Trigeminal inputs to reticulospinal neurones in lampreys are mediated by excitatory and inhibitory amino acids

Reticulospinal (RS) neurones integrate sensory inputs from several modalities to generate appropriate motor commands for maintaining body orientation and initiation of locomotion in lampreys. As in other vertebrates, trigeminal afferents convey sensory inputs from the head region. The in vitro brainstem/spinal cord preparation of the lamprey was used for characterizing trigeminal inputs to RS neurones as well as the transmitter systems involved. The trigeminal nerve on each side was electrically stimulated and synaptic responses, which consisted of mixed excitation and inhibition, were recorded intracellularly in the middle and posterior rhombencephalic reticular nuclei. The EPSPs were mediated by activation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate receptors. An increase in the late phase of the excitatory response occurred when Mg²⁺ ions were removed from the Ringer's solution. This effect was antagonized by 2-amino-5-phosphonopentanoate (2-AP5) or reversed by restoring Mg²⁺ ions to the perfusate suggesting the activation of N-methyl-d-aspartate (NMDA) receptors. IPSPs were mediated by glycine. These findings are similar to those reported for other types of sensory inputs conveyed to RS neurones, where excitatory and inhibitory amino acid transmission is also involved.     

Visual Potentiation of Vestibular Responses in Lamprey Reticulospinal Neurons

The lamprey normally swims with the dorsal side up. Illumination of one eye shifts the set-point of the vestibular roll control system, however, so that the animal swims with a roll tilt towards the source of light (the dorsal light response). A tilted orientation is often maintained for up to 1 min after the stimulation. In the present study, the basis for this behaviour was investigated at the neuronal level. The middle rhombencephalic reticular nucleus (MRRN) is considered a main nucleus for the control of roll orientation in lampreys. Practically all MRRN neurons receive vestibular and visual input and project to the spinal cord. Earlier extracellular experiments had shown that optic nerve stimulation potentiates the response to vestibular stimulation in the ipsilateral MRRN. This most likely represents a neural correlate of the dorsal light response. Experiments were carried out in vitro on the isolated brainstem of the silver lamprey (Ichthyomyzon unicuspis). MRRN cells were recorded intracellularly, and the overall activity of descending systems was monitored with bilateral extracellular electrodes. The responses to 10 Hz optic nerve stimulation and 1 Hz vestibular nerve stimulation, and the influence of optic nerve stimulation on the vestibular responses, were investigated. In most preparations, optic nerve stimulation excited practically all ipsilateral MRRN cells. After stimulation, the cell was typically depolarized and showed an increased level of synaptic noise for up to 80 s. In contralateral MRRN neurons, optic nerve stimulation usually evoked hyperpolarization or no response. Vestibular nerve stimulation evoked compound excitatory postsynaptic potentials (EPSPs) or spikes in -90% of the cells, both ipsilaterally and contralaterally. A smaller subpopulation of MRRN cells (-10%) received vestibular inhibition. In 26 of 48 recorded MRRN cells, the response to vestibular stimulation was potentiated after ipsilateral optic nerve stimulation. The potentiation was seen in cells receiving either excitatory or inhibitory vestibular input as an increase in EPSP amplitudelspiking (85%) and a decrease in inhibitory postsynaptic potential amplitude (15%) respectively. In most cases the vestibular responses did not return to control levels during the testing period (10–30 min), and thus the visual stimulation most likely induced long-lasting changes in the functional connectivity of the roll control network, in addition to the short-lasting afteractivity. In four of the 11 cells recorded contralateral to the stimulated optic nerve, a depression of the vestibular response could be seen. In potentiated cells, single vestibular pulses often evoked longer episodes of large synaptic noise and sometimes spiking. In the latter case, the action potentials appeared with highly variable latency after each stimulation pulse. This indicates that an important mechanism underlying the potentiation may be a long-lasting increase in excitability in a pool of unidentified interneurons located either upstream of the MRRN cells, relaying vestibular and visual inputs, or downstream, providing positive feedback.     

