Brainstem projections to the facial nucleus of the opossum. A study using axonal transport techniques

Modulation of reflex responses of the anterior and posterior bellies of the digastric muscle in freely moving rats

BACKGROUND

Chewing and licking are primarily activated by central pattern generator (CPG) neuronal circuits in the brainstem and when activated trigger repetitive rhythmic orofacial movements such as chewing, licking and swallowing. These CPGs are reported to modulate orofacial reflex responses in functions such as chewing.

OBJECTIVE

This study explored the modulation of reflex responses in the anterior and posterior bellies (ant-Dig and post-Dig, respectively) of the digastric muscle evoked by low-intensity trigeminal stimulation in conscious rats.

METHODS

The ant-Dig and post-Dig reflexes were evoked by using low-intensity electrical stimulation applied to either the right or left inferior alveolar nerve. Peak-to-peak amplitudes and onset latencies were measured.

RESULTS

No difference was observed between threshold and onset latency for evoking ant-Dig and post-Dig reflexes, suggesting that the latter was also evoked disynaptically. The peak-to-peak amplitude of both reflexes was significantly reduced during chewing, licking and swallowing as compared to resting period and was lowest during the jaw-closing phase of chewing and licking. Onset latency was significantly largest during the jaw-closing phase. Inhibitory level was similar between the ant-Dig and post-Dig reflex responses and between the ipsilateral and contralateral sides.

CONCLUSION

These results suggest that both the ant-Dig and post-Dig reflex responses were significantly inhibited, probably due to CPG activation during feeding behaviours to maintain coordination of jaw and hyoid movements and hence ensure smooth feeding mechanics.

Neuroprosthetics for Auricular Muscles: Neural Networks and Clinical Aspects

The mammalian external ear houses extrinsic and intrinsic auricular muscles. There are three extrinsic auricular muscles-the posterior, superior, and anterior auricular muscles-and six intrinsic muscles-the helicis major and minor, tragicus, anti-tragicus, transverse and oblique muscles. These muscles have been considered vestigial in humans. However, numerous therapeutic and diagnostic wearable devices are designed to monitor and alleviate the symptoms of neurological disorders, brainstem injuries, emotional states, and auditory functions, by making use of the neural networks of the auricular muscles and their locations, which are easily accessible for ergonomic wearable biomedical devices. They can also serve as a bio-controller of human neuroprosthetics. The functionality of these auricular muscles remains elusive and requires further experimentation for a more in-depth understanding of their anatomy. The aims of this review are (1) to provide a detailed account of the neural networks of the extrinsic and intrinsic auricular muscles, (2) to describe diagnostic and therapeutic functions of these muscles as demonstrated in the current literature, and (3) to outline existing and potential neuroprosthetic applications making use of the auricular muscles and their neural networks.

Somatotopy in the Medullary Dorsal Horn As a Basis for Orofacial Reflex Behavior

The somatotopy of the trigeminocervical complex of the rat was defined as a basis for describing circuitry for reflex behaviors directed through the facial motor nucleus. Thus, transganglionic transport of horseradish peroxidase conjugates applied to individual nerves/peripheral receptive fields showed that nerves innervating oropharyngeal structures projected most rostrally, followed by nerves innervating snout, periocular, and then periauricular receptive fields most caudally. Nerves innervating mucosae or glabrous receptive fields terminated densely in laminae I, II, and V of the trigeminocervical complex, while those innervating hairy skin terminated in laminae I-V. Projections to lamina II exhibited the most focused somatotopy when individual cases were compared. Retrograde transport of FluoroGold (FG) deposited into the facial motor nucleus resulted in labeled neurons almost solely in lamina V of the trigeminocervical complex. The distribution of these labeled neurons paralleled the somatotopy of primary afferent fibers, e.g., those labeled after FG injections into a functional group of motoneurons innervating lip musculature were found most rostrally while those labeled after injections into motoneurons innervating snout, periocular and preauricular muscles, respectively, were found at progressively more caudal levels. Anterograde transport of injections of biotinylated dextran amine into lamina V at different rostrocaudal levels of the trigeminocervical complex confirmed the notion that the somatotopy of orofacial sensory fields parallels the musculotopy of facial motor neurons. These data suggest that neurons in lamina V are important interneurons in a simple orofacial reflex circuit consisting of a sensory neuron, interneuron and motor neuron. Moreover, the somatotopy of primary afferent fibers from the head and neck confirms the "onion skin hypothesis" and suggests rostral cervical dermatomes blend seamlessly with "cranial dermatomes." The transition area between subnucleus interpolaris and subnucleus caudalis is addressed while the paratrigeminal nucleus is discussed as an interface between the somatic and visceral nervous systems.

Kölliker-Fuse GABAergic and glutamatergic neurons project to distinct targets

The Kölliker-Fuse nucleus (KF) is known primarily for its respiratory function as the "pneumotaxic center" or "pontine respiratory group." Considered part of the parabrachial (PB) complex, KF contains glutamatergic neurons that project to respiratory-related targets in the medulla and spinal cord (Yokota, Oka, Tsumori, Nakamura, & Yasui, 2007). Here we describe an unexpected population of neurons in the caudal KF and adjacent lateral crescent subnucleus (PBlc), which are γ-aminobutyric acid (GABA)ergic and have an entirely different pattern of projections than glutamatergic KF neurons. First, immunofluorescence, in situ hybridization, and Cre-reporter labeling revealed that many of these GABAergic neurons express FoxP2 in both rats and mice. Next, using Cre-dependent axonal tracing in Vgat-IRES-Cre and Vglut2-IRES-Cre mice, we identified different projection patterns from GABAergic and glutamatergic neurons in this region. GABAergic neurons in KF and PBlc project heavily and almost exclusively to trigeminal sensory nuclei, with minimal projections to cardiorespiratory nuclei in the brainstem, and none to the spinal cord. In contrast, glutamatergic KF neurons project heavily to the autonomic, respiratory, and motor regions of the medulla and spinal cord previously identified as efferent targets mediating KF cardiorespiratory effects. These findings identify a novel, GABAergic subpopulation of KF/PB neurons with a distinct efferent projection pattern targeting the brainstem trigeminal sensory system. Rather than regulating breathing, we propose that these neurons influence vibrissal sensorimotor function.

Feedback in the brainstem: an excitatory disynaptic pathway for control of whisking

Sensorimotor processing relies on hierarchical neuronal circuits to mediate sensory-driven behaviors. In the mouse vibrissa system, trigeminal brainstem circuits are thought to mediate the first stage of vibrissa scanning control via sensory feedback that provides reflexive protraction in response to stimulation. However, these circuits are not well defined. Here we describe a complete disynaptic sensory receptor-to-muscle circuit for positive feedback in vibrissa movement. We identified a novel region of trigeminal brainstem, spinal trigeminal nucleus pars muralis, which contains a class of vGluT2+ excitatory projection neurons involved in vibrissa motor control. Complementary single- and dual-labeling with traditional and virus tracers demonstrate that these neurons both receive primary inputs from vibrissa sensory afferent fibers and send monosynaptic connections to facial nucleus motoneurons that directly innervate vibrissa musculature. These anatomical results suggest a general role of disynaptic architecture in fast positive feedback for motor output that drives active sensation.

The facial motor system

Facial movements support a variety of functions in human behavior. They participate in automatic somatic and visceral motor programs, they are essential in producing communicative displays of affective states and they are also subject to voluntary control. The multiplicity of functions of facial muscles, compared to limb muscles, is reflected in the heterogeneity of their anatomical and histological characteristics that goes well beyond the conventional classification in single facial muscles. Such parcellation in different functional muscular units is maintained throughout the central representation of facial movements from the brainstem up to the neocortex. Facial movements peculiarly lack a conventional proprioceptive feedback system, which is only in part vicariated by cutaneous or auditory afferents. Facial motor activity is the main marker of endogenous affective states and of the affective valence of external stimuli. At the cortical level, a complex network of specialized motor areas supports voluntary facial movements and, differently from upper limb movements, in such network there does not seem to be a prime actor in the primary motor cortex.

