Afferents to the periaqueductal gray in the rat. A horseradish peroxidase study

The anterior cingulate cortex directly enhances auditory cortical responses in air-puffing-facilitated flight behavior

For survival, animals encode prominent events in complex environments, which modulates their defense behavior. Here, we design a paradigm that assesses how a mild aversive cue (i.e., mild air puff) interacts with sound-evoked flight behavior in mice. We find that air puffing facilitates sound-evoked flight behavior by enhancing the auditory responses of auditory cortical neurons. We then find that the anterior part of the anterior cingulate cortex (ACC) encodes the valence of air puffing and modulates the auditory cortex through anatomical examination, physiological recordings, and optogenetic/chemogenetic manipulations. Activating ACC projections to the auditory cortex simulates the facilitating effect of air puffing, whereas inhibiting the ACC or its projections to the auditory cortex neutralizes this facilitating effect. These findings show that the ACC regulates sound-evoked flight behavior by potentiating neuronal responses in the auditory cortex.

Dorsal periaqueductal gray ensembles represent approach and avoidance states

Animals must balance needs to approach threats for risk assessment and to avoid danger. The dorsal periaqueductal gray (dPAG) controls defensive behaviors, but it is unknown how it represents states associated with threat approach and avoidance. We identified a dPAG threatavoidance ensemble in mice that showed higher activity farther from threats such as the open arms of the elevated plus maze and a predator. These cells were also more active during threat avoidance behaviors such as escape and freezing, even though these behaviors have antagonistic motor output. Conversely, the threat approach ensemble was more active during risk assessment behaviors and near threats. Furthermore, unsupervised methods showed that avoidance/approach states were encoded with shared activity patterns across threats. Lastly, the relative number of cells in each ensemble predicted threat avoidance across mice. Thus, dPAG ensembles dynamically encode threat approach and avoidance states, providing a flexible mechanism to balance risk assessment and danger avoidance.

Fish as a model in social neuroscience: conservation and diversity in the social brain network

Mechanisms for fish social behaviours involve a social brain network (SBN) which is evolutionarily conserved among vertebrates. However, considerable diversity is observed in the actual behaviour patterns amongst nearly 30000 fish species. The huge variation found in socio-sexual behaviours and strategies is likely generated by a morphologically and genetically well-conserved small forebrain system. Hence, teleost fish provide a useful model to study the fundamental mechanisms underlying social brain functions. Herein we review the foundations underlying fish social behaviours including sensory, hormonal, molecular and neuroanatomical features. Gonadotropin-releasing hormone neurons clearly play important roles, but the participation of vasotocin and isotocin is also highlighted. Genetic investigations of developing fish brain have revealed the molecular complexity of neural development of the SBN. In addition to straightforward social behaviours such as sex and aggression, new experiments have revealed higher order and unique phenomena such as social eavesdropping and social buffering in fish. Finally, observations interpreted as 'collective cognition' in fish can likely be explained by careful observation of sensory determinants and analyses using the dynamics of quantitative scaling. Understanding of the functions of the SBN in fish provide clues for understanding the origin and evolution of higher social functions in vertebrates.

The Neural Basis of Escape Behavior in Vertebrates

Escape is one of the most studied animal behaviors, and there is a rich normative theory that links threat properties to evasive actions and their timing. The behavioral principles of escape are evolutionarily conserved and rely on elementary computational steps such as classifying sensory stimuli and executing appropriate movements. These are common building blocks of general adaptive behaviors. Here we consider the computational challenges required for escape behaviors to be implemented, discuss possible algorithmic solutions, and review some of the underlying neural circuits and mechanisms. We outline shared neural principles that can be implemented by evolutionarily ancient neural systems to generate escape behavior, to which cortical encephalization has been added to allow for increased sophistication and flexibility in responding to threat.

Midbrain control of breathing and blood pressure: The role of periaqueductal gray matter and mesencephalic collicular neuronal microcircuit oscillators

