Caudal Pontine Reticular Formation Research Articles

Brainstem sites for the carbachol elicitation of the hippocampal theta rhythm in the rat.

The effects of brainstem microinjections of carbachol on the hippocampal theta rhythm were examined in urethane anesthetized rats. The two most effective theta-eliciting sites with carbachol were the nucleus pontis oralis (RPO) and the acetylcholine-containing pedunculopontine tegmental nucleus (PPT) of the dorsolateral pontine tegmentum. RPO injections generated theta at mean latencies of 38.5 +/- 70.8 s and for mean durations of 12.9 +/- 5.1 min. Five of seven RPO injections gave rise to theta virtually instantaneously, i.e., before the completion of the injection. PPT injections generated theta at mean latencies of 1.7 +/- 1.1 min and for mean durations of 11.9 +/- 6.0 min. Injections rostral or caudal to RPO in the caudal midbrain reticular formation (RF) or the caudal pontine RF (nucleus pontis caudalis) generated theta at considerably longer latencies (generally greater than 5 min) or were without effect. Medullary RF injections essentially failed to alter the hippocampal EEG. The finding that theta was produced at very short latencies at RPO suggests that RPO, the putative brainstem source for the generation of theta, is modulated by a cholinergic input. The further demonstration that theta was also very effectively elicited with PPT injections suggests this acetylcholine-containing nucleus of the dorsolateral pons may be a primary source of cholinergic input to RPO in the generation of theta. The hippocampal theta rhythm is a major event of REM sleep. The present results are consistent with earlier work showing that each of the other major events of REM sleep, as well as the REM state, are cholinergically activated at the level of the pontine tegmentum.

Oblique misdirection and slowing of vertical saccades after unilateral lesions of the pontine tegmentum

Three patients with unilateral lesions of the pontine tegmentum, identified by CT and MRI, had abnormal vertical saccades and slowed ipsilateral horizontal saccades. Attempted vertical saccades were misdirected obliquely, away from the side of the lesion, and their vertical components were prolonged. Oblique saccades had curved trajectories and prolonged durations of their vertical components. Unilateral damage to excitatory burst neurons and pause cells in the medial part of the caudal paramedian pontine reticular formation may cause these abnormal vertical and oblique saccades. Misdirection and slowing of vertical saccades can accompany the paralysis or slowing of ipsilateral horizontal saccades caused by pontine damage.

Crossed reticular formation connections that mediate the startle reflex in rats

The startle response is a bilateral response even when elicited by unilateral acoustic or tactile stimuli. Similarly, unilateral electrical stimulation of the reticular formation also elicits a bilateral startle-like response. To examine whether crossed reticular formation connections can distribute the effects of unilateral stimulation across the midline, we delivered one pulse to the caudal pontine (RPC) or medullary reticular formation (MRF) and a second pulse to the opposite side of the brain, at various interpulse intervals. The symmetric collision effects 33 suggest that axons which produce at least 37% (range 23–53%) of the startle response efficacy cross from RPC to RPC with a mean conduction velocity of 13 m/s. Similar collision effects were observed between RPC and MRF site but at shoter conduction times. To examine which axoms might cause these collision effect, the axonally transported label DiI was injected post mortem into 37 RPC sites. Many coarse axons were observed to cross in fascicles between bilateral RPC sites and then separate in the contralateral RPC. The fiber diameters and trajectories of these DiI-labelled axons are consistent with the conduction velocities and trajectories of the substrates mediating the startle-like response determined in collision tests.

Giant neurons in the caudal pontine reticular formation receive short latency acoustic input: an intracellular recording and HRP-study in the rat.

