Direct and indirect pathways to lamina I in the medulla oblongata and spinal cord of the cat

A computationally informed distinction of interoception and exteroception

While interoception is of major neuroscientific interest, its precise definition and delineation from exteroception continue to be debated. Here, we propose a functional distinction between interoception and exteroception based on computational concepts of sensor-effector loops. Under this view, the classification of sensory inputs as serving interoception or exteroception depends on the sensor-effector loop they feed into, for the control of either bodily (physiological and biochemical) or environmental states. We explain the utility of this perspective by examining the perception of skin temperature, one of the most challenging cases for distinguishing between interoception and exteroception. Specifically, we propose conceptualising thermoception as inference about the thermal state of the body (including the skin), which is directly coupled to thermoregulatory processes. This functional view emphasises the coupling to regulation (control) as a defining property of perception (inference) and connects the definition of interoception to contemporary computational theories of brain-body interactions.

Microstimulation in Different Parts of the Periaqueductal Gray Generates Different Types of Vocalizations in the Cat

In the cat four different types of vocalization, mews, howls, cries, and hisses were generated by microstimulation in different parts of the periaqueductal gray (PAG). While mews imply positive vocal expressions, howls, hisses, and cries represent negative vocal expressions. In the intermediate PAG, mews were generated in the lateral column, howls, and hisses in the ventrolateral column. Cries were generated in two other regions, the lateral column of the rostral PAG and the ventrolateral column of the caudal PAG. In order to define the specific motor patterns of the mews, howls, and cries, the following muscles were recorded during these vocalizations; larynx (cricothyroid, thyroarytenoid, and posterior cricoarytenoid), tongue (genioglossus), jaw (digastric), and respiration muscles (diaphragm, internal intercostal, external, and internal abdominal oblique). During these mews, howls, and cries we analyzed the frequency, intensity, activation cascades power density, turns, and amplitude analysis of the electromyograms (EMGs). It appeared that each type of vocalization consists of a specific circumscribed motor coordination. The nucleus retroambiguus (NRA) in the caudal medulla is known to serve as the final premotor interneuronal output system for vocalization. Although neurochemical microstimulation in the NRA itself also generated vocalizations, they only consisted of guttural sounds, the EMGs of which involved only small parts of the EMGs of the mews, howls, and cries generated by neurochemical stimulation in the PAG. These results demonstrate that positive and negative vocalizations are generated in different parts of the PAG. These parts have access to different groups of premotoneurons in the NRA, that, in turn, have access to different groups of motoneurons in the brainstem and spinal cord, resulting in different vocalizations. The findings would serve a valuable model for diagnostic assessment of voice disorders in humans.

Central neural substrates involved in temperature discrimination, thermal pain, thermal comfort, and thermoregulatory behavior

A phylogenetically novel pathway that emerged with primate encephalization is described, which conveys high-fidelity cutaneous thermosensory activity in "labeled lines" to a somatotopic map in the dorsal posterior insular cortex. It originates in lamina I of the superficial dorsal horn and ascends by way of the lateral spinothalamic tract and a distinct region in posterolateral thalamus. It evolved from the homeostatic sensory activity that represents the physiologic (interoceptive) condition of the body and drives the central autonomic network, which underlies all affective feelings from the body. Accordingly, human discriminative thermal sensations are accompanied by thermally motivated behaviors and thermal feelings of comfort or discomfort (unless neutral), which evidence suggests are associated with activity in the insular, cingulate, and orbitofrontal cortices, respectively. Yet, the substrates for thermoregulatory behavior have not been established, and several strong candidates (including the hypothalamus and the bed nucleus of the stria terminalis) are discussed. Finally, the neural underpinnings for relationships between thermal affect and social feelings (warm-positive/cold-negative) are addressed, including the association of hyperthermia with clinical depression.

Ventromedial Hypothalamus and the Generation of Aggression

Aggression is a costly behavior, sometimes with severe consequences including death. Yet aggression is prevalent across animal species ranging from insects to humans, demonstrating its essential role in the survival of individuals and groups. The question of how the brain decides when to generate this costly behavior has intrigued neuroscientists for over a century and has led to the identification of relevant neural substrates. Various lesion and electric stimulation experiments have revealed that the hypothalamus, an ancient structure situated deep in the brain, is essential for expressing aggressive behaviors. More recently, studies using precise circuit manipulation tools have identified a small subnucleus in the medial hypothalamus, the ventrolateral part of the ventromedial hypothalamus (VMHvl), as a key structure for driving both aggression and aggression-seeking behaviors. Here, we provide an updated summary of the evidence that supports a role of the VMHvl in aggressive behaviors. We will consider our recent findings detailing the physiological response properties of populations of VMHvl cells during aggressive behaviors and provide new understanding regarding the role of the VMHvl embedded within the larger whole-brain circuit for social sensation and action.