Stimulation of the mesencephalic locomotor region elicits controlled swimming in semi-intact lampreys

The role of the mesencephalic locomotor region (MLR) in initiating and controlling the power of swimming was studied in semi-intact preparations of larval and adult sea lampreys. The brain and the rostral portion of the spinal cord were exposed in vitro, while the intact caudal two-thirds of the body swam freely in the Ringer's-containing chamber. Electrical microstimulation (2-10 Hz; 0. 1-5.0 microA) within a small periventricular region in the caudal mesencephalon elicited well-coordinated and controlled swimming that began within a few seconds after the onset of stimulation and lasted throughout the stimulation period. Swimming stopped several seconds after the end of stimulation. The power of swimming, expressed by the strength of the muscle contractions and the frequency and the amplitude of the lateral displacement of the body or tail, increased as the intensity or frequency of the stimulating current were increased. Micro-injection of AMPA, an excitatory amino acid agonist, into the MLR also elicited active swimming. Electrical stimulation of the MLR elicited large EPSPs in reticulospinal neurons (RS) of the middle rhombencephalic reticular nucleus (MRRN), which also displayed rhythmic activity during swimming. The retrograde tracer cobalt-lysine was injected into the MRRN and neurons (dia. 10-20 microm) were labelled in the MLR, indicating that this region projects to the rhombencephalic reticular formation. Taken together, the present results indicate that, as higher vertebrates, lampreys possess a specific mesencephalic region that controls locomotion, and the effects onto the spinal cord are relayed by brainstem RS neurons.     

Initiation of locomotion in lampreys

The spinal circuitry underlying the generation of basic locomotor synergies has been described in substantial detail in lampreys and the cellular mechanisms have been identified. The initiation of locomotion, on the other hand, relies on supraspinal networks and the cellular mechanisms involved are only beginning to be understood. This review examines some of the findings relative to the neural mechanisms involved in the initiation of locomotion of lampreys. Locomotion can be elicited by sensory stimulation or by internal cues associated with fundamental needs of the animal such as food seeking, exploration, and mating. We have described mechanisms by which escape swimming is elicited in lampreys in response to mechanical skin stimulation. A rather simple neural connectivity is involved, including sensory and relay neurons, as well as the brainstem rhombencephalic reticulospinal cells, which act as command neurons. We have shown that reticulospinal cells have intrinsic membrane properties that allow them to transform a short duration sensory input into a long-lasting excitatory command that activates the spinal locomotor networks. These mechanisms constitute an important feature for the activation of escape swimming. Other sensory inputs can also elicit locomotion in lampreys. For instance, we have recently shown that olfactory signals evoke sustained depolarizations in reticulospinal neurons and chemical activation of the olfactory bulbs with local injections of glutamate induces fictive locomotion. The mechanisms by which internal cues initiate locomotion are less understood. Our research has focused on one particular locomotor center in the brainstem, the mesencephalic locomotor region (MLR). The MLR is believed to channel inputs from many brain regions to generate goal-directed locomotion. It activates reticulospinal cells to elicit locomotor output in a graded fashion contrary to escape locomotor bouts, which are all-or-none. MLR inputs to reticulospinal cells use both glutamatergic and cholinergic transmission; nicotinic receptors on reticulospinal cells are involved. MLR excitatory inputs to reticulospinal cells in the middle (MRRN) are larger than those in the posterior rhombencephalic reticular nucleus (PRRN). Moreover at low stimulation strength, reticulospinal cells in the MRRN are activated first, whereas those in the PRRN require stronger stimulation strengths. The output from the MLR on one side activates reticulospinal neurons on both sides in a highly symmetrical fashion. This could account for the symmetrical bilateral locomotor output evoked during unilateral stimulation of the MLR in all animal species tested to date. Interestingly, muscarinic receptor activation reduces sensory inputs to reticulospinal neurons and, under natural conditions, the activation of MLR cholinergic neurons will likely reduce sensory inflow. Moreover, exposing the brainstem to muscarinic agonists generates sustained recurring depolarizations in reticulospinal neurons through pre-reticular effects. Cells in the caudal half of the rhombencephalon appear to be involved and we propose that the activation of these muscarinoceptive cells could provide additional excitation to reticulospinal cells when the MLR is activated under natural conditions. One important question relates to sources of inputs to the MLR. We found that substance P excites the MLR, whereas GABA inputs tonically maintain the MLR inhibited and removal of this inhibition initiates locomotion. Other locomotor centers exist such as a region in the ventral thalamus projecting directly to reticulospinal cells. This region, referred to as the diencephalic locomotor region, receives inputs from several areas in the forebrain and is likely important for goal-directed locomotion. In summary, this review focuses on the most recent findings relative to initiation of lamprey locomotion in response to sensory and internal cues in lampreys.     