Nociceptive afferents to the premotor neurons that send axons simultaneously to the facial and hypoglossal motoneurons by means of axon collaterals

It is well known that the brainstem premotor neurons of the facial nucleus and hypoglossal nucleus coordinate orofacial nociceptive reflex (ONR) responses. However, whether the brainstem PNs receive the nociceptive projection directly from the caudal spinal trigeminal nucleus is still kept unclear. Our present study focuses on the distribution of premotor neurons in the ONR pathways of rats and the collateral projection of the premotor neurons which are involved in the brainstem local pathways of the orofacial nociceptive reflexes of rat. Retrograde tracer Fluoro-gold (FG) or FG/tetramethylrhodamine-dextran amine (TMR-DA) were injected into the VII or/and XII, and anterograde tracer biotinylated dextran amine (BDA) was injected into the caudal spinal trigeminal nucleus (Vc). The tracing studies indicated that FG-labeled neurons receiving BDA-labeled fibers from the Vc were mainly distributed bilaterally in the parvicellular reticular formation (PCRt), dorsal and ventral medullary reticular formation (MdD, MdV), supratrigeminal nucleus (Vsup) and parabrachial nucleus (PBN) with an ipsilateral dominance. Some FG/TMR-DA double-labeled premotor neurons, which were observed bilaterally in the PCRt, MdD, dorsal part of the MdV, peri-motor nucleus regions, contacted with BDA-labeled axonal terminals and expressed c-fos protein-like immunoreactivity which induced by subcutaneous injection of formalin into the lip. After retrograde tracer wheat germ agglutinated horseradish peroxidase (WGA-HRP) was injected into VII or XII and BDA into Vc, electron microscopic study revealed that some BDA-labeled axonal terminals made mainly asymmetric synapses on the dendritic and somatic profiles of WGA-HRP-labeled premotor neurons. These data indicate that some premotor neurons could integrate the orofacial nociceptive input from the Vc and transfer these signals simultaneously to different brainstem motonuclei by axonal collaterals.

Comparative anatomy of the facial motor nucleus in mammals, with an analysis of neuron numbers in primates

The facial motor nucleus (VII) contains motoneurons that innervate the facial muscles of expression. In this review, the comparative anatomy of this brainstem nucleus is examined. Several aspects of the anatomical organization of the VII appear to be common across mammals, such as the distribution of neuron types, general topography of muscle representation, and afferent connections from the midbrain and brainstem. Phylogenetic specializations are apparent in the proportion of neurons allocated to the representation of subsets of muscles and the degree of differentiation among subnuclei. These interspecific differences may be related to the elaboration of certain facial muscles in the context of socioecological adaptations such as whisking behavior, sound localization, vocalization, and facial expression. Furthermore, current evidence indicates that direct descending corticomotoneuron projections in the VII are present only in catarrhine primates, suggesting that this connectivity is an important substrate for the evolution of enhanced mobility and flexibility in facial expression. Data are also presented from a stereologic analysis of VII neuron numbers in 18 primate species and a scandentian. Using phylogenetic comparative statistics, it is shown that there is not a correlation between group size and VII neuron number (adjusted for medulla volume) among primates. Great apes and humans, however, display moderately more VII neurons that expected for their medulla size.

Trigemino-solitarii-facial pathway in rats

This study was undertaken to identify premotor neurons in the nucleus tractus solitarii (NTS) serving as relay neurons between the sensory trigeminal complex (STC) and the facial motor nucleus in rats. Trigemino-solitarii connections were first investigated following injections of anterograde and/or retrograde (biotinylated dextran amine, biocytin, or gold-HRP) tracers in STC or NTS. Trigemino-solitarii neurons were abundant in the ventral and dorsal parts of the STC and of moderate density in its intermediate part. They project throughout the entire rostrocaudal extent of the NTS with a strong lateral preponderance. Solitarii-trigeminal neurons were observed mostly in the rostral and rostrolateral NTS. They mainly project to the ventral and dorsal parts of the spinal trigeminal nucleus but not to the principal nucleus. Additional neurons located in the middle NTS were found to project exclusively to the spinal trigeminal nucleus pars caudalis. No solitarii-trigeminal cells were observed in the caudal NTS. In addition, evidence was obtained of NTS retrogradely labeled neurons contacted by anterogradely labeled trigeminal terminals. Second, solitarii-facial projections were analyzed following injections of anterograde and retrograde tracers into the NTS and the facial nucleus, respectively. NTS neurons, except those of the rostrolateral part, reached the dorsal aspect of the facial nucleus. Finally, simultaneous injections of anterograde tracer in the STC and retrograde tracer in the facial nucleus gave retrogradely labeled neurons in the NTS receiving contacts from anterogradely labeled trigeminal boutons. Thus, the present data demonstrate for the first time the existence of a trigemino-solitarii-facial pathway. This could account for the involvement of the NTS in the control of orofacial motor behaviors.

Defining projections from the caudal pressor area of the caudal ventrolateral medulla

We previously defined a functional area in the caudal medulla oblongata that elicits an increase in arterial pressure when stimulated (Sun and Panneton [2002] Am. J. Physiol. 283:R768-R778). In the present study, anterograde and retrograde tracing techniques were used to investigate the projections of this caudal pressor area (CPA) to the medulla and pons. Injections of biotinylated dextran amine into the CPA resulted in numerous labeled fibers with varicosities in the ipsilateral subnucleus reticularis dorsalis, commissural subnucleus of the nucleus tractus solitarii, lateral medulla, medial facial nucleus, A5 area, lateral vestibular nucleus, and internal lateral subnucleus of the parabrachial complex. Sparser projections were found ipsilaterally in the pressor and depressor areas of the medulla and the spinal trigeminal nucleus and contralaterally in the CPA. Injections of the retrograde tracer Fluoro-Gold into these areas labeled neurons in the CPA as well as the nearby medullary dorsal horn and reticular formation. However, we conclude that the CPA projects preferentially to the subnucleus reticularis dorsalis, commissural nucleus tractus solitarii, lateral medulla, A5 area, and internal lateral parabrachial nucleus. Weaker projections were seen to the CVLM and RVLM and to the contralateral CPA. The projection to the facial nucleus arises from nearby reticular neurons, whereas projections to the vestibular nucleus arise from the lateral reticular nucleus. Labeled neurons in the CPA consisted mostly of small bipolar and some triangular neurons. The projection to the CVLM, or to A5 area, may provide for the increase in arterial pressure with CPA stimulation. However, most of the projections described herein are to nuclei implicated in the processing of noxious information. This implies a unique role for the CPA in somatoautonomic regulation.

Organization of parabrachial projections from the spinal trigeminal nucleus oralis: An anterograde tracing study in the rat

In recent years, we have accumulated data showing that the spinal trigeminal nucleus oralis (Sp5O) contributes to the processing of somatosensory inputs from the orofacial region. Although the parabrachial area (PB) represents the main brainstem relay for autonomic, nociceptive, and gustatory afferents, few data are available regarding the topographical distribution of the efferent projections from the Sp5O to the PB. We have addressed this question with the rat, by using the anterograde tracer Phaseolus vulgaris leucoagglutinin. A dense trigeminoparabrachial pathway from the Sp5O toward, predominantly, the ipsilateral PB was revealed. Projections come mainly from the dorsal part of the Sp5O that was found to innervate densely the medial, external medial, and ventral lateral subnuclei. In contrast, the ventral part of the Sp5O projected almost exclusively to an as yet not formally described region, located dorsally and laterally to the lateral tip of the brachium conjunctivum, close to the Kölliker-Fuse nucleus. These results suggest that distinct regions within the Sp5O may be involved in the processing of gustatory and nociceptive information.

Descending Projections from the Substantia Nigra and Retrorubral Field to the Medullary and Pontomedullary Reticular Formation

We have investigated the projection from the substantia nigra to the pontomedullary reticular formation in the rat using both retrograde and anterograde neuroanatomical tracers. Injections of a conjugate of wheatgerm agglutinin with horseradish peroxidase into the medullary or pontomedullary reticular formation resulted in the retrograde labelling of a continuous band of cells extending from the caudal half of the dorsolateral substantia nigra into the retrorubral field. Injections of the anterograde tracer Phaseolus vulgaris leukoagglutinin (PHA-L) into either the dorsolateral substantia nigra or the caudally adjacent retrorubral field revealed a descending projection to the lateral medullary and pontomedullary brainstem, which terminated mainly within the lateral (parvicellular) reticular formation. The anterograde PHA-L fibre labelling ran throughout the rostrocaudal extent of the parvicellular reticular formation and extended into the caudally continuous region, the medullary dorsal and medullary ventral reticular formation, where it tapered off. Also labelled, although more lightly, were the rostral and ventrolateral regions of the nucleus of the solitary tract and the magnocellular reticular formation. Electron microscopy established that the PHA-L-labelled fibres formed synaptic contacts with nerve cell bodies and dendrites in the parvicellular reticular formation. It is suggested that one role of this nigroreticular pathway might be to connect the basal ganglia with brainstem premotor neurons that influence orofacial musculature.

