Abducens Motoneurons Research Articles

Electrotonic clusters in the dendritic arborization of abducens motoneurons of the rat.

Following reconstruction with high spatial resolution of the 3-D geometry of the dendritic arborizations of two abducens motoneurons, we simulated the distribution of electronic voltage over the whole dendritic tree. Here, we demonstrate that the complex stochastic electronic structure of both motoneurons can be reduced to a statistically significant small set of well discriminated clusters. These clusters are formed by dendritic branches belonging to different dendrites of the neuron but with similar electronic properties. A cluster analysis was performed to estimate quantitatively the partition of the branches between the dendritic clusters. The contents of the clusters were analysed in relation to their stability under different values of specific membrane resistivity (Rm), to their remoteness from the soma and their location in 3-D space. The cluster analysis was executed in a 2-D parameter space in which each dendritic branch was described by the mean electrotonic voltage and gradient. The number of clusters was found to be four for each motoneuron when computations were made with Rm = 3 k omega.cm2. An analysis of the cluster composition under different Rm revealed that each cluster contained invariant and variant branches. Mapping the clusters upon the dendritic geometry of the arborizations allowed us to describe the cluster distribution in terms of the 3-D space domain, the 2-D path distance domain and the total surface area of the tree. As the cluster behaviour reflects both the geometry and the changes in the neuronal electrotonic structure, we conclude that cluster analysis provides a tool to handle the functional complexity of the arborizations without losing relevant information. In terms of synaptic activities, the stable dendritic branches in each cluster may process the synaptic inputs in a similar manner. The high percentage of stable branches indicates that geometry is a major factor of stability for the electrotonic clusters. Conversely, the variant branches introduce the conditions for mechanisms of functional postsynaptic plasticity.

Botulinum neurotoxin alters the discharge characteristics of abducens motoneurons in the alert cat.

1. The effects of botulinum neurotoxin (BoTx) injected into the lateral rectus muscle were examined in alert cats by recording the extracellular activity of abducens motoneurons during spontaneous eye movements. 2. A single high dose (3 ng/kg) of BoTx produced a complete paralysis of abduction that lasted for more than 2 mo. In addition, changes were found in the discharge pattern of abducens motoneurons. Motoneurons discharged steadily at a low firing rate (15-50 spikes/s), which in some instances showed a complete independence of eye position. Their increases in activity during ON-directed saccades were markedly reduced with respect to controls. The loss of inhibitory signals for OFF-directed saccades was even more evident. 3. A low dose (0.3 ng/kg) of BoTx also produced a paralysis of the lateral rectus muscle that lasted for approximately 1 mo. In this case, only minor modifications in the firing characteristics of abducens motoneurons were observed. 4. The present findings indicate that the effects of BoTx observed in the discharge pattern of abducens motoneurons might be due not only to target disconnection, but also to a central action of the neurotoxin on the motoneuron.

Monosynaptic and disynaptic connections in the utriculo-ocular reflex arc of the cat.

1. Connections from the utricular (UT) nerve to motoneurons and interneurons in the ipsilateral abducens (AB) nucleus were studied in anesthetized and decerebrated cats. Bipolar electrodes were fixed on the left UT nerve under visual observation. The other branches of the vestibular nerve and the facial nerve were transected in the left inner ear. 2. Stimulation of the UT nerve evoked a small positive-negative (P/N) deflection and a negative (N1) potential in the vestibular nuclei, with mean latencies of 0.56 and 0.84 ms, respectively. In the AB nucleus a small P/N deflection with a mean latency of 0.72 ms was recorded, which was considered as a incoming volley of the UT nerve. 3. Excitatory postsynaptic potentials (EPSPs) were recorded from AB motoneurons with short latencies after UT nerve stimulation. They were classified into two types, M and D. M-type EPSPs, which followed repetitive high-frequency stimuli and were recorded from the majority of AB motoneurons, had latencies ranging from 0.9 to 1.2 ms. Double shocks to the UT nerve evoked EPSPs that had the same latency. It was suggested that the AB motoneurons had monosynaptic connections with the UT nerve. D-type EPSPs, which were recorded from most of the AB motoneurons, had slightly longer latencies ranging from 1.2 to 1.8 ms. They showed temporal facilitation when double shocks were provided to the UT nerve. They did not follow repetitive high-frequency stimuli (< or = 2.5-ms interval). It was suggested that D-type EPSPs were di-synaptically evoked via secondary vestibular neurons. Interneurons in the AB nucleus had the same characteristics as AB motoneurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of target depletion on adult mammalian central neurons: Functional correlates

