The fixation area of the cat superior colliculus: effects of electrical stimulation and direct connection with brainstem omnipause neurons

The superior colliculus has long been recognized as an important structure in the generation of saccadic displacements of the visual axis. Neurons with presaccadic activity encoding saccade vectors are topographically organized and form a motor map. Recently, neurons with fixation-related activity have been recorded at the collicular rostral pole, at the area centralis representation or fixation area. Another collicular function which deals with the maintenance of fixation behavior by means of active inhibition of orientation commands was then suggested. We tested that hypothesis as it relates to the suppression of gaze saccades (gaze = eye in space = eye in head + head in space) in the head-free cat by increasing the activity of the fixation cells at the rostral pole with electrical microstimulation. Long stimulation trains applied before gaze saccades delayed their initiation. Short stimuli, delivered during the gaze saccades, transiently interrupted both eye and head components. These results provide further support for a role in fixation behavior for collicular fixation neurons. Brainstem omnipause neurons also exhibit fixation-related activity and have been shown to receive a direct excitatory input from the superior colliculus. To determine whether the collicular projection to omnipause neurons arises from the fixation area, the deep layers of the superior colliculus were electrically stimulated either at the rostral pole including the fixation area or in more caudal regions where stimulation evokes orienting responses. Forty-nine neurons were examined in three cats. 61% of the neurons were found to be orthodromically excited by single-pulse stimulation of the rostral pole, whereas only 29% responded to caudal stimulation. In addition, stimuli delivered to the rostral pole activated, on average, omnipause neurons at shorter latencies and with lower currents than those applied in caudal regions. These results suggest that excitatory inputs to omnipause neurons from the superior colliculus are principally provided by the fixation area, via which the superior colliculus could play a role in suppression of gaze shifts.     

Comparison of saccades evoked by visual stimulation and collicular electrical stimulation in the alert monkey

Summary In the alert monkey we have compared the properties of saccades elicited by a visual stimulus (V-saccades) with those generated by electrical stimulation in the superior colliculus (E-saccades). We found that whereas there exists a graded relation betweenE-saccade amplitude and current strength,E-saccade direction is remarkably independent of electrical stimulation parameters. At sufficiently high current strengths (about 20 A),E-saccades are consistently directed toward the center of the movement field of nearby cells, except when stimulation is performed at sites near the collicular borders. Further interesting differences between the amplitude and direction behaviour were observed when the variability inE-saccade vectors, obtained with fixed stimulation parameters, was analyzed. In all cases,E-saccade amplitude scatter exceeds direction scatter, suggesting the possibility of a polar coordinate organization for the coding of saccade metrics. These data are compared withV-saccade scatter data, recently obtained in the human (Van Opstal and Van Gisbergen 1989 c). Finally, an analysis of saccade dynamics shows thatE-saccades can reachV-saccadic velocities at higher current strengths. However, at near-threshold current strengths, whereE-saccade amplitude decreases (see above), we found at most stimulation sites (22/37) thatE-saccades are consistently slower thanV-saccades of the same amplitude. Possible mechanisms underlying the collicular role in saccade generation are discussed.     

Gaze shifts evoked by stimulation of the superior colliculus in the head-free cat conform to the motor map but also depend on stimulus strength and fixation activity

In our previous paper we demonstrated that electrical microstimulation of the fixation area at the rostral pole of the cat superior colliculus (SC) elicits no gaze movement but, rather, transiently suppresses eye-head gaze saccades. In this paper, we investigated the more caudal region of the SC and its interaction with the fixation area. In the alert head-free cat, supra-threshold stimulation in the anterior portion of the SC but outside the fixation area evoked small saccadic shifts of gaze consisting mainly of an eye movement, the head's contribution being small. Stimulating more posteriorly elicited large gaze saccades consisting of an ocular saccade combined with a rapid head movement. At these latter stimulation sites, craniocentric (goal-directed) eye movements were evoked when the cat's head was restrained. The amplitude of eye-head gaze saccades elicited at a particular stimulation site increased with stimulus duration, current strength, and pulse rate, until a constant or unit value was reached. The peak velocity of gaze shifts depended on both pulse rate and current strength. The movement direction was not affected by stimulus parameters. The unit gaze vector evoked, in the head-free condition, by stimulating one collicular site was similar to that coded by efferent neurons recorded at that site, thereby indicating a retinotopically coded gaze error representation on the collicular motor map which is not revealed by stimulating the head-fixed animal. Evoked gaze saccades were found to be influenced by fixation behavior. The amplitude of evoked gaze shifts was reduced if stimulation occurred when the hungry animal fixated a food target. Electrical activation of the collicular fixation area was found to mimic well the effects of natural fixation on evoked gaze shifts. Taken together, our results support the view that the overall distribution and level of collicular activity contributes to the encoding of the metrics of gaze saccades. We suggest that the combined levels of activity at the site being stimulated and at the fixation area influence the amplitude of evoked gaze saccades through competition. When stimulation is at low intensities, fixation-related activity reduces the amplitude of evoked gaze saccades. At high activation levels, the site being stimulated dominates and the gaze vector is specified only by that site's collicular output neurons, from which arises the close correspondence between the unit-evoked gaze saccades and the neurally coded gaze vector at that site.     