The reticular formation is composed of heterogeneous cell populations with multiple functions. Among these multiple functions is the processing of sensory information in the context of behavior. The purpose of the present study was to identify and characterize neurons in the reticular formation of the rat that receive auditory input. In order to do so, we combined intracellular electrophysiology in vivo with intracellular injection of horseradish peroxidase, enabling us to correlate electrophysiology unequivocally with anatomy at the single cell level. We found that many neurons in the caudal pontine reticular nucleus (PnC), which we analyzed intracellularly, responded to acoustic stimuli and were excited at short latency (mean EPSP latency: 2.6 ms; mean spike latency: 5.2 ms). This short latency suggests a direct input from the cochlear nucleus, the first central nucleus of the auditory pathway. The morphology revealed that the acoustically driven PnC neurons have very large somata (mean diameter: 44.0 microns). They can therefore be referred to as "giant PnC neurons." Complex dendritic arbors extended from these neurons into the reticular formation and thus formed a large membrane surface for the integration of multimodal inputs. Most of the giant PnC neurons sent their axons caudally into the medial longitudinal fasciculus and can therefore be regarded as reticulospinal neurons. The results demonstrate that the giant reticulospinal PnC neurons are in a position to transmit acoustic information very quickly to spinal cord neurons and to receive converging input from other parts of the brain. They are thus good candidates for participation in the mediation and modulation of acoustically elicited behaviors, such as the short latency acoustic startle response.

Descending pathways mediating disynaptic excitation of dorsal neck motoneurones in the cat: brain stem relay

The location of intercalated neurones mediating disynaptic excitation from tectum, tegmentum and pyramids to dorsal neck motoneurones has been investigated by: (a) recording field potentials in the lower brain stem evoked from the above systems, (b) systematic stimulation in the brain stem during intracellular recording from motoneurones innervating the splenius, biventer cervicis and complexus muscles, and (c) comparing the effects of lesions of the brain stem with kainic acid on the disynaptic EPSPs elicited from the above three systems. Electrical stimulation of the contralateral superior colliculus evoked monosynaptic field potentials which were largest in the caudal pontine reticular formation rostral to the abducens nucleus and in the rostral part of the medullary reticular formation caudal to the abducens nucleus. Likewise, stimulation of the ipsilateral tegmentum (the cuneiform and subcuneiform nucleus) evoked field potentials which were large in the caudal medulla and small in the pons. In contrast, stimulation of the contralateral tegmentum was ineffective in evoking field potentials. Stimulation of the pyramid 2–3 mm rostral to the obex elicited monosynaptic field potentials in the reticular formation of the lower brain stem that were only about 25% of those from the superior colliculus. In contrast to the field potentials from the superior colliculus, the pyramidal ones were large in the medulla and small in the pons. Lesions of the reticular formation in the lower brain stem by unilateral kainic acid injection caused disappearance of disynaptic EPSPs in motoneurones from the above three systems. These results strongly suggest that the intercalated neurones mediating pyramidal, tectal and tegmental EPSPs are reticulospinal neurones in the lower brain stem. Systematic stimulation in various locations of the lower brain stem showed that monosynaptic EPSPs were evoked from the regions of the reticular formation which received projection from the above three descending systems. The effective regions for evoking the EPSPs in splenius (SPL) were located somewhat more dorsally than for biventer cervicis and complexus (BCC) motoneurones. The descending axons of presumed reticulospinal neurones were stimulated with electrodes placed in medial, middle and lateral positions at the spinomedullary junction. Monosynaptic EPSPs in SPL and BCC motoneurones were evoked from the medial and middle electrodes but not from the lateral electrode.

Loss of the acoustic startle response following neurotoxic lesions of the caudal pontine reticular formation: Possible role of giant neurons

The effect of the excitotoxic N-methyl- d-aspartate agonist quinolinic acid in the caudal pontine reticular formation on the acoustic startle response was investigated in rats. Bilateral injections of 90 nmol of quinolinic acid led to large lesions in the reticular formation characterized by the loss of all neurons and a marked reduction or even abolition of the acoustic startle response; 18 nmol of quinolinic acid led to smaller lesions characterized by a selective loss of giant neurons within the caudal pontine reticular formation and a reduction of the startle amplitude. The partial correlation analysis revealed that the reduction of the amplitude of the acoustic startle response can be correlated with the loss of the giant neurons ( r = 0.575; d.f.= 29; P < 0.001) but not with the reduction of the number of all neurons ( r = 0.207; d.f.= 29; P > 0.2) in the caudal pontine reticular formation. These findings were reconciled with electrophysiological and anatomical data indicating that the giant neurons in the caudal pontine reticular formation receive acoustic input and project to motoneurons of the spinal cord. It is concluded that the caudal pontine reticular formation is an important element of the startle pathway and that the giant reticulospinal neurons constitute an important part of the sensorimotor interface mediating this response.

