Connections of the caudal ventrolateral medullary reticular formation in the cat brainstem

Pedunculopontine Chx10+ neurons control global motor arrest in mice

Arrest of ongoing movements is an integral part of executing motor programs. Behavioral arrest may happen upon termination of a variety of goal-directed movements or as a global motor arrest either in the context of fear or in response to salient environmental cues. The neuronal circuits that bridge with the executive motor circuits to implement a global motor arrest are poorly understood. We report the discovery that the activation of glutamatergic Chx10-derived neurons in the pedunculopontine nucleus (PPN) in mice arrests all ongoing movements while simultaneously causing apnea and bradycardia. This global motor arrest has a pause-and-play pattern with an instantaneous interruption of movement followed by a short-latency continuation from where it was paused. Mice naturally perform arrest bouts with the same combination of motor and autonomic features. The Chx10-PPN-evoked arrest is different to ventrolateral periaqueductal gray-induced freezing. Our study defines a motor command that induces a global motor arrest, which may be recruited in response to salient environmental cues to allow for a preparatory or arousal state, and identifies a locomotor-opposing role for rostrally biased glutamatergic neurons in the PPN.

Responses of Neurons in the Medullary Lateral Tegmental Field and Nucleus Tractus Solitarius to Vestibular Stimuli in Conscious Felines

Considerable evidence shows that the vestibular system contributes to adjusting sympathetic nervous system activity to maintain adequate blood pressure during movement and changes in posture. However, only a few prior experiments entailed recordings in conscious animals from brainstem neurons presumed to convey baroreceptor and vestibular inputs to neurons in the rostral ventrolateral medulla (RVLM) that provide inputs to sympathetic preganglionic neurons in the spinal cord. In this study, recordings were made in conscious felines from neurons in the medullary lateral tegmental field (LTF) and nucleus tractus solitarius (NTS) identified as regulating sympathetic nervous system activity by exhibiting changes in firing rate related to the cardiac cycle, or cardiac-related activity (CRA). Approximately 38% of LTF and NTS neurons responded to static 40° head up tilts with a change in firing rate (increase for 60% of the neurons, decrease for 40%) of ~50%. However, few of these neurons responded to 10° sinusoidal rotations in the pitch plane, in contrast to prior findings in decerebrate animals that the firing rates of both NTS and LTF neurons are modulated by small-amplitude body rotations. Thus, as previously demonstrated for RVLM neurons, in conscious animals NTS and LTF neurons only respond to large rotations that lead to changes in sympathetic nervous system activity. The similar responses to head-up rotations of LTF and NTS neurons with those documented for RVLM neurons suggest that LTF and NTS neurons are components of the vestibulo-sympathetic reflex pathway. However, a difference between NTS/LTF and RVLM neurons was variability in CRA over time. This variability was significantly greater for RVLM neurons, raising the hypothesis that the responsiveness of these neurons to baroreceptor input is adjusted based on the animal's vigilance and alertness.

Afferents of the mouse linear nucleus

The linear nucleus (Li) was identified in 1978 from its projections to the cerebellum. However, there is no systematic study of its connections with other areas of the central nervous system possibly due to the challenge of injecting retrograde tracers into this nucleus. The present study examines its afferents from some nuclei involved in motor and cardiovascular control with anterograde tracer injections. BDA injections into the central amygdaloid nucleus result in labeled fibers to the ipsilateral Li. Bilateral projections with an ipsilateral dominance were observed after injections in a) jointly the paralemniscal nucleus, the noradrenergic group 7/ Köllike -Fuse nucleus/subcoeruleus nucleus, b) the gigantocellular reticular nucleus, c) and the solitary nucleus/the parvicellular/intermediate reticular nucleus. Retrogradely labeled neurons were observed in Li after BDA injections into all these nuclei except the central amygdaloid and the paralemniscal nuclei. Our results suggest that Li is involved in a variety of physiological functions apart from motor and balance control it may exert via its cerebellar projections.

Activation of Brainstem Neurons During Mesencephalic Locomotor Region-Evoked Locomotion in the Cat

The distribution of locomotor-activated neurons in the brainstem of the cat was studied by c-Fos immunohistochemistry in combination with antibody-based cellular phenotyping following electrical stimulation of the mesencephalic locomotor region (MLR) – the anatomical constituents of which remain debated today, primarily between the cuneiform (CnF) and the pedunculopontine tegmental nuclei (PPT). Effective MLR sites were co-extensive with the CnF nucleus. Animals subject to the locomotor task showed abundant Fos labeling in the CnF, parabrachial nuclei of the subcuneiform region, periaqueductal gray, locus ceruleus (LC)/subceruleus (SubC), Kölliker–Fuse, magnocellular and lateral tegmental fields, raphe, and the parapyramidal region. Labeled neurons were more abundant on the side of stimulation. In some animals, Fos-labeled cells were also observed in the ventral tegmental area, medial and intermediate vestibular nuclei, dorsal motor nucleus of the vagus, n. tractus solitarii, and retrofacial nucleus in the ventrolateral medulla. Many neurons in the reticular formation were innervated by serotonergic fibers. Numerous locomotor-activated neurons in the parabrachial nuclei and LC/SubC/Kölliker–Fuse were noradrenergic. Few cholinergic neurons within the PPT stained for Fos. In the medulla, serotonergic neurons within the parapyramidal region and the nucleus raphe magnus were positive for Fos. Control animals, not subject to locomotion, showed few Fos-labeled neurons in these areas. The current study provides positive evidence for a role for the CnF in the initiation of locomotion while providing little evidence for the participation of the PPT. The results also show that MLR-evoked locomotion involves the parallel activation of reticular and monoaminergic neurons in the pons/medulla, and provides the anatomical and functional basis for spinal monoamine release during evoked locomotion. Lastly, the results indicate that vestibular, cardiovascular, and respiratory centers are centrally activated during MLR-evoked locomotion. Altogether, the results show a complex pattern of neuromodulatory influences of brainstem neurons by electrical activation of the MLR.