The role of the claustrum in the bilateral control of frontal oculomotor neurons in the cat

Summary The effect of claustrum (CL) stimulation on the spontaneous unitary activity of ipsi and contralateral frontal oculomotor neurons, was studied in chloralose-anaesthetized cats. A total of 205 units was bilaterally recorded in the medial oculomotor area, homologous of the primate frontal eye fields 127 neurons were identified as projecting to the superior colliculus; for 33 of these last units stimulation of the ipsilateral CL provoked an excitatory effect lasting 10–25 ms and appearing with a latency of 5–15 ms; on 8 units the excitatory effect was followed by an inhibition lasting 100–250 ms. Ninety-eight of the 127 neurons were also tested through activation of the contralateral CL: 13 cells showed an excitatory effect lasting 10–35 ms and appearing with a latency of 20–50 ms. In three of the thirteen units the excitatory effect was followed by an inhibition lasting 100–150 ms. Complete section of the corpus callosum abolished the contralateral CL effect, suggesting the existence of a direct claustro-contralateral oculomotor cortex pathway running through the corpus callosum. The results could support the hypothesis that the CL may play a role in the bilateral control of the visuomotor performance.     

The effect of microstimulation of the oculomotor vermis on discharges of fastigial neurons and visually-directed saccades in macaques

The present study confirmed our previous reports that neurons in the fastigial oculomotor region (FOR) of the macaque show presaccadic bursts during contralateral saccades and that the burst duration is closely related to the duration of the accompanying saccade. Furthermore, when the burst duration was reduced by subthreshold electrical stimulation applied to the oculomotor vermis prior to the onset of the burst, the impending visually-directed saccade became hypometric. The reduction in the burst duration was closely related to the degree of the hypometria. Since saccadic burst neurons in the FOR constitute the sole output channel for saccadic signals of the oculomotor vermis, the findings support the hypothesis that the cerebellum can regulate the amplitude of eye movements.     

Gaze-related activity of brainstem omnipause neurons during combined eye-head gaze shifts in the alert cat

Summary Studies undertaken in head-restrained animals have long implicated the omnipause neurons (OPNs) in the initiation of saccadic eye movements. These inhibitory neurons discharge tonically but cease firing just before and during saccades in all directions. By recording from OPNs in alert behaving head-unrestrained cats, we have demonstrated that the activity of these cells is related to the displacement of the visual axis in space (gaze), which is the sum of the eye movement relative to the head and head movement relative to space. OPNs were found to exhibit a complete cessation of discharge for a period equivalent to the duration of the gaze shift, and not to the duration of either the rapid eye movement or the head movement components. In large gaze shifts, OPNs were silent even when the eye was immobile in the orbit, as long as the gaze shift was not completed. The results of this study show that OPNs are controlled by neural elements that take into account the actual position of the visual axis relative to its final desired position, irrespective of the trajectory of the eye in the orbit or of whether the head is moving or not.     

Lattice of high oxidative metabolism in the intermediate grey layer of the rat and hamster superior colliculus

Patches of high oxidative metabolic activity were observed in sagittal sections through the rat and hamster colliculus. These patches were demonstrated with the histochemical markers succinate dehydrogenase and cytochrome oxidase. In surface parallel sections the patches were found to form a continuous lattice in both species. The lattice was composed of strips of high enzyme activity, about 60–200 μm wide, that surrounded islands of low activity. This lattice occurs at the same depth as the lattice of acetylcholinesterase activity, which also occurs in the intermediate grey layer, but the two lattices do not correspond.     

