Medullary Projections Research Articles

Essential Role of the cVRG in the Generation of Both the Expiratory and Inspiratory Components of the Cough Reflex.

As stated by Korpáš and Tomori (1979), cough is the most important airway protective reflex which provides airway defensive responses to nociceptive stimuli. They recognized that active expiratory efforts, due to the activation of caudal ventral respiratory group (cVRG) expiratory premotoneurons, are the prominent component of coughs. Here, we discuss data suggesting that neurons located in the cVRG have an essential role in the generation of both the inspiratory and expiratory components of the cough reflex. Some lines of evidence indicate that cVRG expiratory neurons, when strongly activated, may subserve the alternation of inspiratory and expiratory cough bursts, possibly owing to the presence of axon collaterals. Of note, experimental findings such as blockade or impairment of glutamatergic transmission to the cVRG neurons lead to the view that neurons located in the cVRG are crucial for the production of the complete cough motor pattern. The involvement of bulbospinal expiratory neurons seems unlikely since their activation affects differentially expiratory and inspiratory muscles, while their blockade does not affect baseline inspiratory activity. Thus, other types of cVRG neurons with their medullary projections should have a role and possibly contribute to the fine tuning of the intensity of inspiratory and expiratory efforts.

CO2-Induced ATP-Dependent Release of Acetylcholine on the Ventral Surface of the Medulla Oblongata.

Complex mechanisms that detect changes in brainstem parenchymal PCO2/[H+] and trigger adaptive changes in lung ventilation are responsible for central respiratory CO2 chemosensitivity. Previous studies of chemosensory signalling pathways suggest that at the level of the ventral surface of the medulla oblongata (VMS), CO2-induced changes in ventilation are (at least in part) mediated by the release and actions of ATP and/or acetylcholine (ACh). Here we performed simultaneous real-time biosensor recordings of CO2-induced ATP and ACh release from the VMS in vivo and in vitro, to test the hypothesis that central respiratory CO2 chemosensory transduction involves simultaneous recruitment of purinergic and cholinergic signalling pathways. In anaesthetised and artificially ventilated rats, an increase in inspired CO2 triggered ACh release on the VMS with a peak amplitude of ~5 μM. Release of ACh was only detected after the onset of CO2-induced activation of the respiratory activity and was markedly reduced (by ~70%) by ATP receptor blockade. In horizontal slices of the VMS, CO2-induced release of ATP was reliably detected, whereas CO2 or bath application of ATP (100 μM) failed to trigger release of ACh. These results suggest that during hypercapnia locally produced ATP induces or potentiates the release of ACh (likely from the medullary projections of distal groups of cholinergic neurones), which may also contribute to the development and/or maintenance of the ventilatory response to CO2.

Hypovolemic shock: Critical involvement of a projection from the ventrolateral periaqueductal gray to the caudal midline medulla

Previous research has suggested that the ventrolateral column of the periaqueductal gray (vlPAG) plays a crucial role in triggering a decompensatory response (sympathoinhibition, hypotension, bradycardia) to severe blood loss. vlPAG excitation triggers also quiescence, decreased vigilance and decreased reactivity, the behavioral response which usually accompanies hypovolemic shock. The aim of this study was to identify, in unanesthetized rats, the main descending pathway(s) via which vlPAG neurons trigger sympathoinhibition and bradycardia in response to severe blood loss. Firstly, immediate early gene (c-Fos) expression was used to identify vlPAG neurons selectively activated by severe blood loss. Subsequently, the specific medullary projections of these vlPAG neurons were defined by combined c-Fos, retrograde tracing (double-label) experiments. It was found that vlPAG neurons selectively activated by severe hemorrhage project overwhelmingly to the vasodepressor portion of the caudal midline medulla (CMM). Previous studies indicate that this CMM region mediates behaviorally-coupled cardiovascular adjustments and the findings described here fit with the idea that CMM neurons are uniquely recruited by salient challenges, the adaptive responses to which require more than reflexive homeostatic cardiovascular adjustments.