Adrenergic agonist induces rhythmic firing in quiescent cardiac preganglionic neurons in nucleus ambiguous via activation of intrinsic membrane excitability

Cholinergic vagal nerves projecting from neurons in the brain stem nucleus ambiguus (NAm) play a predominant role in cardiac parasympathetic pacemaking control. Central adrenergic signaling modulates the tone of this vagal output; however, the exact excitability mechanisms are not fully understood. We investigated responses of NAm neurons to adrenergic agonists using in vitro mouse brain stem slices. Preganglionic NAm neurons were identified by ChAT-tdTomato fluorescence in young adult transgenic mice, and their cardiac projection was confirmed by retrograde dye tracing. Juxtacellular recordings detected sparse or absent spontaneous action potentials (AP) in NAm neurons. However, bath application of epinephrine or norepinephrine strongly and reversibly activated most NAm neurons regardless of their basal firing rate. Epinephrine was more potent than norepinephrine, and this activation largely depends on α₁-adrenoceptors. Interestingly, adrenergic activation of NAm neurons does not require an ionotropic synaptic mechanism, because postsynaptic excitatory or inhibitory receptor blockade did not occlude the excitatory effect, and bath-applied adrenergic agonists did not alter excitatory or inhibitory synaptic transmission. Instead, adrenergic agonists significantly elevated intrinsic membrane excitability to facilitate generation of recurrent action potentials. T-type calcium current and hyperpolarization-activated current are involved in this excitation pattern, although not required for spontaneous AP induction by epinephrine. In contrast, pharmacological blockade of persistent sodium current significantly inhibited the adrenergic effects. Our results demonstrate that central adrenergic signaling enhances the intrinsic excitability of NAm neurons and that persistent sodium current is required for this effect. This central balancing mechanism may counteract excessive peripheral cardiac excitation during increased sympathetic tone.

NEW & NOTEWORTHY Cardiac preganglionic cholinergic neurons in the nucleus ambiguus (NAm) are responsible for slowing cardiac pacemaking. This study identified that adrenergic agonists can induce rhythmic action potentials in otherwise quiescent cholinergic NAm preganglionic neurons in brain stem slice preparation. The modulatory influence of adrenaline on central parasympathetic outflow may contribute to both physiological and deleterious cardiovascular regulation.

Descending Influences on Vestibulospinal and Vestibulosympathetic Reflexes

This review considers the integration of vestibular and other signals by the central nervous system pathways that participate in balance control and blood pressure regulation, with an emphasis on how this integration may modify posture-related responses in accordance with behavioral context. Two pathways convey vestibular signals to limb motoneurons: the lateral vestibulospinal tract and reticulospinal projections. Both pathways receive direct inputs from the cerebral cortex and cerebellum, and also integrate vestibular, spinal, and other inputs. Decerebration in animals or strokes that interrupt corticobulbar projections in humans alter the gain of vestibulospinal reflexes and the responses of vestibular nucleus neurons to particular stimuli. This evidence shows that supratentorial regions modify the activity of the vestibular system, but the functional importance of descending influences on vestibulospinal reflexes acting on the limbs is currently unknown. It is often overlooked that the vestibulospinal and reticulospinal systems mainly terminate on spinal interneurons, and not directly on motoneurons, yet little is known about the transformation of vestibular signals that occurs in the spinal cord. Unexpected changes in body position that elicit vestibulospinal reflexes can also produce vestibulosympathetic responses that serve to maintain stable blood pressure. Vestibulosympathetic reflexes are mediated, at least in part, through a specialized group of reticulospinal neurons in the rostral ventrolateral medulla that project to sympathetic preganglionic neurons in the spinal cord. However, other pathways may also contribute to these responses, including those that dually participate in motor control and regulation of sympathetic nervous system activity. Vestibulosympathetic reflexes differ in conscious and decerebrate animals, indicating that supratentorial regions alter these responses. However, as with vestibular reflexes acting on the limbs, little is known about the physiological significance of descending control of vestibulosympathetic pathways.

Glutamate and GABA in Vestibulo-Sympathetic Pathway Neurons

The vestibulo-sympathetic reflex (VSR) actively modulates blood pressure during changes in posture. This reflex allows humans to stand up and quadrupeds to rear or climb without a precipitous decline in cerebral perfusion. The VSR pathway conveys signals from the vestibular end organs to the caudal vestibular nuclei. These cells, in turn, project to pre-sympathetic neurons in the rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively). The present study assessed glutamate- and GABA-related immunofluorescence associated with central vestibular neurons of the VSR pathway in rats. Retrograde FluoroGold tract tracing was used to label vestibular neurons with projections to RVLM or CVLM, and sinusoidal galvanic vestibular stimulation (GVS) was employed to activate these pathways. Central vestibular neurons of the VSR were identified by co-localization of FluoroGold and cFos protein, which accumulates in some vestibular neurons following galvanic stimulation. Triple-label immunofluorescence was used to co-localize glutamate- or GABA- labeling in the identified VSR pathway neurons. Most activated projection neurons displayed intense glutamate immunofluorescence, suggestive of glutamatergic neurotransmission. To support this, anterograde tracer was injected into the caudal vestibular nuclei. Vestibular axons and terminals in RVLM and CVLM co-localized the anterograde tracer and vesicular glutamate transporter-2 signals. Other retrogradely-labeled cFos-positive neurons displayed intense GABA immunofluorescence. VSR pathway neurons of both phenotypes were present in the caudal medial and spinal vestibular nuclei, and projected to both RVLM and CVLM. As a group, however, triple-labeled vestibular cells with intense glutamate immunofluorescence were located more rostrally in the vestibular nuclei than the GABAergic neurons. Only the GABAergic VSR pathway neurons showed a target preference, projecting predominantly to CVLM. These data provide the first demonstration of two disparate chemoanatomic VSR pathways.

