L. Herrero

Tail and eye movements evoked by electrical microstimulation of the optic tectum in goldfish

Abstract This work studies the tail and eye co-ordinated movements evoked by the focal electrical stimulation of the tectum in goldfish. The aim of the study is to understand better those tectal sites and mechanisms that either remain functionally unaltered or are adaptively modified across vertebrates. Stimulation was applied in various tectal zones, and the characteristics of evoked tail and eye movements were examined as a function of the stimulation site over tectal surface and the stimulus parameters. Two types of response were electrically evoked: the former turned the body and the eyes contraversively towards the source of natural stimulus; the second produced initial ipsiversive turning of the body and eyes, followed by several tail beats. Evoking one or other response depended on both the site and parameters of stimulation, and responses were interpreted as orienting- and escape-like, respectively. Depending on the stimulation site, four different zones in the tectum were distinguished: in the medial zone the stimulus elicited eye and tail movements whose size increased with the distance to the rostral pole. The stimulation of the antero-medial zone evoked contraversive or ipsiversive eye saccades but tail movements were similar, irrespective of eye movements. Stimulation within the extreme antero-medialzone evoked convergent eye movements, and tail displacements turning the body either ipsiversively or contraversively. Stimulation of the posterior zone often evoked complex tail movements and pure horizontal eye saccades. Both orienting- and escape-like responses were also dependent on the stimulus parameters. The relationships between stimulus parameters and tail- and eye-orienting movement characteristics suggest that the velocity and duration might be encoded in different aspects of the tectal activity. Current strength also modified the number of tail beats that appeared during escape-like response. In conclusion, the present data suggest the involvement of the optic tectum not only in orienting but also in escape responses and that movements of eye and tail mediating such responses depend on the tectal active locus together with its level of activity.

Differential effects of trans-crotononitrile and 3-acetylpyridine on inferior olive integrity and behavioural performance in the rat

The inferior olive climbing fibre projection is key to cerebellar contributions to motor control. Here we present evidence for a novel tool, trans‐crotononitrile (TCN), to selectively inactivate the olive to study its functions. Anatomical, electrophysiological and behavioural techniques have been used in rats to assess the CNS effects of TCN, with a focus on the olivocerebellar projection. These findings were compared with those obtained with 3‐acetylpyridine (plus nicotinamide administered 3.5 h later, 3AP + 3.5 h). Fluoro‐Jade B cell labelling showed that TCN and 3AP + 3.5 h induce neurodegeneration primarily within the inferior olive, with no other targets in common. Recordings of evoked field potentials on the cerebellar cortical surface showed that both neurotoxins can reduce transmission in climbing fibre but not mossy fibre pathways. Both histological and electrophysiological differences suggest that TCN and 3AP have distinct mechanisms of action. Estimates of the numbers of surviving cells within individual subdivisions of the olive indicate that TCN and 3AP + 3.5 h cause different patterns of subtotal olivary lesion: most surviving neurons are present in the rostral (TCN) or caudal (3AP + 3.5 h) parts of the medial accessory olive, which are associated with two different cerebellar modules: the C2 and A modules, respectively. In behavioural studies, TCN and 3AP + 3.5 h produced differences in motor deficits consistent with the notion that these cerebellar modules have distinct functional responsibilities. Thus, studies using TCN as compared with 3AP + 3.5 h have the potential to shed light on the contributions of different cerebellar modules in motor control.