Effects of stimulation of frontal cortex,superior colliculus,and neck muscle afferents on interstitiospinal neurons in the cat

Summary Interstitiospinal neurons were activated by antidromic stimulation of the spinal cord ventromedial funiculus at C₁ and C₄ in cerebellectomized cats under chlor alose anesthesia. Neurons responding only to C₁ were classified as N cells and those responding both to C₁ and C₄ were classified as D cells, as in previous experiments (Fukushima et al. 1980a). Vestibular branching interstitiospinal and reticulospinal neurons were also identified as in the previous experiments.Stimulation of the ipsilateral pericruciate cortex evoked firing in 31% of N cells, 41% of D cells and 35% of vestibular branching neurons, while stimulation of the contralateral cortex excited 6% of N cells, 29% of D cells and 14% of vestibular branching neurons. Response latencies ranged from 2 to 15 ms after the effective pulse. By measuring the thresholds of activation of these neurons while changing the depth of the stimulating electrodes, and by mapping the cortical areas, it was shown that the lowest threshold areas were in the frontal eye fields and the anterior sigmoid gyrus near the presylvian sulcus (Area 6). Stimulation of the latter area often evoked neck or shoulder muscle contraction.Stimulation in the deep layers of the ipsilateral superior colliculus evoked firing in about 20% of interstitiospinal neurons and about 42% of vestibular branching neurons, with typical latencies 2–3 ms after the effective pulse, while stimulation of the contralateral superior colliculus was rarely effective. N cells and D cells responded similarly. Thresholds for activation were high in the intermediate tectal layers and declined as the electrodes entered the underlying tegmentum. This suggests that the superior colliculus is not the main source of synaptic inputs to these neurons. Low threshold points were found above the deep fiber layer when stimulating electrodes were inserted into the pretectum.Stimulation of the C₂ biventer cervicis nerve excited about 8% of N cells, 18% of D cells, and 15% of vestibular branching neurons bilaterally with typical latencies around 10 ms. Similar results were obtained when C₂ splenius nerves were stimulated. The fibers responsible for such excitation are probably group II, since stimuli stronger than 1.8 times threshold of the lowest threshold fibers were needed to evoke excitation. Response decrement was often observed when stimuli were repeated at 1/s, while no such decrement was observed at the rate of 1/3 s.When the convergence of cortical and labyrinthine excitatory inputs was studied, 36% of interstitiospinal neurons received single inputs either from the pericruciate cortex or from the labyrinth, 22% of neurons received convergent excitation from both and the remaining 42% did not respond to either stimulus. Although vestibular branching neurons rarely received labyrinthine inputs, they frequently showed convergence of excitation to stimulation of the frontal cortex, superior colliculus and vestibular nuclei.Supported in part by a Grant-in-Aid for Scientific Research (No. 477063) from The Ministry of Education, Science, and Culture of Japan     

Spatial and temporal visual properties of the neurons in the intermediate layers of the superior colliculus

Although the visual perception depends on the integration of spatial and temporal information, no knowledge is available concerning the responsiveness of neurons in the intermediate layers of the superior colliculus (SCi) to extended visual grating stimuli. Accordingly, we set out to investigate the responsiveness of these neurons in halothane-anesthetized cats to drifting sinewave gratings at various spatial and temporal frequencies. The SCi units responded optimally to gratings of low spatial frequencies (none of the analyzed SCi units exhibited maximal activity to spatial frequencies higher than 0.3 c/deg) and exhibited low spatial resolution and narrow spatial frequency tuning. On the other hand, the SCi neurons preferred high temporal frequencies and exhibited high temporal resolution. Thus, the SCi neurons seem to be good spatio–temporal filters of visual information in the low spatial and high temporal frequency domain. Based upon the above summarized results we suggest that the SCi units can detect large contours moving at high velocities well, but are unable to distinguish small details. This is in line with the generally held view that the SCi could possess visuomotor function, such as organizing the complex, sensory-guided oculomotor and skeletomotor responses during the self-motion of the animal.     

Activity of dorsal respiratory group inspiratory neurons during laryngeal-induced fictive coughing and swallowing in decerebrate cats