Functional circuitry involved in the regulation of whisker movements

Neuroanatomical tract-tracing methods were used to identify the oligosynaptic circuitry by which the whisker representation of the motor cortex (wMCx) influences the facial motoneurons that control whisking activity (wFMNs). Injections of the retrograde tracer cholera toxin subunit B into physiologically identified wFMNs in the lateral facial nucleus resulted in dense, bilateral labeling throughout the brainstem reticular formation and in the ambiguus nucleus as well as predominantly ipsilateral labeling in the paralemniscal, pedunculopontine tegmental, Kölliker-Fuse, and parabrachial nuclei. In addition, neurons in the following midbrain regions projected to the wFMNs: superior colliculus, red nucleus, periaqueductal gray, mesencephalon, pons, and several nuclei involved in oculomotor behaviors. Injections of the anterograde tracer biotinylated dextran amine into the wMCx revealed direct projections to the brainstem reticular formation as well as multiple brainstem and midbrain structures shown to project to the wFMNs. Regions in which retrograde labeling and anterograde labeling overlap most extensively include the brainstem parvocellular, gigantocellular, intermediate, and medullary (dorsal and ventral) reticular formations; ambiguus nucleus; and midbrain superior colliculus and deep mesencephalic nucleus. Other regions that contain less dense regions of combined anterograde and retrograde labeling include the following nuclei: the interstitial nucleus of medial longitudinal fasciculus, the pontine reticular formation, and the lateral periaqueductal gray. Premotoneurons that receive dense inputs from the wMCx are likely to be important mediators of cortical regulation of whisker movements and may be a key component in a central pattern generator involved in the generation of rhythmic whisking activity.

Reticular premotor neurons projecting to both facial and hypoglossal nuclei receive trigeminal afferents in rats

The distribution of premotor neurons projecting to motor nuclei of both the VIIth (VII) and XIIth (XII) nerves was examined in the pontomedullary reticular formation (RF) of the rat by using retrograde double labeling. After injection of two different tracers in the VII and the XII, most of the double labeled neurons were found caudally in the dorsal RF whereas rostrally they were located in the ventral RF. In some experiments, additional injections of an anterograde tracer were made in the sensory trigeminal nuclei. Anterogradely labeled trigeminal boutons were found in contact with retrogradely double labeled neurons throughout the pontomedullary RF. These neurons were mainly encountered ventral to the trigeminal motor nucleus and dorsal to the VII. Functionally, this region is known to be involved in eye protection mechanisms.

Trigemino-reticulo-facial and trigemino-reticulo-hypoglossal pathways in the rat

This study was undertaken to identify premotor neurons in the pontomedullary reticular formation serving as relay neurons between the sensory trigeminal complex and the motor nuclei of the VIIth and XIIth nerves. Trigeminoreticular projections were first investigated after injections of anterogradely transported tracers (biotinylated dextran amine, biocytin) into single subdivisions of the sensory trigeminal complex. The results show that the trigeminoreticular projections were abundant from the pars interpolaris (5i) and caudalis (5c) and moderate from pars oralis (5o) of the spinal trigeminal nucleus. Injections into the 5i and 5c produce dense anterograde labeling (1) in the dorsal medullary reticular field; (2) in the parvocellular reticular field, medially adjacent to the 5i; and (3) more rostral in the region dorsal and lateral to the superior olivary nucleus. Some labeled terminals were also found in the intermediate reticular field, whereas only light anterograde labeling was observed in the gigantocellular and oral pontine reticular formation. The 5o sends fibers and terminals throughout the whole reticular formation, with no clear preferential projections within a particular field. Only light projections originated from the principal nucleus (5P). In a second series of experiments, we examined whether premotor neurons in the reticular formation are afferented by trigeminal fibers. Double labeling was performed by injection of an anterograde tracer in the 5i and 5c and retrograde tracer (gold-horseradish peroxidase complex) into the VII or the XII motor nucleus on the same side. Retrogradely labeled neurons in contact with anterogradely labeled boutons were found throughout the reticular formation with predominance in the parvocellular and intermediate reticular fields. These experiments demonstrate the existence of trigeminal disynaptic influences, via reticular neurons of the pontomedullary reticular formation, in the control of orofacial motor behaviors.

Animal models of deficient sensorimotor gating: what we know, what we think we know, and what we hope to know soon

Sensorimotor gating of the startle reflex can be studied in humans and laboratory animals using measures of prepulse inhibition (PPI) of the startle reflex. PPI is reduced in patients with specific neuropsychiatric disorders and in rats after manipulation of the limbic cortex, striatum, pallidum or pontine tegmentum. Studies are rapidly identifying the neurochemical and neuroanatomical substrates regulating PPI in laboratory animals; this detailed circuit information has been used as a 'blueprint' to identify possible candidate substrates responsible for PPI deficits in psychiatrically disordered humans. In parallel, studies have also begun to assess the homology of pharmacological effects on PPI across species, as an initial step towards translating detailed neural circuit information from rats to humans. Despite this rapid progress, there is an increasing danger of overlooking important methodological and interpretative issues that could impact either positively or negatively on the ultimate utility of models based on measures of PPI. Some of these issues--ranging from the cross-species methods for quantifying specific variables to the relevance of genetic drift to animal and human studies of PPI--and their implications for future studies are the focus of this review.

Trigeminal projections to hypoglossal and facial motor nuclei in the rat

This study was undertaken to identify the trigeminal nuclear regions connected to the hypoglossal (XII) and facial (VII) motor nuclei in rats. Anterogradely transported tracers (biotinylated dextran amine, biocytin) were injected into the various subdivisions of the sensory trigeminal complex, and labeled fibers and terminals were searched for in the XII and VII. In a second series of experiments, injections of retrogradely transported tracers (biotinylated dextran amine, gold-horseradish peroxidase complex, fluoro-red, fluoro-green) were made into the XII and the VII, and labeled cells were searched for in the principal sensory trigeminal nucleus, and in the pars oralis, interpolaris, and caudalis of the spinal trigeminal nucleus. Trigeminohypoglossal projections were distributed throughout the ventral and dorsal region of the XII. Neurons projecting to the XII were found in all subdivisions of the sensory trigeminal complex with the greatest concentration in the dorsal part of each spinal subnucleus and exclusively in the dorsal part of the principal nucleus. Trigeminofacial projections reached all subdivisions of the VII, with a gradual decreasing density from lateral to medial cell groups. They mainly originated from the ventral part of the principal nucleus. In the spinal nucleus, most of the neurons projecting to the VII were in the dorsal part of the nucleus, but some were also found in its central and ventral parts. By using retrograde double labeling after injections of different tracers in the XII and VII on the same side, we examined whether neurons in the trigeminal complex project to both motor nuclei. These experiments demonstrate that in the spinal trigeminal nucleus, neurons located in the pars caudalis and pars interpolaris project by axon collaterals to XII and VII.

Trigeminal-reticular connections: possible pathways for nociception-induced cardiovascular reflex responses in the rat

Cardiovascular regulatory neurons of the ventral medulla and pons are thought to have an important role in the mediation of trigeminal nociception-induced reflex cardiovascular responses. However, the neural pathways that link the spinal trigeminal nucleus with ventral medullary and pontine autonomic cell groups are poorly understood. The present study utilized injections of the highly sensitive anterograde tracer substance biotinylated dextran combined with immunocytochemistry for tyrosine hydroxylase, the synthesizing enzyme for catecholamines, to investigate the distribution and morphology of projections from the spinal trigeminal subnucleus caudalis to ventral medullary and pontine catecholaminergic cell groups. Injection of biotylinated dextran into the dorsal subnucleus caudalis produced dense anterograde labeling in dorsal regions of the medullary and pontine reticular formation including the dorsal medullary reticular field, the parvicellular reticular field, and the parvicellular reticular field pars anterior. In the ventral medullary and pontine reticular formation, light anterograde labeling tended to be distributed in close proximity to the distal dendrites of catecholaminergic neurons located in the C1, A1, and A5 regions. Injections of anterograde tracer into the dorsal medullary reticular field produced dense anterograde labeling in the ventral medullary and pontine reticular formation. Numerous terminal-like varicosities were observed in close proximity to catecholaminergic neurons located in the C1, A1, and A5 regions. These data suggest that trigeminal pain-induced reflex cardiovascular responses involve indirect projections that terminate in the dorsal medullary and pontine reticular formation before reaching ventral medullary and pontine catecholaminergic cell groups known to be involved in cardiovascular regulation.