The physiological signals and patterns of synaptic connectivity that CNS neurons display after the loss of their target cells were evaluated in adult cats for one year. Abducens internuclear neurons were chosen as the experimental model because of their highly specific projection onto the medial rectus motoneurons of the oculomotor nucleus. Selective death of medial rectus motoneurons was induced by the injection into the medial rectus muscle of ricin, a potent cytotoxic lectin that leaves the presynaptic axons intact. The electrical activity of antidromically identified abducens internuclear neurons was recorded in chronic alert animals, during both spontaneous and vestibularly induced eye movements, before and after target removal. During the three weeks that followed ricin injection, abducens internuclear neurons exhibited several firing-related abnormal properties. There was an overall reduction in firing rate with a corresponding increase in the eye position threshold for recruitment. In addition, neuronal sensitivities to eye position and velocity were significantly decreased with respect to control data. Bursting activity was also altered since low-frequency delayed burst accompanied the saccades in the on-direction and, occasionally, internuclear neurons exhibited low-frequency discharges associated with off-directed saccades. Intracellular recordings carried out seven and 15 days after ricin injection demonstrated no significant changes in their electrical properties, although a marked depression of synaptic transmission was evident. The amplitude of both excitatory and inhibitory postsynaptic potentials of vestibular origin was reduced by 60–85% with respect to controls. However, postsynaptic potentials recorded one month after ricin injection showed normal amplitude values which persisted unaltered one year after target loss. Recovery of synaptic transmission occurred at the same time as the re-establishment of normal eye-related signals in the discharge pattern of abducens internuclear neurons recorded in alert cats from days 25–30 post lesion. The functional restoration of firing properties was maintained in the long term (one year). Conversely, abducens motoneurons showed normal firing and synaptic patterns at all time intervals analysed. These results demonstrate that, after an initial period of altered physiological properties, abducens internuclear neurons survive the loss of their target motoneurons and regain a normal discharge pattern and afferent synaptic connections.

Synaptic Excitation Triggers Oscillations During NMDA Receptor Activation in Rat Abducens Motoneurons

Rat abducens motoneurons were intracellularly recorded in vivo during synaptic excitation and extracellular microionophoretic application of N-methyl-D-aspartate (NMDA). Trigeminal excitatory post-synaptic potentials (EPSPs) evoked in abducens motoneurons were studied during intracellular current injection. They were not sensitive to hyperpolarization or depolarization of the membrane potential in the range of -75 mV to -55 mV using current pulse intensities between -3 nA and +1 nA. Microionophoretic applications of aminophosphonovalerate (APV), MK801 and i.v. injections of MK801 (1-3 mg/kg) or ketamine (10 mg/kg) did not modify trigeminal EPSPs, suggesting that NMDA receptors are not involved in this synaptic transmission. However, microionophoretic applications of NMDA on abducens motoneurons enhanced trigeminal EPSPs and gave rise to regenerative oscillations. The co-activation of NMDA receptors and trigeminal synapses induced these oscillations. The trigeminal EPSP may delay and reset the oscillations depending on where it was evoked in the oscillatory cycle. Depolarizing current pulses intracellularly applied to abducens motoneurons could trigger a post-hyperpolarization followed by rebound depolarization during NMDA application, confirming the activation of active membrane properties. However, depolarizing current pulses could not trigger oscillations similar to those entrained by the EPSPs. The importance of the location of trigeminal synapses in relation to those of NMDA receptors in the dendritic arborization of abducens motoneurons is discussed. Our results show that the same sensory stimulus may have different post-synaptic effects on abducens motoneurons during the co-activation of NMDA receptors. A complete modification of the motor output during NMDA receptor activation strongly supports an active role of abducens motoneurons provided that NMDA receptors are physiologically activated during motor pattern generation.

Testing the common neural integrator hypothesis at the level of the individual abducens motoneurones in the alert cat.

