S.M. Highstein

Depletion of vesicles and fatigue of transmission at a vertebrate central synapse

Synapses from Mauthner to giant fibers in the hatchetfish are chemically transmitting excitatory axo-axonic synapses located in the medulla. The synapses are 4--10 mum in diameter and easily identified for electron microscopy. Presynaptic vesicles are clustered near the contact regions and are round, clear and 40-60 nm in diameter. Stimulation of the Mauthner fiber at 10/sec for 10 min greatly reduces PSP amplitude and causes profound changes in presynaptic structures. Synaptic vesicles become few in number and there is a marked accumulation of irregular membranous structures. These changes are reversible. During the recovery period, the number of synaptic vesicles progressively increases to control values, and the number of irregular membranous structures declines. Further, stimulation during cooling induces depletion of vesicles together with a great increase in the surface area of the presynaptic membrane and in the number of coated vesicles. Internal irregular membranous structures are few. Our data provide evidence for the vesicular release of transmitter and are consistent with there being a mechanism of membrane recycling in which vesicle membrane fuses with the presynaptic membrane and is reclaimed from it by coated vesicles that then coalesce to form irregular membranous structures from which new synaptic vesicles are formed.

Vestibular and medullary brain stem afferents to the abducens nucleus in the cat

Brain stem neurons that project to the abducens nucleus (nVI) were labeled by the technique of retrograde transport of horseradish peroxidase (HRP). Following iontophoresis of HRP into nVI a large number of labeled cells are found in the ipsilateral vestibular nuclear complex, extending from the rostral medial vestibular nucleus into the ventral lateral vestibular nucleus. A smaller number of HRP-positive cells are also found in the contralateral medial vestibular nucleus. In addition, labeled cells are localized to the contralateral dorsomedial gigantocellular tegmental field as well as the nucleus praepositus hypoglossi of both sides, evidence that these neuronal groups may also be involved in eye movement control.

Vestibular nucleus neurons relaying excitation from the anterior canal to the oculomotor nucleus

A morphological approach was undertaken to determine which vestibular nucleus neurons relay excitation from the anterior canal to the IIIrd nucleus. In anesthetized rabbits HRP was iontophoresed into the IIIrd nucleus and cells filled with HRP reaction product (positive cells) searched for within the vestibular nuclear complex. By lesioning the MLF or brachium conjunctivum immediately after iontophoresis it was demonstrated that positive cells in the dorsum of the superior vestibular nucleus are backfilled via their axons which ascend in the brachium conjunctivum. By contrast positive cells in the center of the superior nucleus are backfilled via their axons in the MLF. In electrophysiological experiments in the presence of a severed MLF the anterior canal was selectively stimulated for orthodromic, and the 3rd nucleus stimulated for antidromic, activation of vestibular nucleus neurons. Recording extracellularly with glass microelectrodes filled with fast green FCF the only cells both ortho- and antidromically activated were localized to the dorsum of the superior vestibular nucleus. It is concluded that cells dorsally located in the superior nucleus relay the disynaptic excitatory vestibulo-ocular reflex from the anterior canal to the contralateral 3rd nucleus via their axons which ascend in the brachium conjunctivum.

The superior vestibular nucleus: An intracellular hrp study in the cat. II. Non‐vestibulo‐ocular neurons

Superior vestibular neurons were penetrated with horseradish perox‐idase (HRP)‐loaded glass microelectrodes in anesthetized cats. Responses to electrical stimulation of the oculomotor complex and the vestibular nerves were characterized and selected neurons were injected with HRP. Neurons antidromically activated by oculomotor complex stimulation were generally monosynaptically excited by the ipsilateral vestibular nerve. Notable was the absence of strong commissural inhibition by stimulation of the contra‐lateral vestibular nerve. Light microscopy of antidromically identified injected cells demonstrated that these cells are predominantly located at the central levels of the superior vestibular nucleus along the incoming vestibular nerve fibers but a few are found at more caudal levels. Cell bodies, elongated or pyramidal, are mainly medium‐sized to large (30–50 μm). Dendritic trees extend in a plane at an acute angle to the collaterals of the vestibular nerve fibers. Dendrites remain within the nuclear territory and generally display an isodendritic branching pattern. Dendritic spines and appendages are mainly distributed on secondary and distal dendrites. A few terminal enlargements similar to growth cones are observed in these neurons.

Fatigue and recovery of transmission at the Mauthner fiber-giant fiber synapse of the hatchetfish

When the Mauthner fiber-giant fiber synapse of the hatchetfish is activated at gradually increasing frequencies, postsynaptic potentials (PSPs) in the giant fiber become progressively smaller, but complete failures of transmission are not observed even when PSP size is as small or smaller than miniature PSPs (mPSPs) simultaneously recorded. On the assumption of a Poisson distribution of amplitudes, calculations from the absence of failures and from variance suggest that guantum number remains at least as high as 5--10 and that quantal size is greatly reduced. During tetanic stimulation the frequency of mPSPs first increases and then decreases again, sometimes to a very low frequency. However, mPSP amplitude is reduced by no more than about 50%, which indicates that quanta giving rise to mPSPs come from a different population of vesicles than those comprising evoked PSPs. During rest following a tetanus, calculated quantal size in evoked PSPs recovers within several hundred milliseconds to mPSP size simultaneouly recorded. Most of this recovery time represents time for filling, since vesicles can be supplied at much higher rates during tetanic stimulation. After one second rest PSP amplitude exceeds threshold but recovery for later PSPs in a short train requires many minutes. The slowness of this recovery is consistent with the morphological demonstration of slow recovery of the vesicle population after depletion. These data are interpreted in terms of vesicle release, depletion and membrane recycling. Following depletion new vesicles are released after only partial filling which accounts for small quanta. Very small mPSPs are not seen because filling time is short compared to time for release as mPSPs. Since quantal size can be gradually reduced, release can interrupt filling, and filling and release sites are likely to be the same. The data in combination with the morphological observations support the hypothesis of vesicular release of transmitter and provide new evidence as to rates and sites for filling of vesicles.