Cochlear efferent neurons projecting to both ears in the chicken, Gallus domesticus

Different retrograde neuroanatomical tracers were injected into each cochlea of adult chicken. The number of cells labeled in the cochlear efferent cell group found bilaterally within the caudal pontine reticular formation depended upon the tracer, with True Blue and Fluoro Gold yielding maximal average counts of 332 efferent neurons per injection. Double labeling of less than 1% of these cells was possible with the combination of True Blue and Diamidino Yellow. Thus the contribution of efferent neurons with axon collaterals projecting to both ears is not fundamentally different in birds and other vertebrates.

Descending inhibitory pathway responsible for simultaneous suppression of postural tone and respiration in decerebrate cats

The present study was performed in order to elucidate whether the suppressive effects on postural tone and respiration evoked by stimulation of the dorsal tegmental field (DTF) of the pons are relayed through the neurons in the ponto-medullary reticular formation. First, the DTF was functionally identified, and then a microelectrode was inserted into the caudal pontine and medullary reticular formation to investigate the distribution of the neurons monosynaptically activated by stimulation of the DTF. The monosynaptically activated neurons were distributed within the nucleus reticularis gigantocellularis (NRGc) in the caudal pons and medulla. The spinal cord (L 1) was stimulated to study whether such monosynaptically activated neurons project to the spinal cord. Almost all the neurons monosynaptically activated by DTF stimulation were antidromically activated by spinal stimulation. This result indicates that the neurons in the NRGc activated monosynaptically by DTF stimulation send axons to the spinal cord. Tonic micro-stimulation was then delivered to the site in the NRGc, from which the monosynaptically driven units were recorded by DTF stimulation. The stimulation resulted in the parallel suppression of postural tone and respiration, similar to the suppressive effects elicited by DTF stimulation. The present study suggests the possible existence of a descending inhibitory pathway through the reticulospinal neurons in the NRGc responsible for parallel suppression of postural tone and respiration.

Double-pulse stimulation of startle-like responses in rats: refractory periods and temporal summation

A startle-like response was evoked by electrical stimulation with one pulse in several brainstem sites of the primary acoustic startle circuit. If a second pulse was delivered 0.4–10 ms after the first pulse, a stronger response or a decreased current threshold resulted. The facilitatory effect of the second pulse increased as interpulse (C-T) interval increased from 0.4 to 2.0 ms in cochlear nucleus or ventral lateral lemniscus sites. In caudal pontine reticular formation sites, the effect of the second pulse increased sharply from 0.3 to 0.5 ms. These results suggest that very short refractory period axons mediate electrically elicited startle in reticular formation, and that longer refractory period axons mediate startle in cochlear nucleus or ventral lateral lemniscus. In reticular formation sites, the effect of the second pulse declined nearly exponentially from 2.0 to 50 ms with a time constant of about 4 ms. Stimulation of similar reticular formation sites in cats evokes monosynaptic EPSPs in spinal motoneurons with an almost identical time course, as reported by other investigators. This suggests that the startle response evoked from the reticular formation results from monosynaptic activation of spinal motoneurons. Temporal summation declined more slowly and irregularly in cochlear nucleus and ventral lateral lemniscus sites, suggesting that these sites are not monosynaptically connected with spinal motoneurons, a conclusion consistent with anatomical data. In reticular formation sites near the facial nerve, a second peak in the two-pulse curve was observed at a C-T interval of 10 ms. The second peak was blocked by local anesthesia of the face ipsilateral to the stimulating electrode, suggesting that a single twitch of facial muscles facilitates startle.

Effects of pontomedullary reticular formation stimulation on the neuronal networks responsible for rhythmical jaw movements in the guinea pig.