1. As far as horizontal eye movements are concerned, the well-known hypothesis of a common neural integrator states that the eye-position signal is generated by a common network, regardless of the type of versional movement. The aim of this study was to evaluate the validity of this hypothesis by analysing the behaviour of the abducens motoneurones, the system into which the horizontal neural integrator(s) project(s). If there were a common neural integrator, the different motoneurones would receive the eye position signal through the same pathway and the sensitivity to eye position would be the same regardless of the type of versional movement. If there were multiple integrators, the sensitivity to eye position in one type of versional movement might be different from the sensitivity to eye position in another type of versional movement, at least for occasional motoneurones. 2. The discharge of thirty-one antidromically identified abducens motoneurones was recorded in the alert cat during spontaneous eye movements made in the light and in response to sinusoidal rotations of the head in complete darkness. 3. All of the abducens motoneurones exhibited a burst of action potentials for lateral saccades. During fixation between saccades, they maintained a steady firing rate that increased as the cat fixated increasingly lateral eye positions. 4. For each abducens motoneurone, the sensitivity to eye position (Kf) was determined from measurements carried out during intersaccadic fixations. Kf was calculated from the slope of the firing rate-eye position linear regression line. 5. The discharge rate of the identified motoneurones was observed during four sinusoidal vestibular stimulations (+/- 10 deg, 0.10 Hz; +/- 20 deg, 0.10 Hz; +/- 30 deg, 0.10 Hz; +/- 40 deg, 0.10 Hz). The motoneurones exhibited a burst of activity during fast phases in the lateral direction and paused during fast phases in the opposite direction. During slow phases, motoneurones modulated their activity as a function of the vestibularly induced eye movements except for slow phases that occurred in position ranges below their recruitment threshold. In these cases their activity was cut off. 6. A new method was developed to measure the sensitivity to eye position of neurones during vestibular slow phases. The difficulty came from the fact that, during slow phases, eye velocity and eye position changed simultaneously and that each of those two variables could influence neuronal activity.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological and pharmacological characterization of vestibular inputs to identified frog abducens motoneurons and internuclear neurons in vitro.

Synaptic vestibular inputs of antidromically identified motoneurons and internuclear neurons in the abducens nucleus were studied electrophysiologically and pharmacologically in the isolated brain of grass frogs (Rana temporaria). The prevailing response pattern of abducens motoneurons (AbMOT) following stimulation of the VIIIth nerve was crossed disynaptic excitation and uncrossed disynaptic inhibition. A few AbMOT (five of 46), however, exhibited uncrossed excitation instead of inhibition. Abducens internuclear neurons (AbINT), identified by antidromic activation following stimulation of the contralateral medial longitudinal fascicle, exhibited disynaptic response patterns to stimulation of the VIIIth nerve that were very similar in latency and rise time to those of AbMOT except for the absence of uncrossed disynaptic inhibition. Bath application of strychnine (50 microM), a glycine antagonist, blocked the uncrossed inhibitory vestibular input to AbMOT and AbINT completely and reversibly, whereas picrotoxin (100 microM), a GABA (gamma-aminobutyric acid) antagonist, had no detectable effect on these disynaptic potentials. These results suggest glycine as the transmitter of inhibitory vestibular projections onto AbMOT and AbINT. The pharmacology of the excitatory vestibular input of these neurons was studied by electrical stimulation of the vestibular nuclear complex. Crossed monosynaptic excitatory inputs in AbMOT and AbINT were blocked completely by CNQX (6-cyano-7-nitroquinoxaline-2,3-dione) (10 microM), an antagonist of AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptors, indicating glutamatergic excitation. Comparison of these results with those in the cat suggests the presence of a basic horizontal vestibulo-ocular reflex that is very similarly organized, and corroborates the hypothesis that major behavioural differences in the performance of compensatory eye movements between species result from the properties of supplementary networks and not from differences in a common 'three-neuron' vestibulo-ocular arc.

A morphological study of the principal and accessory abducens nuclei in the caspian terrapin (Mauremys caspica).