Spinal and medullary projections to the parabrachial nuclear complex

Spinal and medullary projections to the parabrachial nuclear complex

Cardiovascular Responses to Somatosensory Stimulation and Their Modulation by Baroreflex Mechanisms

Exposure of experimental animals to noxious somatic stimulations sometimes induces sustained hypertension. Information regarding the medullary projections of somatic afferents and the neurotransmitters involved in them is incomplete. The present investigation in urethane-anesthetized, artificially ventilated, adult male Wistar rats was undertaken to clarify some of these issues. It was observed that the inhibition of contralateral, ipsilateral, or bilateral rostral ventrolateral medullary pressor area (RVLM) with muscimol attenuated the pressor and tachycardic responses to sciatic nerve stimulation. Similar inhibition of the medial subnucleus of the solitary tract (mNTS) exaggerated the cardiovascular responses to sciatic nerve stimulation. Interruption of the baroreflex by microinjections of ionotropic glutamate receptor antagonists into the mNTS or barodenervation also exaggerated the responses to sciatic nerve stimulation. Unilateral stimulation of the aortic nerve blocked the cardiovascular responses to the sciatic nerve stimulation. These results indicated that in the rat, the ascending afferents in the sciatic nerve project bilaterally to the RVLM as well as mNTS; an excitatory amino acid, probably glutamate, is released in the mNTS in response to the sciatic nerve stimulation; and barodenervation or blockade of baroreflex in the mNTS exaggerates, while baroreceptor stimulation inhibits, cardiovascular responses to somatosensory stimulation.

Organization of immune-responsive medullary projections to the bed nucleus of the stria terminalis, central amygdala, and paraventricular nucleus of the hypothalamus: Evidence for parallel viscerosensory pathways in the rat brain

Immune-responsive neurons in the brainstem, primarily in the nucleus of the solitary tract (NTS) and ventrolateral medulla (VLM), contribute to a significant drive on forebrain nuclei responsible for brain-mediated host defense responses. The current study investigated the relative contribution of brainstem-derived ascending pathways to forebrain immune-responsive nuclei in the rat by means of retrograde tract tracing and c-Fos immunohistochemistry. Fluorogold was iontophoresed into the bed nucleus of stria terminalis (BST), central nucleus of the amygdala (CEA), paraventricular nucleus of the hypothalamus (PVN), and the pontine lateral parabrachial nucleus (PBL; an important component of ascending viscerosensensory pathways) followed 2 weeks later by intraperitoneal injection of lipopolysaccharide (LPS, 0.1 mg/kg) or saline. The NTS and VLM provide immune-responsive input to all four regions, via direct, predominantly catecholaminergic, projections to the PVN, the lateral BST, and the CEA, and mostly non-catecholaminergic projections to the PBL. The PBL provides a major LPS-activated input to the BST and CEA. The pattern of LPS-activated catecholaminergic projections from the VLM and NTS to the forebrain is characterized by a strong predominance of VLM input to the PVN, whereas the NTS provides a greater contribution to the BST. These findings indicate that direct and indirect pathways originate in the caudal brainstem that propagate immune-related information from the periphery with multiple levels of processing en route to the forebrain nuclei, which may allow for integration of brain responses to infection.

Distribution and medullary projection of respiratory neurons in the dorsolateral pons of the rat

The dorsolateral pons around the parabrachial nucleus including the Kölliker-Fuse nucleus is closely linked with the medullary respiratory center and plays an important role in respiratory control. We aimed to elucidate the firing properties, detailed distributions, and medullary projections of pontine respiratory neurons in pentobarbitone-anesthetized, paralyzed, and artificially ventilated rats with intact vagi. A total of 235 respiratory neurons were recorded from the dorsolateral pons in and around the Kölliker-Fuse nucleus. Six types of firing patterns were identified: inspiratory, expiratory–inspiratory phase spanning, inspiratory–expiratory phase spanning, decrementing expiratory, augmenting expiratory, and whole-phase expiratory patterns. Of these, the inspiratory neurons and the expiratory–inspiratory phase spanning neurons, which constituted the largest population (61%), were characterized most carefully by changing lung inflation levels, since under some conditions both showed similar firing patterns. Many (58%) of the 133 respiratory neurons examined were antidromically activated by electrical stimulation of the medulla. They were activated from the ventrolateral medulla around the ventral respiratory group and the Bötzinger complex and from the dorsomedial medulla around the nucleus tractus solitarii and the hypoglossal nucleus. The projections to the dorsomedial medulla were bilateral in many cases, and those to the ventrolateral medulla were unilateral. Of these medullary projections, two specific projections could be characterized in detail. First, many expiratory–inspiratory phase spanning neurons projected to the hypoglossal nucleus, suggesting that these pontine neurons are important premotor neurons of the hypoglossal motoneurons. This projection explains well the hypoglossal inspiratory activity, which is often dissociated from the phrenic inspiratory activity. Second, most whole-phase expiratory neurons that were distributed medially to the KF nucleus sent their axons toward the spinal cord via the midline medulla. These findings provide a new insight into the pontine control of medullary and spinal respiratory function.