1. In the ketamine-anesthetized guinea pig, electromyographic (EMG) responses of the digastric muscle and vertical and horizontal movements of the mandible were studied when loci within the caudal pontine and rostral medullary reticular formation were stimulated during rhythmic jaw movements (RJMs) evoked by stimulation of the masticatory area of the cortex. 2. Within these regions electrical brain stem stimulation of the pontis nucleus caudalis and nucleus gigantocellularis (PnC-Gi) of the reticular formation completely blocked RJMs at stimulus intensities as low as 10 microA while suppressing the short-latency digastric EMG response that was time locked to each cortical stimulus in the train. PnC-Gi stimulation did not, however, reduce the excitability of the short-latency corticotrigeminal excitatory pathway to digastric motoneurons when tested by short pulse train stimulation at 2 Hz (3 pulses, 500 Hz, 0.3 ms) in the absence of RJMs. 3. Short trains (80 ms) of PnC-Gi stimuli delivered at various phases of the RJM cycle produced a permanent phase shift of the RJM rhythm. If the stimulus train was delivered at an early phase of the cycle (8-40%) the next cycle onset was advanced; if the train was delivered later in the cycle (60-80%) the next cycle onset was delayed. Long trains of PnC-Gi stimuli (100, 200, 300, and 400 ms) increased the time of onset of the next cycle by an amount directly proportional to the duration of the stimulus train. 4. Digastric EMG activity occurring during cortically evoked RJMs occupied nearly 50% of the cycle. If a short train of PnC-Gi stimuli was delivered between approximately 5 and 125 ms after the onset of the burst, the duration of the burst was significantly shortened. 5. These results demonstrate that the suppression of cortically evoked RJMs resulting from PnC-Gi stimulation is due to direct effects on central circuits responsible for the production of the RJM behavior and not on the motoneurons themselves. The evidence presented is consistent with our previously presented hypothesis that the neurons involved in mediating the short-latency corticotrigeminal pathway to digastric motoneurons are separate and distinct from those neurons comprising the central networks responsible for the production of the fundamental jaw oscillation during RJMs.

Reticulo-spinal neurons participating in the control of synergic eye and head movements during orienting in the cat. I. Behavioral properties.

The activity of 24 reticulo-spinal neurons (RSN) identified by antidromic stimulation at the C1-C2 level has been recorded intra-axonally in the pons of alert head-fixed cats during spontaneous gaze shifts and orienting towards novel targets. Relationship of neuronal discharge to saccadic eye movements, positions of fixation and EMG of dorsal neck muscles were analysed. The present report describes behavioral properties of a group of 14 RSN showing similar types of correlations with motor parameters during eye-head synergies. These "eye-neck" RSN (EN-RSN) generate bursts in synchrony with phasic components of ipsilateral neck EMG and leading ipsiversive saccades by a variable lead time. Bursts are followed by a prolonged discharge whose frequency decays even when eccentric eye position is maintained constant and accompanied by sustained neck muscle activity. The firing rate of EN-RSNs depends on eye position: they are silent with saccades in their ON-direction when the eyes are deviated towards the contralateral half of the oculomotor range and the ipsilateral neck muscles are relaxed. When the eyes cross the vertical meridian, the frequency of phasic and tonic components related to eye-head synergies increase proportionally to ipsilateral eye position. Ten of the 14 EN-RSNs, located in the pontine reticular formation, received monosynaptic input from the contralateral superior colliculus. Two were labeled by intra-axonal injection of HRP which revealed extensive branching in the abducens, facial, medial and lateral vestibular, prepositus and intercalatus nuclei and in the caudal pontine and bulbar reticular formation. It is concluded that the caudal pontine tegmentum, including the region just anterior to the abducens nucleus, contains RSNs whose signals seem appropriate to control phasic neck muscle activity and which also project to structures related to ocular and facial movements. Comparisons with the perisaccadic activity of tectal neurons projecting in the predorsal bundle reveals a profound transformation of the descending signal at the level of EN-RSNs which represent first order relay neurons of the tecto-reticulo-spinal pathway.

Retrograde labeling of neurones in the brain stem following injections of [3H]choline into the forebrain of the rat.