The ascending tract of Deiters' conveys a head velocity signal to medial rectus motoneurons

Horizontal conjugate gaze is organized in the ponto-medullary brain stem by neuronal signals relayed to the medial rectus (MR) extra ocular muscle of one eye and the lateral rectus of the other,13,aa,x6,19, 20. In the cat, there are two distinct excitatory projections to the MR motoneuron pool from the posterior brain stem. Firstly, abducens internuclear neurons send their axons into the contralateral median longitudinal fasciculus (MLF) to terminate upon MR motoneurons x6-1s. Eye position and eye velocity related signals are carried by these internuclear cellsS,18,22,2~, 26. The second projection, the ascending tract of Deiters (ATD) 24 is formed by the axons of neurons located in the ventrolateral vestibular nucleusZ,~3,14. These ventrolateral vestibular neurons also terminate upon medial rectus neurons, however, signals carried by the ATD have not yet been elucidated. Qualitative analysis of the signals carried by the abducens internuclear neurons indicates a similarity to the discharges of medial rectus motoneurons during gaze. (The internuclear neurons behave like medial rectus motoneurons during gaze emitting a burst of spikes with saccades in the on direction of the contralateral m~dial rectus muscle, pausing for off direction saccades and firing at tonic frequencies proportional to eye positionS.) This implies that an integrated head velocity signal is already contained in the first projection to the medial rectus motoneurons (the M LF) and on this basis it was suggested z6 that an additional head velocity signal transmitted by the second pathway (the ATD)would be superfluous. As macular and canal input terminates in the ventrolateral vestibular nucleus12, 27 ATD neurons could conceivably carry macular signals. This suggestion is, however, not borne out by the shape, amplitude and rise time of the EPSPs evoked in medial ~ectus motoneurons by ATD activationL These EPSPs have the characteristics of semicircular canal evoked PSPs in other oculomotor neurons rather than those evoked by macular stimulation. We have studied ATD neurons with intracellular recording to determine connectivity, and with extracellular techniques to determine sensitivity to head motion. The response of ATD neurons to a head velocity stimulus is the basis of this report.

The vestibulothalamic pathway: Contribution of the ascending tract of deiters

Axoplasmic transport techniques were used to determine the contribution of the ascending tract of Deiters (ATD) to the vestibulothalamic projection in cats. Large injections of HRP into the thalamus centered on the border region between the ventrobasal complex and the caudal ventrolateral nucleus resulted in bilateral retrograde labeling of cells in the vestibular nuclear complex and the nucleus prepositus hypoglossi (PH). Similar thalamic injections were also made in animals with extensive bilateral lesions of the medial longitudinal fasciculus (MLF) and the brachium conjunctivum (BC). HRP-positive neurons in these cases were localized principally to the ventral lateral vestibular nucleus and adjacent superior vestibular nucleus ipsilateral to the thalamic injection, evidence that vestibulothalamic neurons in these nuclei may project to the thalamus over the unlesioned ATD. Injections of [35S]methionine into the rostral vestibular nuclear complex in animals with MLF and BC lesions confirmed these findings, demonstrating orthograde transport of radiolabel in the ATD with termination in thalamus. These experiments document a contribution of the ATD to the ipsilateral vestibulothalamic projection; other sources of the vestibulothalamic pathway (PH, Y group) likely travel through projection systems destroyed in the lesions made in the present study.

Eye position and head velocity signals are conveyed to medial rectus motoneurons in the alert cat by the ascending tract of Deiters

Eye position and head velocity signals are conveyed to medial rectus extraocular motoneurons in the alert cat by the ascending tract of Deiters. Physiologically and behaviorally identified ascending Deiters neurons have been injected intra-axonally and their morphology studied.

Abducens to Medial Rectus Pathway in the MLF: A Possible Cellular Basis for the Syndrome of Internuclear Ophthalmoplegia

The neurological syndrome of internuclear ophthalmoplegia consists of defects in horizontal and vertical eye movements: a prominent feature is the loss of conjugate horizontal gaze (Bender & Weinstein, 1944; Cogan, 1948; Cogan et al., 1960; Christoff et al., 1960; Carpenter & McMasters, 1963; Carpenter & Strominger, 1965; Cohen, 1971). Horizontal gaze deficits are caused by an apparent palsy of the medial rectus extraocular muscle which is manifested as a weakness of medially directed gaze in the affected eye. With interruption of/or damage to the medial longitudinal fasciculus (MLF) on one side, the ipsilateral eye is abducted at rest and does not cross into the nasal or medial field of gaze except during convergence movements. Medially directed saccades in the affected eye are slowed (Evinger et al., in press) and neither vestibular, visual nor voluntarily induced eye movement can cause the eye to deviate into the medial hemifield (Evinger et al., in press; Cohen, 1971). It is noted that the adductive paresis is also present during electrical stimulation of the pontine reticular formation, a powerful stimulus which usually produces ipsilaterally directed conjugate horizontal eye movement (Cohen, 1974). The profound nature of this adductive paresis implies the necessity of an intact MLF for the production of all conjugate horizontal eye movements (Cohen, 1971; Cohen, 1974).