Highway to thermosensation: a traced review, from the proteins to the brain

Temperature maintenance and detection are essential for the survival and perpetuation of any species. This review is focused on thermosensation; thus a detailed and traced explanation of the anatomical and physiological characteristics of each component of this sensation is given. First, the proteins that react to temperature changes are identified; next, the nature of the neurons involved in thermosensation is described; and then, the pathways from the skin through the spinal cord to the brain are outlined. Finally, the areas of the brain and their interconnections where thermoperception arises are explained. Transduction of the external and internal temperature information is essentially mediated by the transient receptor potential ion channels (TRPs). These proteins are embedded in the neurons' membrane and they hyper- or de-polarize neurons in function of the intrinsic voltage and the temperature changes. There are distinct TRP sensors for different temperature ranges. Interestingly, the primary afferent neurons have either cold or hot receptors, so they are dedicated separately to cold or hot sensation. The information is transmitted by different pathways from the skin to the brain, where it either remains separated or is integrated to generate a response. It seems that both the determination of how thermoperception is produced and how we interact with the world are dependent on the particular arrangement and nature of the components, the way of transduction of information and the communication between these elements.

The neural circuits of mating and fighting in male mice

Tinbergen proposed that instinctive behaviors can be divided into appetitive and consummatory phases. During mating and aggression, the appetitive phase contains various actions to bring an animal to a social target and the consummatory phase allows stereotyped actions to take place. Here, we summarize recent advances in elucidating the neural circuits underlying the appetitive and consummatory phases of sexual and aggressive behaviors with a focus on male mice. We outline the role of the main olfactory inputs in the initiation of social approach; the engagement of the accessory olfactory system during social investigation, and the role of the hypothalamus and its downstream pathways in orchestrating social behaviors through a suite of motor actions.

Motor organization of positive and negative emotional vocalization in the cat midbrain periaqueductal gray

Neurochemical microstimulation in different parts of the midbrain periaqueductal gray (PAG) in the cat generates four different types of vocalization, mews, howls, cries, and hisses. Mews signify positive vocal expression, whereas howls, hisses, and cries signify negative vocal communications. Mews were generated in the lateral column of the intermediate PAG and howls and hisses in the ventrolateral column of the intermediate PAG. Cries were generated in two regions, the lateral column of the rostral PAG and the ventrolateral column of the caudal PAG. To define the specific motor patterns belonging to mews, howls, and cries, the following muscles were recorded during these vocalizations: larynx (cricothyroid, thyroarytenoid, and posterior cricoarytenoid), tongue (genioglossus), jaw (digastric), and respiration (diaphragm, internal intercostal, external abdominal oblique, and internal abdominal oblique) muscles. Furthermore, the frequency, intensity, activation cascades, and turns and amplitude analyses of the electromyograms (EMGs) during these vocalizations were analyzed. The results show that each type of vocalization consists of a specific, circumscribed motor coordination. The nucleus retroambiguus (NRA) in the caudal medulla serves as the final premotor interneuronal output system for vocalization. NRA neurochemical microstimulation also generated vocalizations (guttural sounds). Analysis of the EMGs demonstrated that these vocalizations consist of only small parts of the emotional voalizations generated by neurochemical stimulation in the PAG. These results demonstrate that motor organization of positive and negative emotional vocal expressions are segregated in the PAG and that the PAG uses the NRA as a tool to gain access to the motoneurons generating vocalization.

Central neuropathic pain in MS is due to distinct thoracic spinal cord lesions

Objective

To determine a neuro-anatomic cause for central neuropathic pain (CNP) observed in multiple sclerosis (MS) patients.

Methods

Parallel clinical and neuro-anatomical studies were performed. A clinical investigation of consecutively acquired MS patients with and without CNP (i.e. cold allodynia or deep hyperesthesia) within a single MS center was pursued. A multivariate logistic regression model was used to assess the relationship between an upper central thoracic spinal cord focus to central pain complaints. To identify the hypothesized autonomic interneurons with bilateral descending projections to lumbosacral sensory neurons, retrograde single- and double-labeling experiments with CTb and fluorescent tracers were performed in three animal species (i.e. rat, cat, and monkey).

Results

Clinical data were available in MS patients with (n = 32; F:23; median age: 34.6 years (interquartile range [IQR]: 27.4–45.5)) and without (n = 30; F:22; median age: 36.6 years [IQR: 31.6–47.1]) CNP. The value of a central focus between T1–T6 in relation to CNP demonstrated a sensitivity of 96.9% (95% confidence interval [CI]: 83.8–99.9) and specificity of 83.3% (95% CI: 65.3–94.4). A significant relationship between CNP and a centrally located focus within the thoracic spine was also observed (odds ratio [OR]: 155.0 [95% CI lower limit: 16.0]; P < 0.0001, two-tailed Fisher exact test). In all animal models, neurons with bilateral descending projections to the lumbosacral superficial dorsal horn were concentrated in the autonomic intermediomedial nucleus surrounding the mid-thoracic central canal.

Interpretation

Our observations provide the first evidence for the etiology of CNP. These data may assist with the development of refined symptomatic therapies and allow for insights into unique pain syndromes observed in other demyelinating subtypes.