In an attempt to identify cholinergic neurons of the brain stem which project to the forebrain, retrograde labeling of neurons in the brain stem was examined by autoradiography following injections of 20 microCi [3H]choline into the thalamus, hypothalamus, basal forebrain and frontal cortex. After injections into the thalamus, retrogradely labeled neurons were evident within the lateral caudal mesencephalic and dorsolateral oral pontine tegmentum (particularly in the laterodorsal and pedunculopontine tegmental nuclei) and in smaller number within the latero-medial caudal pontine (Reticularis pontis caudalis, Rpc) and medullary (Reticularis gigantocellularis, Rgc) reticular formation. Following [3H]choline injections into the lateral hypothalamus and into the basal forebrain, retrogradely labeled neurons were localized in the dorsolateral caudal midbrain and oral pontine tegmentum and in smaller number in the medial medullary reticular formation (Rgc), as well as in the midbrain, pontine and medullary raphe nuclei. After injections into the anterior medial frontal cortex, a small number of retrogradely labeled cells were found in the brain stem within the laterodorsal tegmental nucleus and the dorsal raphe nucleus. In a parallel immunohistochemical study, choline acetyltransferase (ChAT)-positive neurons were found to be located in most of the regions of the reticular formation where cells were retrogradely labeled from the forebrain following [3H]choline injections. These results suggest that multiple cholinergic neurons within the lateral caudal midbrain and dorsolateral oral pontine tegmentum and a few within the caudal pontine and medullary reticular formation project to the thalamus, hypothalamus and basal forebrain and that a limited number of pontine cholinergic neurons project to the frontal cortex.

An autoradiographic analysis of ascending projections from the medullary reticular formation in the rat

Ascending projections from the several nuclei of the medullary reticular formation were examined using the autoradiographic method. The majority of fibers labeled after injections of [ 3Hleucine into nucleus gigantocellularis ascended within Forel's tractus fasciculorum tegmenti which is located ventrolateral to the medial longitudinal fasciculus. Nucleus gigantocellularis injections produced heavy labeling in the pontomesencephalic reticular formation, the intermediate layers of the superior colliculus, the pontine and midbrain central gray, the anterior pretectal nucleus, the ventral midbrain tegmentum including the retrorubral area, the centromedian-parafascicular complex, the fields of Forel/zona incerta, the rostral intralaminar nuclei and the lateral hypothalamic area. Nucleus gigantocellularis projections to the rostral forebrain were sparse. Labeled fibers from nucleus reticularis ventralis, like those from nucleus gigantocellularis, ascended largely in the tracts of Forel and distributed to the pontomedullary reticular core, the facial and trigeminal motor nuclei, the pontine nuclei and the dorsolateral pontine tegmentum including the locus coerulus and the parabrachial complex. Although projections from nucleus reticularis ventralis diminished significantly rostral to the pons, labeling was still demonstrable in several mesodiencephalic nuclei including the cuneiform-pedunculopontine area, the mesencephalic gray, the superior colliculus, the anterior pretectal nucleus, the zona incerta and the paraventricular and intralaminar thalamic nuclei. The main bundle of fibers labeled by nucleus gigantocellularis-pars alpha injections ascended ventromedially through the brainstem, just dorsal to the pyramidal tracts, and joined Forel's tegmental tract in the midbrain. With the brainstem, labeled fibers distributed to the pontomedullary reticular formation, the locus coerulus, the raphe pontis, the pontine nuclei, and the dorsolateral tegmental nucleus and adjacent regions of the pontine gray. At mesodiencephalic levels, labeling was present in the rostral raphe nuclei (dorsal, median and linearis), the mesencephalic gray, the deep and intermediate layers of the superior colliculus, the medial and anterior pretectal nuclei, the ventral tegmental area, zona incerta as well as the mediodorsal and reticular nuclei of the thalamus. Injections of the parvocellular reticular nucleus labeled axons which coursed through the lateral medullary tegmentum to heavily innervate lateral regions of the medullary and caudal pontine reticular formation, cranial motor nuclei (hypoglossal, facial and trigeminal) and the parabrachial complex. Parvocellular projections to the medial pontomedullary reticular formation were light and few ascended rostrally beyond the mid-pons. Labeled fibers from reticular formation injections that included the A1 noradrenergic area coursed laterally through the brainstem and distributed significantly to the locus coeruleus, the parabrachial complex, the dorsolateral tegmental nucleus and mid-lateral regions of the pontine gray. Labeling was light throughout the core of the pontomedullary reticular formation. Labeling in the diencephalon was pronounced. Several cell groups of the hypothalamus were heavily labeled including the median eminence, the arcuate nucleus, the periventricular nucleus, the supraoptic nucleus, the paraventricular nucleus and the lateral hypothalamic area. In the thalamus, light labeling was observed in the rostral intralaminar nuclei and midline nuclei (reuniens and rhomboid) moderate labeling was present in the paraventricular thalamic nucleus. In contrast to the other cases, significant numbers of labeled fibers reached the rostral forebrain and distributed to parts of the amygdaloid complex and the ventral striatum as well as to the medial septum and the diagonal band nucleus. The present results indicate that the ascending projections from the bulbar reticular formation are connectionally heterogeneous. We predict that this heterogeneity will be manifested by differences in function.