Spinal inputs from lateral columns to reticulospinal neurons in lampreys

This study characterizes the inputs from the lateral columns of the spinal cord to reticulospinal neurons in the lampreys, using the in vitro isolated brainstem and spinal cord preparation. Synaptic responses to the electrical stimulation of the lateral columns were recorded in reticulospinal neurons of the posterior and middle rhombencephalic reticular nuclei. The responses consisted of a mixture of excitation and inhibition. They were markedly potentiated when using trains of two to five pulses, suggesting that the larger part of these synaptic responses was mediated via an oligosynaptic pathway. An early component, however, persisted when using twin pulses at 10–20 Hz on the ipsilateral side, suggesting the presence of an early mono- or disynaptic component. When increasing the stimulation strength, an early fast rising excitatory component appeared. It most likely resulted from an antidromic activation of vestibulospinal axons in the lateral tracts, which make en passant synaptic contacts with reticulospinal neurons. Responses were practically abolished by adding CNQX and AP5 to the Ringer's solution. The late component of excitatory responses was decreased by AP5, suggesting that NMDA receptors were activated. The NMDA receptor-mediated component was larger when using trains of stimuli or in Mg²⁺-free Ringer's. The application of NMDA depolarized reticulospinal neurons. The glycinergic inhibitory component was markedly increased in Mg²⁺-free Ringer's. Moreover, GABA_B-receptor activation with (−)-baclofen abolished both excitatory and inhibitory responses. Taken together, the present results indicate that ascending lateral column axons generate large excitatory and inhibitory synaptic potentials in reticulospinal neurons. The possible role of these inputs in modulating the activity of reticulospinal neurons during locomotion is discussed.     

Further evidence for excitatory amino acid transmission in lamprey reticulospinal neurons: selective retrograde labeling with (3H)D-aspartate

The distribution of radiolabeled neurons in the brain stem of Lampetra fluviatilis was studied following unilateral injections of (3H)D-aspartate in the rostral spinal cord. After survival periods of 1-3 days, labeled perikarya were present within and nearby the posterior, middle, and anterior rhombencephalic reticular nuclei and in the mesencephalic reticular nucleus. The highest number of (3H)D-aspartate labeled cell bodies were present in the posterior rhombencephalic reticular nucleus. The labeled reticulospinal neurons were distributed mainly ipsilateral to the injection site and included the giant Müller cells as well as medium-sized and small neurons. Contralateral labeling occurred in cell bodies scattered along the lateral margin of the rhombencephalic reticular formation, the most rostral of these contralaterally projecting neurons being the Mauthner cell. The (3H)D-aspartate labeling correlates with previous electrophysiological studies showing that lamprey reticulospinal neurons utilize excitatory amino acid transmission.     