The location of principal and accessory motoneurons and principal interneurons of the nucleus abducens was determined in the caspian terrapin (Mauremys caspica) by means of horseradish peroxidase histochemical tracing. Enzyme injections were made into the ipsilateral lateral rectus and retractor bulbi muscles and into the contralateral oculomotor nucleus. Labeled principal abducens motoneurons formed a cluster of cells in the rhombencephalon, under the IVth ventricle and adjacent to the medial longitudinal fascicle. The accessory abducens motoneurons were located more deeply in the rhombencephalon and more ventrolaterally than the principal motoneurons forming a compact aggregation of neurons. The principal interneurons of abducens nucleus were arranged as a cluster of cells under the floor of the IVth ventricle and more lateral than the principal motoneurons, with no intermingling.

The neurotransmitter of inhibitory synaptic inputs on inhibitory burst neurons (IBNs) controlling vestibular nystagmus in the cat.

In the pontine neural networks that govern vestibular nystagmus in the cat, inhibitory burst neurons (IBNs) are known to fire in bursts during the quick phase, and to suppress firing of the abducens motoneurons on the contralateral side. The present experiments were designed to identify the neurotransmitter controlling the burst firing pattern of IBNs. Inhibitory burst neuron activity was recorded extracellularly, and various chemicals were applied to the IBNs iontophoretically through multibarrel micropipettes. GABA and muscimol (a GABAA-receptor agonist) strongly suppressed IBN activity and eliminated the burst pattern. Bicuculline (a GABAA-receptor antagonist) increased the firing of IBNs and suppressed the inhibitory effect of GABA when applied simultaneously. Although baclofen (a GABAB-receptor agonist) had no inhibitory effect, it slightly shortened the duration of burst firing. Neither glycine nor serotonin, other inhibitory transmitter candidates, nor their respective antagonists, strychnine and methysergide, had any effect. Systemically administered picrotoxin prevented the GABA-induced suppression of IBNs. These results suggest that IBNs possess GABAA receptors and are controlled by higher GABA-liberating neurons and Cl- channels.

Physiological and behavioral identification of vestibular nucleus neurons mediating the horizontal vestibuloocular reflex in trained rhesus monkeys.

1. To describe in detail the secondary neurons of the horizontal vestibuloocular reflex (VOR), we recorded the extracellular activity of neurons in the rostral medial vestibular nucleus of alert, trained rhesus monkeys. On the basis of their activity during horizontal head and eye movements, neurons were divided into several different types. Position-vestibular-pause (PVP) units discharged in relation to head velocity, eye velocity, eye position, and ceased firing during some saccades. Eye and head velocity (EHV) units discharged in relation to eye velocity and head velocity in the same direction so that the two signals partially canceled during the VOR. Two cell types discharged in relation to eye position and velocity but not head velocity; other types discharged in relation to head velocity only. 2. The position in the neural path from the primary vestibular afferents to abducens motoneurons was examined for each type. Direct input from the vestibular nerve was indicated if the cell could be activated by shocks to the nerve at latencies less than or equal to 1.4 ms. A projection to abducens motoneurons was indicated if spike-triggered averaging of lateral rectus electromyographic (EMG) activity yielded responses with a sharp onset at monosynaptic latencies. 3. PVP neurons were the principal interneuron in the VOR "three-neuron arc." Eighty percent received primary afferent input, and 66% made excitatory connections with contralateral abducens motoneurons. Surprisingly few, approximately 11%, made inhibitory connections with ipsilateral abducens motoneurons. This imbalance in the ipsi- and contralateral projections was confirmed by measuring the EMG activity evoked by electrical microstimulation in regions where PVP neurons were located. 4. EHV neurons whose activity increased during contralaterally directed head or eye movements were also interneurons in the ipsilateral inhibitory pathway. Eighty-nine percent received ipsilateral primary afferent input, and 25% projected to ipsilateral abducens motoneurons. EHV neurons excited during ipsilateral movements received neither direct primary afferent input nor projected to either abducens nucleus. A small proportion of each of two other cell types having sensitivity to contralateral eye position made excitatory connections with contralateral abducens motoneurons. Other types rarely were activated from the eighth nerve or projected to the abducens nucleus. 5. The significance of the connections of VOR interneurons and the signals they convey is discussed for three situations: smooth pursuit of a moving target, suppression of the VOR, and the VOR itself. PVP neurons convey a signal with a ratio of eye position and velocity components that is inappropriate to drive motoneurons during pursuit or the VOR.(ABSTRACT TRUNCATED AT 400 WORDS)

Discharge patterns in nucleus prepositus hypoglossi and adjacent medial vestibular nucleus during horizontal eye movement in behaving macaques.