Projection neurons of the vestibulo-sympathetic reflex pathway

Changes in head position and posture are detected by the vestibular system and are normally followed by rapid modifications in blood pressure. These compensatory adjustments, which allow humans to stand up without fainting, are mediated by integration of vestibular system pathways with blood pressure control centers in the ventrolateral medulla. Orthostatic hypotension can reflect altered activity of this neural circuitry. Vestibular sensory input to the vestibulo-sympathetic pathway terminates on cells in the vestibular nuclear complex, which in turn project to brainstem sites involved in the regulation of cardiovascular activity, including the rostral and caudal ventrolateral medullary regions (RVLM and CVLM, respectively). In the present study, sinusoidal galvanic vestibular stimulation was used to activate this pathway, and activated neurons were identified through detection of c-Fos protein. The retrograde tracer Fluoro-Gold was injected into the RVLM or CVLM of these animals, and immunofluorescence studies of vestibular neurons were conducted to visualize c-Fos protein and Fluoro-Gold concomitantly. We observed activated projection neurons of the vestibulo-sympathetic reflex pathway in the caudal half of the spinal, medial, and parvocellular medial vestibular nuclei. Approximately two-thirds of the cells were ipsilateral to Fluoro-Gold injection sites in both the RVLM and CVLM, and the remainder were contralateral. As a group, cells projecting to the RVLM were located slightly rostral to those with terminals in the CVLM. Individual activated projection neurons were multipolar, globular, or fusiform in shape. This study provides the first direct demonstration of the central vestibular neurons that mediate the vestibulo-sympathetic reflex.

Vestibulo-sympathetic responses

Evidence accumulated over 30 years, from experiments on animals and human subjects, has conclusively demonstrated that inputs from the vestibular otolith organs contribute to the control of blood pressure during movement and changes in posture. This review considers the effects of gravity on the body axis, and the consequences of postural changes on blood distribution in the body. It then separately considers findings collected in experiments on animals and human subjects demonstrating that the vestibular system regulates blood distribution in the body during movement. Vestibulosympathetic reflexes differ from responses triggered by unloading of cardiovascular receptors such as baroreceptors and cardiopulmonary receptors, as they can be elicited before a change in blood distribution occurs in the body. Dissimilarities in the expression of vestibulosympathetic reflexes in humans and animals are also described. In particular, there is evidence from experiments in animals, but not humans, that vestibulosympathetic reflexes are patterned, and differ between body regions. Results from neurophysiological and neuroanatomical studies in animals are discussed that identify the neurons that mediate vestibulosympathetic responses, which include cells in the caudal aspect of the vestibular nucleus complex, interneurons in the lateral medullary reticular formation, and bulbospinal neurons in the rostral ventrolateral medulla. Recent findings showing that cognition can modify the gain of vestibulosympathetic responses are also presented, and neural pathways that could mediate adaptive plasticity in the responses are proposed, including connections of the posterior cerebellar vermis with the vestibular nuclei and brainstem nuclei that regulate blood pressure.

Cholinergic neurons in the mouse rostral ventrolateral medulla target sensory afferent areas

The rostral ventrolateral medulla (RVLM) primarily regulates respiration and the autonomic nervous system. Its medial portion (mRVLM) contains many choline acetyltransferase (ChAT)-immunoreactive (ir) neurons of unknown function. We sought to clarify the role of these cholinergic cells by tracing their axonal projections. We first established that these neurons are neither parasympathetic preganglionic neurons nor motor neurons because they did not accumulate intraperitoneally administered Fluorogold. We traced their axonal projections by injecting a Cre-dependent vector (floxed-AAV2) expressing either GFP or mCherrry into the mRVLM of ChAT-Cre mice. Transduced neurons expressing GFP or mCherry were confined to the injection site and were exclusively ChAT-ir. Their axonal projections included the dorsal column nuclei, medullary trigeminal complex, cochlear nuclei, superior olivary complex and spinal cord lamina III. For control experiments, the floxed-AAV2 (mCherry) was injected into the RVLM of dopamine beta-hydroxylase-Cre mice. In these mice, mCherry was exclusively expressed by RVLM catecholaminergic neurons. Consistent with data from rats, these catecholaminergic neurons targeted brain regions involved in autonomic and endocrine regulation. These regions were almost totally different from those innervated by the intermingled mRVLM-ChAT neurons. This study emphasizes the advantages of using Cre-driver mouse strains in combination with floxed-AAV2 to trace the axonal projections of chemically defined neuronal groups. Using this technique, we revealed previously unknown projections of mRVLM-ChAT neurons and showed that despite their close proximity to the cardiorespiratory region of the RVLM, these cholinergic neurons regulate sensory afferent information selectively and presumably have little to do with respiration or circulatory control.