Noradrenaline in the ventral forebrain is critical for opiate withdrawal-induced aversion.

Cessation of drug use in chronic opiate abusers produces a severe withdrawal syndrome that is highly aversive, and avoidance of withdrawal or associated stimuli is a major factor contributing to opiate abuse. Increased noradrenaline in the brain has long been implicated in opiate withdrawal, but it has not been clear which noradrenergic systems are involved. Here we show that microinjection of beta-noradrenergic-receptor antagonists, or of an alpha2-receptor agonist, into the bed nucleus of the stria terminalis (BNST) in rats markedly attenuates opiate-withdrawal-induced conditioned place aversion. Immunohistochemical studies revealed that numerous BNST-projecting cells in the A1 and A2 noradrenergic cell groups of the caudal medulla were activated during withdrawal. Lesion of these ascending medullary projections also greatly reduced opiate-withdrawal-induced place aversion, whereas lesion of locus coeruleus noradrenergic projections had no effect on opiate-withdrawal behaviour. We conclude that noradrenergic inputs to the BNST from the caudal medulla are critically involved in the aversiveness of opiate withdrawal.

Intramedullary projections of the rostral nucleus of the solitary tract in the rat: gustatory influences on autonomic output.

The efferent connections of the rostral nucleus of the solitary tract (NTS) in the rat were studied by anterograde transport of Phaseolus vulgaris leucoagglutinin. Rostral to the injection site, fibers travel through the rostral parvocellular reticular formation and deflect medially or laterally around the motor trigeminal nucleus, giving off few terminals in these nuclei and terminate in the parabrachial nucleus. Moderate projections to the peritrigeminal zone, including the intertrigeminal nucleus and the dorsal subcoeruleus nucleus, were observed. Caudally to the injection site, dense innervations from the rostral nucleus of the solitary tract were detected in the parvocellular reticular formation ventral and caudal to the injection site and in the intermediate and ventral medullary reticular formation. The rostral central and ventral subdivisions of the NTS up to the level where the nucleus of the solitary tract abuts the fourth ventricle and the hypoglossal nucleus, receive moderate input from the rostral nucleus of the solitary tract. In general, the projections from the rostral nucleus of the solitary tract were bilateral with an ipsilateral predominance. The caudal part of the nucleus of the solitary tract, the dorsal motor nucleus of the vagus and the facial nucleus were not labeled. It is concluded that medullary rNTS projections participate in oral motor behavior and autonomic control of abdominal organs.

Evidence for medullary projections of the intercostal nerve-elicited inspiratory termination

This study hypothesized that the ICN-elicited inspiratory termination reflex required synaptic activation in two distinct regions of the ventral respiratory group (VRG): (1) transitional (tVRG), and (2) pre-Bötzinger complex (pre-BötC). Data from adult cats indicate that axons of passage associated with the ICN-elicited termination reflex traverse tVRG, but that relevant synaptic processing does not occur in this region. Furthermore, data indicate that neither synaptic nor axonal transmission within the pre-BötC is required for the SLN- or ICN-elicted termination reflex.

Medullary projections of rabbit carotid sinus nerve

In New Zealand white rabbits, cholera-toxin HRP was injected into the carotid sinus nerve just proximal to the carotid sinus. After survival periods of 3–5 days the rabbits were anesthetized and the brain fixed with aldehyde solution. Transverse sections were cut on a sledge microtome and the sections reacted with the tetramethylbenzidine procedure. HRP-positive fibers entered the ipsilateral dorsolateral medulla at the level of the acoustic tubercle, joining the tractus solitarius. Positive fibres were found principally ipsilaterally in four regions of the medulla: in the caudal two thirds of the nucleus tractus solitarius, in its dorsolateral regions and, more caudally, in its commissural subdivision; in the dorsolateral aspect of the spinal nucleus of the trigeminal nerve; in the region ventral and ventrolateral to the tractus solitarius (extending beyond the nucleus tractus solitarius); and in the ventrolateral medulla oblongata.