The periaqueductal gray controls brainstem emotional motor systems including respiration

Respiration is a motor system essential for the survival of the individual and of the species. Because of its vital significance, studies on respiration often assume that breathing takes place independent of other motor systems. However, motor systems generating vocalization, coughing, sneezing, vomiting, as well as parturition, ejaculation, and defecation encompass abdominal pressure control, which involves changes in the respiratory pattern. The mesencephalic periaqueductal gray (PAG) controls all these motor systems. It determines the level setting of the whole body by means of its very strong projections to the ventromedial medullary tegmentum, but it also controls the cell groups that generate vocalization, coughing, sneezing, vomiting, as well as respiration. For this control, the PAG maintains very strong connections with the nucleus retroambiguus, which enables it to control abdominal and intrathoracic pressure. In this same context, the PAG also runs the pelvic organs, bladder, uterus, prostate, seminal vesicles, and the distal colon and rectum via its projections to the pelvic organ stimulating center and the pelvic floor stimulating center. These cell groups, via long descending projections, have direct control of the parasympathetic motoneurons in the sacral cord as well as of the somatic motoneurons in the nucleus of Onuf, innervating the pelvic floor. Respiration, therefore, is not a motor system that functions by itself, but is strongly regulated by the same systems that also control the other motor output systems.

The cerebellum and pain: passive integrator or active participator?

The cerebellum is classically considered to be a brain region involved in motor processing, but it has also been implicated in non-motor, and even cognitive, functions. Though previous research suggests that the cerebellum responds to noxious stimuli, its specific role during pain is unclear. Pain is a multidimensional experience that encompasses sensory discriminative, affective motivational, and cognitive evaluative components. Cerebellar involvement during the processing of pain could thus potentially reflect a number of different functional processes. This review will summarize the animal and human research to date that indicates that (1) primary afferents conduct nociceptive (noxious) input to the cerebellum, (2) electrical and pharmacological stimulation of the cerebellum can modulate nociceptive processing, and (3) cerebellar activity occurs during the presence of acute and chronic pain. Possible functional roles for the cerebellum relating to pain will be considered, including perspectives relating to emotion, cognition, and motor control in response to pain.

The periaqueductal grey modulates sensory input to the cerebellum: a role in coping behaviour?

The paths that link the periaqueductal grey (PAG) to hindbrain motor circuits underlying changes in behavioural responsiveness to external stimuli are unknown. A major candidate structure for mediating these effects is the cerebellum. The present experiments test this directly by monitoring changes in size of cerebellar responses evoked by peripheral stimuli following activation of the PAG. In 22 anaesthetized adult Wistar rats, climbing fibre field potentials were recorded from the C1 zone in the paramedian lobule and the copula pyramidis of the cerebellar cortex evoked, respectively, by electrical stimulation of the ipsilateral fore- and hindlimb. An initial and a late response were attributable to activation of Abeta and Adelta peripheral afferents respectively (hindlimb onset latencies 16.9 and 23.8 ms). Chemical stimulation at physiologically-identified sites in the ventrolateral PAG (a region known to be associated with hyporeactive immobility) resulted in a significant reduction in size of both the Abeta and Adelta evoked field potentials (mean reduction relative to control +/- SEM, 59 +/- 7.5 and 66 +/- 11.9% respectively). Responses evoked by electrical stimulation of the dorsal or ventral funiculus of the spinal cord were also reduced by PAG stimulation, suggesting that part of the modulation may occur at supraspinal sites (including at the level of the inferior olive). Overall, the results provide novel evidence of descending control into motor control centres, and provide the basis for future studies into the role of the PAG in regulating motor activity in different behavioural states and in chronic pain.

Interoception: the sense of the physiological condition of the body

Converging evidence indicates that primates have a distinct cortical image of homeostatic afferent activity that reflects all aspects of the physiological condition of all tissues of the body. This interoceptive system, associated with autonomic motor control, is distinct from the exteroceptive system (cutaneous mechanoreception and proprioception) that guides somatic motor activity. The primary interoceptive representation in the dorsal posterior insula engenders distinct highly resolved feelings from the body that include pain, temperature, itch, sensual touch, muscular and visceral sensations, vasomotor activity, hunger, thirst, and 'air hunger'. In humans, a meta-representation of the primary interoceptive activity is engendered in the right anterior insula, which seems to provide the basis for the subjective image of the material self as a feeling (sentient) entity, that is, emotional awareness.

How do you feel? Interoception: the sense of the physiological condition of the body

As humans, we perceive feelings from our bodies that relate our state of well-being, our energy and stress levels, our mood and disposition. How do we have these feelings? What neural processes do they represent? Recent functional anatomical work has detailed an afferent neural system in primates and in humans that represents all aspects of the physiological condition of the physical body. This system constitutes a representation of 'the material me', and might provide a foundation for subjective feelings, emotion and self-awareness.

Neuropeptide expression in rat paraventricular hypothalamic neurons that project to the spinal cord

The paraventricular hypothalamic nucleus (PVH) exerts many of its regulatory functions through projections to spinal cord neurons that control autonomic and sensory functions. By using in situ hybridization histochemistry in combination with retrograde tract tracing, we analyzed the peptide expression among neurons in the rat PVH that send axons to the spinal cord. Projection neurons were labeled by immunohistochemical detection of retrogradely transported cholera toxin subunit B, and radiolabeled long riboprobes were used to identify neurons containing dynorphin, enkephalin, or oxytocin mRNA. Of the spinally projecting neurons in the PVH, approximately 40% expressed dynorphin mRNA, 40% expressed oxytocin mRNA, and 20% expressed enkephalin mRNA. Taken together with our previous findings on the distribution of vasopressin-expressing neurons in the PVH (Hallbeck and Blomqvist [1999] J. Comp. Neurol. 411:201-211), the results demonstrated that the different PVH subdivisions display distinct peptide expression patterns among the spinal cord-projecting neurons. Thus, the lateral parvocellular subdivision contained large numbers of spinal cord-projecting neurons that express any of the four investigated peptides, whereas the ventral part of the medial parvocellular subdivision displayed a strong preponderance for dynorphin- and vasopressin-expressing cells. The dorsal parvocellular subdivision almost exclusively contained dynorphin- and oxytocin-expressing spinal cord-projecting neurons. This parcellation of the peptide-expressing neurons suggested a functional diversity among the spinal cord-projecting subdivisions of the PVH that provide an anatomic basis for its various and distinct influences on autonomic and sensory processing at the spinal level.