Redefining the components of central CO2 chemosensitivity--towards a better understanding of mechanism

Abstract

The field of CO₂ chemosensitivity has developed considerably in recent years. There has been a mounting number of competing nuclei proposed as chemosensitive along with an ever increasing list of potential chemosensory transducing molecules. Is it really possible that all of these areas and candidate molecules are involved in the detection of chemosensory stimuli? How do we discriminate rigorously between molecules that are chemosensory transducers at the head of a physiological reflexversusthose that just happen to display sensitivity to a chemosensory stimulus? Equally, how do we differentiate between nuclei that have a primary chemosensory function, versusthose that are relays in the pathway? We have approached these questions by proposing rigorous definitions for the different components of the chemosensory reflex, going from the salient molecules and ions, through the components of transduction to the identity of chemosensitive cells and chemosensitive nuclei. Our definitions include practical and rigorous experimental tests that can be used to establish the identity of these components. We begin by describing the need for central CO₂ chemosensitivity and the problems that the field has faced. By comparing chemosensory mechanisms to those in the visual system we suggest stricter definitions for the components of the chemosensory pathway. We then, considering these definitions, re-evaluate current knowledge of chemosensory transduction, and propose the ‘multiple salient signal hypothesis’ as a framework for understanding the multiplicity of transduction mechanisms and brain areas seemingly involved in chemosensitivity.

Direct projections from the caudal vestibular nuclei to the ventrolateral medulla in the rat

While the basic pathways mediating vestibulo-ocular, -spinal, and -collic reflexes have been described in detail, little is known about vestibular projections to central autonomic sites. Previous studies have primarily focused on projections from the caudal vestibular region to solitary, vagal and parabrachial nuclei, but have noted a sparse innervation of the ventrolateral medulla. Since a direct pathway from the vestibular nuclei to the rostral ventrolateral medulla would provide a morphological substrate for rapid modifications in blood pressure, heart rate and respiration with changes in posture and locomotion, the present study examined anatomical evidence for this pathway using anterograde and retrograde tract tracing and immunofluorescence detection in brainstem sections of the rat medulla. The results provide anatomical evidence for direct pathways from the caudal vestibular nuclear complex to the rostral and caudal ventrolateral medullary regions. The projections are conveyed by fine and highly varicose axons that ramify bilaterally, with greater terminal densities present ipsilateral to the injection site and more rostrally in the ventrolateral medulla. In the rostral ventrolateral medulla, these processes are highly branched and extremely varicose, primarily directed toward the somata and proximal dendrites of non-catecholaminergic neurons, with minor projections to the distal dendrites of catecholaminergic cells. In the caudal ventrolateral medulla, the axons of vestibular nucleus neurons are more modestly branched with fewer varicosities, and their endings are contiguous with both the perikarya and dendrites of catecholamine-containing neurons. These data suggest that vestibular neurons preferentially target the rostral ventrolateral medulla, and can thereby provide a morphological basis for a short latency vestibulo-sympathetic pathway.

Anatomical observations of the caudal vestibulo-sympathetic pathway

The vestibular system senses the movement and position of the head in space and uses this information to stabilize vision, control posture, perceive head orientation and self-motion in three-dimensional space, and modulate autonomic and limbic activity in response to locomotion and changes in posture. Most vestibular signals are not consciously perceived and are usually appreciated through effector pathways classically described as the vestibulo-ocular, vestibulo-spinal, vestibulo-collic and vestibulo-autonomic reflexes. The present study reviews some of the recent data concerning the connectivity and chemical anatomy of vestibular projections to autonomic sites that are important in the sympathetic control of blood pressure.

Angiotensin-(1-7) antagonist, A-779, microinjection into the caudal ventrolateral medulla of renovascular hypertensive rats restores baroreflex bradycardia

In the present study we evaluated the effect of caudal ventrolateral medulla (CVLM) microinjection of the main angiotensin (Ang) peptides, Ang II and Ang-(1-7), and their selective antagonists on baseline arterial pressure (AP) and on baroreceptor-mediated bradycardia in renovascular hypertensive rats (2K1C). Microinjection of Ang II and Ang-(1-7) into the CVLM of 2K1C rats produced similar decrease in AP as observed in Sham rats. In both Sham and 2K1C, the hypotensive effect of Ang II and Ang-(1-7) at the CVLM was blocked, for up to 30 min, by previous CVLM microinjection of the Ang II AT1 receptor antagonist, Losartan, and Ang-(1-7) Mas antagonist, A-779, respectively. As expected, the baroreflex bradycardia was lower in 2K1C in comparison to Sham rats. CVLM microinjection of A-779 improved the sensitivity of baroreflex bradycardia in 2K1C hypertensive rats. In contrast, Losartan had no effect on the baroreflex bradycardia in either 2K1C or Sham rats. These results suggest that Ang-(1-7) at the CVLM may contribute to the low sensitivity of the baroreflex control of heart rate in renovascular hypertensive rats.

The trigemino-cardiac reflex: an update of the current knowledge

The trigemino-cardiac reflex (TCR) is clinically defined as the sudden onset of parasympathetic activity, sympathetic hypotension, apnea, or gastric hypermotility during central or peripheral stimulation of any of the sensory branches of the trigeminal nerve. Clinically, the TCR has been reported to occur during craniofacial surgery, manipulation of the trigeminal nerve/ganglion and during surgery for lesion in the cerebellopontine angle, cavernous sinus, and the pituitary fossa. Apart from the few clinical reports, the physiologic function of this brainstem reflex has not yet been fully explored. The manifestation of the TCR can vary from bradycardia and hypotension to asystole. From the experimental findings, the TCR represents an expression of a central reflex leading to rapid cerebrovascular vasodilatation generated from excitation of oxygen-sensitive neurons in the rostral ventro-lateral medulla oblongata. By this physiologic response, the systemic and cerebral circulations may be adjusted in a way that augments cerebral perfusion. This review summarizes the current state of knowledge about TCR.