Identification of rat brainstem multisynaptic connections to the oral motor nuclei using pseudorabies virus. I. Masticatory muscle motor systems

Oromotor behavior results from the complex interaction between jaw, facial, and lingual muscles. The experiments in this and subsequent papers identify the sources of multisynaptic input to the trigeminal, facial, and hypoglossal motor nuclei. In the current experiments, pseudorabies virus (PRV-Ba) was injected into the jaw-opening (anterior digastric and mylohyoid) and jaw-closing muscles (masseter, medial pterygoid, and temporalis) in bilaterally sympathectomized rats. Injection volumes ranged from 2 to 21 microl with average titers of 2.8 x 10(8) pfu/ml and maximum survival times of 96 h. The labeling patterns and distributions were consistent between each of the individual muscles and muscle groups. A predictable myotopic labeling pattern was produced in the trigeminal motor nucleus (Mo 5). Transneuronally labeled neurons occurred in regions known to project directly to Mo 5 motoneurons including the principal trigeminal sensory and supratrigeminal areas, Kölliker-Fuse region, nucleus subcoeruleus, and the parvicellular reticular formation. Maximum survival times revealed polysynaptic connections from the periaqueductal gray, laterodorsal and pedunculopontine tegmental areas, and the substantia nigra in the midbrain, ventromedial pontine reticular regions including the gigantocellular region and pars alpha and ventralis in the pons and medulla, and the nucleus of the solitary tract, paratrigeminal region, and paramedian field in the medulla. Thus, the results define the structure of the multisynaptic brainstem neural circuits controlling mandibular movement in the rat.

Identification of rat brainstem multisynaptic connections to the oral motor nuclei in the rat using pseudorabies virus. II. Facial muscle motor systems

The present experiments continue our investigations of the higher order afferent systems controlling the orofacial musculature. Pseudorabies virus (PRV) was injected into the buccinator, platysma, posterior digastric, and zygomatic muscles in bilaterally sympathectomized rats. Injection volumes ranged from 6 to 12 microl with average titers of 7 x 10(8) pfu/ml and maximum survival times of 96 h. The labeling patterns and distributions were similar across the individual muscles and between muscle groups (perioral vs. posterior digastric), as well as in comparison to the results from previous masticatory muscle injections. Injections produced a predictable myotopic labeling pattern in the facial motor nucleus (Mo 7) and transneuronally in regions known to project directly to Mo 7 including the red nucleus, ventrolateral parabrachial region, principal trigeminal sensory nucleus, supratrigeminal area, and the parvicellular reticular formation. Maximum survival times revealed more distant connections from a variety of nuclear zones including the periaqueductal gray, laterodorsal and pedunculopontine tegmental areas, and the substantia nigra in the midbrain, ventromedial reticular regions including the gigantocellular region and pars alpha and ventralis in the pons and medulla, and the nucleus of the solitary tract, spinal trigeminal nucleus caudalis, paratrigeminal region, and paramedian field in the medulla. The similarity of the labeling patterns and distributions of the higher order afferents resulting from PRV facial and masticatory muscle injections identifies the neural circuits that may coordinate the activity of these muscle groups during oral motor behavior.

Distribution of GABAergic and glycinergic premotor neurons projecting to the facial and hypoglossal nuclei in the rat

The distribution of inhibitory premotor neurons for the facial and hypoglossal nuclei was examined in the lower brainstem of the rat. A retrograde axonal tracing method with the fluorescent tracer, tetramethylrhodamine dextran amine (TMR-DA), was combined with immunofluorescence histochemistry for glutamic acid decarboxylase (GAD), i.e., the enzyme involved in gamma-aminobutyric acid synthesis, or glycine. In the rats injected with TMR-DA unilaterally into the facial or hypoglossal nucleus, the distribution of TMR-DA-labeled neurons showing GAD-like immunoreactivity (GAD/TMR-DA neurons) was essentially the same as that of TMR-DA-labeled neurons displaying glycine-like immunoreactivity (Gly/TMR-DA neurons). The distributions of GAD/TMR-DA and Gly/TMR-DA neurons in the rats injected with TMR-DA into the facial nucleus were also similar to those in the rats injected with TMR-DA into the hypoglossal nucleus. These neurons were seen most frequently in the lateral aspect of the pontine reticular formation, the supratrigeminal region, the dorsal aspect of the lateral reticular formation of the medulla oblongata, and the reticular regions around the raphe magnus nucleus and the gigantocellular reticular nucleus pars alpha, bilaterally with a slight dominance on the side ipsilateral to the injection site. A number of GAD/TMR-DA and Gly/TMR-DA neurons were also seen in the oral and interpolar subnuclei of the spinal trigeminal nucleus, bilaterally with a slight ipsilateral dominance. In the rats injected with TMR-DA into the facial nucleus, GAD/TMR-DA and Gly/TMR-DA neurons were also encountered in the paralemniscal zone of the midbrain tegmentum bilaterally with an apparent dominance on the side contralateral to the injection site. A large part of these inhibitory premotor neurons for the facial and hypoglossal nuclei and the excitatory ones may constitute premotor neuron pools common to the orofacial motor nuclei implicated in the control of integrated orofacial movements.

Azimuthal sensitivity of rat pinna reflex: EMG recordings from cervicoauricular muscles

Electromyographic (EMG) responses of the cervicoauricular muscles (CAM) to free-field sounds were recorded in two groups of rats whose brainstems were dissected transversely either at a pretectal or transtectal level. After the rat recovered from anesthesia, wide-band noise pulses were presented and speaker positions were varied systematically in azimuth. Sound levels were set at 10-15 dB above empirically determined threshold for an EMG response to a sound from 0 degree azimuth. In both animal groups, transient CAM EMGs with short latency were produced and three main types of azimuthal sensitivity of CAM EMG response were observed. (1) For the majority of the cases, an inverted "U' type of azimuthal sensitivity was identified: the maximum activity occurred around 0 degree azimuth, but as the speaker was moved toward either the ipsilateral or contralateral fields, the sound-evoked activity declined systematically. This directional tuning is quite different from the passive pinna directionality which is very lateral in the resting positions used in this study. (2) In a small number of cases, the spatial sensitivity curves were not symmetrical about the midline (0 degree azimuth): the EMG response was vigorous in one hemifield and dropped off systematically as the speaker was moved toward extreme positions of the other hemifield. Regardless of shapes of EMG spatial tuning curves, obstruction of either the ipsilateral or contralateral meatus reduced the sound-elicited response dramatically and eliminated the spatial sensitivity. (3) Some cases exhibited an omnidirectional function: the EMG spike rate had no or minor systematical variation as the speaker position was changed in azimuth. The results of this study indicate that with either pretectal or transtectal decerebrate preparations, the acoustically evoked CAM EMG can exhibit an azimuthal sensitivity which is based on binaural processing.

Anatomical basis for audio-vocal integration in echolocating horseshoe bats

Neurophysiological recordings suggest that audio-vocal neurons located in the paralemniscal tegmentum of the midbrain in horseshoe bats provide an interface between the pathways for auditory sensory processing and those for the motor control of vocalization. To verify these physiological results anatomically, the projection pattern of the audio-vocally active area in the paralemniscal tegmentum was investigated by using extracellular tracer injections of wheat germ agglutinin conjugated to horseradish peroxidase. Several nuclei of the lemniscal auditory pathway (dorsal nucleus of the lateral lemniscus, central nucleus of the inferior colliculus, lateral superior olive) as well as the nucleus of the central acoustic tract appear to project to the paralemniscal tegmentum. Other possible sources of afferent projections are a small but distinctly labeled structure within the lateral hypothalamic area, the substantia nigra pars compacta, the deep mesencephalic nucleus, the rostral portion of the inferior colliculus, the deep and intermediate layers of the superior colliculus, and several small areas in the rhombencephalic reticular formation. No direct efferent projection from the audio-vocally active area of the paralemniscal tegmentum to primarily auditory structures was found. Instead, the main targets were structures that are involved in the control of different motor patterns. These targets include the deep and intermediate layers of the superior colliculus and the dorsomedial portion of the facial nucleus, both of which most probably control pinna movements in cats, and the reticular formation medial and caudal to the facial nucleus and rostral to the nucleus ambiguus, which represents an area involved in the control of vocalization. Hence, the anatomical projection pattern suggests that the paralemniscal tegmentum in horseshoe bats serves as a link between the processing of auditory information and the control of vocalization and related motor patterns.

