Frontal eye field projection to the paramedian pontine reticular formation traced with wheat germ agglutinin in the monkey

Injections of the retrograde tracer [125I]wheat germ agglutinin have been placed in different areas of the paramedian pontine reticular formation (PPRF), a well known premotor center for gaze control. Experiments in 5 monkeys revealed 3 major sources of input: (1) bilateral projections from the so-called frontal eye field (FEF), which is situated in the frontal cortex around the arcuate sulcus; (2) the intermediate and deep layers of mainly the contralateral superior colliculus; and (3) ipsilateral projections from brainstem structures such as the accessory oculomotor nuclei (nucleus interstitialis of Cajal, nucleus of Darkschewitsch, and nucleus of the posterior commissure), the mesencephalic reticular formation, the vestibular nuclei, the nucleus prepositus hypoglossi, and the cerebellar fastigial nucleus. The results are compared with previous anatomical investigations and confirm the electrophysiologically demonstrated FEF-PPRF-abducens disynaptic pathway.     

Involvement of the pontine reticular formation in head movements and labyrinthine righting in the rat

Compensatory head movements as responses to passive angular acceleration are obvious in lower vertebrates, but they are usually small and difficult to observe in mammals. The may simply be cut short very early by an inhibitory mechanism linked to the initiation of anticompensatory head movements (quick phases of head nystagmus). After electrolytic lesions of the pontine reticular formation (PRF), rats made no spontaneous lateral head movements, but exhibited large compensatory lateral head movements and abnormal postrotatory circus movements, both of which can be regarded as consequences of a loss of the quick phase of head nystagmus. The quick phase of ocular nystagmus was also lost, corroborating earlier findings of others in primates, cats, and rabbits, and supporting the view that saccadic movements of the head and eyes are generated by closely related neural substrates. Rats with bilateral PRF lesions were unable to right themselves in the air. Together with additional observations, this indicates involvement of the PRF in some functions of the otolith organ.     

Role of pontomedullary reticular formation neurons in horizontal head movements: an ibotenic acid lesion study in the cat

Single-cell recording, electrolytic lesion and electrical stimulation studies have indicated that the pontomedullary reticular formation (PMRF) plays a role in head movement (HM) control. However, recent studies utilizing excitotoxin lesions of the PMRF have reported no effect on HM. In the present study, we have examined the acute and chronic motor effects of injecting ibotenic acid (IBO) into the nucleus reticularis pontis oralis, nucleus reticularis pontis caudalis and rostral medullary nucleus gigantocellularis of the feline PMRF. IBO injections in all of these regions induced tonic flexion of the head toward the ipsilateral side. This effect lasted 4-16 h. It was followed by a second phase in which head flexion and whole body circling were directed toward the contralateral side. Although this forced contralateral head turning disappeared within two days, the tendency to turn contralaterally and the impaired ability to make rapid ipsilateral HMs were present throughout survival periods lasting more than 4 months. Unilateral IBO PMRF lesions reduced the amplitude of vestibular induced quick phase (anti-compensatory) HMs toward the ipsilateral side and resulted in abnormally large and persistent slow compensatory HMs toward the contralateral side. Following IBO injections, the threshold intensity for HMs evoked by electrical stimulation at the injection site was elevated, and the amplitude and velocity of evoked HMs reduced. Histological data indicated that the reticular area involved in HM control was relatively large and probably extended beyond the PMRF region examined here. However, lesions including the nucleus reticularis pontis caudalis (NRPC) produced more severe and persistent HM deficits than those including the nucleus reticularis gigantocellularis. These data together with available anatomical and electrophysiological evidence indicate that PMRF neurons play a critical role in the generation of fast horizontal HMs toward the ipsilateral side.     

Intracellular evidence linking medial pontine reticular formation neurons to PGO wave generation

Intracellular recordings in the medial portion of the pontine reticular formation in the gigantocellular tegmental field and the tegmental reticular nucleus of the naturally sleeping cat have indicated the presence of a class of neurons whose discharges have a long lead time (50–300 ms) prior to, and a high correlation with primary ponto-geniculo-occipital (PGO) waves in the lateral geniculate nucleus (LGN) ipsilateral to the recording site; we term these neurons long lead PGO wave positive neurons. Further correlative evidence for their being a part of the circuitry generating PGO waves was their discharge suppression prior to the occurrence of secondary PGO waves in the ipsilateral LGN, and the presence of a contralateral inhibitory input that may explain the occurrence of PGO waves with lateralized amplitude characteristics.     