Retrograde labeling of neurons in the brain stem following injections of [ 3H]choline into the rat spinal cord

In an attempt to identify cholinergic neurons in the brain stem which project to the spinal cord, [ 3H]choline (100,20, 10, 5 or 1 μ Ci) was injected into the upper cervical spinal cord in 55 rats. The animals were killed 20 h later and the brains processed for autoradiography of diffusible substances. At all doses of [ 3H]choline, cells were consistently, retrogradely labeled in the medial medullary reticular formation, the lateral vestibular nucleus, the dorsolateral pontine tegmentum and the red nucleus. The retrogradely labeled cells were found to be moderately to darkly stained for acetylcholinesterase. Injection of [ 3H]noradrenaline (50 μ Ci) into the upper cervical spinal cord resulted in retrograde labeling of cells in the locus coeruleus, subcoeruleus and the ventrolateral pontine tegmentum, that correspond in position to the neurons of the A6, A7 and A5 catecholamine cell groups, respectively. Injection of [ 3H]serotonin (20 μ Ci) into the upper cervical spinal cord was associated with retrograde labeling of cells in the raphe pallidus, obscurus and magnus nuclei that correspond in position to those of the B1, B2 and B3 serotonin cell groups, respectively. Injection of True Blue into the upper cervical spinal cord was followed by retrograde labeling of a large number of cells located in the areas where cells were retrogradely labeled by [ 3H]choline, [ 3H]noradrenaline and [ 3H]serotonin, and additionally, in the solitary tract nucleus, the lateral, parvicellular medullary reticular formation, the caudal and oral pontine reticular formation, the mesencephalic reticular formation and the superior colliculus. These results indicate that from the cervical spinal cord, [ 3H]choline selectively retrogradely labels a certain population of non-monaminergic, acetylcholinesterase-positive cells localized in the medial medullary, and secondarily the dorsolateral pontine, reticular formation, the lateral vestibular nucleus, and the red nucleus.

Anatomy and physiology of saccadic burst neurons in the alert squirrel monkey. I. Excitatory burst neurons.

Saccadic burst neurons in the pontine reticular formation have been implicated in the generation of saccades in the horizontal plane on the basis of lesion and extracellular recording studies in the cat and monkey. In the present study, saccadic burst neurons were anatomically and physiologically characterized with intraaxonal recording and injection of horseradish peroxidase in the alert squirrel monkey. A population of burst neurons were found that appear analogous to the excitatory burst neurons (EBNs) described previously in the cat. All neurons are located in the caudal pontine reticular formation and have a major axonal projection to the ipsilateral abducens nucleus. Additional projections were found to the medial vestibular nucleus, the nucleus prepositus, and regions of the pontine and medullary reticular formation rostral, ventral, and caudal to the abducens. All neurons fire exclusively during saccades and have a discharge pattern similar to that of medium-lead burst neurons described previously in the cat and monkey. In most neurons the saccadic burst begins 5-15 msec before saccade onset. Linear relationships exist between burst duration and saccade duration, number of spikes in the burst and saccade amplitude, and firing frequency and instantaneous velocity. Physiological activity of each neuron shows the closest relationship with the amplitude of the saccade component in a particular direction. For all neurons, this on-direction is in the ipsilateral hemifield and is predominantly horizontal, but may have either an upward or downward vertical component. These results support a major role for the EBNs in the monkey in generating the saccadic burst in abducens motoneurons, as well as in contributing to the oculomotor activity in other classes of premotor neurons.