Specificity of rabies virus as a transneuronal tracer of motor networks: transfer from hypoglossal motoneurons to connected second-order and higher order central nervous system cell groups

The specificity of transneuronal transfer of rabies virus [challenge virus standard (CVS) strain] was evaluated in a well-characterized neuronal network, i.e., retrograde infection of hypoglossal motoneurons and transneuronal transfer to connected (second-order) brainstem neurons. The distribution of the virus in the central nervous system was studied immunohistochemically at sequential intervals after unilateral inoculation into the hypoglossal nerve. The extent of transneuronal transfer of rabies virus was strictly time dependent and was distinguished in five stages. At 1 day postinoculation, labelling involved only hypoglossal motoneurons (stage 1). Retrograde transneuronal transfer occurred from 2.0-2.5 days postinoculation (stage 2). In stages 2-4, different groups of second-order neurons were labelled sequentially, depending on the strength of their input to the hypoglossal nucleus. In stages 4 and 5, labelling extended to several cortical and subcortical cell groups, which can be regarded as higher order because they are known to control tongue movements and/or to provide input to hypoglossal-projecting cell groups. The pattern of transneuronal transfer of rabies virus resembles that of alpha-herpesviruses with regard to the nonsynchronous labelling of different groups of second-order neurons and the transfer to higher order neurons. In striking contrast to alpha-herpesviruses, the transneuronal transfer of rabies is not accompanied by neuronal degeneration. Moreover, local spread of rabies from infected neurons and axons to adjoining glial cells, neurons, or fibers of passage does not occur. The results show that rabies virus is a very efficient transneuronal tracer. Results also provide a new insight into the organization of cortical and subcortical higher order neurons that mediate descending control of tongue movements indirectly via hypoglossal-projecting neurons.

The role of the periaqueductal grey in vocal behaviour

This is a review of our current knowledge about the role of the periaqueductal grey (PAG) in vocal control. It shows that electrical stimulation of the PAG can evoke species-specific calls with short latency and low habituation in many mammals. The vocalization-eliciting region contains neurones the activity of which is correlated with the activity of specific laryngeal muscles. Lesioning studies show that destruction of the PAG and laterally bordering tegmentum can cause mutism without akinesia. Neuroanatomical studies reveal that the PAG lacks direct connections with the majority of phonatory motoneurone pools but is connected with the periambigual reticular formation, an area which does have direct connections with all phonatory motor nuclei. The PAG receives a glutamatergic input from several sensory areas, such as the superior and inferior colliculi, solitary tract nucleus and spinal trigeminal nucleus. Glutamatergic input, in addition, reaches it from numerous limbic structures the stimulation of which also produces vocalization, such as the anterior cingulate cortex, septum, amygdala, hypothalamus and midline thalamus. Pharmacological blocking of this glutamatergic input causes mutism. The glutamatceptive vocalization-controlling neurones are under a tonic inhibitory control from GABAergic neurones. Removal of this inhibitory input lowers the threshold for the elicitation of vocalization by external stimuli. A modulatory control on vocalization threshold is also exerted by glycinergic, opioidergic, cholinergic, histaminergic and, possibly, noradrenergic and dopaminergic afferents. It is proposed that the PAG serves as a link between sensory and motivation-controlling structures on the one hand and the periambigual reticular formation coordinating the activity of the different phonatory muscles on the other.

Projections of the dorsal raphe nucleus to the brainstem: PHA-L analysis in the rat

Early studies that used older tracing techniques reported exceedingly few projections from the dorsal raphe nucleus (DR) to the brainstem. The present report examined DR projections to the brainstem by use of the anterograde anatomical tracer Phaseolus vulgaris leucoagglutinin (PHA-L). DR fibers were found to terminate relatively substantially in several structures of the midbrain, pons, and medulla. The following pontine and midbrain nuclei receive moderate to dense projections from the DR: pontomesencephalic central gray, mesencephalic reticular formation, pedunculopontine tegmental nucleus, medial and lateral parabrachial nuclei, nucleus pontis oralis, nucleus pontis caudalis, locus coeruleus, laterodorsal tegmental nucleus, and raphe nuclei, including the central linear nucleus, median raphe nucleus, and raphe pontis. The following nuclei of the medulla receive moderately dense projections from the DR: nucleus gigantocellularis, nucleus raphe magnus, nucleus raphe obscurus, facial nucleus, nucleus gigantocellularis-pars alpha, and the rostral ventrolateral medullary area. DR fibers project lightly to nucleus cuneiformis, nucleus prepositus hypoglossi, nucleus paragigantocellularis, nucleus reticularis ventralis, and hypoglossal nucleus. Some differences were observed in projections from rostral and caudal parts of the DR. The major difference was that fibers from the rostral DR distribute more widely and heavily than do those from the caudal DR to structures of the medulla, including raphe magnus and obscurus, nucleus gigantocellularis-pars alpha, nucleus paragigantocellularis, facial nucleus, and the rostral ventrolateral medullary area. A role for the dorsal raphe nucleus in several brainstem controlled functions is discussed, including REM sleep and its events, nociception, and sensory motor control.

Trigeminal projections to the peribrachial region in the muskrat

The anterograde and retrograde transport of wheat germ agglutinin-horseradish peroxidase was used to study the trigeminoperibrachial pathway in the muskrat after injections of tracer into either the medullary dorsal horn or the dorsolateral pons. After injections into the medullary dorsal horn, labeled fibers ascended into the ipsilateral dorsolateral pons via the spinal trigeminal tract, within the neuropil of the trigeminal sensory complex and within the reticular formation adjacent to the spinal trigeminal nucleus. At caudal levels of the ipsilateral peribrachial area, dense terminal-like label distributed in the Kölliker-Fuse nucleus continued into the lateral parabrachial nucleus. At intermediate levels ipsilaterally, the Kölliker-Fuse nucleus again was labeled densely, as were areas analogous to the external lateral and external medial subnuclei of the parabrachial nucleus in the rat. A thin band of label along the ventral spinocerebellar tract outlined an unlabeled area in the central portion of the lateral parabrachial nucleus. Rostrally near the pontomesencephalic junction, the area designated the superior lateral subnucleus in the hamster was labeled, while sparser label was present more dorsally. Contralateral to the injections, caudal and intermediate levels of the peribrachial area contained only scant reaction product. However, the rostral area of the superior lateral subnucleus was labeled densely via fibers ascending in the trigeminothalamic tract. Injections made just rostral to the obex and either centered in or including the dorsal or ventral paratrigeminal nuclei produced similar labeling at caudal and intermediate levels of the peribrachial area. An exception, however, was that the caudal medial parabrachial nucleus was also labeled after the dorsal paratrigeminal injection. Also, only scant label was found in the rostral third of the dorsolateral pons on either side after these injections. Both trigeminothalamic and trigeminolemniscal pathways were labeled contralaterally after these injections. These trigeminal projections to the dorsolateral pons were compared to the projections from the nucleus tractus solitarii and the ventrolateral medulla. Numerous trigeminal neurons were labeled retrogradely after injections of wheat germ agglutinin-horseradish peroxidase into the dorsolateral pons. In the medullary dorsal horn, they were found almost exclusively in laminae I and V. Labeled neurons in lamina I were especially prominent in rostral ventral levels of the medullary dorsal horn. Labeled cells in lamina I were continuous with others found in the displaced band of substantia gelatinosa at the interface of the subnucleus caudalis and subnucleus interpolaris, as well as with those found in the ventral and dorsal paratrigeminal nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

Midbrain tegmentum as an intermediate relay station of the periaqueductal-facial pathway in the cat

Iontophoretic injection of horseradish peroxidase (HRP) into the medial part of the facial nucleus resulted in retrograde labeling in the lateral midbrain tegmentum contralaterally which corresponded to the paralemniscal zone (PL). Further experiments in which HRP was injected iontophoretically into the PL revealed heavy retrograde labeling in the lateral portion of the ipsilateral periaqueductal gray (PAG). These light microscopic studies indicated the possibility of the pathway from the PAG to the facial nucleus via the midbrain PL. The electron microscopic observations were carried out on the lateral midbrain tegmentum containing the PL after kainic acid, and wheat germ agglutinin conjugated HRP injections were made into the PAG and the contralateral facial nucleus in the same animal, respectively. Although in the neuropil many degenerating PAG fibers and retrogradely labeled neurons were observed, it was of particular interest that the degenerating fibers made synaptic contacts with HRP-labeled somata and dendrites.