1. Monkeys were trained to perform a variety of horizontal eye tracking tasks designed to reveal possible eye movement and vestibular sensitivities of neurons in the medulla. To test eye movement sensitivity, we required stationary monkeys to track a small spot that moved horizontally. To test vestibular sensitivity, we rotated the monkeys about a vertical axis and required them to fixate a target rotating with them to suppress the vestibuloocular reflex (VOR). 2. All of the 100 units described in our study were recorded from regions of the medulla that were prominently labeled after injections of horseradish peroxidase into the abducens nucleus. These regions include the nucleus prepositus hypoglossi (NPH), the medial vestibular nucleus (MVN), and their common border (the "marginal zone"). We report here the activities of three different types of neurons recorded in these regions. 3. Two types responded only during eye movements per se. Their firing rates increased with eye position; 86% had ipsilateral "on" directions. Almost three quarters (73%) of these medullary neurons exhibited a burst-tonic discharge pattern that is qualitatively similar to that of abducens motoneurons. There were, however, quantitative differences in that these medullary burst-position neurons were less sensitive to eye position than were abducens motoneurons and often did not pause completely for saccades in the off direction. The burst of medullary burst position neurons preceded the saccade by an average of 7.6 +/- 1.7 (SD) ms and, on average, lasted the duration of the saccade. The number of spikes in the burst was well correlated with saccade size. The second type of eye movement neuron displayed either no discernible burst or an inconsistent one for on-direction saccades and will be referred to as medullary position neurons. Neither the burst-position nor the position neurons responded when the animals suppressed the VOR; hence, they displayed no vestibular sensitivity. 4. The third type of neuron was sensitive to both eye movement and vestibular stimulation. These neurons increased their firing rates during horizontal head rotation and smooth pursuit eye movements in the same direction; most (76%) preferred ipsilateral head and eye movements. Their firing rates were approximately in phase with eye velocity during sinusoidal smooth pursuit and with head velocity during VOR suppression; on average, their eye velocity sensitivity was 50% greater than their vestibular sensitivity. Sixty percent of these eye/head velocity cells were also sensitive to eye position. 5. The NPH/MVN region contains many neurons that could provide an eye position signal to abducens neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Discharge characteristics of medial rectus and abducens motoneurons in the goldfish

1. The discharge of antidromically identified medial rectus and abducens motoneurons was recorded in restrained unanesthesized goldfish during spontaneous eye movements and in response to vestibular and optokinetic stimulation. 2. All medial rectus and abducens motoneurons exhibited a similar discharge pattern. A burst of spikes accompanied spontaneous saccades and fast phases during vestibular and optokinetic nystagmus in the ON-direction. Firing rate decreased for the same eye movements in the OFF-direction. All units showed a steady firing rate proportional to eye position beyond their recruitment threshold. 3. Motoneuronal position (ks) and velocity (rs) sensitivity for spontaneous eye movements were calculated from the slope of the rate-position and rate-velocity linear regression lines, respectively. The averaged ks and rs values of medial rectus motoneurons were higher than those of abducens motoneurons. The differences in motoneuronal sensitivity coupled with structural variations in the lateral versus the medial rectus muscle suggest that symmetric nasal and temporal eye movements are preserved by different motor unit composition. Although the abducens nucleus consists of distinct rostral and caudal subgroups, mean ks and rs values were not significantly different between the two populations. 4. Every abducens and medial rectus motoneuron fired an intense burst of spikes during its corresponding temporal or nasal activation phase of the "eye blink." This eye movement consisted of a sequential, rather than a synergic, contraction of both vertical and horizontal extraocular muscles. The eye blink could act neither as a protective reflex nor as a goal-directed eye movement because it could not be evoked in response to sensory stimuli. We propose a role for the blink in recentering eye position. 5. Motoneuronal firing rate after ON-directed saccades decreased exponentially before reaching the sustained discharge proportional to the new eye position. Time constants of the exponential decay ranged from 50 to 300 ms. Longer time constants after the saccade were associated with backward drifts of eye position and shorter time constants with onward drifts. These postsaccadic slide signals are suggested to encode the transition of eye position to the new steady level. 6. Motoneurons modulated sinusoidally in response to sinusoidal head rotation in the dark, but for a part of the cycle they went into cutoff, dependent on their eye position recruitment threshold. Eye position (kv) and velocity (rv) sensitivity during vestibular stimulation were measured at frequencies between 1/16 and 2 Hz. Motoneuronal time constants (tau v = rv/kv) decreased on the average by 25% with the frequency of vestibular stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Is there a three neuron arc in the cat utriculo-trochlear pathway?