Stimulation of the mesencephalic locomotor region inhibits the discharge of neurons in the superficial laminae of the dorsal horn of cats

In decerebrate cats, we found that stimulation of the mesencephalic locomotor region (MLR) attenuated the responses of neurons in the superficial laminae of the dorsal horn to thin fiber muscle afferent input. The attenuation appeared to be more effective for group III afferent input than for group IV. These findings may shed light on the interaction between central command, (i.e. the MLR) and the muscle reflex, mechanisms which both contribute to the cardiovascular responses to exercise.

Periaqueductal gray matter projections to midline and intralaminar thalamic nuclei of the rat

The periaqueductal gray matter (PAG) projections to the intralaminar and midline thalamic nuclei were examined in rats. Phaseolus vulgaris-leucoagglutinin (PHA-L) was injected in discrete regions of the PAG, and axonal labeling was examined in the thalamus. PHA-L was also placed into the dorsal raphe nuclei or nucleus of Darkschewitsch and interstitial nucleus of Cajal as controls. In a separate group of rats, the retrograde tracer cholera toxin beta-subunit (CTb) was injected into one of the intralaminar thalamic nuclei-lateral parafascicular, medial parafascicular, central lateral (CL), paracentral (PC), or central medial nucleus-or one of the midline thalamic nuclei-paraventricular (PVT), intermediodorsal (IMD), mediodorsal, paratenial, rhomboid (Rh), reuniens (Re), or caudal ventral medial (VMc) nucleus. The distribution of CTb labeled neurons in the PAG was then mapped. All PAG regions (the four columns of the caudal two-thirds of the PAG plus rostral PAG) and the precommissural nucleus projected to the rostral PVT, IMD, and CL. The ventrolateral, lateral, and rostral PAG provided additional inputs to most of the other intralaminar and midline thalamic nuclei. PAG inputs to the VMc originated from the rostral and ventrolateral PAG areas. In addition, the lateral and rostral PAG projected to the zona incerta. No evidence was found for a PAG input to the ventroposterior lateral parvicellular, ventroposterior medial parvicellular, caudal PC, oval paracentral, and reticular thalamic nuclei. PAG --> thalamic circuits may modulate autonomic-, nociceptive-, and behavior-related forebrain circuits associated with defense and emotional responses.

Pain modulation: expectation, opioid analgesia and virtual pain

To summarize, although there are multiple potential target nuclei for modulating pain transmission and several candidate efferent pathways that exert modulatory control, the most completely described pain modulating circuit includes the amygdala, PAG, DLPT and RVM in the brainstem. Through descending projections, this circuit controls both spinal and trigeminal dorsal horn pain transmission neurons and mediates both opioid and stimulation produced analgesia. Several different neurotransmitters are involved in the modulatory actions of this circuit, which exerts bi-directional control of pain through On cells that facilitate and Off cells that inhibit dorsal horn nociceptive neurons. There is evidence that this circuit contributes to analgesia in humans and may be activated by acute stress or the expectation of relief. Conversely, through the facilitating effect of On cells, this circuit is theoretically capable of generating or enhancing perceived pain intensity. Such an effect could provide a physiological mechanism for the pain enhancing actions of mood, attention and expectation.

Presynaptic and postsynaptic relations of mu-opioid receptors to gamma-aminobutyric acid-immunoreactive and medullary-projecting periaqueductal gray neurons

The ventrolateral portion of the periaqueductal gray (PAG) is one brain region in which ligands of the mu-opioid receptor (MOR) produce analgesia. In the PAG, MOR ligands are thought to act primarily on inhibitory [e.g., gamma-aminobutyric acidergic (GABAergic)] neurons to disinhibit PAG output rather than directly on medullary-projecting PAG neurons. In this study, the ultrastructural localization of MOR immunolabeling was examined with respect to either GABAergic PAG neurons or PAG projection neurons that were labeled retrogradely from the rostral ventromedial medulla. Immunoreactivity for MOR and GABA often coexisted within dendrites. Dual-labeled profiles accounted for subpopulations of dendrites containing immunoreactivity for either MOR (65 of 145 dendrites; 45%) or GABA (65 of 183 dendrites; 35%). In addition, nearly half of PAG neuronal profiles (148 of 344 profiles) that were labeled retrogradely from the ventromedial medulla contained MOR immunoreactivity. MOR was distributed equally among retrogradely labeled neuronal profiles in the lateral and ventrolateral columns of the caudal PAG. With respect to the presynaptic distribution of MOR, approximately half of MOR-immunolabeled axon terminals (35 of 69 terminals) also contained GABA. Some MOR and GABA dual-immunolabeled axon terminals contacted unlabeled dendrites (11 of 35 terminals), whereas others contacted GABA-immunoreactive dendrites (15 of 35 terminals). Furthermore, axon terminals synapsing on medullary-projecting PAG neurons sometimes contained immunoreactivity for MOR. These data support the model that MOR ligands can act by inhibiting GABAergic neurons, but they also provide evidence that MOR ligands may act directly on PAG output neurons. In addition, MOR at presynaptic sites could affect both GABAergic neurons and output neurons. Thus, the disinhibitory model represents only partially the potential mechanisms by which MOR ligands can modulate output of the PAG.