Vestibular effects in long C3-C5 propriospinal neurones

The effects of stimulation of the vestibular nerve and of regions in and around the vestibular nuclei on long C3-C5 propriospinal neurones (PNs) were investigated with intracellular recording. Disynaptic excitatory postsynaptic potentials were evoked from the contralateral (co) or ipsilateral (i) vestibular nerve in many long PNs but mainly in crossed PNs from the co and in uncrossed from the i nerve. Disynaptic inhibitory postsynaptic potentials were evoked more rarely, mainly from the i vestibular nerve. Threshold mapping revealed an excitatory relay from the co nerve in the medial vestibular nucleus (MVN) and also that the excitatory MVN neurones projecting to the long PNs send collaterals to the abducens and interstitial nucleus of Cajal. Excitation from the i vestibular nerve was relayed in the lateral vestibular nucleus (LVN) and in the MVN. Also, non-second order LVN neurones project to the long PNs. Monosynaptic IPSPs were evoked from the i MVN and i LVN.     

Long C3-C5 propriospinal neurones in the cat

Intracellular recording was made in the C3-C5 segments of cats from cells identified as long propriospinal neurones (PNs) by antidromic activation from the lower thoracic segments. The cell bodies were in laminae VII and VIII and their ventrally located axons were either uncrossed or crossed. Stimulation of higher motor centres revealed monosynaptic excitatory postsynaptic potentials (EPSPs) from cortico-, rubro-, tecto-, reticulo-, interstitio-, fastigio- and trigeminospinal fibres. Monosynaptic inhibitory postsynaptic potentials (IPSPs) were evoked from reticulospinal fibres. These PSPs were in addition to the separately described effects from the vestibular nuclei. Monosynaptic EPSPs were also evoked in some cells from neck or forelimb afferents and disynaptic EPSPs or IPSPs from forelimb afferents.     

Afferents to the cerebellar lateral nucleus. Evidence from retrograde transport of horseradish peroxidase after pressure injections through micropipettes.

HRP was injected by pressure from glass capillary micropipettes unilaterally into the lateral nucleus of rat so as to encompass the entire nucleus, but without spread into the interpositus nuclei. The cells of origin of the afferents to the lateral nucleus were studied after retrograde transport of the HRP. The reticulotegmental nucleus of the pons was labelled bilaterally and is the major source of crossed and uncrossed reticular imputs. The pontine nuclei also provide extensive crossed and uncrossed afferents. The inferior olive gives a large crossed olivo-lateral nucleus projection and a minor uncrossed input. The trigeminal nuclear complex--the nucleus of the spinal tract and the mesencephalic, principal sensory, and motor nuclei--all provide uncrossed afferents. The rostral portion of the lateral reticular nucleus gives a small crossed and uncrossed projection while the perihypoglossal nuclei and the dorsal parabrachial body give crossed afferents to the lateral nucleus. The norepinephrine afferent system from the locus coeruleus is represented by one or two heavily labelled cells and the serotonin raphe systems come from at least five raphe subgroups, the dorsal, superior centralis, pontis, obscurus and magnus nuclei. No evidence was found for commissural fibers between ipsilateral or contralateral cerebellar nuclei, or afferent axons from the spinocerebellar nuclei and the paramedian retricular nucleus. The significance of these sources of afferent imputs to the lateral cerebellar nucleus is discussed. The question is raised of the direct relationship between size of terminal axonal arborization and the quantity of HRP granules present in a cell retrograde transport. The limitations of the HRP method for detecting subtle local differences in the distribution of afferents within the heterogeneous groups of neurons in the lateral nucleus are discussed.     

Descending brain neurons in larval lamprey: Spinal projection patterns and initiation of locomotion