Physiology and pharmacology of the postinjury pain state

CUTE ACTIVATION of small sensory afferent axons evokes autonomic responses and pain behavior in humans and animals. This effect is mediated by the activation of spinofugal projection neurons. These projection systems traveling both ipsilaterally and contralaterally in the ventrolateral aspect of the spinal cord, projeering supraspinally into the medulla, mesencephalon, and diencephalon. Medullary projections serVe to activate spino-bulbo-spinal reflexes that influence autonomic tone. Other projections into the mesencephalon and thalamus are assumed to contribute to the perceptual and complex emotive and discriminative components of the pain state. Thus, the somatotopie association between spinal segment and supraspinal projections to the ventrobasal thalamus, which itself sends projections to the sensory cortex, has been well described. The mapping of the sensory homunculus by the hard-wired projections is believed to serve in establishing the precise localization of the stimulus event in the uninjured nervous system. More recently, work with positron emission tomography has indicated that areas in the anterior cingulate cortex known to receive input from several thalamic nuclei, notably the nucleus submedius, may be activated in the presence of noxious, but not nonnoxious sensory input. 2 Importantly, the submedius is itself the recipient of nociceptive-specific input. Details of these ascending systems are discussed in detail elsewhere) '4 However, it is important to appreciate that the encoding by the sensory afferent and the spinal dorsal horn of the nociceptive stimulus is the first step in nociceptive processing, and the properties of this encoding process contribute properties that are important for the understanding of the behavioral correlates of nociception. In this article, we consider aspects of the mechanisms whereby tissue injury leads to an ongoing pain state from the perspective of the organization of the sensory afferents and the spinal dorsal horn.

Mapping of carotid baroreceptor subtype projections to the nucleus tractus solitarius using c- fos immunohistochemistry

This study has combined physiological pressure stimulation of carotid baroreceptors via a vascularly isolated carotid sinus and anodal block of baroreceptor afferent fibers in the carotid sinus nerve to examine the medullary projections of type I vs. type II (large A- vs. small A- and C-fiber afferent) baroreceptors. The control distribution of cells in the nucleus tractus solitarius expressing c- fos in response to physiological activation of carotid baroreceptors in the isolated sinus was compared to that during anodal block of large A-fibers in the carotid sinus nerve. Carotid baroreceptor stimulation primarily activated neurons in the ipsilateral commissural and medial subnuclei of the caudal nucleus tractus solitarius and the dorsolateral, dorsomedial and medial subnuclei in the intermediate and rostral levels of the nucleus tractus solitarius. Elimination of large A-fiber carotid baroreceptor afferents, during similar carotid baroreceptor stimulation resulted in a decrease in the nmber of cells expressing c- fos in the dorsomedial subnucleus of the rostral nucleus tractus solitarius. These data indicate that projections of larger A-fiber (type I) carotid baroreceptors are localized primarily to the rostral dorsomedial subnucleus, while those of smallr A- and C-fiber baroreceptors are more widely distributed to the commissural, medial and dorsal subnuclei of the nucleus tractus solitarius.

Pontine and medullary projections to the nucleus retroambiguus: a wheat germ agglutinin-horseradish peroxidase and autoradiographic tracing study in the cat.

The nucleus retroambiguus (NRA) in the caudal medulla oblongata plays a role in expiration, vocalization, vomiting, and possibly lordosis. The present study tried to determine which structures, in turn, control the NRA. One cell group is the periaqueductal gray (PAG), which is considered to be the final integrator of defensive and aggressive behaviors, micturition, vocalization, and lordosis. Structures rostral to the PAG seem to bypass the NRA. With respect to the existence of cell groups caudal to the PAG projecting to the NRA, the situation is less clear. Therefore, in five adult female cats, injections of wheat germ agglutinin-horseradish peroxidase were centered on the NRA, and the resulting retrogradely labeled neurons were plotted. In the areas containing retrogradely labeled cells, the anterograde autoradiographic tracer [3H]-leucine was injected in 66 cats. The combined results demonstrated that NRA afferents not only originate from the PAG but also from specific cell groups in the pontine and medullary lateral tegmental field, i.e., the ventrolateral parabrachial nucleus, the nucleus Kölliker-Fuse, the retrotrapezoid nucleus, and the ventrolateral part of the medulla caudal to the facial nucleus including the Bötzinger and pre-Bötzinger complex and the periambigual region. Afferents also originate from the solitary nucleus and two cell groups in the ventral part of the medullary medial tegmental field, one at the level of facial nucleus and one just rostral to the hypoglossal nucleus. It can be concluded that many respiratory-related cell groups have direct access to the NRA. The cell groups in the medial tegmental field, which have not yet been found to play an important role in respiration, might serve as relay for certain limbic system cell groups to reach the NRA in the context of specific emotional behavior.