Redirection of the hypoglossal nerve to facial muscles alters central connectivity in human brainstem

Functional motor control requires perfect matching of central connectivity of motoneurones with their peripheral connections. However, it is not known to what extent central circuitry is influenced by target muscles, either during development or following a lesion. Surgical interventions aimed at restoring function following peripheral nerve lesions provide an opportunity for studying this interaction in the mature human nervous system. We have followed 8 patients in whom the hypoglossal nerve was anastomosed into a lesioned facial nerve, allowing voluntary contractions of the previously paralyzed muscles. We show that, in addition to replacing the facial neurons at peripheral synapses, a new short-latency trigemino-hypoglossal reflex, of the R1 blink reflex type, can be demonstrated in patients showing recovery, implying a sprouting of trigeminal neurons towards hypoglossal motoneurones, over a distance of at least 0.5 cm. These surprising results show an unexpected influence of the periphery in remodelling central connectivity in man.

Descending brainstem projections of the pedunculopontine tegmental nucleus in the rat

Descending brainstem projections from the pedunculopontine tegmental nucleus (PPN) were studied in the rat by use of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) and the retrograde tracer lectin-conjugated horseradish peroxidase (HRP-WGA). Results of these experiments demonstrated prominent bilateral projections to the pontomedullary reticular nuclei, but direct connections to the motor and sensory nuclei of the cranial nerves could not be ascertained. The PPN fibers terminated mainly in the pontine reticular nuclei oralis and caudalis and in ventromedial portions (pars alpha and pars ventralis) of the gigantocellular reticular nucleus. A smaller number of labeled fibers distributed to more dorsal regions of the gigantocellular nucleus, lateral para-gigantocellular, ventral reticular nucleus of the medulla and lateral reticular nucleus. Although a significant number of PHA-L labeled fibers was seen in two cases in the contralateral medial portion of the facial nucleus, and all cases exhibited a sparse predominantly ipsilateral projection to the lateral facial motor neurons, the retrograde tracing experiments have revealed that these facial afferents originated in the nuclei surrounding the PPN. The results are discussed in the context of PPN involvement in motor functions. It is suggested that the PPN may participate in a complex network involved in the orienting reflex.

A comparison of the distribution and morphology of thalamic, cerebellar and spinal projection neurons in rat trigeminal nucleus interpolaris

The retrograde transport of horseradish peroxidase was used to examine and compare the distribution and morphology of thalamic, cerebellar and spinal projecting neurons in rat trigeminal nucleus interpolaris following large injections into their respective targets. The regional distribution of these three populations was evaluated in relation to the six cytoarchitecturally distinct regions which characterize the nucleus. Cerebellar projecting neurons were distributed throughout the rostrocaudal extent of trigeminal nucleus interpolaris, but were infrequently present in its dorsolateral region and in the rostral pole of the nucleus. Thalamic projecting neurons exhibited a distribution pattern that extensively overlapped with that of the trigeminocerebellar neurons: however, they were particularly concentrated in caudal, dorsomedial and rostral, ventrolateral regions of the nucleus. Trigeminospinal projecting neurons exhibited a more restricted distribution within ventral and lateral regions of trigeminal nucleus interpolaris. Although the three populations of projection neurons could not be distinguished solely on the basis of somatic size or shape, distinct regional variations in the distribution and somatodendritic and axonal morphology of these neurons indicated that they arise largely from independent cell populations. However, several regions were identified in which specific cell types were likely to contribute to axonal collaterilization among these pathways. In the ventrolateral magnocellular region of the nucleus, for example, more than half of the large multipolar-shaped neurons were retrogradely labeled after injections into each of the three target sites. The results of the present study indicate that the thalamic, cerebellar and spinal projections of trigeminal nucleus interpolaris arise from a morphologically heterogeneous group of neurons. In addition, regional variations in the distribution and morphology of these neurons provide evidence for the existence of functionally distinct regions that parallel the cytoarchitecturally defined regions of the nucleus. This study also provides indirect evidence for and against collateralization among these three projections within specific regions of the nucleus.

Projections from the rostral parvocellular reticular formation to pontine and medullary nuclei in the rat: involvement in autonomic regulation and orofacial motor control

The efferent connections of the rostral parvocellular reticular formation to pontine and medullary nuclei in the rat were studied with anterogradely transported Phaseolus vulgaris leucoagglutinin. Dense innervations from the rostral parvocellular reticular formation were found in the mesencephalic trigeminal nucleus, the supratrigeminal area, the motor trigeminal nucleus, the motor trigeminal nucleus, the facial, hypoglossal and parabrachial nuclei and specific parts of the caudal parvocellular reticular formation, including nucleus linearis and the dorsal reticular nucleus of the medulla. The raphe nuclei, nucleus of the solitary tract, inferior olive, dorsal principal sensory, spinal trigeminal nuclei and gigantocellular reticular nucleus and the ventral reticular nucleus of the medulla received moderate projections. In general, the projections from the rostral parvocellular reticular formation were bilateral with an ipsilateral dominance. The dorsal motor vagus and the ambiguus nuclei were not labeled. It is concluded that the rostral parvocellular reticular formation participates in regulation of orofacial motor control and in neural networks for limbic control of metabolic homeostasis.

Diverse afferents converge on the nucleus paragigantocellularis in the rat ventrolateral medulla: retrograde and anterograde tracing studies

The nucleus paragigantocellularis in the ventrolateral medulla has been implicated in cardiovascular, pain, and analgesic functions; and it has also been found to be a major afferent to the pontine nucleus locus coeruleus. In the present study, afferents to the nucleus paragigantocellularis were identified in the rat by means of the retrograde tracers wheat germ agglutinin-conjugated horseradish peroxidase or Fluoro-Gold. Projections to the nucleus paragigantocellularis arise from a wide variety of nuclei with autonomic, visceral, and sensory-related functions. Major afferents with consistent and robust retrograde labeling include most laminae of the spinal cord, the caudal lateral medulla, the contralateral paragigantocellularis, the nucleus of the solitary tract, the A1 area, the lateral parabrachialis, the Kölliker-Fuse nucleus, the periaqueductal gray, and a preoculomotor nucleus in the ventral central gray, the supraoculomotor nucleus. Other notable afferents, seen only after large caudal injections into the nucleus paragigantocellularis, include the lateral hypothalamus, the paraventricular nucleus of the hypothalamus, and the medial prefrontal cortex. Minor afferents include the gigantocellular nucleus, the area postrema, the caudal raphe groups, the inferior colliculus, the A5 area, and the locus coeruleus. The projection from the supraoculomotor nucleus, not previously reported as an afferent to the ventrolateral medulla, was confirmed with anterograde tracing by means of Phaseolus vulgaris-leucoagglutinin. Iontophoretic deposits of Phaseolus vulgaris-leucoagglutinin into the nucleus of the solitary tract (commissuralis level) or into the periaqueductal gray also yielded terminal fiber labeling in the nucleus paragigantocellularis. Fibers from the supraoculomotor nucleus and the nucleus of the solitary tract were densest in the lateral aspect of the nucleus paragigantocellularis (corresponding to the rostroventrolateral reticular nucleus), while fibers from the periaqueductal gray were more medially located. Previous studies have defined inputs to the rostral ventrolateral medulla from the cochlear nucleus as well as from the colliculi. In the present study, deposits of wheat germ agglutinin-conjugated horseradish peroxidase or Phaseolus vulgaris-leucoagglutinin into the cochlear nucleus or the superior colliculus yielded only sparse anterograde labeling in the nucleus paragigantocellularis, but heavily labeled adjacent areas. The inferior collicular injections yielded strong but restricted anterograde labeling in the rostromedial paragigantocellularis, medial to the facial nucleus. These results indicate that the paragigantocellularis area receives inputs from diverse brain structures. Neurons in the nucleus paragigantocellularis afferent to the locus coeruleus, being distributed throughout this region, may provide a channel where several types of information are integrated and transmitted to the extensive locus coeruleus noradrenergic efferent network...