Projections of the lateral reticular nucleus to the cochlear nucleus in rats

The lateral reticular nucleus (LRN) resides in the rostral medulla and caudal pons, is implicated in cardiovascular regulation and cranial nerve reflexes, and gives rise to mossy fibers in the cerebellum. Retrograde tracing data revealed that medium-sized multipolar cells from the magnocellular part of the LRN project to the cochlear nucleus (CN). We sought to characterize the LRN projection to the CN using BDA injections. Anterogradely labeled terminals in the ipsilateral CN appeared as boutons and mossy fibers, and were examined with light and electron microscopy. The terminal field in the CN was restricted to the granule cell domain (GCD), specifically in the superficial layer along the anteroventral CN and in the granule cell lamina. Electron microscopy showed that the smallest LRN boutons formed 1-3 synapses, and as boutons increased in size, they formed correspondingly more synapses. The largest boutons were indistinguishable from the smallest mossy fibers, and the largest mossy fiber exhibited 15 synapses. Synapses were asymmetric with round vesicles and formed against thin dendritic profiles characterized by plentiful microtubules and the presence of fine filopodial extensions that penetrated the ending. These structural features of the postsynaptic target are characteristic of the terminal dendritic claw of granule cells. LRN projections are consistent with known organizational principles of non-auditory inputs to the GCD.

Vestibular neurons in the rat contain imidazoleacetic acid-ribotide, a putative neurotransmitter involved in blood pressure regulation

A substantial body of research has led to the recognition that the vestibular system participates in blood pressure modulation during active movements and changes in posture, and that this modulation is effected at least partly by the caudal vestibular nuclei. The I-4 isomer of imidazoleacetic acid-ribotide (IAA-RP) is a putative neurotransmitter/modulator that is thought to be an endogenous regulator of general sympathetic drive, particularly systemic blood pressure. The present study employed immunofluorescence and light and electron microscopic immunocytochemistry to visualize IAA-RP in the vestibular nuclei of adult male rats. The results demonstrate IAA-RP immunolabeling of subpopulations of vestibular neurons in the descending nucleus and the caudal half of the medial nucleus, with scattered immunostained vestibular neurons also present more rostrally. On the basis of double immunofluorescence staining for IAA-RP and calbindin, many of these ribotide-immunoreactive neurons appear to be innervated by cerebellar Purkinje cell afferents. Ultrastructural observations in the caudal vestibular nuclei confirm the IAA-RP immunolocalization in cell bodies and dendritic processes, and in some myelinated axons and presynaptic boutons. The regional distribution of IAA-RP immunoreactivity corresponds to the location of vestibular neurons involved in autonomic functions. The presence of IAA-RP in those neurons suggests that they participate specifically in vestibulo-autonomic regulation of blood pressure. The localization of immunostain in processes and terminals suggests that vestibulo-autonomic activity is subject to local feedback control. Overall, the observations offer a chemoanatomic basis for understanding the vestibular side effects commonly experienced by patients treated with clonidine and other imidazoline-related drugs.

RVLM glycine receptors mediate GABAA and GABAB)independent sympathoinhibition from CVLM in rats

The caudal ventrolateral medulla (CVLM) provides tonic inhibitory and also excitatory inputs to the rostral ventrolateral medulla (RVLM). These experiments evaluated the role of RVLM gamma-amino butyric acid (GABA) receptor subtypes and glycine receptors in mediating CVLM sympathoinhibition. In Inactin anesthetized female rats, the CVLM and RVLM were functionally defined by pressor and depressor responses to microinjected GABA (500 pmol, 50 nl). Although reduced, pressor and sympathoexcitatory responses due to inhibition of the CVLM with GABA persisted following ipsilateral RVLM GABA(A) receptor blockade (bicuculline, BIC, 400 pmol, 100 nl; n=12) in rats with contralateral nucleus tractus solitarius (NTS) lesion. In the presence of either ipsilateral (+contralateral NTS lesion; n=8) or bilateral (n=6) GABA(A) and GABA(B) receptor blockade of the RVLM (400 pmol BIC+400 pmol CGP35348, 100 nl), inhibition of the CVLM still increased MAP and renal sympathetic nerve activity (RSNA). Thus neither GABA(B) receptors nor a contralateral CVLM to RVLM GABAergic pathway explains residual responses to CVLM blockade. The addition of strychnine (300 pmol, 100 nl) to the RVLM eliminated responses to CVLM inhibition, suggesting that a GABA(A) and GABA(B) independent sympathoinhibitory influence from CVLM to RVLM is mediated by glycine receptors. Decreases in MAP and RSNA due to activation of the CVLM with glutamate (500 pmol, 50 nl) were reversed to increases in the presence of RVLM GABA(A) receptor blockade (n=7). Thus, a sympathoexcitatory pathway from the CVLM can be activated in the presence of RVLM GABA receptor blockade, but sympathoinhibitory influences from the CVLM predominate.

Baroreflex modulation by angiotensins at the rat rostral and caudal ventrolateral medulla