The utriculo-ocular pathway was examined in decerebrated and anesthetized cats, in which all the vestibular afferents in the labyrinth, except for those innervating the utricular (UT) macula, had been transected. The UT nerve was stimulated with tungsten electrodes which were insulated except for 200 microns at the tips. Stimulation of the UT nerve evoked a small negative (N1) potential in the vestibular nuclei, with a threshold (N1T) less than 25 microA. The stimulus evoked disynaptic EPSPs in ipsilateral abducens (AB) motoneurons. The threshold and latency of the excitatory postsynaptic potentials (EPSPs) was 1.3 x N1T and 1.2 ms, respectively, in accordance with the data of Schwindt et al. (1973). On the other hand, EPSPs with a clear rising phase and short latency, suggesting the existence of a disynaptic pathway, were never observed in any contralateral trochlear (TR) motoneurons, even when triple shocks at intensities of up to 4 x N1T were applied. This stimulus strength was strong enough to activate the UT nerve. Thus it seems very likely that a disynaptic pathway from the UT nerve to contralateral TR motoneurons, is absent or very poorly organized.

The neuronal organization of horizontal semicircular canalactivated inhibitory vestibulocollic neurons in the cat

1. The somatic location and axonal projections of inhibitory vestibular nucleus neurons activated by the horizontal semicircular canal nerve (HCN) were studied in anesthetized cats. Cats were anesthetized with ketamine hydrochloride and pentobarbital sodium. 2. Intracellular recordings were obtained from 11 neck extensor motoneurons which were identified by antidromic activation from the dosal rami (DR) in the C1 segment. Stimulation of the ipsilateral (i-) HCN and the ipsilateral abducens (AB) nucleus evoked IPSPs in the motoneurons. These IPSPs were fully or partially occluded when they were evoked simultaneously. 3. Intracellular recordings were obtained from 8 AB motoneurons. Stimulation of the i-HCN and the i-C1DR motoneuron pool evoked IPSPs in the AB motoneurons. These IPSPs were also partially occluded when they were evoked simultaneously, which implied that some HCN-activated neurons inhibit both i-AB motoneurons and ipsilateral neck motoneurons. 4. Unit activity was extracellularly recorded from 30 vestibular neurons that were activated monosynaptically by i-HCN stimulation. Their axonal projections were determined by stimulating the i-AB nucleus and the i-C1DR motoneuron pool. Eight neurons were activated by both stimuli, and were termed vestibulooculo-collic (VOC) neurons. Their axonal branching was examined by means of local stimulation in and around the i-AB nucleus and the i-C1DR motoneuron pool. Eighteen neurons were antidromically activated from the i-C1DR motoneuron pool but not from the i-AB nucleus. These were termed vestibulo-collic (VC) neurons. Four neurons were activated from the i-AB nucleus but not from the ventral funiculus in the C1 segment, and were termed vestibulo-ocular (VO) neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA Actions on Rat Abducens Motoneurons.