Monoaminergic, peptidergic, and cholinergic afferents to the cat facial nucleus as evidenced by a double immunostaining method with unconjugated cholera toxin as a retrograde tracer

Using a sensitive double immunostaining technique with unconjugated cholera-toxin B subunit as a retrograde tracer, the authors determined the nuclei of origin of monoaminergic, peptidergic, and cholinergic afferent projections to the cat facial nucleus (FN). The FN as a whole receives substantial afferent projections, with relative subnuclear differences, from the following areas: 1) the perioculomotor areas, the contralateral paralemniscal region, and the mesencephalic reticular formation dorsal to the red nucleus; 2) the ipsilateral parabrachial region and the nucleus reticularis pontis, pars ventralis; and 3) the nuclei reticularis parvicellularis, magnocellularis, ventralis, and dorsalis of the medulla. In addition, the present study demonstrated that the lateral portion of the FN receives specific projections from the contralateral medial and olivary pretectal nuclei and the ipsilateral reticular formation of the pons. It was also found that the FN receives: 1) serotoninergic inputs mainly from the nuclei raphe obscurus, pallidus, magnus, and the caudal ventrolateral bulbar reticular formation; 2) catecholaminergic afferent projections from the A7 noradrenaline cell group located in the Kölliker-Fuse, parabrachialis lateralis, and locus subcoeruleus nuclei; 3) methionin-enkephalin-like inputs originating in the pretectal complex, the nucleus paragigantocellularis lateralis and the caudal raphe nuclei; 4) substance P-like afferent projections mainly from the Edinger-Westphal complex and the caudal raphe nuclei; and 5) cholinergic afferents from an area located ventral to the nucleus of the solitary tract at the level of the obex. In the light of these anatomical data, the present report discusses the physiological significance of FN inputs relevant to tonic and phasic events occurring at the level of the facial musculature during the period of paradoxical sleep in the cat.

An analysis of the cyto- and myeloarchitectonic organization of trigeminal nucleus interpolaris in the rat

The cyto- and myeloarchitectonic organization of trigeminal nucleus interpolaris (Vi) was examined in the rat using correlated Nissl- and myelin-stained sections. The caudal boundary of Vi is marked by a spatial overlap with the rostral pole of the medullary dorsal horn (MDH), where there is a dorsal and medial displacement of the substantia gelatinosa (SG, lamina II) layer of MDH. This spatial displacement was further documented using cytochrome-oxidase-reacted sections through the periobex region (POR) of the medulla, where the relatively unstained SG contrasts sharply with the intensely stained Vi neuropil. The rostral boundary of Vi is characterized partly by a distinct overlap with the caudal pole of the dorsomedial region (DM) of trigeminal nucleus oralis (Vo), and partly by a more gradual transition with ventral and lateral regions of Vo. The presence of the distinct MDH-Vi overlap is discussed in terms of its impact on the widespread contention that Vi is involved in the processing of dental pain afferents in the POR. Six separate and distinct regions of rat Vi can be distinguished on the basis of differences in their overall cyto- and myeloarchitecture: (1) a ventrolateral parvocellular region (vlVipc), which occupies the ventrolateral caudal half of Vi; (2) a ventrolateral magnocellular region (vlVimc), which occupies a similar region in the rostral half of the nucleus; (3) a border region (brVi), interposed between the spinal trigeminal tract (SVT) and vlVipc and vlVimc; (4) a dorsolateral region (dlVi), which lies predominantly in the rostral two-thirds of Vi subjacent to the dorsal half of SVT; (5) a dorsal cap region (dcVi), occupying the dorsomedial aspect of the nucleus throughout its entire rostrocaudal extent; and (6) an intermediate region (irVi), which lies immediately ventral to dcVi within the concavity formed by the medial borders of vlVipc and vlVimc. It is proposed that these cyto- and myeloarchitecturally distinct regions of Vi may largely represent functionally distinct regions, based on reported differences in the organization of afferent and efferent projections within the nucleus.

Invasion of cranial nerves and brain stem by herpes simplex virus inoculated into the mouse tongue

Herpes simplex virus type 1 (HSV) of a highly or slightly neuropathogenetic strain was inoculated into the tongues of mice. After appropriate survival times, the animals were killed and portions of the facial and trigeminal nerves as well as the brain stem were examined with virus isolation and immunocytochemistry for the presence of HSV. Both virus strains were demonstrated in the geniculate and trigeminal ganglia and in the CNS portion of the facial and trigeminal nerves, as well as in various brain stem areas including the trigeminal tract and nucleus, the facial and hypoglossal nuclei, and nuclei in the reticular formation. The results indicate that HSV is transported to the CNS by efferent and afferent axons of the tongue and subsequently via transneuronal passage to various brain stem nuclei. These findings provide an anatomic basis for a putative relationship between HSV and cranial nerve dysfunction.

Distribution of glucagonlike peptide I (GLP-I), glucagon, and glicentin in the rat brain: an immunocytochemical study

Although glucagonlike immunoreactants (GLIs) are present in the central nervous system of several mammalian species, their structural relationship with pancreatic proglucagon is not defined, and their precise anatomical distribution has not been studied extensively. To obtain further information about the structure and biological significance of brain GLIs, the anatomical distribution of three different antigenic determinants of pancreatic proglucagon--glucagonlike peptide I (GLP-I), glucagon, and glicentin--was mapped in the brain of colchicine-treated rats by immunocytochemistry using the avidin-biotin-peroxidase method. Neuronal cell bodies immunoreactive with antisera specific for GLP-I, glucagon, and glicentin were found only in the caudal medulla oblongata. Within the caudal medulla immunostained cell bodies were found at levels from approximately 0.55 mm rostral to the obex to 0.45 mm caudal to the obex, and were located within the nucleus of the solitary tract (NTS) and the dorsal (MdD) and ventral (MdV) parts of the medullary reticular nucleus. The NTS contained three times more immunoreactive cell bodies than the MdD and MdV, and these cell bodies were located in the midline, medial, and lateral subnuclei of the caudal third of the NTS. Immunostaining of the same cell bodies in paired adjacent sections incubated with GLP-I and glucagon antisera or glucagon and glicentin antisera provided evidence for coexistence of the three antigens within the same neurons of the NTS. Nerve fibers and terminals immunoreactive with GLP-I, glucagon, and glicentin antisera were widely distributed throughout the rat brain and there was no discernible difference in the distribution of fibers and terminals immunoreactive with each of the three antisera. The highest densities of immunostained fibers and terminals were observed in the hypothalamus, thalamus, and septal regions, and the lowest in the cortex and hindbrain. The localization of neuronal cell bodies containing GLP-I, glucagon, and glicentin within the NTS and the MdD and MdV, and the extensive distribution of immunoreactive fibers and terminals throughout the rat brain suggest a role for these peptides in the integration of autonomic as well as central nervous system functions.