We determined the effect of microinjection of ANG-(1–7) and ANG II into two key regions of the medulla that control the circulation [rostral and caudal ventrolateral medulla (RVLM and CVLM, respectively)] on baroreflex control of heart rate (HR) in anesthetized rats. Reflex bradycardia and tachycardia were induced by increases and decreases in mean arterial pressure produced by intravenous phenylephrine and sodium nitroprusside, respectively. The pressor effects of ANG-(1–7) and ANG II (25 pmol) after RVLM microinjection (11 ± 0.8 and 10 ± 2 mmHg, respectively) were not accompanied by consistent changes in HR. In addition, RVLM microinjection of these angiotensin peptides did not alter the bradycardic or tachycardic component of the baroreflex. CVLM microinjections of ANG-(1–7) and ANG II produced hypotension (−11 ± 1.5 and −11 ± 1.9 mmHg, respectively) that was similarly not accompanied by significant changes in HR. However, CVLM microinjections of angiotensins induced differential changes in the baroreflex control of HR. ANG-(1–7) attenuated the baroreflex bradycardia (0.26 ± 0.06 ms/mmHg vs. 0.42 ± 0.08 ms/mmHg before treatment) and facilitated the baroreflex tachycardia (0.86 ± 0.19 ms/mmHg vs. 0.42 ± 0.10 ms/mmHg before treatment); ANG II produced the opposite effect, attenuating baroreflex tachycardia (0.09 ± 0.06 ms/mmHg vs. 0.31 ± 0.07 ms/mmHg before treatment) and facilitating the baroreflex bradycardia (0.67 ± 0.16 ms/mmHg vs. 0.41 ± 0.05 ms/mmHg before treatment). The modulatory effect of ANG II and ANG-(1–7) on baroreflex sensitivity was completely abolished by peripheral administration of methylatropine. These results suggest that ANG II and ANG-(1–7) at the CVLM produce a differential modulation of the baroreflex control of HR, probably through distinct effects on the parasympathetic drive to the heart.

Adenosine A2A receptors are expressed by GABAergic neurons of medulla oblongata in developing rat

During early development, adenosine contributes to the occurrence of respiratory depression and recurrent apneas. Recent physiological studies indicate that GABAergic mechanisms may be involved in this inhibitory action of adenosine, via their A(2A) receptors. In the present study, in situ hybridization with ribonucleotide probes for A(2A) receptor (A(2A)R) mRNA was combined with the immunolabeling technique for parvalbumin and transneuronal retrograde tracing method using green fluorescent protein expressing pseudorabies virus (GFP-PRV) to (1) characterize age-dependent changes in the expression of adenosine A(2A)Rs mRNA in brain stem regions where GABAergic neurons are located; (2) determine whether GABA-containing neurons express A(2A)R mRNA traits, and (3) identify whether bulbospinal GABAergic neurons projecting to phrenic nuclei contain A(2A)R mRNA. Results revealed expression of A(2A) receptors in regions of medulla oblongata containing GABAergic neurons, namely in the ventral aspect of the medulla, within the Bötzinger region and caudal to it, the gigantocellular reticular nucleus, midline neurons and the caudal ventrolateral medulla oblongata. Furthermore, a subpopulation of identified GABAergic neurons, projecting to the phrenic motor nuclei, possess A(2A)R mRNA. It is concluded that adenosine A(2A)Rs expressed by GABAergic neurons are likely to play a role in mediating adenosine-induced respiratory depression.

Cardiovascular modules in the cerebellum

Mapping with local lesions, electrical or chemical stimulation, or recording evoked field potentials or unit spikes revealed localized representations of cardiovascular functions in the cerebellum. In this review, which is based on literatures in the field (including our own publications), I propose that the cerebellum contains five distinct modules (cerebellar corticonuclear microcomplexes) dedicated to cardiovascular control. First, a discrete rostral portion of the fastigial nucleus and the overlying medial portion of the anterior vermis (lobules I, II and III) conjointly form a module that controls the baroreflex. Second, anterior vermis also forms a microcomplex with the parabrachial nucleus. Third, a discrete caudal portion of the fastigial nucleus and the overlying medial portion of the posterior vermis (lobules VII and VIII) form another module controlling the vestibulosympathetic reflex. Fourth, the medial portion of the uvula may form a module with the nucleus tractus solitarius and parabrachial nucleus. Fifth, the lateral edge of the nodulus and the uvula, together with the parabrachial nucleus and vestibular nuclei, forms a cardiovascular microcomplex that controls the magnitude and/or timing of sympathetic nerve responses and stability of the mean arterial blood pressure during changes of head position and body posture. The lateral nodulus-uvula appears to be an integrative cardiovascular control center involving both the baroreflex and the vestibulosympathetic reflex.

Adenosine A2A receptors interact with GABAergic pathways to modulate respiration in neonatal piglets

GABA and adenosine contribute to respiratory inhibition in early postnatal life. In this study the adenosine A2A receptor agonist CGS21680 was used to evaluate adenosine receptor specificity and the interrelation of adenosine and GABA in the inhibition of inspiratory drive. In neonatal piglets (n = 10), CGS21680 was injected into the fourth ventricle resulting in apnea and/or decreased burst area and frequency of phrenic discharge. Phrenic burst area decreased to 58.9 +/- 8.6% (S.E.M.) after CGS21680 injection (control = 91.8 +/- 1.0%). Expiratory time increased 261.0 +/- 59.9% after CGS21680 from control (87.7 +/- 2.7%). When bicuculline was injected locally within the rostral ventrolateral medulla (n = 5), or into the fourth ventricle (n = 5), the CGS21680 induced inhibition of phrenic was abolished. To define expression of A2A receptor at the message level (mRNA), we employed in situ hybridization with a digoxigenin-coupled oligonucleotide. Adenosine A2A receptor mRNA was expressed in regions of the medulla oblongata known to contain GABAergic neurons. We conclude that GABAergic inputs affecting respiratory timing and inspiratory drive are modulated by activation of A2A receptors. These findings offer new insight into the mechanism whereby xanthine therapy diminishes apnea of prematurity.