Bilateral horizontal gaze paralysis due to pontine hemorrhage: A case report

In a case of bilateral horizontal gaze paralysis, vertical gaze was clinically intact but eye movement recordings demonstrated a transient reduction of vertical saccadic velocities. Horizontal caloric vestibulo-ocular responses were absent. CT scanning and NMR imaging showed a hematoma in the median pontine tegmentum. This case provides additional evidence that vertical and horizontal saccade genesis may be independent of caudal paramedian pontine reticular formation lesions.

A lectin horseradish peroxidase study of the origin of ascending fibers in the medial forebrain bundle of the rat. The lower brainstem

The origins of projections within the medial forebrain bundle from the lower brainstem were examined with the horseradish peroxidase technique. Labeled cells were found in at least 15 lower brainstem nuclei following injections of a conjugate or horseradish peroxidase and wheat germ agglutinin at various levels of the medial forebrain bundle. Dense labeling was observed in the following cell groups (from caudal to rostral): A1 (above the lateral reticular nucleus); A2 (mainly within the nucleus of the solitary tract); a distinct group of cell trailing ventrolaterally from the medial longitudinal fasciculus at the level of the rostral pole of the inferior olive; raphe magnus; nucleus incertus; dorsolateral tegmental nucleus (of Castaldi); locus coeruleus; nucleus subcoeruleus; caudal part of the dorsal (lateral) parabrachial nucleus; and raphe pontis. Distinct but light labeling was seen in raphe pallidus and obscurus, nucleus prepositus hypoglossi, nucleus gigantocellularis pars ventralis, and the ventral (medial) parabrachial nucleus. Sparse labeling was observed throughout the medullary and caudal pontine reticular formation. Several lower brainstem nuclei were found to send strong projections along the medial forebrain bundle to very anterior levels of the forebrain. They were: A1, A2, raphe magnus (rostral part), nucleus incertus, dorsolateral tegmental nucleus, raphe pontis and locus coeruleus. With the exception of the locus coeruleus, attention has only recently been directed to the ascending projections of most of the nuclei mentioned above. Evidence was reviewed indicating that fibers from lower brainstem nuclei with ascending medial forebrain bundle projections distribute to widespread regions of the forebrain. It is concluded from the present findings that several medullary cell groups are capable of exerting a direct effect on the forebrain and that the medial forebrain bundle is the major ascending link between the lower brainstem and the forebrain.

A comparison of the distribution of central cholinergic neurons as demonstrated by acetylcholinesterase pharmacohistochemistry and choline acetyltransferase immunohistochemistry