Membrane potential changes and/or discharges from 36 inspiratory neurons were recorded intracellularly in the dorsal respiratory group (DRG; i.e., the ventrolateral subdivision of the nucleus tractus solitarii) in decerebrate, paralyzed, and ventilated cats. Electrical activities were recorded from both somata (n=10) and axons (n=26). Activities during quiet breathing were compared with those observed during fictive coughing and swallowing evoked by repetitive electrical stimulation of afferent fibers of the superior laryngeal nerve (SLN). These nonrespiratory behaviors were evident in paralyzed animals as characteristic discharge patterns of the phrenic, abdominal, and hypoglossal nerves. Twenty-six neurons exhibiting antidromic action potentials in response to electrical stimuli applied to the cervical (C3–5) spinal cord were classified as inspiratory bulbospinal neurons (IBSNs). These neurons were considered as premotoneurons. The remaining 10 inspiratory neurons (INAA) were not antidromically activated by electrical stimuli applied to either cervical spinal cord or ipsilateral cervical vagus. These neurons are thought to be propriobulbar neurons. We recorded the activity of 31 DRG inspiratory neurons (24 IBSNs and 7 I-NAA) during coughing. All but one (a late-recruited IBSN) discharged a burst of action potentials during the coughing-related phrenic nerve activity. Typically, ramp-like membrane depolarization trajectories and discharge frequencies during coughing were similar to those observed during inspiration. We recorded the activity of 33 DRG inspiratory neurons (23 IBSNs and 10 I-NAA) during swallowing. Most (28/33) neurons were briefly activated, i.e., discharged a burst of action potentials during swallowing, but peak discharge frequency decreased compared with that measured during inspiration. The membrane potentials of nine somata exhibited a brief bell-shaped depolarization during swallowing, the amplitude of which was similar to that observed during inspiration. These results suggest that some inspiratory premotoneurons and propriobulbar neurons of the DRG might be involved in nonrespiratory motor activities, even if clearly antagonistic to breathing (e.g., swallowing). We postulate the existence in the medulla oblongata of adult mammals of neurons exhibiting a functional flexibility.     

The control of slow orienting eye movements by tectoreticulospinal neurons in the cat: behavior,discharge patterns and underlying connections

The activity of tectoreticulospinal neurons (TRSN) during orienting gaze shifts was studied in alert, head-fixed cats by intra-axonal recordings. The scope of the study was to evaluate the role of this class of superior colliculus neurons in the generation of slow eye movements (drifts) which often follow main-sequence saccades and sometimes appear as an independent motor event of orienting. The parameters of such movements are described in the first part of the paper. The organization of underlying pathways in the lower brainstem has been studied by intra-axonal horseradish peroxidase (HRP) tracing. The mean amplitude of postsaccadic drifts (PSD) is 1.21° (SD 0.63), but it can eventually reach 6–8°. PSDs have mean velocity of 14.9°/s (SD 4.28) and mean duration of 104.2 ms (SD 50.8). These two parameters are positively correlated with PSD amplitude. The presence of PSDs is usually associated with an increased neck muscle activity on the side toward which the eyes move. The durations of these two motor events show a reliable positive correlation. PSDs appear to occur when gaze error persists after a saccade and a correction is attempted by means of a slow eye movement and a head turn. The durations of TRSN bursts are, on average, longer than the sum of the lead time and the saccade duration. Bursts associated with combinations of saccades and PSD are significantly longer than those recorded in the absence of PSDs. The probability of occurrence of PSDs is higher when firing of TRSNs continues after saccade termination. Such prolonged discharges usually coincide with a combination of PSDs and phasic activation of the neck electromyogram. The mean firing rate of TRSNs during PSDs is 62% of that during saccade-related portions of the burst and declines to 45% after the end of PSDs. According to its timing and intensity, postsaccadic firing of TRSNs is appropriate as a signal underlying slow, corrective eye movements and later portions of phasic neck muscle contractions during orienting. Intraaxonal HRP labeling showed that visuomotor TRSNs of the X type (n = 3) terminate in the abducens nucleus, with 145–331 boutons terminaux and en passant. Average bouton densities in the nucleus are lower than in the periabducens reticular formation, but higher than in more rostral paramedian pontine reticular formation (PPRF) regions. Terminal fields in the PPRF match the locations of eye-neck reticulospinal neurons (RSNs) and exitatory burst neurons. Termination densities comparable with those in the caudal PPRF are found also in the rostral nucleus reticularis gigantocellularis, which contains phasic RSNs (neck bursters) and inhibitory burst neurons. Morphological observations alone do not exclude firing rate modulation of abducens motoneurons through the monosynaptic tectal pathway. However, the available physiological data point to a major role of a multiple convergent connection involving the eye-neck RSNs. In conclusion, the signals of X type TRSNs, reinforced by parallel connection through RSNs, encode mainly the intended head movement. Collateral actions of these two populations may be sufficient to induce slow, orienting eye movements, independently of the burst output from the classic saccadic generator.     