The contribution of the rostral ventromedial medulla to the antinociceptive effects of systemic morphine in restrained and unrestrained rats

Although there are numerous opioid-sensitive structures in the central nervous system, the contribution of each to the analgesic effect of systemically administered morphine is controversial. One such structure is the rostral ventromedial medulla. In the present study, we tested the hypothesis that the rostral ventromedial medulla is necessary for the full expression of systemic morphine-induced antinociception. Additionally, we examined whether the modulatory effect of the rostral ventromedial medulla on tail-flick latency is dependent on the behavioral state of the animal. In unrestrained rats, inactivation of the rostral ventromedial medulla with either lidocaine (0.5 microl of 4%) or muscimol (50 ng) had no effect on tail-flick latency. In contrast, in restrained rats, inactivation of the rostral ventromedial medulla with either lidocaine (0.5 microl of 4%) or muscimol (50 ng) significantly decreased tail-flick latency. In both conditions, microinjection of morphine (5 microg) into this region significantly increased tail-flick latency. Additionally, in unrestrained rats, muscimol (50 ng) and cholecystokinin tetrapeptide (0.5 ng) infusion into the rostral ventromedial medulla completely reversed systemic morphine-induced analgesia, while lidocaine (0.5 microl of 4%) and cholecystokinin octapeptide (0.25 ng) infusion partially reversed systemic morphine-induced analgesia. These findings demonstrate that the rostral ventromedial medulla does not tonically modulate tail-flick latency in unrestrained rats, but does modulate tail-flick latency when animals are stressed via restraint. These findings also strongly support the hypothesis that the rostral ventromedial medulla is necessary for the full analgesic effects of systemically administered morphine.

Dorsal border periaqueductal gray neurons project to the area directly adjacent to the central canal ependyma of the C4-T8 spinal cord in the cat

In a previous study horseradish peroxidase (HRP) injections in the upper thoracic and cervical spinal cord revealed some faintly labeled small neurons at the dorsal border of the periaqueductal gray (PAG). The present light microscopic and electronmicroscopic tracing study describes the precise location of these dorsal border PAG-spinal neurons and their terminal organization. Wheat germ agglutinin-conjugated HRP (WGA-HRP) injections into cervical and upper thoracic spinal segments resulted in several hundreds of small retrogradely labeled neurons at the dorsal border of the ipsilateral caudal PAG. These neurons were not found after injections in more caudal segments. WGA-HRP injections in the dorsal border PAG region surprisingly resulted in anterogradely labeled fibers terminating in the area dorsally and laterally adjoining the central canal ependyma of the C4-T8 spinal cord. No anterogradely labeled fibers were found more caudal in the spinal cord. The labeled fibers found in the upper cervical cord were not located in the area immediately adjoining the ependymal layer of the central canal, but in the lateral part of laminae VI, VII and VIII and in area X bilaterally. Electronmicroscopic results of one case show that the dorsal border PAG-spinal neurons terminate in the neuropil of the subependymal area and in the vicinity of the basal membranes of capillaries located laterally to the central canal. The terminal profiles contain electron-lucent and densecored vesicles, suggesting a heterogeneity of possible transmitters. A striking observation was the lack of synaptic contacts, suggesting nonsynaptic release from the profiles. The function of the dorsal border PAG-spinal projection is unknown, but considering the termination pattern of the dorsal border PAG neurons on the capillaries the intriguing similarity between this projection system and the hypothalamohypophysial system is discussed.

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.

Differential expression of fos-like immunoreactivity in the descending projections of superior colliculus after electrical stimulation in the rat

In rodent, there is evidence that the orienting behaviour elicited by direct stimulation of the superior colliculus (SC) is partly mediated by contralateral descending projections, while avoidance-type behaviour is associated with ipsilateral descending projections. However, the identity of target structures in the brainstem which mediate these different behavioural responses is unknown. The c-fos immediate early gene is expressed polysynaptically in neurons in response to a wide range of extracellular stimuli, and hence has been proposed as a technique for mapping functional pathways. The purpose of this study was, therefore, to use the c-fos technique to investigate the functional specificity of brainstem regions which are innervated by the two main descending projections of the SC. Patterns of fos-like immunoreactivity (FLI) were observed throughout the brainstem following electrical stimulation of the SC in Urethane-anaesthetized rats. Previously, the electrical stimulation had been shown to elicit either approach-like or avoidance-like movement. The main results of this experiment were; (i) animals in which the stimulation elicited defensive behaviour had elevated levels of immunostaining in specific terminal areas of the ipsilateral descending projections, e.g. the ventrolateral midbrain/pontine reticular formation, the cuneiform area and rostral periaqueductal grey; (ii) there was no FLI expression in any of the terminal areas of the crossed descending projection, even in animals where the electrical stimulation elicited approach. Control experiments showed that the lack of expression in the crossed descending pathway was not due to the restricted range of stimulation parameters used in the main study, or to the effects of the anaesthetic. In conclusion, this experiment was able to identify likely substrates for the mediation of defensive reactions elicited by tectal stimulation. However, given the total lack of expression in a pathway which is known to be activated, it also provides further evidence that c-fos cannot simply be used as a high resolution neuronal activity marker for mapping functional pathways.