Tectotectal connectivity in goldfish

The vertebrate optic tectum is a functionally coupled bilateral structure which plays a major role in the generation of motor commands for orienting responses. However, the characteristics of the tectotectal connectivity are unknown in fish, and have been reported only to a limited extent in other vertebrates. The purpose of the present study was to determine the anatomical basis underlying the functional coupling between tecta in goldfish, and to identify both similarities and differences to those features reported in other vertebrate species. The present experiments used the bidirectional tracer biotinylated dextran amine to map the distribution of labeled cells and synaptic boutons in the contralateral tectum following injections into identified tectal sites. Fibers that interconnect both tecta coursed through the tectal commissure. The cells of origin of these fibers, the tectotectal cells, and their synaptic endings were located in the deep layers, mainly in the strata periventricular and griseum central, respectively. Corresponding sites throughout the two tecta were interconnected in a symmetrical point‐to‐point fashion. The tectal commissure was composed of at least two distinct bundles of axons, which differed in their dorsoventral location, fiber diameter, and projection targets. The dorsal axons were tectotectal axons, they were thinner in diameter and profusely branched, and gave off en passant and terminal boutons in the deep layers of the contralateral tectum. The ventral axons were thicker in diameter, and formed the contralateral tectofugal‐descending tract. Such fibers had few axon collaterals and boutons in the contralateral tectum. Boutons adjacent to retrogradely labeled tectotectal cells were very scarce. The data are discussed in terms of the coupling between tecta generating the motor commands required for orienting movements. J. Comp. Neurol. 411:455–471, 1999.

Afferent connectivity to different functional zones of the optic tectum in goldfish.

This work studies the afferent connectivity to different functionally identified tectal zones in goldfish. The sources of afferents contributed to different degrees to the functionally defined zones. The dorsocentral area of the telencephalon was connected mainly with the ipsilateral anteromedial tectal zone. At diencephalic levels, neurons were found in three different regions: preoptic, thalamic, and pretectal. Preoptic structures (suprachiasmatic and preoptic nuclei) projected mainly to the anteromedial tectal zone, whereas thalamic (ventral and dorsal) and pretectal (central, superficial, and posterior commissure) nuclei projected to all divisions of the tectum. In the mesencephalon, the mesencephalic reticular formation, torus longitudinalis, torus semicircularis, and nucleus isthmi were, in the anteroposterior axis, topographically connected with the tectum. In addition, neurons in the contralateral tectum projected to the injected zones in a symmetrical point-to-point correspondence. At rhombencephalic levels, the superior reticular formation was connected to all studied tectal zones, whereas medial and inferior reticular formations were connected with medial and posterior tectal zones. The present results support a different quantitative afferent connectivity to each tectal zone, possibly based on the sensorimotor transformations that the optic tectum carries out to generate orienting responses.

Connectivity of the goldfish optic tectum with the mesencephalic and rhombencephalic reticular formation

The optic tectum of goldfish, as in other vertebrates, plays a major role in the generation of orienting movements, including eye saccades. To perform these movements, the optic tectum sends a motor command through the mesencephalic and rhombencephalic reticular formation, to the extraocular motoneurons. Furthermore, the tectal command is adjusted by a feedback signal arising from the reticular targets. Since the features of the motor command change with respect to the tectal site, the present work was devoted to determining, quantitatively, the particular reciprocal connectivity between the reticular regions and tectal sites having different motor properties. With this aim, the bidirectional tracer, biotin dextran amine, was injected into anteromedial tectal sites, where eye movements with small horizontal and large vertical components were evoked, or into posteromedial tectal sites, where eye movements with large horizontal and small vertical components were evoked. Labeled boutons and somas were then located and counted in the reticular formation. Both were more numerous in the mesencephalon than in the rhombencephalon, and ipsilaterally than contralaterally, with respect to the injection site. Furthermore, the somas showed a tendency to be located in the area containing the most dense labeling of synaptic endings. In addition, labeled boutons were often observed in close association with retrogradely stained neurons, suggesting the presence of a tectoreticular feedback circuit. Following the injection in the anteromedial tectum, most of the boutons and labeled neurons were found in the reticular formation rostral to the oculomotor nucleus. Conversely, following the injection in the posteromedial tectum, most of the boutons and neurons were also located in the caudal mesencephalic reticular formation. Finally, boutons and neurons were found in the rhombencephalic reticular formation surrounding the abducens nucleus. They were more numerous following the injection in the posteromedial tectum. These results demonstrate characteristic patterns of reciprocal connectivity between physiologically different tectal sites and the mesencephalic and rhombencephalic reticular formation. These patterns are discussed in the framework of the neural substratum that underlies the codification of orienting movements in goldfish.