Rostrocaudal and lateromedial density distributions of superior colliculus neurons projecting in the predorsal bundle and to the spinal cord: a retrograde HRP study in the cat

Summary Efferent neurons of the cat superior colliculus (SC) which project in the predorsal bundle (PDB) and to the spinal cord (PDB neurons) form a major pathway by which the SC controls the changes of the direction of gaze in response to stimuli of visual and other modalities. Knowledge of rostrocaudal and lateromedial density distributions of different groups of PDB neurons within the SC is necessary to analyse their relationships with the topography of sensory and motor maps. Density gradients may also bear on the efficacy of connections originating from topographically different collicular regions. In the present study, large injections of HRP/ WGA-HRP were made in the C1 segment of the spinal cord and in the pontobulbar tegmentum. Judged by several morphological criteria, axons of passage, including those not subjected to a direct mechanical damage, were participating in the uptake of tracers. Therefore, labeled SC neurons corresponded to the nearly total populations of contralaterally projecting tectospinal neurons (TSNs) and neurons projecting in the PDB, respectively. Subtraction of the TSN density map from that of the whole PDB population was used to infer the distribution of tectal neurons terminating in the rhombencephalic tegmentum (TRhN). This subtotal labeling method proved useful in resolving the contradictions between the earlier HRP studies on the TSN and TRhN topography. The following density distributions were obtained for different groups of PDB neurons: 1) The mean TSN density is more than two times higher in the lateral half of the SC, representing the lower visual field. In this region the density remains constant from rostral to caudal, i.e., from the representation of vertical meridian to large contralateral azimuths. In the medial half, the average density decreases from rostral to caudal. Consequently, TSNs do not show the caudalward increment predicted by the higher efficacy of caudal stimulation points in eliciting head movements. 2) The distribution of PDB neurons is symmetrical with respect to the representation of the horizontal meridian. It is close to homogeneous at all azimuths of the retinotopic map and within the zone limited by small (10–15°) upward and downward elevations. There are clear density decrements towards the representation of greater elevations. We conclude that the assumption of homogeneous distribution of efferent neurons, made in the models of the primate SC, is valid in the cat, but only for the whole population of neurons projecting in the PDB and within the area representing the proximity of the horizontal meridian. The same restrictions apply to the validity of the translation invariance principle. 3) The distribution of TRhNs, obtained by subtraction, shows a tendency to higher densities in the caudal half of the SC, and a clear bias towards its medial zone, including the representation of the horizontal meridian and upper visual field. The different types of asymmetry in TRhN and TSN populations may be related to different types of head movements depending, respectively, on distant and near stimuli in the upper and lower visual fields.Abbreviations NRTP nucl. reticularis tegmenti pontis - PDB predorsal bundle - PPRF paramedian pontine reticular formation - RGc nucl. reticularis gigantocellularis - Rmc nucl. reticularis magnocellularis - RPc nucl. reticularis pontis caudalis - RPo nucl. reticularis pontis oralis - SGI stratum griseum intermediale - SGP deeper layers, beginning from stratum album intermediale - SC superior colliculus - TRhN tectorhombencephalic neuron with crossed projection - TSN tectospinal neuron On leave from: Laboratoire de Neurophysiologie, Faculté de Médecine, Université de Louvain, Belgium     

Subcellular localization of GABAA/benzodiazepine receptor-like immunoreactivity in the superficial gray layer of the rat superior colliculus

The monoclonal antibody bd-17, which recognizes the β₂ and β₃-subunits of GABA_A/benzodiazepine receptors, was used to determine the cellular and subcellular localization of receptor-like immunoreactivity in the superficial gray layer of the rat superior colliculus. In numerous dendrites, very strong immunostaining was present in the cytoplasm and on the postsynaptic dendritic membrane of synaptic junctions. The extrasynaptic portion of the dendritic membrane also very often showed [β₂ + β₃]-like immunoreactivity. However, due to methodological limitations, it could not be stated with certainty whether presynaptic β₂- and β₃-subunits of GABA_A/benzodiazepine receptors actually occur in this mesencephalic visual structure. In conclusion, these results strongly suggest that synaptic and non-synaptic GABA_A/benzodiazepine receptors are present in the superficial gray layer of the rat superior colliculus. These receptors may modulate neuronal cell activity in different ways, depending on their location.     