Pump neurons of the nucleus of the solitary tract project widely to the medulla

Axonal projections of pump neurons (P-cells) of the nucleus of the solitary tract were studied in Nembutal-anesthetized, paralyzed, and artificially ventilated cats. Extracellular recordings were made from single P-cells which were monosynaptically activated by electrical stimulation of the ipsilateral vagal afferents and distributed in close proximity to the solitary tract. Antidromic stimulation of the medulla showed that all P-cells examined projected their axons to the ipsilateral ventrolateral medulla. Axonal arborizations were found in respiration-related areas and their vicinity between the level of the retrofacial nucleus and the level a few millimeters caudal to the obex. Some of these P-cells sent axon collaterals to the contralateral nucleus of the solitary tract (NTS) as well. This study is the first to show wide spread medullary projections of P-cell axons.

1618 Medullary projections of neurons of the solitary tract nucleus which receive input from pulmonary slowly adapting stretch receptors

1618 Medullary projections of neurons of the solitary tract nucleus which receive input from pulmonary slowly adapting stretch receptors

Chapter 10 The ventro-medial medullary projections to spinal motoneurons: ultrastructure, transmitters and functional aspects

Publisher Summary This chapter considers the ventro-medial medullary projections to spinal motoneurons in detail with respect to their ultrastructure, transmitters and function. In general terms, the ventro-medial medulla comprises the area ranging from the caudal part of the inferior olive to the caudal part of the trapezoid body. At the level of the inferior olive it also comprises the nucleus raphe obscurus, the ventral part of the nucleus raphe pallidus and the area immediately overlying the inferior olive and immediately laterals to it. More rostrally it comprises the nucleus raphe magnus and laterally adjacent areas up to the medial border of the facial nucleus. In the rat, according to the nomenclature of Andrezik and Beitz, the ventro-medial medulla includes the ventral part of the paramedian reticular nucleus, the gigantocellular nucleus-ventral part and the ventro-medial part of the paragigantocellular nucleus at the level of the inferior olive. It is important to note that the more dorsally located gigantocellular nucleus, which comprises many spinal projecting cells, including giant cells, is not considered part of the ventro-medial medulla.

Medullary projections to the vagus nerve and posterolateral hypothalamus.

Vagal visceromotor reflexes are dependent upon reciprocal neural connections existing between the medulla and the hypothalamus. Medullohypothalamic neurons may provide feedback cues to the hypothalamus regarding the activity of vagal motor neurons. As yet, however, studies investigating the spatial relationships between medullohypothalamic neurons and vagal motor neurons have not been performed. A variety of retrogradely transported tracers were used for the purpose of mapping the relative locations of medullovagal and medullohypothalamic neurons. Tracers were injected into the cervical vagus nerve and/or the posterolateral hypothalamus, and subsequently the retrogradely labeled medullary neurons were plotted. Labeling of the two neuronal populations was primarily observed within the ventrolateral and dorsomedial medulla. Within the ventrolateral medulla, medullovagal neurons were found within the retrofacial nucleus and nucleus retroambiguus, whereas medullohypothalamic neurons were located subjacent to these nuclei. Within the dorsomedial medulla, labeling of the two neuronal populations was primarily limited to the vagal-solitary complex. At this location medullovagal neurons were found within the dorsal vagal nucleus, whereas medullohypothalamic neurons were largely confined to the caudal aspect of the solitary nucleus. Because of the spatial proximity existing between medullovagal and medullohypothalamic neurons, it is suggested that functional interrelationships may exist between these two neuronal populations. Specifically, it is suggested that the medullohypothalamic neurons identified in this study may support vagal-related functions by providing feedback cues to the posterolateral hypothalamus.

Direct projections from the anterior pretectal nucleus to the ventral medulla oblongata in rats.