The topographical distribution of cholinergic cell bodies has been studied in the rat brain and spinal cord by choline acetyltransferase (ChAT)-immunohistochemistry and acetylcholinesterase (AChE)-pharmacohistochemistry using diisopropylfluorophosphate (DFP). The ChAT-containing cells and the cells that stained intensely for AChE 4–8 hr after DFP were mapped in detail on an atlas of the forebrain (telencephalon, diencephalon) hindbrain (mesencephalon, rhombencephalon) and cervical cord (C2, C6). Striking similarities were observed between ChAT-positive cells and neuronal soma that stained intensely for AChE both in terms of cytoarchitectural characteristics, and with respect to the distribution of the labelled cells in many areas of the central nervous system (CNS). In the forebrain these areas include the caudatoputamen, nucleus accumbens, medial septum, nucleus of the diagonal band, magnocellular preoptic nucleus and nucleus basalis magnocellularis. In contrast, a marked discrepancy was observed in the hypothalamus and ventral thalamus where there were many neurons that stained intensely for AChE, but where there was an absence of ChAT-positive cells. No cholinergic perikarya were detected in the cerebral cortex, hippocampus, amygdala and dorsal diencephalon by either histochemical procedure. In the hindbrain, all the motoneurons constituting the well-established cranial nerve nuclei (III–VII, IX–XII) contained ChAT and exhibited intense staining for AChE. Further, a close correspondence was observed in the distribution of labeled neurons obtained by the two histochemical procedures in the midbrain and pontine tegmentum, including the laterodorsal tegmental nucleus, some areas in the caudal pontine and bulbar reticular formation, and the central gray of the closed medulla oblongata. On the other hand, AChE-intense cells were found in the nucleus raphe magnus, ventral part of gigantocellular reticular nucleus, and flocculus of the cerebellum, where ChAT-positive cells were rarely observed. According to both techniques, no positive cells were seen in the cerebellar nuclei, the pontine nuclei, or the nucleus reticularis tegmenti pontis. Large ventral horn motoneurons and, occasionally, cells in the intermediomedial zone of the cervical cord displayed ChAT-immunoreactivity and intense AChE staining. On the other hand, AChE-intense cells were detected in the dorsal portion of the lateral funiculus, but immunoreactive cells were not found in any portion of the spinal cord white matter. The present observations indicate that there is an excellent correspondence, both in terms of distribution and morphology, between ChAT-containing and AChE-intense neurons in most regions of the CNS. AChE pharmacohistochemistry can, therefore, be used as a reliable method to detect cholinergic neurons in many, but not all regions of the CNS. Finally, it is proposed that central cholinergic neurons can be subdivided conveniently into four major cell groups: (1) the rostral column (basal forebrain); (2) the caudal column (pons); (3) local circuit neurons in dopamine-rich regions of the forebrain; and (4) somatic and visceral motoneurons.

Parabrachial nucleus neurons projecting to the lower brain stem and the spinal cord. A study in the cat by the Fink-Heimer and the horseradish peroxidase methods

Direct projections from the parabrachial nucleus (PBN) to the lower brain stem and the spinal cord were examined in the cat by the Fink-Heimer and the horseradish peroxidase (HRP) methods. After placing lesions in the PBN, many fine degenerated fibers were seen contralaterally in the ventromedial portions of the caudal pontine reticular formation, and ipsilaterally in the lateral portions of the facial nucleus, the regions around the hypoglossal nucleus, and the regions around the ambiguus nucleus; some degenerated fibers were traced ipsilaterally down to the spinal cord. Subsequently, HRP injections were attempted into these regions where many fine degenerated fibers were observed. In cats injected with HRP into the lateral portions of the facial nucleus, the regions around the hypoglossal nucleus, the regions around the ambiguus nucleus, or the first cervical cord segment, many HRP-labeled neurons were seen in the ventral portions of the PBN. The mean of the average soma diameters of the PBN neurons labeled with HRP injected into the regions around the hypoglossal nucleus or the first cervical cord segment was significantly larger than that of the PBN neurons labeled with HRP injected into the lateral portions of the facial nucleus or the regions around the ambiguus nucleus.

A cell group associated with vertical eye movements in the rostral mesencephalic reticular formation of the monkey

A cytologically distinct group of cells in the rostral mesencephalic reticular formation was strongly labelled by injections of anterograde tracer substances into the caudal paramedian pontine reticular formation (PPRF) of the monkey. The cell group lies ventral to nucleus of Darkschewitsch (nD), rostral to the interstitial nucleus of Cajal (iC) and the tractus retroflexus (TR). It receives inputs from areas which control eye movements, PPRF and the vestibular nuclei, and sends efferents to the oculomotor nucleus. Physiological, anatomical and clinical evidence support the conclusion that this cell group is involved in the generation of vertical eye movements. In an attempt to be anatomically specific the name rostral interstitial nucleus of the medial longitudinal fasciculus ( rostral iMLF) has been used.