Pontine and lateral reticular projections to the c1 zone in lobulus simplex and paramedian lobule of the rat cerebellar cortex.

Spatial localization and axonal branching in mossy fiber projections to two rostrocaudally-separated regions of the ‘forelimb’ c1 zone in lobulus simplex and paramedian lobule were studiend in rats using a retrograde double-labelling tracer technique. In four animals the two cortical regions were localized electrophysiologically and each was micro-injected with tracer material, yielding a total of eight different cases. Single- and double-labelled cell bodies were plotted in the basal pontine nucleus (BPN), nucleus reticularis tegmenti pontis (NRTP), and the lateral reticular nucleus (LRN). As a control, cells labelled in the contralateral inferior olive were also counted. The parts of the c1 zone in lobulus simplex and the paramedian lobule were found to receive mossy fiber inputs from similar regions of BPN, NRTP and LRN. Double-labelled cells were not found in NRTP but were present in BPN and LRN (on average 6% and 25% of the smaller single-labelled population, respectively). The incidence of double-labelled cells in the olive and LRN was positively correlated, but no relation was found between olive and BPN, suggesting a zonal organization within the mossy fiber projections from LRN, but not from the pons. In quantitative terms, the c1 zone in lobulus simplex received a greater density of mossy fiber projections from BPN, NRTP and LRN than the c1 zone in the paramedian lobule. This suggests that the two parts of the same cerebellar cortical zone differ, at least partially, in regard to their inputs from three major sources of mossy fibers. This is consistent with the modular hypothesis and could enable a higher degree of parallel processing and integration of information within different parts of the same zone.

Eye movements evoked by electrical microstimulation of the mesencephalic reticular formation in goldfish

Anatomical studies in goldfish show that the tectofugal axons provide a large number of boutons within the mesencephalic reticular formation. Electrical stimulation, reversible inactivation and cell recording in the primate central mesencephalic reticular formation have suggested that it participates in the control of rapid eye movements (saccades). Moreover, the role of this tecto-recipient area in the generation of saccadic eye movements in fish is unknown. In this study we show that the electrical microstimulation of the mesencephalic reticular formation of goldfish evoked short latency saccadic eye movements in any direction (contraversive or ipsiversive, upward or downward). Movements of the eyes were usually disjunctive. Based on the location of the sites from which eye movements were evoked and the preferred saccade direction, eye movements were divided into different groups: pure vertical saccades were mainly elicited from the rostral mesencephalic reticular formation, while oblique and pure horizontal were largely evoked from middle and caudal mesencephalic reticular formation zones. The direction and amplitude of pure vertical and horizontal saccades were unaffected by initial eye position. However the amplitude, but not the direction of most oblique saccades was systematically modified by initial eye position. At the same time, the amplitude of elicited saccades did not vary in any consistent manner along either the anteroposterior, dorsoventral or mediolateral axes (i.e. there was no topographic organization of the mesencephalic reticular formation with respect to amplitude). In addition to these groups of movements, we found convergent and goal-directed saccades evoked primarily from the anterior and posterior mesencephalic reticular formation, respectively. Finally, the metric and kinetic characteristics of saccades could be manipulated by changes in the stimulation parameters. We conclude that the mesencephalic reticular formation in goldfish shares physiological functions that correspond closely with those found in mammals.

Influence of the tectal zone on the distribution of synaptic boutons in the brainstem of goldfish.