Projections from facial nucleus interneurons to the respiratory groups of brainstem in the rat

Projections of the interneurons of the facial nucleus to respiration-related areas in the brainstem of the rat were revealed by unilateral, iontophorectic injection of the anterograde neuronal tracer, Phaseolus vulgaris leucoagglutinin (PHA-L), into the facial nucleus after motor neurons degeneration had been induced by axotomy of the facial nerve. Anterogradely labeled fibers and terminals were found bilaterally in the nucleus tractus solitarius, nucleus ambiguous and ventrolateral reticular formation in the medulla oblongata and ipsilaterally in the nucleus parabrachialis and nucleus Kolliker-Fuse in the pons. These results revealed that the interneurons in the facial nucleus have widespread projections to the respiratory groups in the brainstem, and suggest that the facial nucleus is involved not only in the control of muscles for facial expression but also in the regulation of functional respiratory activity.

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.

Differentiation of lamina I spinomedullary and spinothalamic neurons in the cat

We characterized spinomedullary neurons that project to the ventrolateral portion of the medulla that receives lamina I terminations in two sets of experiments in the cat. First, their distribution was examined using single unilateral iontophoretic injections of cholera toxin subunit B. The injection sites were characterized by microelectrode recordings from nociceptive- and thermoreceptive-specific units, indicative of lamina I input. The spinomedullary neurons were symmetrically distributed bilaterally, predominantly (63-69%) in lamina I but also in laminae V-VIII and the thoracic lateral horn (intermediolateral cell column). In horizontal sections, spinomedullary lamina I neurons included all three main morphological types described earlier. Second, spinomedullary and spinothalamic neurons were compared in retrograde double-labeling experiments. Different combinations of tracers were injected in the right thalamus and the left or right ventrolateral medulla (guided by recordings). The numbers of spinomedullary and spinothalamic neurons on the left side were comparable, and the segmental and laminar distributions were similar, except that a greater proportion of spinomedullary neurons originated from thoracic segments. However, the proportion of double-labeled neurons was consistently approximately 1%, indicating that spinomedullary and spinothalamic pathways arise from separate subpopulations. Spinomedullary neurons were more ventrally located within lamina I than spinothalamic neurons. A significantly greater proportion of spinomedullary neurons had fusiform somata (49% vs. 36%). These observations indicate that lamina I is the major source of spinal input to this portion of the ventrolateral medulla, that the projection includes several morphological types of inputs, and that this projection is distinct from the spinothalamic projection. These findings are consistent with the concept that lamina I projections constitute an ascending homeostatic afferent pathway relating the physiological condition of the body.

Autonomic nervous system activity associated with postural disturbances in patients with perilymphatic fistula: sympathetic or vagal origin?

The study focused on patients suffering from perilymphatic fistula (PLF), whether they had undergone surgery or not. Vestibular disturbances can be harmless but are associated with varying symptoms, demonstrating disorders within the autonomic nervous system (ANS). The aim was to test whether the orthosympathetic is involved as the vagal part is often suspected of eliciting a feeling of sickness. Non-invasive and uninterrupted recording of ANS activity represents an objectivation technique to evidence such disturbances. Electrodermal activity, thermovascular variables, instantaneous cardiac rate and blood pressure were recorded. Discomfort was triggered experimentally by applying various stimulations successively to the intact ear then to the PLF (or operated) side. Twelve subjects took part in the experiment. Two types of ANS activity were distinguished: (1) phasic responses during stimulation and (2) tonic evolution thereafter. Results show strong activation in orthosympathetic variables when the PLF side was stimulated. No further significant difference between the two sides was to be observed following surgery. After stimulation, a sudden increase in skin resistance was observed, associated with slight bradycardia. No vagal signs having been evidenced, actual nausea may result from brief inhibition of sympathetic activation resulting, in turn from primary over-activation of this system.

Trigemino-autonomic connections in the muskrat: the neural substrate for the diving response

Stimulation of the anterior ethmoidal nerve of the muskrat produces a cardiorespiratory depression similar to the diving response. This includes an apnea, a parasympathetic bradycardia, and a selective increase in sympathetic vascular tone. However, the brainstem circuitry that links the afferent stimulus to the efferent autonomic responses is unknown. We used the anterograde transneuronal transport of the herpes simplex virus (HSV-1), strain 129, after its injection into the anterior ethmoidal nerve to determine the primary, secondary, and tertiary brainstem relays responsible for this cardiorespiratory response. In an effort to check the validity of this relatively untested tracer, we also injected the medullary dorsal horn with biotinylated dextran amine to determine the secondary trigemino-autonomic projections. Approximately 1 microl (6x10(6) PFU) of the HSV-1 virus was injected directly into the anterior ethmoidal nerve of muskrats. After 2-6 days, their trigeminal ganglions, spinal cords and brainstems were cut and immunohistologically processed for HSV-1. Initially (2 days), HSV-1 was observed only in the trigeminal ganglion. After approximately 3 days, HSV-1 was observed first in many brainstem areas optimally labeled between 4 and 4.5 days. In these cases, the ventrolateral superficial medullary dorsal horn, the ventral paratrigeminal nucleus and the interface between the interpolar and caudal subnuclei were labeled ipsilaterally. The nucleus tractus solitarius (NTS), especially its ventrolateral, dorsolateral, and commissural subnuclei were labeled as well as the caudal, intermediate and rostral ventrolateral medulla. Within the pons, the superior salivatory nucleus, the A5 area, the ventrolateral part of the parabrachial nucleus and the Kölliker-Fuse nucleus were labeled. Only after a survival of 4 days or more, the locus coeruleus, the nucleus raphe magnus, the nucleus paragigantocellularis, pars alpha, and the pontine raphe nucleus were labeled. Injections of biotinylated dextran amine were made into the medullary dorsal horn (MDH) in a location similar to that labeled after the viral injections. Fine fibers and terminals were labeled in the same brainstem areas labeled after injections of HSV-1 into the anterior ethmoidal nerve. This study outlines the potential brainstem circuit for the diving response, the most powerful autonomic reflex known. It also confirms the efficacy for using HSV-1, strain 129, as an anterograde transneuronal transport method.