In larval lamprey, partial lesions were made in the rostral spinal cord to determine which spinal tracts are important for descending activation of locomotion and to identify descending brain neurons that project in these tracts. In whole animals and in vitro brain/spinal cord preparations, brain-initiated spinal locomotor activity was present when the lateral or intermediate spinal tracts were spared but usually was abolished when the medial tracts were spared. We previously showed that descending brain neurons are located in eleven cell groups, including reticulospinal (RS) neurons in the mesenecephalic reticular nucleus (MRN) as well as the anterior (ARRN), middle (MRRN), and posterior (PRRN) rhombencephalic reticular nuclei. Other descending brain neurons are located in the diencephalic (Di) as well as the anterolateral (ALV), dorsolateral (DLV), and posterolateral (PLV) vagal groups. In the present study, the Mauthner and auxillary Mauthner cells, most neurons in the Di, ALV, DLV, and PLV cell groups, and some neurons in the ARRN and PRRN had crossed descending axons. The majority of neurons projecting in medial spinal tracts included large identified Müller cells and neurons in the Di, MRN, ALV, and DLV. Axons of individual descending brain neurons usually did not switch spinal tracts, have branches in multiple tracts, or cross the midline within the rostral cord. Most neurons that projected in the lateral/intermediate spinal tracts were in the ARRN, MRRN, and PRRN. Thus, output neurons of the locomotor command system are distributed in several reticular nuclei, whose neurons project in relatively wide areas of the cord.     

Phasic modulation of reticulospinal neurones during fictive locomotion and other types of spinal motor activity in lamprey

The intracellular activity of different types of reticulospinal neurones was studied during fictive locomotion and other types of spinal motor activity in an in vitro preparation of the lamprey brainstem-spinal cord. The examined neurones included large Müller cells of the rhombencephalic and mesencephalic reticular formation, the Mauthner cell, and neurones in the posterior rhombencephalic reticular nucleus with different sizes and conduction velocities. During bouts of fictive swimming initiated spontaneously or by stimulation of the trigeminal nerve or spinal cord, the Müller cells were depolarized and fired action potentials. Bulbar Müller cells in addition showed a phasic modulation of membrane potential with excitation in phase with ipsilateral motoneurones of the rostral spinal cord. The Mauthner cell was depolarized in phase with contralateral motoneurones. Many neurones in the posterior rhombencephalic reticular nucleus showed modulation in phase with ipsilateral motoneurones during fictive swimming. Such oscillations were observed in both fast-conducting neurones, located mainly in the medial part of the nucleus, and slower conducting cells with a more lateral distribution. All examined reticulospinal neurones showed a strong coupling also with other types of spinal motor activity, such as slow alternating bursting and synchronous bilateral ventral root bursts, but the reticulospinal activity had no correlation with respiratory activity recorded from the Xth nerve. The consequences of a phasic reticulospinal activity during locomotion are discussed.     

Rostro-caudal distribution of reticulospinal projections from different brainstem nuclei in the lamprey

The reticulospinal (RS) system in the lamprey is responsible for the control of locomotion, postural corrections and steering. To perform these functions, the RS system has to affect different muscular compartments along the body axis selectively. In this study, the possibility that RS neurones in different nuclei may project to different parts of the spinal cord, was investigated. The rostro-caudal extent of single RS axons was defined by stimulating them antidromically while recording from their cell body. All recorded mesencephalic RS neurones projected to the caudal tip of the spinal cord. Of the rhombencephalic RS neurones, 26% of the recorded neurones did not reach the caudalmost fourth of the spinal cord and this proportion varied between the anterior (18%), middle (17%) and posterior (36%) rhombencephalic reticular nuclei. For these RS axons, the level of termination covered the whole rostro-caudal extent of the spinal cord. No correlation was found between the length of an axon and its conduction velocity or between the length of an axon and the rostro-caudal position of its cell body in the nuclei.     

Serotoninergic modulation of sensory transmission to brainstem reticulospinal cells

