal absorbance were measured at 29 wavelengths. For all of the conditions studied (fractional bleaches of from 65 to 100%), the regeneration of rhodopsin to its prebleach levels required greater than 60 min in darkness. After the 1-min exposures, the difference spectra recorded during the first 10 min of dark adaptation were dominated by photoproduct absorption, and rhodopsin regeneration kinetics were obscured by these intermediate processes. Extending the bleaching duration to 20 min gave the products of photolysis an opportunity to dissipate, and it was possible to follow the regenerative process over its full time-course. It was not possible, however, to fit these data with the simple exponential function predicted by first-order reaction kinetics. Other possible mechanisms were considered and are presented in the text. Nevertheless, the kinetics of regeneration compared favorably with the temporal changes in log sensitivity determined electrophysiologically by other investigators. Based on the bleaching curve for cat rhodopsin, the photosensitivity was determined and found to approximate closely the value obtained for human rhodopsin; i.e., the energy Ec required to bleach 1-e-1 of the available rhodopsin was 7.09 log scotopic troland-seconds (corrected for the optics of the cat eye), as compared with approximately 7.0 in man.

Longitudinal spread of adaptation in the rods of the frog's retina

1. The stimulus-response function of the red rods in the retina of the common frog (Rana temporaria) was determined in different adaptational states by measuring aspartate-isolated receptor responses. 2. Flash stimuli, background adaptations and bleaches were delivered through the same optical channel forming an oblique light-beam striking the receptor side of the isolated and flat-mounted retina at an angle of 10 degrees. 3. When the light was blue-green and optimally polarized the absorbance of the receptor layer was about 2, from which follows that 70-80% of the light was absorbed in the distal third of the rod outer segments, i.e. the exposure was local. Homogeneous exposures of the whole rod outer segments were obtained with orange and red lights. 4. Combinations of homogeneous and local stimuli with homogeneous and local adaptations were used to investigate the longitudinal spread of background, intermediate and opsin adaptation, i.e. the sensitivity-reducing effect of a background light, and the transient and permanent sensitivity losses following a bleach isomerizing 3.5-26% (usually 10%) of the rhodopsin in the retina. 5. The results obtained were related to predictions based both on the assumption that the adaptation effects spread longitudinally within the rod outer segments and the assumption that they are strictly confined to the disks absorbing the adapting lights. 6. These comparisons reveal that all three types of adaptation spread longitudinally. It is for instance clear that the sensitivity loss observed with homogeneous stimuli and local adaptation (as compared to homogeneous adaptation) is larger than that predicted by the non-spreading hypothesis. 7. The longitudinal spread of background adaptation is largely finished within 10 sec after turning on the background light, while an efficient spread of the intermediate adaptation effect may require minutes. 8. A background light decreasing the sensitivity by about one log unit decreases the time from flash to response maximum from 5 to 1 sec (small responses). Corresponding opsin adaptation effects are accompanied by less dramatic changes in response kinetics. 9. Independent of adaptation type - homogeneous or local, background, intermediate or opsin - it was found that local stimuli are less efficient that homogeneous stimuli in light-adapted retinae. This effect can be explained assuming that the sensitivity-reducing effects are pronounced in the distal than in the proximal parts of the rod outer segments. 10. The opsin adaptation effect following 10% local bleaches decreases the sensitivity to both homogeneous and local stimuli 2-3 times more than corresponding homogeneous bleaches. This means that the strength of the opsin effect is not related to the average percentage bleached but to the fraction bleached in the distal part of the rod, or generally to the fraction bleached in the most affected region. 11...