Intracellular recordings were made from rat abducens motoneurons in vivo during local extracellular micro-ionophoretic application of N-methyl-d-aspartate (NMDA) and NMDA receptor antagonists. Typical NMDA responses, at a resting potential of -60 mV, consisted of a slow depolarization with an apparent increase in membrane resistance, bursts of action potentials followed by stable repetitive firing. Ionophoretic applications of aminophosphonovalerate (APV), kynurenate or MK801 reduced or blocked the NMDA-induced responses. The NMDA responses were voltage-dependent. NMDA responses induced by short (< 30 s) NMDA application pulses were blocked by hyperpolarizing the neuron. Long duration (> 30 s) NMDA applications induced rhythmic plateau potentials in hyperpolarized abducens motoneurons. The rhythmic depolarizations (15 - 30 mV) were modulated in both frequency and duration by current injection. They were abolished by further hyperpolarization or replaced by stable repetitive firing when hyperpolarization was removed. Our data show that NMDA receptors are present in rat abducens motoneurons and may be involved in the induction of rhythmic activities. The voltage-dependent blockade of somatic NMDA receptor-associated ion channels by cell hyperpolarization may be important for these oscillations. It is suggested that the rhythmic behaviour is due to the activation of dendritic NMDA receptors.

GYKI 52466 antagonizes glutamate responses but not NMDA and kainate responses in rat abducens motoneurones

A new non-N- methyl- d-aspartate (non-NMDA) glutamate receptor antagonist, GYKI 52466, was tested on l-glutamate (Glu)-, kainate (KAI)- and NMDA-induced responses in vivo, using both extracellular recording of antidromic field potentials and intracellular recording from rat abducens motoneurones. Intravenous (5–10 mg/kg) or iontophoretic applications of GYKI 52466 blocked the Glu-induced depression of antidromic field potentials only. Furthermore, intravenous application of ketamine blocked the NMDA-induced depression only. Iontophoretic application of GYKI 52466 reduced the Glu-induced neuronal depolarization but not those induced by NMDA and KAI. Our results show a selective blockade of Glu responses by GYKI 52466, probably by acting at the AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptor subtype in rat abducens motoneurones.

Behavior of cat abducens motoneurons following the injection of toxic ricin into the lateral rectus muscle

The aim of this investigation was to study the behavior of identified abducens motoneurons in the chronic cat following a single injection of toxic ricin into the lateral rectus muscle. Lateral rectus electromyographic potentials induced by VIth nerve stimulation disappeared, and abducens antidromic field potentials decreased by 90% 3 days following ricin injection. Several abnormalities and a significant decrease in eye position and velocity sensitivities were observed in motoneuron activity up to 8–10 days following ricin injection. Contrary to a previous report for axotomized abducens motoneurons, no functional sign of recovery was observed. Histological analysis showed a survival of 10–15% of the abducens motoneuron population 10 days following ricin injection. From this time on, recorded motoneurons behaved like controls, but showed a specific retraction signal suggesting an exclusive projection onto the retractor bulbi muscle. Although intermingled in the nucleus with motoneurons, no recorded abducens internuclear interneuron was affected by the ricin during one month following the injection.

Behavior of accessory abducens and abducens motoneurons during eye retraction and rotation in the alert cat

1. The activity of both accessory abducens (Acc Abd) and abducens (Abd) motoneurons (Mns) was recorded in the alert cat during eye retraction and rotational eye movements. Cats were fitted with two scleral coils, one measured rotational eye movements directly and the other retraction by distinguishing the translational component. 2. Acc Abd and Abd Mns were identified following antidromic activation from electrical stimulation of the ipsilateral VIth nerve. 3. In response to corneal air puffs, bursts of spikes were produced in all (n = 30) Acc Abd Mns. The burst began 7.2 +/- 1.2 (SD) ms after onset of the air puff and 8.9 +/- 1.9 ms before eye retraction. 4. Acc Abd Mns were silent throughout all types of rotational eye movements, and tonic activity was not observed during intervals without air-puff stimulation. 5. In contrast, all (n = 50) identified Abd Mns exhibited a burst and/or pause in activity preceding and during horizontal saccades as well as a tonic activity proportional to eye position. 6. Only 10% of Abd Mns fired a weak burst of spikes in response to air-puff stimulation. 7. We conclude that Acc Abd Mns are exclusively involved in eye retraction in the cat and that only a few Abd Mns have an eye-retraction signal added to their eye position and velocity signals. Thus any rotational eye-movement response described in retractor bulbi muscle must result from innervation by Mns located in the Abd and/or the oculomotor nuclei. 8. The organization of the prenuclear circuitry and species variation are discussed in view of the nictiating membrane extension response measured in associative learning.

Neural correlates of horizontal vestibulo-ocular reflex cancellation during rapid eye movements in the cat.