Arm-movement-related neurons in the primate superior colliculus and underlying reticular formation: comparison of neuronal activity with EMGs of muscles of the shoulder, arm and trunk during reaching

 Neuronal activity was recorded from the superior colliculus (SC) and the underlying reticular formation in two monkeys during an arm reaching task. Of 744 neurons recorded, 389 (52%) clearly modulated their activity with arm movements. The temporal activity patterns of arm-movement-related neurons often had a time course similar to rectified electromyograms (EMGs) of particular muscles recorded from the shoulder, arm or trunk. These reach cells, as well as the muscles investigated, commonly exhibited mono- or biphasic (less frequently tri- or polyphasic) excitatory bursts of activity, which were related to the (pre-)movement period, the contact phase and/or the return movement. The vast majority of reach cells exhibited a consistent activity pattern from trial to trial as did most of the muscles of the shoulder, arm and trunk. Similarities between the activity patterns of the neurons and the muscles were sometimes very strong and were especially notable with the muscles of the shoulder girdle (e.g. trapezius descendens, supraspinatus, infraspinatus or the anterior and medial deltoids). This high degree of co-activation suggests a functional linkage, though not direct, between the collicular reach cells and these muscles. Neuronal activity onset was compared with that of 25 muscles of the arms, shoulders and trunk. The majority of cells (78.5%) started before movement onset with a mean lead time of 149±90 ms, and 36.5% were active even before the earliest EMG onset. The neurons exhibited the same high degree of correlation (r=0.97, Spearman rank) between activity onset and the beginning of the arm movement as did the muscles (r=0.98) involved in the task. The mean neuronal reach activity (background subtracted) ranged between 7 and 193 impulses/s (mean 40.5±24.2). The mean modulation index calculated [(reach activity −background activity)/reach activity+background activity)] was 0.75±0.23 for neurons (n=358) and 0.87±0.14 for muscles (n=25). As the monkeys fixated the reach target constantly during an arm movement, neuronal activity which was modulated in this period was not related to eye movements. The three neck muscles investigated in the reach task exhibited no reach-related activity modulation comparable to that of either the reach cells or the muscles of the shoulder, arm and trunk. However, tonic neck muscle EMG was monotonically related to horizontal eye position. The clear skeletomotor discharge characteristics of arm-movement-related SC neurons revealed in this study agree with those already known from other sensorimotor regions (for example the primary motor, the premotor and parietal cortex, the basal ganglia or the cerebellum) and are consistent with the possible role of this population of reach cells in the control of arm movements. Received: 17 June 1996 / Accepted: 24 December 1996     

Activity of substantia nigra neurons in the cat brain during a self-initiated behavioral act

Cats were trained to perform a self-initiated behavioral act in the form of an operant food-obtaining reflex with defined time requirements. Activity was recorded from 50 dopaminergic neurons (identified in terms of their low frequency of background activity and long action potentials) and 67 nondopaminergic neurons of the substantia nigra and adjacent region. Dopaminergic neurons were the more responsive. Prior to EMG activation, the activity of 33 (66%) of these cells changed, and 44 (88%) showed changes in activity on movement. Dopaminergic neurosn showed increased activity during the period of waiting for the conditioned stimulus, predicting the release of reinforcement or its absence. These cells were more frequently activated in response to a positive signal and reinforcement and were more frequently inhibited in the absence of reinforcement. The high reactivity of dopaminergic neurons during execution of a movement task could be explained by the involvement of a cognitive component, i.e., determining the point at which the movement should start. Department of Human and Animal Physiology and Biophysics, Simferopol' State University, 4 Yaltinskaya, 333036 Simferopol', Ukraine. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova Vol. 83, No. 1–2, pp. 28–34, January–February, 1997. Original article     

Subcellular localization of GABAA/benzodiazepine receptor-like immunoreactivity in the superficial gray layer of the rat superior colliculus

The monoclonal antibody bd-17, which recognizes the β₂ and β₃-subunits of GABA_A/benzodiazepine receptors, was used to determine the cellular and subcellular localization of receptor-like immunoreactivity in the superficial gray layer of the rat superior colliculus. In numerous dendrites, very strong immunostaining was present in the cytoplasm and on the postsynaptic dendritic membrane of synaptic junctions. The extrasynaptic portion of the dendritic membrane also very often showed [β₂ + β₃]-like immunoreactivity. However, due to methodological limitations, it could not be stated with certainty whether presynaptic β₂- and β₃-subunits of GABA_A/benzodiazepine receptors actually occur in this mesencephalic visual structure. In conclusion, these results strongly suggest that synaptic and non-synaptic GABA_A/benzodiazepine receptors are present in the superficial gray layer of the rat superior colliculus. These receptors may modulate neuronal cell activity in different ways, depending on their location.