Neural control of vocalization: respiratory and emotional influences

Previous research has shown that a region of the midbrain, the periaqueductal gray matter (PAG), is critical for vocalization. In this review, we describe the results of previous investigations in which we sought to find out how PAG neurons integrate the activity and precise timing of respiratory, laryngeal, and oral muscle activity for natural-sounding vocalization using the technique of excitatory amino acid microinjections in cats. In these studies, all surgical procedures were carried out under deep anaesthesia. In the precollicular decerebrate cat two general types of vocalization, classified as voiced and unvoiced, could be evoked by exciting neurons in the lateral part of the intermediate part of the PAG. The patterns of evoked electromyographic activity were strikingly similar to previously reported patterns of human muscle activity. Coordinated patterns of activity were evoked with just-threshold excitation leading to the conclusion that patterned muscle activity corresponding to the major categories of voiced and voiceless sound production are represented in the PAG. In a parallel series of human and animal experiments, we also determined that the speech and vocalization respiratory patterns are integrated and coordinated with afferent signals related to lung volume. These data have led to the proposal of a new hypothesis for the neural control of vocalization: that the PAG is a crucial brain site for mammalian voice production, not only in the production of emotional or involuntary sounds, but also as a generator of specific respiratory and laryngeal motor patterns essential for human speech and song.

Distribution of brainstem projections from spinal lamina I neurons in the cat and the monkey

The distribution of terminal projections in the brainstem from lamina I neurons in the spinal dorsal horn was investigated with the anterograde tracer Phaseolus vulgaris-leucoagglutinin in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-III in the cervical (C6-8) or lumbar (L6-7) enlargement. The distribution of terminal labeling was essentially identical in the cat and the monkey, although consistently of greater intensity in the monkey. Terminations were observed in the solitary nucleus, the dorsomedial medullary reticular formation, the entire rostrocaudal extent of the ventrolateral medulla, the locus coeruleus, the subcoerulear region and the Kölliker-Fuse nucleus, the lateral and medial portions of the parabrachial nucleus, the cuneiform nucleus, the ventrolateral and lateral portions of the periaqueductal gray, and the intercollicular nucleus. Lamina I terminations were generally bilateral in the medulla but more dense contralaterally in the pons and mesencephalon. The density and laterality of labeling in the medulla varied between cases independently from that in the pons and mesencephalon, suggesting that the lamina I projections to these regions may originate from different subsets of neurons. A clear topographic organization was observed only in the lateral column of the periaqueductal gray, where lumbar lamina I terminations were found caudal to cervical terminations. These observations indicate that spinal lamina I neurons project to a variety of brainstem sites involved in autonomic (cardiovascular, respiratory) and homeostatic processing and the control of behavioral state. These projections provide an afferent substrate for spino-bulbo-spinal somatoautonomic reflex arcs activated by nociceptive, thermoreceptive activity and for a spino-bulbo-hypothalamic relay of such activity by cells in the caudal ventrolateral medulla. These observations support the general concept that lamina I projections distribute modality-selective sensory information relevant to the physiological status and maintenance of the tissues and organs of the entire organism.

Caudal medullary pathways to lumbosacral motoneuronal cell groups in the cat: evidence for direct projections possibly representing the final common pathway for lordosis

The nucleus retroambiguus (NRA) projects to distinct brainstem and cervical and thoracic cord motoneuronal cell groups. The present paper describes NRA projections to distinct motoneuronal cell groups in the lumbar enlargement. Lumbosacral injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made to localize and quantify the retrogradely labeled neurons in the caudal medullary lateral tegmentum. These injections were combined with spinal hemisections to distinguish between neurons having ipsi-or contralaterally descending axons. The NRA-lumbosacral fibers descend almost exclusively contralaterally, but neurons in areas surrounding the NRA project mainly ipsilaterally. In an anterograde tracing study, injections of WGA-HRP or tritiated leucine were made in the region of the NRA to determine the NRA targets in the lumbosarcral cord. Hemisections in C2 made it possible to distinguish between NRA projections and projections from neurons in the adjoining lateral tegmentum. The results show delicate NRA projections to distinct lumbosacral motoneuronal cell groups innervating specific hindlimb muscles (iliopsoas, adductors, and hamstrings) as well as axial muscles (medial longissimus and proximal tail muscles). The projection is bilateral, with a contralateral predominance. Ipsilaterally terminating fibers are derived from NRA neurons whose axons cross the midline at the level of the obex, descend through the contralateral spinal white matter, and recross at the level of termination. A conceptual description is presented in which the periaqueductal gray-NRA-lumbosacral projections form the final common pathway for lordosis in the cat.