Sensory inputs are subjected to modulation by central neural networks involved in controlling movements. It has been shown that serotonin (5-HT) modulates sensory transmission. This study examines in lampreys the effects of 5-HT on sensory transmission to brainstem reticulospinal (RS) neurons and the distribution of 5-HT cells that innervate RS cells. Cells were recorded intracellularly in the in vitro isolated brainstem of larval lampreys. Trigeminal nerve stimulation elicited disynaptic excitatory responses in RS neurons, and bath application of 5-HT reduced the response amplitude with maximum effect at 10 μ m . Local ejection of 5-HT either onto the RS cells or onto the relay cells decreased sensory-evoked excitatory postsynaptic potentials (EPSPs) in RS cells. The monosynaptic EPSPs elicited from stimulation of the relay cells were also reduced by 5-HT. The reduction was maintained after blocking either N -methyl- d -aspartate (NMDA) or α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors. The local ejection of glutamate over RS cells elicited excitatory responses that were only slightly depressed by 5-HT. In addition, 5-HT increased the threshold for eliciting sustained depolarizations in response to trigeminal nerve stimulation but did not prevent them. Combined 5-HT immunofluorescence with axonal tracing revealed that the 5-HT innervation of RS neurons of the middle rhombencephalic reticular nucleus comes mainly from neurons in the isthmic region, but also from neurons located in the pretectum and caudal rhombencephalon. Our results indicate that 5-HT modulates sensory transmission to lamprey brainstem RS cells.     

Dorsal root and dorsal column mediated synaptic inputs to reticulospinal neurons in lampreys: Involvement of glutamatergic,glycinergic, and GABaergic transmission

This study was aimed at characterizing the inputs from dorsal roots and dorsal columns to reticulospinal neurons within the posterior rhombencephalic reticular nucleus in the lamprey. The in vitro isolated brainstem and spinal cord preparation was used. Microstimulation of dorsal roots and columns on both sides induced, in identified reticulospinal neurons, synaptic responses which consisted of large IPSPs mixed with excitation, particularly from stimulation on the ipsilateral side. When the spinal cord was selectively exposed to kynurenic acid or to Ca²⁺, synaptic responses to stimulation of dorsal roots and columns were not modified, whereas the same responses were abolished when the brainstem was exposed selectively to kynurenic acid, thus suggesting that the responses were carried by long fibres ascending directly to the brainstem. The excitatory and inhibitory synaptic responses are relayed by interneurons located in the brainstem. The ascending excitatory inputs to inhibitory interneurons and, most likely, also to excitatory interneurons, use excitatory amino acid transmission. Inhibitory responses were abolished by adding the glycinergic antagonist strychnine (5 μM) to the physiological solution, thus suggesting that inhibitory interneurons use glycine transmission. The synaptic transmission was depressed by (?)-baclofen, a GABA_u agonist, probably acting at a presynaptic site. Taken together, the present results suggest that dorsal root and dorsal column stimulations give rise to disynaptic inhibition and excitation of reticulospinal neurons mediated by excitatory and inhibitory amino acid transmission via brainstem interneurons. © 1993 Wiley-Liss, Inc.     

Origins of the descending spinal projections in petromyzontid and myxinoid agnathans

The origins of the descending spinal pathways in sea lampreys (Petromyzon marinus), silver lampreys (Ichthyomyzon unicuspis), and Pacific hagfish (Eptatretus stouti) were identified by the retrograde transport of horseradish peroxidase (HRP) placed in the rostral spinal cord. In lampreys, the majority of HRP-labeled cells were located along the length of the brainstem reticular formation in the inferior, middle, and superior reticular nuclei of the medulla, mesencephalic tegmentum, and nucleus of the medial longitudinal fasciculus. Labeled reticular cells included the Mauthner and Müller cells. Horseradish-peroxidase-filled cells were also present in the descending trigeminal tract, intermediate and posterior octavomotor nuclei, and a diencephalic cell group, the nucleus of the posterior tubercle. As in lampreys, the reticular formation of the Pacific hagfish was the largest source of descending afferents to the spinal cord. Labeled cells were found in the dorsolateral and ventromedial reticular nuclei, the dorsal tegmentum at the juncture of the medulla and midbrain, and the nucleus of the medial longitudinal fasciculus. Additional medullary cells projecting to the cord were located in the perivagal nucleus, the central gray, and the anterior and posterior magnocellular octavolateralis nuclei. The existence of reticulospinal and possible vestibulo-, trigemino-, and solitary spinal projections in lampreys and hagfishes and the wide distribution of these pathways in jawed vertebrates suggest that they evolved in the common ancestor of gnathostomes and both groups of jawless fishes. However, descending spinal pathways from the cerebellum, red nucleus, and telencephalon appear to be gnathostome characters.     