Direct projection of pause neurons to nystagmus-related excitatory burst neurons in the cat pontine reticular formation

Brain-stem pause neurons (PNs) are inhibitory neurons which cease their tonic firing about 20 ms prior to the quick phase of horizontal vestibular nystagmus in either direction. One group of nystagmus-related burst neurons just rostral to the abducens nucleus exhibits a burst of spikes before and during the quick phase to the ipsilateral side--excitatory burst neurons (EBNs). The present study supported the conclusion that PNs project to, and tonically inhibit EBNs during the slow phase and that the burst of activity of EBNs at the quick phase is partly caused by the abrupt release from pauser inhibition. The evidence leading to this conclusion is: simultaneous recording of PNs and EBNs showed close alternation of firing; PNs were antidromically activated from the EBN region; systematic microstimulation tracks within the EBN region showed an antidromic activation pattern of low threshold sites separated by high threshold sites consistent with PN axonal branching in the EBN region; during the nystagmus slow phase there were positive field potentials in the EBN region, followed by an abrupt negative deflection whose onset was synchronous with the last pauser spike; when single PN spikes were used to trigger averages of extracellular field potentials in the EBN region (postspike averaging), a consistent short-latency positivity was observed. This study shows an additional connection in the premotor neural network responsible for the generation of the quick phase of horizontal vestibular nystagmus.     

Functional localization in the three floccular zones related to eye movement control in the cat

Anatomically, the cat's cerebellar flocculus can be divided into 3 zones on the basis of differences in their efferent projection sites^13,14. The functional differences of these 3 zones in relation to eye movement control were investigated by observing the eye movements evoked by electric stimulation of each zone of the flocculus in ketamine-anesthetized cats. Stimulation of the flocculus elicited a slow eye movement. The direction of the slow eye movement was mapped. A downward eye movement was evoked by stimulation of the caudal zone. An ipsilateral horizontal eye movement was induced from the middle zone. An upward eye movement was elicited from the rostral zone. When prolonged stimulation was applied to the flocculus, the slow eye movement was followed by nystagmus in the opposite direction. This nystagmus persisted for many seconds after cessation of stimulation (afternystagmus). Nystagmus and afternystagmus could not be elicited in deeply anesthetized cats. Possibilities as to how the stimulation leads to various eye movements are discussed.     

The origin of brainstem afferents of the paramedian pontine reticular formation in the cat

Transcannular microinjections of horseradish peroxidase (HRP) were made into the paramedian pontine reticular formation (PPRF) in adult cats to determine the origin of the principal sources of inputs to this important preoculomotor center for the production of saccadic eye movements. Retrogradely labeled cells were observed in numerous oculomotor-related structures, including the prerubral field (rostral interstitial nucleus of the medial longitudinal fasciculus), nucleus of Darkschewitsch, nucleus of the posterior commissure, deep superior colliculus, supraoculomotor ventral periaqueductal gray, contralateral paramedian pontine reticular formation, pontine raphe and dorsal medial pontine tegmentum medial to the abducens nucleus (purported to contain omnipause neurons), cell group Y, and the perihypoglossal complex (nucleus prepositus hypoglossi). Other sources of afferents to the region included the zona incerta, lateral and medial habenular nuclei, medial hypothalamus, medial mammillary nucleus, nucleus cuneiformis, medial medullary reticular formation, and the medial and lateral cerebellar nuclei. The results are discussed in terms of the potential influence of these nuclei on the control of eye movement.     

Morphological and physiological identification of excitatory pontine reticular neurons projecting to the cat abducens nucleus and spinal cord

The current study provides strong morphological and physiological evidence for identifying reticular neurons which project to the ipsilateral abducens nucleus. In conjunction with recent work in the alert cat, these neurons are believed to be excitatory and are implicated to play a role in the generation of saccadic and/or vestibular fast phase eye movements.     

Activity of medullary reticular formation neurons in the unrestrained cat during waking and sleep

Single units, recorded in the medial medullary reticular formation (RF) in unrestrained, behaving cats, discharged in conjunction with specific movements and postures. Most cells were also active during REM sleep. Discharge rates in active waking and REM sleep were positively correlated and discharge patterns in these states were similar. We conclude that activity in these cells is related to the motor activation occurring in both active waking and REM sleep. We found no cells whose discharge was related in a non-specific way to motor tone or to REM sleep atonia. We discuss mechanisms by which medullary units with specific motor relations may give rise when stimulated to the relatively non-specific motor effects previously reported.     

Direct projection of type II vestibular neurons to eye movement-related pause neurons in the cat pontine reticular formation

Brain stem pause neurons play an important role in the regulation of rapid eye movements. However, the input sources that drive pause neurons are uncertain. In the present study, horizontal canal type II neurons in the medial vestibular nucleus were antidromically activated by electrical stimulation of the pause neuron region. Systematic microstimulation tracks within that region showed an antidromic activation pattern of low-threshold sites separated by high-threshold sites consistent with axonal branching of type II neurons in the pause neuron region. Spike-triggered averaging of single spontaneously firing type II vestibular neuronal discharges in the pause neuron region resulted in short-latency, positive field responses. These results supported the conclusion that horizontal canal type II neurons of the medial vestibular nucleus project to and inhibit pause neurons monosynaptically.     