Correlation of vasomotor- and respiratory-controlling mechanisms around the caudal ventrolateral medulla in cats

We examined the involvement of caudal ventrolateral medulla (CVLM) in respiratory control. Microinjection of glutamate (Glu) into CVLM decreased systemic arterial blood pressure (SAP) and altered phrenic nerve activities (PNA). Among 143 depressor sites, 55% (78/143) increased respiratory frequency (Rf), while 72% altered PNA amplitude (36% increased and 36% decreased). A small but significant positive correlation was observed between the magnitudes of depressor responses and inhibition of PNA amplitude (r = 0.1718, n = 143), indicating a substantial cross-talk between depressor and PNA inhibitory neurons. Furthermore, microinjections of acetylcholine (ACh) mimicked the Glu-induced depressor responses. However, ACh did not alter Rf, but still reduced PNA amplitude. Our findings suggest that Rf-regulating and depressor neurons are two separate neuronal populations, coexisting in CVLM. The PNA inhibitory and depressor neurons, in contrast, could have stronger correlation.

Forebrain pathways mediating stress-induced hormone secretion

Exposure to hostile conditions initiates the secretion of several hormones, including corticosterone/cortisol, catecholamines, prolactin, oxytocin, and renin, as part of the survival mechanism. Such conditions are often referred to as "stressors" and can be divided into three categories: external conditions resulting in pain or discomfort, internal homeostatic disturbances, and learned or associative responses to the perception of impending endangerment, pain, or discomfort ("psychological stress"). The hormones released in response to stressors often are referred to as "stress hormones" and their secretion is regulated by neural circuits impinging on hypothalamic neurons that are the final output toward the pituitary gland and the kidneys. This review discusses the forebrain circuits that mediate the neuroendocrine responses to stressors and emphasizes those neuroendocrine systems that have previously received little attention as stress-sensitive hormones: renin, oxytocin, and prolactin. Anxiolytic drugs of the benzodiazepine class and other drugs that affect catecholamine, GABAA, histamine, and serotonin receptors alter the neuroendocrine stress response. The effects of these drugs are discussed in relation to their effects on forebrain neural circuits that regulate stress hormone secretion. For psychological stressors such as conditioned fear, the neural circuits mediating neuroendocrine responses involve cortical activation of the basolateral amygdala, which in turn activates the central nucleus of the amygdala. The central amygdala then activates hypothalamic neurons directly, indirectly through the bed nucleus of the stria terminalis, and/or possibly via circuits involving brainstem serotonergic and catecholaminergic neurons. The renin response to psychological stress, in contrast to those of ACTH and prolactin, is not mediated by the bed nucleus of the stria terminalis and is not suppressed by benzodiazepine anxiolytics. Stressors that challenge cardiovascular homeostasis, such as hemorrhage, trigger a pattern of neuroendocrine responses that is similar to that observed in response to psychological stressors. These neuroendocrine responses are initiated by afferent signals from cardiovascular receptors which synapse in the medulla oblongata and are relayed either directly or indirectly to hypothalamic neurons controlling ACTH, prolactin, and oxytocin release. In contrast, forebrain pathways may not be essential for the renin response to hemorrhage. Thus current evidence indicates that although a diverse group of stressors initiate similar increases in ACTH, renin, prolactin, and oxytocin, the specific neural circuits and neurotransmitter systems involved in these responses differ for each neuroendocrine system and stressor category.

Regional and functional differences in the distribution of vestibulosympathetic reflexes

Although considerable evidence suggests that the vestibular system regulates sympathetic outflow during movement and changes in posture, little is known about relative vestibular influences on activity of different sympathetic nerves and sympathetic efferents with different functions. In the present study, we demonstrated that electrical stimulation of the vestibular nerve in the cat elicited responses in sympathetic nerves innervating the head and abdominal viscera. This observation suggests that activity of sympathetic efferents innervating multiple body regions is affected by vestibular signals. These responses were attenuated by >80% when blood pressure was increased to >160 mmHg. Because raising blood pressure decreases the responsiveness of vasoconstrictor fibers, the simplest explanation for these data is that the vestibular system provides particularly strong inputs to components of the sympathetic nervous system that regulate peripheral vascular resistance. Furthermore, the relative magnitude of vestibulosympathetic reflexes was over four times larger in one sympathetic nerve composed mainly of vasoconstrictor efferents (renal nerve) than another nerve (external carotid nerve) containing similar types of fibers. Collectively, these data indicate that the vestibular system has selective influences on sympathetic outflow to particular tissues and body regions.

Role of nucleus retroambigualis in respiratory reflexes evoked by superior laryngeal and vestibular nerve afferents and in emesis

An ascending projection from the medullary nucleus retroambigualis (NRA) has recently been described as important for the control of the upper airway during vocalization. We evaluated the importance of this projection in other behaviors by making localized injections of the neurotoxin kainic acid in the NRA in decerebrate cats, most of which were paralyzed and artificially ventilated. In contrast to its importance for vocalization, the NRA is not essential for activation of upper airway musculature during respiration, swallowing, vomiting, or reflexes elicited by superior laryngeal or vestibular nerve afferents. However, kainic acid injections in the NRA and adjacent reticular formation prolonged the inhibitory phrenic motoneuronal response to superior laryngeal nerve stimulation and abolished or reduced abdominal motoneuronal responses during respiration, vomiting, and superior laryngeal nerve stimulation. Thus, of the behaviors we investigated, the importance of the ascending projection from the NRA appears to be limited to vocalization, while descending projections from the NRA region are important in a number of behaviors.