Development of brainstem and cerebellar projections to the diencephalon with notes on thalamocortical projections: studies in the North American opossum

The North American opossum is born in a very immature state, 12 days after conception, and climbs into an external pouch where it remains attached to a nipple for an extended period of time. We have taken advantage of the opossum's embryology to study the development of brainstem and cerebellar projections to the diencephalon as well as the timing of diencephalic projections to somatosensory motor areas of neocortex. The techniques employed included immunocytochemistry for serotonin, the retrograde and orthograde transport of wheat germ agglutinin conjugated to horseradish peroxidase, and the selective impregnation of degenerating axons. Our results suggest that serotoninergic axons, presumably from the dorsal raphe and superior central nuclei, are present in the diencephalon at birth. Axons from the bulbar reticular formation, the vestibular complex, the trigeminal sensory nuclei, and the dorsal column nuclei reach at least mesencephalic (and probably diencephalic) levels by postnatal day (PND) 3, whereas those from the cerebellar nuclei may not grow into comparable levels until PND 5. The dorsal column and cerebellar nuclei innervate the ventral nuclei of the thalamus by estimated postnatal day (EPND) 17 and all of the diencephalic nuclei supplied in the adult animal by EPND 26. Diencephalic axons enter ventrolateral (face) areas of presumptive somatosensory motor cortex by PND 12, but do not reach dorsomedial (limb) regions until EPND 21. At both ages, diencephalic axons are limited to the cortical subplate and marginal zone; they do not innervate an identifiable internal granular layer until considerably later. Our results suggest that axons from the brainstem and cerebellum grow into the diencephalon early in development, but that they do not influence the cerebral cortex until relatively late. When the results of the present study are compared with those reported previously on the development of ascending spinal (Martin et al., '83) and corticofugal (Martin et al., '80; Cabana and Martin, '85b,c) projections, it appears that specific components of major somatosensory and motor circuits develop according to different timetables.

Common input of the cranial motor nuclei involved in phonation in squirrel monkey

Our aim was to locate brain regions projecting to all cranial motor nuclei involved in phonation simultaneously, that is, the ambiguus, trigeminal motor, facial, and hypoglossal nuclei. For this purpose, four squirrel monkeys (Saimiri sciureus) were injected with horseradish peroxidase, each of the four nuclei in a different animal. Those regions retrogradely labeled in all four cases then were injected in another 29 animals with [3H]leucine for anterograde tracing. We found that the only region connected directly with all phonatory motor nuclei is a restricted portion of the pontine and medullary reticular formation, including the nucl. subceruleus ventralis, nucl. parvocellularis and nucl. centralis myelencephali. It is assumed that these nuclei are involved in the integration of vocal fold adduction, articulation, and respiration during vocal utterances.

Difference in projections to the lateral and medial facial nucleus: anatomically separate pathways for rhythmical vibrissa movement in rats

The present paper demonstrates that the lateral and medial subdivisions of the rat facial motor nucleus (NVII) receive differing mesencephalic and metencephalic projections. In order to study brain projections to facial nucleus, horseradish peroxidase (HRP) was injected iontophoretically into the entire facial nucleus or the following subdivisions: lateral, dorsolateral, medial, intermediate, and ventral. In the mesencephalic region, the retrorubral nucleus was found to project to the contralateral medial subdivision of NVII, while the red nucleus was found to project to the contralateral lateral subdivision of NVII. Other mesencephalic projections to the facial nucleus arose from the deep mesencephalic nucleus, oculomotor nucleus, central gray including interstitial nucleus of Cajal and nucleus Darkschewitsch, superior colliculus and substantia nigra (reticular). In the mesencephalic region, the Kölliker-Fuse nucleus, parabrachial nucleus, and the ventral nucleus of the lateral lemniscus projected mainly to the ipsilateral lateral subdivision of NVII. In addition, the trapezoid, pontine reticular, vestibular, and motor trigeminal nuclei were observed to have predominantly ipsilateral connections to the facial nucleus. In contrast, projections from the myelencephalic region were to both the lateral and medial subdivision of NVII. The medullary reticular nucleus, ambiguus nucleus, spinal trigeminal nucleus and parvocellular reticular nucleus projected to both lateral and medial subdivisions of NVII with an ipsilateral predominance. The gigantocellular and paragigantocellular reticular nuclei, raphe magnus, external cuneate nucleus and the nucleus of the solitary tract also projected to the facial motor nucleus. Surprisingly, no direct projections to the NVII were observed from diencephalic and telencephalic regions. Our findings that the lateral subdivision of NVII which innervates vibrissa-pad-muscles (Dom et al. 1973; Martin and Lodge 1977; Watson et al. 1982) receives different metencephalic and mesencephalic projections than medial subdivision which controls pinna movement (Henkel and Edwards 1978), suggest that the functional difference between these subdivisions is mediated by the anatomically separate pathways. We confirmed our anatomical findings by eliciting exclusively vibrissa responses by electrical stimulation of the nuclei which project to the lateral subdivision of NVII.

Morphology of motoneurons in different subdivisions of the rat facial nucleus stained intracellularly with horseradish peroxidase

Horseradish peroxidase was injected into single facial motoneurons of the rat. Neurons were identified by antidromic stimulation of either the buccal or the marginal mandibular or the posterior auricular nerve branches. Motoneuronal cell bodies supplying the buccal branch were located in the lateral subdivision of the facial nucleus, those supplying the marginal mandibular branch were in the intermediate subdivision, and those supplying the posterior auricular branch were in the medial subdivision. Eleven motoneurons were reconstructed with a computer-assisted technique. Their soma diameters averaged 20 microns; the average number of primary dendrites was 7.9 and the combined lengths of the dendritic trees averaged 17,650 microns. There was no distinction between the three motoneuron groups in terms of these and other quantitative data. However, on the basis of reconstructed dendritic tree orientation (i.e., dendritic distribution), major differences were observed between motoneurons of the three groups. Dendrites from all groups extended beyond the boundaries of the facial nucleus into the reticular formation. The border between the intermediate and the lateral subdivision was crossed by some dendrites but the overlap was small. In contrast, no dendrite of a motoneuron in the medial subdivision entered the intermediate subdivision and vice versa. The dendritic extent was totally restricted by the borders between these two subdivisions. Outside the Nissl-defined nuclear border, however, dendrites from cells in adjacent subdivisions overlapped. It is concluded that the medial subdivision of the facial nucleus can be distinguished from the intermediate and lateral subdivisions not only by its sharp Nissl-defined border but also by the discrete organization of its dendritic field.

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 [3H]leucine 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 coeruleus 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 coeruleus, 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.(ABSTRACT TRUNCATED AT 400 WORDS)

Classical conditioning using stimulation of the inferior olive as the unconditioned stimulus

We show that conditioned eyelid responses develop when the unconditioned stimulus is electrical stimulation of the dorsal accessory nucleus of the inferior olive. When compared to conditioning using a standard unconditioned stimulus (air puff), the conditioning produced in this manner appears quite normal: the responses develop at a similar rate, are of comparable magnitude and topography, and demonstrate a steep interstimulus interval function; and response topography varies according to the interstimulus interval. These data indicate that activation of neurons in the dorsal accessory olive is a sufficient condition for a stimulus to be an effective unconditioned stimulus. Previous experiments indicate the dorsal accessory olive is necessary in that lesions have effects functionally equivalent to removal of the unconditioned stimulus. These data indicate that the dorsal accessory olive forms a portion of the pathway conveying information about the occurrence of an unconditioned stimulus to sites of synaptic plasticity responsible for conditioning.

Afferent projections to the orbicularis oculi motoneuronal cell group. An autoradiographical tracing study in the cat

The motoneurons innervating the orbicularis oculi muscle from a subgroup within the facial nucleus, called the intermediate facial subnucleus. This makes it possible to study afferents to these motoneurons by means of autoradiographical tracing techniques. Many different injections were made in the brainstem and diencephalon and the afferent projections to the intermediate facial subnucleus were studied. The results indicated that these afferents were derived from the following brainstem areas: the dorsal red nucleus and the mesencephalic tegmentum dorsal to it; the olivary pretectal nucleus and/or the nucleus of the optic tract; the dorsolateral pontine tegmentum (parabrachial nuclei and nucleus of Kölliker-Fuse) and principal trigeminal nucleus; the ventrolateral pontine tegmentum at the level of the motor trigeminal nucleus; the caudal medullary medial tegmentum; the lateral tegmentum at the level of the rostral pole of the hypoglossal nucleus and the ventral part of the trigeminal nucleus and the nucleus raphe pallidus and caudal raphe magnus including the adjoining medullary tegmentum. These latter projections probably belong to a general motoneuronal control system. The mesencephalic projections are mainly contralateral, the caudal pontine and upper medullary lateral tegmental projections are mainly ipsilateral and the caudal medullary projections are bilateral. It is suggested that the different afferent pathways subserve different functions of the orbicularis oculi motoneurons. Interneurons in the dorsolateral pontine and lateral medullary tegmentum may serve as relay for cortical and limbic influences on the orbicularis oculi musculature, while interneurons in the ventrolateral pontine and caudal medullary tegmentum may take part in the neuronal organization of the blink reflex.