Afferents to the oculomotor nucleus in the goldfish (Carassius auratus) as revealed by retrograde labeling with horseradish peroxidase.

The goal of this work was to compare the distribution and morphology of neurons projecting to the oculomotor nucleus in goldfish with those previously described in other vertebrate groups. Afferent neurons were revealed by retrograde labeling with horseradish peroxidase. The tracer was electrophoretically injected into the oculomotor nucleus. The location of the injection site was determined by the antidromic field potential elicited in the oculomotor nucleus by electrical stimulation of the oculomotor nerve. Labeled axons whose trajectories could be reconstructed were restricted to the medial longitudinal fasciculus. In order of quantitative importance, the afferent areas to the oculomotor nucleus were: (1) the ipsilateral anterior nucleus and the contralateral tangential and descending nuclei of the octaval column. Furthermore, a few labeled cells were found dorsomedially to the caudal pole of the unlabeled anterior octaval nucleus; (2) the contralateral abducens nucleus. The labeled internuclear neurons were arranged in two groups within and 500 microns behind the caudal subdivision of the abducens nucleus; (3) a few labeled cells were observed in the rhombencephalic reticular formation near the abducens nucleus, most of which were contralateral to the injection site. Specifically, stained cells were found in the caudal pole of the superior reticular nucleus, throughout the medial reticular nucleus and in the rostral area of the inferior reticular nucleus; (4) eurydendroid cells of the cerebellum, located close to the contralateral eminentia granularis pars lateralis, were also labeled; and (5) a small and primarily ipsilateral group of labeled cells was located at the mesencephalic nucleus of the medial longitudinal fasciculus. The similarity in the structures projecting to the oculomotor nucleus in goldfish to those in other vertebrates suggests that the neural network involved in the oculomotor system is quite conservative throughout phylogeny. Nevertheless, in goldfish these projections appeared with some specific peculiarities, such as the cerebellar and mesencephalic afferents to the oculomotor nucleus.     

Alterations in membrane potential and excitability of cat medial pontine reticular formation neurons during changes in naturally occurring sleep-wake states

Intracellular recordings of medial pontine reticular formation (mPRF) neurons in behaving cats show the following distinctive desynchronized sleep (D) state characteristics: excitability is greater in D than in waking (W) or synchronized sleep (S) and in D there is a tonic depolarization that persists throughout the state and upon which are superimposed phasic runs of further depolarization. In contrast, the background, tonic level of membrane potential in both W and S is more negative than in D, which phasic depolarizations associated with motor activity occurring in W but not in S. These data are compatible with a generating role for mPRF neurons in D sleep phenomena.     

Brainstem afferents to the oculomotor omnipause neurons in monkey

To determine how saccade-related areas in the brainstem address the saccade generator, we examined the afferents to the nucleus raphe interpositus. This region contains the omnipause neurons, which are pivotal in the generation of saccades. Horseradish peroxidase injected iontophoretically into the nucleus raphe interpositus retrogradely labeled a variety of brainstem nuclei. The greatest numbers of labeled neurons were in the paramedian pontomedullary reticular formation, in the nuclei reticularis gigantocellularis, and paragigantocellularis lateralis. Labeling was more modest but consistent in the interstitial nucleus of Cajal and the adjacent mesencephalic reticular formation, the middle gray of the superior colliculi, the region dorsolateral to the nucleus reticularis tegmenti pontis, and the medial vestibular nucleus. A few neurons were labeled around the habenulopeduncular tract and in the medial portion of the nucleus of the fields of Forel, in the nucleus reticularis medullaris ventralis, and in the spinal nucleus of the trigeminal nerve, the cochlear nucleus, and the superior olivary complex. The distribution and density of labeling suggest that omnipause neurons in the monkey are more intimately connected with other oculomotor structures than those in the cat. In addition, the rhombencephalic reticular afferents to the monkey omnipause neurons are more concentrated in their immediate vicinity than in the cat. The label consistently found dorsolateral to the nucleus reticularis tegmenti pontis may be a newly discovered link in saccade generation.     