Effects of stimulating the reticular formation during fictive locomotion in lampreys

The effects of stimulating the reticular formation were studied during fictive locomotion in lampreys (Ichthyomyzon unicuspis). The in vitro isolated preparation of the brainstem and spinal cord was used and fictive locomotion was induced by bath application of N-methyl-

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-aspartate (NMDA; 50–100 μM). During different phases of the locomotor cycle, short trains of stimuli (10 pulses at 80–100 Hz; 10 μA) were delivered through glass-coated tungsten microelectrodes positioned within the middle rhombencephalic reticular nucleus (MRRN) and their effects were studied on ipsi- and contralateral ventral root locomotor discharges. Irrespective of the locomotor phase during which the stimulation train was delivered, a resetting effect occurred. It was characterized by a re-synchronization of the locomotor discharges with a constant latency for each ventral root on the ipsilateral side. The latency increased as the recorded root was located further caudally. This increase in latency was in the range of the phase lag observed between roots during control bouts of locomotion. These results suggest that reticulospinal neurones exert strong resetting effects on spinal locomotor networks. These effects may play a significant role with respect to changes of direction during swimming.     

Connectivity of the tectal zones coding for upward and downward oblique eye movements in goldfish

Deep layers of the goldfish tectum code movements in a topographically ordered motor map. This work studies the relationship between tectal sites (coding eye movements with different vertical directions) and the distributions of boutons (left by their projections), within rostral mesencephalic structures and rhombencephalic reticular formations. These regions have been involved in the generation of the vertical and horizontal components of eye movement, respectively, as suggested by the Cartesian hypothesis of de-codification of tectal signal. With this aim, discrete injections of biotinylated dextran amine (BDA) and Fluoro-Ruby (FR) were made into functionally identified tectal sites, coding oblique eye movements with similar amplitude of the horizontal component but opposite upward and downward vertical directions, and the distribution of synaptic endings was determined. The main findings of the present work were as follows: 1) within the tectal descending tract, axons were organized according to the location of injected sites within the tectum; 2) BDA and FR boutons were distributed in separate clusters within the medial longitudinal fasciculus and oculomotor nuclei, as well as in the nearby mesencephalic reticular formation; and 3) the regions containing both types of bouton overlapped moderately within the mesencephalic reticular formation at the isthmus level. Overlapping was more extended at the different levels of the rhombencephalic reticular formation, although a shift in the distribution of both types of bouton was always observed. These results suggest that, within the vertical generator, the endings were separated to contact the different neuronal population that codes the upward and downward components of movements. In contrast, in the horizontal generator, tectal endings more likely converge on the same neuronal population to code the horizontal component of movements, irrespective of whether the oblique movements were directed upward or downward.     

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. © 1998 Wiley-Liss, Inc.     

Efferent tectal cells of crucian carp: physiology and morphology

Tectal cells of the crucian carp (Carassius ararssius) showing antidromic responses evoked by rhombencephalic electrical stimulation were physiologically studied and subsequently stained with Lucifer Yellow CH. The stained efferent tectal cells were fusiform, horizontal, and multipolar. The main axon of these efferent tectal cells descended along the wall of the deep tegmentum and could be traced to the motor area below the cerebellum. The axons gave off their collaterals in several brain areas: 1) descending collaterals in the torus semicircularis, dorso-lateral tegmental area and mesencephalic reticular formation and 2) an ascending collateral in the area between the hypothalamus and tegmentum. Fifty percent of the efferent cells were unresponsive to visual stimuli, but some of these cells were activated by visual or tactile stimulation in conjunction with rhombencephalic electrical stimulation. On the other hand, most of the visually active cells were On-transient and movement sensitive with habituation and some were bimodal.