Functional characteristics of the midbrain periaqueductal gray

The major functions of the midbrain periaqueductal gray (PAG), including pain and analgesia, fear and anxiety, vocalization, lordosis and cardiovascular control are considered in this review article. The PAG is an important site in ascending pain transmission. It receives afferents from nociceptive neurons in the spinal cord and sends projections to thalamic nuclei that process nociception. The PAG is also a major component of a descending pain inhibitory system. Activation of this system inhibits nociceptive neurons in the dorsal horn of the sinal cord. The dorsal PAG is a major site for processing of fear and anxiety. It interacts with the amygdala and its lesion alters fear and anxiety produced by stimulation of amygdala. Stimulation of PAG produces vocalization and its lesion produces mutism. The firing of many cells within the PAG correlates with vocalization. The PAG is a major site for lordosis and this role of PAG is mediated by a pathway connecting the medial preoptic with the PAG. The cardiovascular controlling network within the PAG are organized in columns. The dorsal column is involved in pressor and the ventrolateral column mediates depressor responses. The major intrinsic circuit within the PAG is a tonically-active GABAergic network and inhibition of this network is an important mechanism for activation of outputs of the PAG. The various functions of the PAG are interrelated and there is a significant interaction between different functional components of the PAG. Using the current information about the anatomy, physiology, and pharmacology of the PAG, a model is proposed to account for the interactions between these different functional components.

General and segmental reduced pain thresholds in juvenile chronic arthritis

The objective of this work was to investigate the pattern of pain threshold (PT) alterations in paraspinal soft tissues as related to inflamed joints in patients with juvenile chronic arthritis (JCA). A pressure algometer was used in JCA patients with inflamed knee(s) (n = 16), with inflamed ankle(s) (n = 17), and in healthy controls (n = 69), all aged 6-17 years. Local pressure was applied to the joint capsules of the knees and ankles and the soft paraspinal tissues, and PTs were recorded. JCA patients showed not only a significantly lower PT at the inflamed knee and ankle joints, but also in the non-inflamed paraspinal areas. All paraspinal PTs showed a significant negative correlation with the inflamed knee or ankle joint. The correlation rank order showed that the paraspinal L1 area had the highest negative correlation with the inflamed knee joint and the paraspinal L3 area with the inflamed ankle joint. JCA coincides with generally diminished pain thresholds in the paraspinal region. Nociception from inflamed joints may have established changes in the peripheral as well as central nociceptive processing system in JCA.

Periaqueductal gray area and cardiovascular function

The periaqueductal gray (PAG) area seems to play an important role in modulating several biological functions such as the triggering of stereotyped defence and reproductive behaviour, pain, anxiety and cardiovascular and respiratory activities. Anatomically this midbrain area is made up of symmetric neuronal columns arranged along the long axis of the aqueduct. In this paper we review the most important findings of the last 10-15 years about the interaction between the PAG area and the cardiovascular function. It is shown that these neuronal columns within the PAG area exhibit a viscerotropic organization which elicits both hypertensive and hypotensive responses. In particular, the stimulation of the ventral neuronal column evokes a hypotensive response associated with a regional decrease in the vascular resistance. On the contrary, the stimulation of the dorsal and lateral neuronal columns evokes arterial hypertension associated with specific changes of the vascular resistance. Recently the authors demonstrated that the glutamergic system in the PAG area (prevalently through NMDA subtype receptor) may also be involved in the control of cardiovascular system. Moreover, the involvement of the arginine vasopressin neuropeptide in the hypertension induced by administration of excitatory amino acids into the PAG area has been demonstrated.

Propriospinal input to thoracolumbar sympathetic nuclei from cervical and lumbar lamina I neurons in the cat and the monkey

The possibility that specific thermoreceptive and nociceptive influences on sympathetic outflow are conveyed directly to spinal sympathetic regions by lamina I neurons was investigated anatomically with the immunofluorescent PHA-L technique in the cat and the cynomolgus monkey. Iontophoretic injections made with physiological guidance were restricted to lamina I or to laminae I-II in the cervical (C6-8) or lumbar (L6-7) enlargement. Bilateral (symmetric) terminal arborizations were observed (with an ipsilateral predominance) in the intermediolateral, intermediomedial, and intervening regions of the thoracolumbar intermediate zone. In serial horizontal sections, patches of terminal labeling appeared at regular longitudinal intervals in the intermediolateral region. Longitudinally coursing fibers that had multiple varicosities and gave off small terminal branches were observed in the intermediolateral and the intermediomedial regions. Mediolateral strips of labeling that extended from labeling in the intermediolateral region to labeling in the intermediomedial region occurred at fairly regular longitudinal intervals. Because the longitudinal distribution of these terminations corresponds very well with the characteristic (ladder-like) longitudinal pattern of organization of the neuropil of the thoracolumbar sympathetic nuclei, i.e., the principal part of the intermediolateral cell column, the central autonomic n., and the intervening n. intercalatus, it is inferred that these lamina I terminations occur within these nuclei. After cervical injections, the labeling was most dense in the upper thoracic T2-4 spinal cord segments in both the cat and the monkey; labeling was also present in the T10-12 segments. After lumbar injections, labeling in the cat was located in the L4 segment; labeling in the monkey was present in the T4-6 and T10-12 segments. The labeling obtained was much more dense in the monkey than in the cat. These observations reveal a spinal lamina I projection that could provide a direct pathway for the somatosympathetic reflex effects of thermal and noxious stimuli. Considered together with reports that lamina I and the sympathetic nuclei both receive descending input from certain key autonomic regions, this result emphasizes the importance of lamina I for homeostasis, in addition to its probable roles in behavioral arousal, affect, and sensation. These observations thus support the proposed concept that lamina I processes and distributes in a functionally specific manner the sensory input relevant to the physiological status of the tissues and organs of the entire organism.