Amygdaloid pathway to the trigeminal motor nucleus via the pontine reticular formation in the rat

The connections of the amygdala with the trigeminal motor nucleus were studied by light and electron microscopy. Horseradish peroxidase (HRP) experiments showed that the pontine reticular formation, ventromedial to the spinal trigeminal nucleus at the level rostral to the genu of the facial nerve, receives fibers from the central nucleus of the amygdala ipsilaterally and sends fibers to the trigeminal motor nucleus contralaterally. Electron microscopic observations were carried out on the pontine reticular formation after electrolytic lesions in the central nucleus of the amygdala and HRP injections into the contralateral trigeminal motor nucleus were made on the same animal. These experiments using the combined degeneration and HRP technique clearly demonstrated that degenerating amygdaloid fibers made synaptic contacts with retrogradely labeled neurons.     

Physiological studies of brainstem reticular connectivity. I. Responses of mPRF neurons to stimulation of bulbar reticular formation

The connectivity between medial pontine reticular formation (mPRF) and bulbar reticular formation (BRF) was studied by intracellular recordings of mPRF neuronal responses to microstimulation of BRF in unanesthetized, undrugged cats. There was a very high percentage (75-90%) of monosynaptic latency postsynaptic potentials (PSPs) in mPRF neurons in response to microstimulation of 3 BRF areas: the magnocellular tegmental field (FTM), the bulbar gigantocellular tegmental field (BFTG), and bulbar lateral tegmental field (BFTL). The type of initial orthodromic response produced in mPRF neurons by BRF stimulation was predominantly (75-95%) a monosynaptic excitatory PSP (EPSP) which was characterized by a rapid rise time, a nearly constant latency, and often led to spike potential generation. In contrast, the percentage of initial monosynaptic inhibitory PSPs (IPSPs) was much lower for FTM (12.3%), for BFTG (12.5%) and was zero for BFTL. While microstimulation techniques alone cannot differentiate between excitation of fibers of passage and neuronal somata, the very high percentage of initial EPSPs in our data and the anatomical evidence for dense BRF to mPRF neuronal projections as compared with less dense projections from fibers passing through BRF to mPRF suggest that excitatory BRF-mPRF connections are predominant. The high degree of connectivity between BRF and mPRF may furnish an important substrate for functional interaction. Comparison of the mPRF neuronal population that was not antidromically activated by FTM microstimulation vs the mPRF neuronal population that was antidromically activated from FTM and also studied for orthodromic responsiveness showed no statistically significant differences between these populations on the parameters of percentage of monosynaptic input, monosynaptic initial EPSPs, monosynaptic initial IPSPs and presence of a PSP with a latency of less than 5 ms. For BRF connectivity this suggests an identity of mPRF input and output neurons with respect to synaptic response properties.     

Effect upon eye movements of rabbits induced by severance of mossy fiber visual pathway to the cerebellar flocculus

In albino rabbits the visual mossy fiber pathway to the cerebellar flocculus was interrupted by placing lesions in the nucleus reticularis tegmenti pontis (NRTP). After recovery of more than 3 days, eye movements were tested by means of a television eye tracking system. The optokinetic response (OKR) in one eye induced by sinusoidally moving a vertical slit light on the horizontal plane (2.5° peak-to-peak amplitude) at 0.17-0.033 Hz in front of that eye. In rabbits with unilateral NRTP lesions, the OKR gain was reduced significantly in the eye contralateral to lesions, whereas that in the ipsilateral eye did not differ from control rabbits. The horizontal vestibulo-ocular reflex (HVOR) exhibited no change attributable to NRTP lesions. The operated rabbits were rotated (5° peak-to-peak amplitude) at 0.1 Hz continuously for 3 h, while the slit light was presented to the eye contralateral to the NRTP lesions. During the rotation, the HVOR gain in the test eye increased adaptively as in control rabbits. It is concluded that the visual mossy fiber pathway to the flocculus contributes to the OKR, but not to visually-guided adaptive modification of the HVOR.     

REM sleep enhancement induced by sensory stimulation is prevented by kainic acid lesion of the pontine reticular formation

It has been shown that auditory or somatic stimulation during rapid eye movement (REM) sleep is capable of producing a significant increase in ponto-geniculo-occipital (PGO) spike density as well as in REM sleep duration. The purpose of this study was to determine the role of the medial pontine reticular formation (PRF) in mediating such increase of REM sleep duration. After a baseline recording whereby on the same recording day the control and the stimulus (auditory or somatic) alternated with each REM, a group of cats was lesioned with kainic acid in the PRF. The sleep-wake cycle was recorded again on days 15, 30 and 45 post-lesion, following the same procedure. The results showed no changes in REM sleep duration and PGO spike density in the lesioned animals. However, when sensory stimulation was applied it was ineffective in producing REM sleep enhancement, although it was able to increase PGO spike density. These findings suggest that the effects of sensory stimulation on REM sleep duration are accomplished through the PRF, probably by inducing an increase in the excitability levels of such neurons, and further suggests that PGO spike density and REM duration are independent of each other.