This study investigated whether the topographic differences in the functional properties of the tectal motor map of goldfish are related to particular patterns of connections with downstream structures. With this aim, the distribution of synaptic boutons in the mesencephalic and rhombencephalic structures was studied after discrete injections of the tracer biotinylated dextran amine were placed at separate sites along the tectal anteroposterior axis. Irrespective of the location of the injection site, the boutons were more abundant in the mesencephalon than in the rhombencephalon, and they were located chiefly ipsilaterally all throughout the brainstem. In the mesencephalon, the boutons were found in its ventrolateral reticular formation and, to a lesser extent, in the nucleus of the medial longitudinal fasciculus, the oculomotor and isthmi nuclei, and the torus semicircularis. In the mesencephalic reticular formation, the bouton location was distributed topographically with respect to the injection site. Terminals were also observed in the nucleus of the medial longitudinal fasciculus after injections into anteromedial or middle tectal zones. In the oculomotor nucleus, boutons were present exclusively in the case of the anteromedial injection. In the rhombencephalon, most boutons were found in the superior reticular formation, and their number decreased in the medial and inferior reticular formations. A topographic distribution could be observed within the superior reticular formation, although its density was attenuated compared with that observed in the mesencephalic reticular formation. The domains of synaptic endings on the ipsilateral side were different from those on the contralateral side: The ipsilateral synaptic endings were located more medially. Finally, a few boutons were also found in the vestibulocerebellar area on either the ipsilateral or the contralateral side, depending on the injection site. From these data, the authors conclude that, in goldfish, irrespective of the tectal injection site, the endings are in similar nuclei in the brainstem; however, the distribution of synaptic boutons within such nuclei can be related to the functional properties of each tectal zone. J. Comp. Neurol. 401:411–428, 1998.

Visual orienting response in goldfish: a multidisciplinary study.

The neural basis underlying the orienting response has been thoroughly studied in frontal-eyed mammals. However, in non-mammalian species, including fish, it remains almost unknown. Therefore, we studied the contribution of the optic tectum and the mesencephalic reticular formation to the performance of the orienting response in goldfish, using behavioural, physiological, and anatomical tracer techniques. The appearance of a visual stimulus (a pellet of food) in the environment of a goldfish evoked a turn of the body to reorient the line of sight. Left-tectal lobe ablation abolished the orienting turn response towards the contralateral hemifield. Electrical microstimulation of the optic tectum suggested the presence of a motor map, which is in correspondence with the overlying visual representation, as previously reported in other vertebrates. The tracer biotin-dextran amine was injected into different functionally identified tectal zones. The results showed that rostral and caudal poles of the mesencephalic reticular formation receive outflow mainly from the rostral and caudal tectal poles, respectively. This suggests that the tectal wiring with downstream structures is site-dependent. Furthermore, the electrical activation of rostral and caudal mesencephalic reticular formation revealed a different contribution to vertical and horizontal orienting eye movements. We conclude that the basic neural system coding the orienting response appears early in phylogenesis, although some specific characteristics are selected by adaptive pressure.

Neural substrata underlying tectal eye movement codification in goldfish.

The optic tectum encodes orienting eye saccades in a spatially ordered map. To investigate whether the functional properties of each tectal site are related to a particular pattern of connectivity with downward structures in the brainstem, two sets of experiments were carried out. First, biotinylated dextran amine (BDA) was injected at different tectal sites along the anteroposterior axis. Electrical stimulation at these sites evoked saccades whose horizontal component amplitudes increased with the distance to the rostral pole. In the second experiment, BDA and fluoro-ruby (FR) were injected at different tectal sites along the mediolateral axis. Electrical stimulation here evoked saccades with different upward and downward directions, but similar horizontal component amplitudes. A major finding of the first experiment was that a topographic link of the tectum exists with the mesencephalic reticular formation, but that such a connection was absent or very attenuated for the rhombencephalic reticular formation. In the second set of experiments, the clusters of BDA and FR boutons left by the mediolateral tectal sites were separated in the rostral mesencephalon, at the level of the nucleus of the medial longitudinal fasciculus, but overlapped in the caudal mesencephalon and rhombencephalon. These data provide evidence that decodification of tectal motor commands is based, at least in part, on the connectivity of each tectal locus on downward structures with the brainstem.