1. The aim of the present study is to describe the behaviour of identified second-order vestibular neurones in the alert cat during eye saccades. A selection of neurones which are involved in horizontal eye movements has been made. The activity has been compared with a selected sample of abducens motoneurones recorded in the same animals. 2. Alert head-fixed cats were used for this study. Eye movements were recorded by the scleral search coil technique. Abducens motoneurones were identified by antidromic stimulation from the VIth nerve with chronically implanted electrodes. They were recorded extracellularly. 3. Second-order vestibular neurones were identified by orthodromic stimulation from the vestibular organs. They were recorded intra-axonally and injected with horseradish peroxidase after recording of their physiological characteristics. Their morphology was reconstructed from frozen sections. 4. All the recorded vestibular neurones showed various amounts of eye position sensitivity. The firing rate (F) - horizontal eye position (H) characteristics are compared for abducens and vestibular neurones. The population average values are F = 33 + 4 H for motoneurones and F = 51 + 2.4 H for vestibular neurones. 5. All recorded vestibular neurones showed an increase of discharge rate during contralateral horizontal saccades and a strong decrease or pause during ipsilateral saccades. Firing rate - horizontal eye velocity sensitivity has been calculated. 6. Results suggest a strong inhibitory input on vestibular neurones from the saccadic generator. This mechanism underlies the suppression of the vestibulo-ocular reflex during saccades. Our results suggest that in the cat, for saccades of amplitude smaller than 20 deg, there is a variable degree of suppression which is provided by a projection of excitatory bursters (EBNs) on second-order vestibular neurones through inhibitory type II neurones. 7. We also conclude from this study that the eye position sensitivity of vestibular second-order neurones is in fact a motor signal indicating a motor error, i.e. the amount of head or eye movement which remains to be done in order to align gaze on target with the eyes centred in the orbit.

Evidence for glycine as an inhibitory neurotransmitter of vestibular, reticular, and prepositus hypoglossi neurons that project to the cat abducens nucleus

The localization and distribution of brain-stem afferent neurons to the cat abducens nucleus has been examined by high-affinity uptake and retrograde transport of 3H-glycine. Injections of 3H-glycine selectively labeled (by autoradiography) only neurons located predominantly in the ipsilateral medial vestibular and contralateral prepositus hypoglossi nuclei, and in the contralateral dorsomedial reticular formation, the latter corresponding to the location of inhibitory burst neurons. The specificity of uptake and retrograde transport of 3H-glycine was indicated by the absence of labeling of the dorsomedial medullary reticular neurons ipsilateral and in close proximity to the injection site, where local uptake by diffusion could have occurred. The selectivity of uptake and transport was demonstrated by the absence of retrograde labeling following injections of 3H-GABA or 3H-leucine into the abducens nucleus. The immunohistochemical localization of glycine and GABA revealed a differential distribution of the 2 inhibitory neurotransmitter candidates in the extraocular motor nuclei. Glycine-immunoreactive staining of synaptic endings in the abducens nucleus was dense with a widespread soma-dendritic distribution but was sparse in the trochlear and oculomotor nuclei. By contrast, GABA-immunoreactive staining within the oculomotor and trochlear nuclei was associated with synaptic endings that were particularly prominent on the somata of motoneurons. GABA-immunoreactive staining in the abducens nucleus, however, was sparse. These differences between glycine- and GABA-immunoreactive staining in the extraocular motor nuclei were correlated with differences in the immunoreactivity of axons in the descending (glycine) and ascending (GABA) limbs of the medial longitudinal fasciculus. Glycine-immunoreactive neurons, furthermore, were observed in the same locations as neurons that were labeled autoradiographically by retrograde transport of 3H-glycine from the abducens nucleus. Electrophysiological recordings from abducens motoneurons and internuclear neurons revealed a marked reduction in the slow positivity of the orthodromic extracellular potential elicited by ipsilateral vestibular nerve stimulation following systemic administration of strychnine, an antagonist of glycine. Intracellular recordings demonstrated that the vestibular-evoked disynaptic inhibitory postsynaptic potentials in abducens neurons were effectively blocked by strychnine but were unaffected by picrotoxin, an antagonist of GABA.(ABSTRACT TRUNCATED AT 400 WORDS)