Ascending inhibition of nociceptive neurons in the nucleus ventralis posterolateralis following conditioning stimulation of the nucleus raphe magnus

Recordings were made from neurons in the nucleus ventralis posterolateralis (VPL) of urethane-chloralose-anesthetized cats, following both noxious mechanical stimulation of the integument and electrical stimulation of the greater splanchnic nerve (SPL). The effects of stimulating the nucleus raphe magnus (NRM) on responses obtained from these units were investigated. Units responding to noxious mechanical stimulation of the integument with SPL input were found in the posterior shell region of the VPL. Responses elicited from these units by electrical stimulation of the SPL were inhibited following conditioning stimulation in or near the NRM. Inhibition could still be demonstrated after bilateral section of the dorsolateral funiculi at the level of C3-C4. Responses of these units to electrical stimulation of the ventrolateral funiculus (VLF) of the cervical cord were also inhibited following conditioning stimulation in or near the NRM. These results suggest that inhibition of these units produced by conditioning NRM stimulation may be partially mediated by an ascending pathway, in addition to the well-known descending spinal pathways. Glutamate stimulation of the NRM inhibited responses of nociceptive VPL units to SPL stimulation, but responses of the same units to VLF stimulation were little affected by the glutamate stimulation of the NRM. Inhibition of responses of nociceptive VPL units to SPL stimulation may be due to anti-dromic excitation of brainstem neurons having efferent connection with the NRM.

Subcortical projections to the centromedian and parafascicular thalamic nuclei in the cat

The primary objective of this study is to identify the totality of input to the centromedian and parafascicular (CM-Pf) thalamic nuclear complex. The subcortical projections upon the CM-Pf complex were studied in the cat with three different retrograde tracers. The tracers used were unconjugated horseradish peroxidase (HRP), horseradish peroxidase conjugated to wheat germ agglutinin (WGA-HRP), and rhodamine-labeled fluorescent latex microspheres (RFM). Numerous subcortical structures or substructures contained labeled neurons with all three tracing techniques. These labeled structures included the central nucleus of the amygdala; the entopeduncular nucleus; the globus pallidus; the reticular and ventral lateral geniculate nuclei of the thalamus; parts of the hypothalamus including the dorsal, lateral, and posterior hypothalamic areas and the ventromedial and parvicellular nuclei; the zona incerta and fields of Forel; parts of the substantia nigra including the pars reticularis and pars lateralis, and the retrorubral area; the pretectum; the intermediate and deep layers of the superior colliculus; the periaqueductal gray; the dorsal nucleus of the raphe; portions of the reticular formation, including the mesencephalic, pontis oralis, pontis caudalis, gigantocellularis, ventralis, and lateralis reticular nuclei; the nucleus cuneiformis; the marginal nucleus of the brachium conjunctivum; the locus coeruleus; portions of the trigeminal complex, including the principal sensory and spinal nuclei; portions of the vestibular complex, including the lateral division of the superior nucleus and the medial nucleus; deep cerebellar nuclei, including the medial and lateral cerebellar nuclei; and lamina VII of the cervical spinal cord. Moreover, the WGA-HRP and rhodamine methods (known to be more sensitive than the HRP method) revealed several afferent sources not shown by HRP: the anterior hypothalamic area, ventral tegmental area, lateral division of the superior vestibular nucleus, nucleus interpositus, and the nucleus praepositus hypoglossi. Also, the rhodamine method revealed labeled neurons in laminae V and VI of the cervical spinal cord.

Viscerotopic organization of neurons subserving hypotensive reactions within the midbrain periaqueductal grey: a correlative functional and anatomical study

Microinjection of the excitatory amino acid D,L-homocysteic acid (40 nmol, in 200 nl) made into the ventrolateral part of the caudal half (A2.5-P1.5) of the midbrain periaqueductal gray (PAG) of the decerebrate cat evoked a hypotensive reaction associated with a slowing of the heart and a decrease in either external iliac or renal vascular resistance. The decrease in iliac vascular resistance was elicited from the pretentorial portion (A2.5-A0.6) of the PAG hypotensive area, whereas the decrease in renal vascular resistance was elicited from the subtentorial portion (A0.6-P1.5). Anatomical experiments using the method of retrograde transport of rhodamine-labelled microspheres or wheat germ agglutinin-horseradish peroxidase demonstrated topographically organized projections from the ventrolateral PAG to the subretrofacial (SRF) pressor nucleus in the rostral ventrolateral medulla. The pretentorial part of the ventrolateral PAG projected mainly to the caudal part of the SRF nucleus, which preferentially controls iliac vascular resistance. The subtentorial part of the ventrolateral PAG projected mainly to the rostral part of the SRF nucleus, which preferentially controls renal vascular resistance. Taken together, these findings suggest: (i) that neurons within the ventrolateral PAG are viscerotopically organized; and (ii) that their hypotensive function may be mediated by an inhibition of SRF pressor neurons. The results are discussed in relation to the recently described PAG hypertensive area which also is viscerotopically organized and projects to the SRF nucleus.