The anterior pretectal nucleus has recently been implicated in the descending modulation of nociception. Electrical stimulation of the nucleus was found to reduce the nociceptive responses of deep dorsal horn neurons and to inhibit spinally integrated withdrawal reflexes. It is believed that at least part of the descending inhibitory effects of the anterior pretectal nucleus are mediated by reticulospinal cells of the ventrolateral medulla. The purpose of the present study was to trace the direct medullary projections of the anterior pretectal nucleus, to describe their topographical organization and to reveal the chemical nature of some of their putative target cells. The connections were studied using anterograde tract-tracing with Phaseolus vulgaris leucoagglutinin. Direct projections from the anterior pretectal nucleus to the ipsilateral rostral ventral medulla were found in all cases. A dense innervation of the dorsal inferior olive, the gigantocellular reticular nucleus pars ventralis and pars alpha and the ventral pontine reticular nucleus was found from all aspects of the anterior pretectal nucleus. Descending labelled terminals were also observed in the gigantocellular reticular nucleus proper and, laterally, in the lateral paragigantocellular nucleus and in the region of the A5 noradrenergic cell group. A relatively lower density of labelled terminals was noted in the medullary raphe nuclei and in the rostroventrolateral reticular nucleus. Following tract-tracer injections into five distinct subregions of the anterior pretectal nucleus, the topographical organization of the projection was examined and the relatively highest density and most widespread projection was found to originate from the caudoventral part of the anterior pretectal nucleus. A combined tract-tracing and immunolabelling study revealed that some of the descending, labelled terminals were in close proximity of tyrosine hydroxylase-immunoreactive dendrites in the C1 and A5 cell groups. Some labelled fibres were also noted among the serotonin-immunoreactive cells in the lateral extension of the B3 cell population. The existence of direct projections to the ventral medulla and pons correlates well with physiological data which showed that the descending, antinociceptive effects of the anterior pretectal nucleus are relayed via the rostral ventrolateral medulla. The data are also in keeping with pharmacological studies that suggested the role of catecholaminergic cells in the mediation of these descending effects. It is proposed that the rostral ventral medullary projections provide a path through which antinociceptive effects of the anterior pretectal nucleus are mediated to the spinal cord.

Direct projections from the ventrolateral medulla oblongata to the limbic forebrain: anterograde and retrograde tract-tracing studies in the rat.

Neurons in the ventrolateral medulla oblongata, a brain region implicated in central vasomotor regulation, have previously been reported to project to some forebrain limbic structures. The aim of the present study was (1) to describe the termination pattern of ventral medullary afferents in forebrain limbic areas using anterograde tract tracing, and (2) to determine the location and some morphological characteristics of the projection neurons using retrograde tract tracing from selected forebrain sites. Following ionophoretic microinjections of the anterograde tract tracer Phaseolus vulgaris leucoagglutinin into the rostral ventrolateral medulla, labelled afferents were observed in the hippocampus, entorhinal and retrosplenial cortices, dorsal septum, nucleus accumbens, and the medial prefrontal cortex. Anterogradely labelled axons, ascending from the caudal ventrolateral medulla, could be traced only to the rostral aspects of the investigated forebrain limbic structures. Here, the main target of the ascending projection was in the ventral septum. However, labelled terminals were also present in the nucleus accumbens, the dorsolateral septum, and in the infralimbic cortex. The density of the ventrolateral medullary projections into all examined forebrain areas was low. The location of the cells in the ventral medulla oblongata which give rise to direct forebrain projections was examined using retrograde tract tracing with wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Following WGA-HRP injections into the septo-accumbens region, retrogradely labelled cells were present in both the rostral and caudal ventrolateral medulla. When the tract tracer injection was restricted to the ventral region of the septal complex, the labelled cells were concentrated in the caudal aspects of the ventrolateral medulla (and the nucleus of the solitary tract). Following tracer injections into the anterior cingulate cortex or the hippocampus or the entorhinal cortex, retrogradely labelled cells in the medulla oblongata were predominantly in the rostral ventrolateral medulla. As a first attempt to reveal the chemical nature of the projection cells, the contribution of tyrosine hydroxylase-immunoreactive cells to the innervation of the septo-accumbens area was also investigated: tyrosine hydroxylase-immunoreactive cells of both the caudal ventrolateral medulla and the nucleus of the solitary tract were found to contribute to the innervation of the septo-accumbens area. The distribution of retrogradely labelled cells as well as the termination pattern of the anterogradely labelled terminals indicated that the innervation of the various forebrain limbic areas arises from cells, diffusely distributed in the rostral and/or the caudal ventrolateral medulla oblongata.(ABSTRACT TRUNCATED AT 400 WORDS)