Receptive fields of frog retinal ganglion cells: response formation and light-dark-adaptation

1. The excitatory and inhibitory receptive field mechanisms of retinal ganglion cells were studied by extracellular recording from the eyecup of Rana temporaria in order to elucidate the nature of adaptational changes in the functioning of the receptive field. 2. The responses to large stimuli were always strongly depressed relative to responses evoked by smaller spots. This was true even in the fully dark-adapted state and at the very lowest stimuli intensities. 3. Threshold measurements confirmed earlier findings, usually revealing the surround only in light-adapted states. However, in more than 10% of fully dark-adapted cells thresholds to large stimuli were significantly elevated. 4. The central summation area of the receptive field was found to shrink with light-adaptation. There was a gradual decrease in diameters, amounting to some 20-30%, from the dark-adapted, rod-determined receptive fields to the cone-determined ones. 5. Adaptation by bleaching and adaptation by backgrounds changed the effects of the surround in different ways. After a rhodopsin bleach the transition from a light-adapted to a dark-adapted situation was seen as an abrupt drop of large-stimulus thresholds at some time during adaptation. Steady backgrounds produced no such dramatic changes, but the increment threshold lines were somewhat steeper with test spots stimulated the surround than with smaller spots. 6. Although the discharge patterns generally show the strength of the surround influence, they underwent no qualitative change at the time of the drop of large-stimulus thresholds after a bleach. 7. It is suggested that the drop does not reflect a sudden reorganization of the receptive field, but is the consequence of the different ways the response to large stimuli are formed in different ranges of stimulus intensity (pre-inhibitory at high intensities, post-inhibitory at low intensities), and of gradual changes in signal dynamics.

Interaction of light with the organ of Corti. I. Light guide effects in cochlear hair cells

A cochlear postmortem preparation has been developed which allows for hydromechanical studies of the transilluminated organ of Corti. It was found that cochlear hair cells act as optical waveguides. This poperty of hair cells is important for optical cochlear investigations. It may be used in the study of the motion of single hair cells.

Effects of alcohol on ganglion cell receptive field properties and sensitivity in the frog retina

Previous results have shown that alcohol has an effect on vision and on the excitability of retinal neurons. Action potentials of single ganglion cells were recorded by microelectrodes in opened and excised eyes from frogs (Rana temporaria L.). Histologically two types of synapses have been described in the retina: conventional synapses and synapses with a ribbon or bar shaped component surrounded by a rather uniform layer of synaptic vesicles. The "ribbon synapses" are presynaptic contacts in receptor and bipolar cells while horizontal and amacrine cells have conventional synapses. Tests with ethanol doses up to 0.2% indicated stronger effects on the conventional synapses than on the ribbon synapses. Alcohol decreased or abolished the lateral inhibition ( inhibitory surround) mediated by the amacrine cells and depressed the signals from the green rods, which apparently are mediated by horizontal cells. Further alcohol decreased the sensitivity of the signals from the completely dark-adapted red rods in the retina, and increased the sensitivity of the cone-mediated responses for class 3 and deviating class 4 cells, when measured against a background light. Alcohol also increased the latency of the response up to 55 msec. depending on the size of the stimulus field.

Receptive field organization of ganglion cells in the frog retina: contributions from cones, green rods and red rods

1. The impulse discharge of ganglion cells was recorded with extracellular micro-electrodes in the excised and opened eye of the common frog, Rana temporaria. 2. When a single unit was isolated, the cell type was first determined according to the Maturana, Lettvin, McCulloch & Pitts (1960) classification with the aid of varying moving and stationary stimuli. 3. Class 4 cells respond only to a decrease of light when cones are stimulated but respond to an increase of light when green rods are stimulated. A distinct class of deviating class 4 cells was found that give a brief high frequency burst at 'off' from their small excitatory receptive fields (ERF); unlike typical class 4 cells they possess a purely inhibitory surrounding field (IRF).4. The contributions from the cones and the green and red rods were isolated by measuring the thresholds of the discharges with on-off stimuli of varying wave-lengths against strong yellow backgrounds, or against a very weak background or no background at all. The spatial distribution of the contributions to the ERF was determined by mapping threshold profiles, and additional information about ERF and IRF was obtained from area-threshold curves. 5. The cone-mediated ERFs were found to be 0-06-0-50 mm wide (1-5-12 degrees of visual field), which agrees well with the sizes of the dendritic trees of the ganglion cells. The green rod-mediated ERFs can be 0-5-1-5 mm wide and have less distinct boundaries than the cone-mediated. The green rod-mediated ERF of an individual ganglion cell is always larger than the cone-mediated ERF of the same cell. The red rod-mediated ERFs seem to be somewhat larger than the cone-mediated but smaller than the green rod-mediated. 6. The green rods contribute only to the on thresholds of class 1, 2 and 4 cells, but both to on and off in typical class 3 cells, while the cones contribute to on and off in classes 1-3 and only to off in class 4.7. When the red rods begin to contribute during dark adaptation they seem to enter the cone but not the green rod channels. 8. All three receptor types contribute to the IRF surrounding the ERF of classes 1, 2, 3 and deviating class 4 cells. Normal class 4 cells have no IRF. 9. The organization of the receptive fields is discussed in relation to the anatomy and electrophysiology of the cell types transmitting the signals from the receptors to the ganglion cells.

Visual adaptation in the retina of the skate

The electroretinogram (ERG) and single-unit ganglion cell activity were recorded from the eyecup of the skate (Raja erinacea and R. oscellata), and the adaptation properties of both types of response compared with in situ rhodopsin measurements obtained by fundus reflectometry. Under all conditions tested, the b-wave of the ERG and the ganglion cell discharge showed identical adaptation properties. For example, after flash adaptation that bleached 80% of the rhodopsin, neither ganglion cell nor b-wave activity could be elicited for 10-15 min. Following this unresponsive period, thresholds fell rapidly; by 20 min after the flash, sensitivity was within 3 log units of the dark-adapted level. Further recovery of threshold was slow, requiring an additional 70-90 min to reach absolute threshold. Measurements of rhodopsin levels showed a close correlation with the slow recovery of threshold that occurred between 20 and 120 min of dark adaptation; there is a linear relation between rhodopsin concentration and log threshold. Other experiments dealt with the initial unresponsive period induced by light adaptation. The duration of this unresponsive period depended on the brightness of the adapting field; with bright backgrounds, suppression of retinal activity lasted 20-25 min, but sensitivity subsequently returned and thresholds fell to a steady-state value. At all background levels tested, increment thresholds were linearly related to background luminance.

Visual adaptation of the rhodopsin rods in the frogs retina

1. The threshold of the discharge from single ganglion cells in the excised and opened frog's eye has been measured with on/off stimuli and test parameters that make it possible to activate the rhodopsin rods only. The test stimuli have been restricted to the central part of the receptive field, where no nervous reorganization can be observed with changes in the state of adaptation.2. When such thresholds and the intensities of the background lights are expressed in terms of the number of quanta absorbed per unit time, it is found that three factors can be correlated with the thresholds measured in various states of light- and dark-adaptation: (i) the intensity of a steady background, (ii) the rate of regeneration of rhodopsin, and (iii) the amount of metarhodopsin II present in the rods.3. The threshold is found to be proportional both to the intensity of a background and to the rate of regeneration, whereas there is a linear relationship between the logarithm of the threshold and the amount of metarhodopsin II.4. The presence of metarhodopsin elevates all thresholds, the absolute threshold, increment thresholds and the thresholds elevated by regenerating rhodopsin in the same way.5. The saturation of the rods at high background intensities is found to be correlated with the accumulation of significant amounts of metarhodopsin in the rods, caused by the bleaching effect of the background.6. The effect of metarhodopsin on the threshold is independent of the amount of rhodopsin present in the rods.7. The combined effect of all three factors can be expressed in a general formula, given as eqn. (7) on p. 74.8. A background not only reduces the signals from the rods illuminated, but also those from neighbouring unilluminated rods. This effect is rapidly decreased with increasing distance from rods covered by the background. This kind of lateral spread in the retina probably occurs also when the rate of regeneration affects the threshold. The effect of metarhodopsin, on the other hand, appears restricted to those receptors that contain this substance.