Neural circuitry residing within the medullary ventral respiratory column nuclei and dorsal respiratory group interact with the Kölliker-Fuse and medial parabrachial nuclei to generate the core breathing rhythm and pattern during resting conditions. Triphasic eupnea consists of inspiratory [I], post-inspiratory [post-I], and late-expiratory [E2] phases. Mesencephalic zones exert modulatory influences upon respiratory rhythm-generating circuitry, sympathetic oscillators, and parasympathetic nuclei. The earliest evidence supporting the existence of midbrain control of breathing derives from studies conducted by Martin and Booker in 1878. These authors demonstrated electrical stimulation of the deep layers of the mesencephalic colliculi in the rabbit augmented ventilation and sequentially elicited chest wall tremors and tetany. Investigations performed during the past several decades would demonstrate stimlation of distributed zones within the midbrain reticular formation elicits starkly disparate effects upon respiratory phase switching. Schmid, Böhmer, and Fallert demonstrated electrical stimulation of the nucleus rubre and emanating axon bundles alternately elicits or inhibits the activity of medullary expiratory- or inspiratory-related units and phrenic nerve discharge with differential latency. A series of studies would later indicate the red nucleus mediates hypoxic ventilatory depression. Periaqueductal gray matter neurons exhibit extensive afferent and efferent interconnectivity with suprabulbar, brainstem, and spinal cord zones aptly positioning these units to modulate breathing, autonomic outflow, nociception locomotion, micturtion, and sexual behavior. Experimental stimulatory activation of the tectal colliculi and periaqueductal gray matter via electrical current or glutamate, D,L-homocysteinic acid, or bicuculline microinjections coordinately modulates neuromotor inspiratory bursting frequency and amplitude and discharge of pre-Bötzinger complex, ventrolateral medullary late-I and post-I, and ventrolateral nucleus tractus solitarius decrementing early-I and augmenting and decrementing late-I neurons, elicits expiratory outflow and vocalization, and blunt the Hering-Breuer reflex in unanesthetzed decerebrate and anesthetized preprations of the cat and rat. Stimulation of the mesencephalic colliuli or dorsal divisions of the PAG potently amplifes renal sympathetic neural efferent activity, dynamic arterial pressure magnitude, and myocardial contraction frequency and elicits various behavioral defense responses. Elicited physiological effects exhibit extensive locoregional heterogeneity and variably enlist requisite contributions from the dorsomedial hypothalamus and/or lateral parabrachial nuclei. Stimulation of the dorsal mesencephalon occasionally elicits dynamic increases of arterial pressure magnitude exhibiting prominent oscillatory variability coherent with phrenic nerve discharge, perhaps by generating intra-neuraxial hysteresis, serving to intermittently deliver blood to organ vascular beds under high pressure in order to prevent organ edema, microcirculatory dysfunction, and downregulation of vascular smooth muscle alpha adrenergic receptors. Chemosensitive mesencephalic caudal raphé units and projections of hypoxia-sensitive units in the caudal hypothalamus to the periaqueductal gray matter may imply the existence of a diencephalo-smesencephalic chemosensitive network modulating breathing and sympathetic discharge.

The role of the periaqueductal gray in escape behavior

Escape behavior is a defensive action deployed by animals in response to imminent threats. In mammalian species, a variety of different brain circuits are known to participate in this crucial survival behavior. One of these circuits is the periaqueductal gray, a midbrain structure that can command a variety of instinctive behaviors. Recent experiments using modern systems neuroscience techniques have begun to elucidate the specific role of the periaqueductal gray in controlling escape. These have shown that periaqueductal gray neurons are crucial units for gating and commanding the initiation of escape, specifically activated in situations of imminent, escapable threat. In addition, it is becoming clear that the periaqueductal gray integrates brain-wide information that can modulate escape initiation to generate flexible defensive behaviors.

A cross-modality enhancement of defensive flight via parvalbumin neurons in zona incerta

The ability to adjust defensive behavior is critical for animal survival in dynamic environments. However, neural circuits underlying the modulation of innate defensive behavior remain not well-understood. In particular, environmental threats are commonly associated with cues of multiple sensory modalities. It remains to be investigated how these modalities interact to shape defensive behavior. In this study, we report that auditory-induced defensive flight behavior can be facilitated by somatosensory input in mice. This cross-modality modulation of defensive behavior is mediated by the projection from the primary somatosensory cortex (SSp) to the ventral sector of zona incerta (ZIv). Parvalbumin (PV)-positive neurons in ZIv, receiving direct input from SSp, mediate the enhancement of the flight behavior via their projections to the medial posterior complex of thalamus (POm). Thus, defensive flight can be enhanced in a somatosensory context-dependent manner via recruiting PV neurons in ZIv, which may be important for increasing survival of prey animals.

Noradrenergic Transmission at Alpha1-Adrenergic Receptors in the Ventral Periaqueductal Gray Modulates Arousal

BACKGROUND

Dysregulation of arousal is symptomatic of numerous psychiatric disorders. Previous research has shown that the activity of dopamine (DA) neurons in the ventral periaqueductal gray (vPAG) tracks with arousal state, and lesions of vPAGDA cells increase sleep. However, the circuitry controlling these wake-promoting DA neurons is unknown.

METHODS

This study combined designer receptors exclusively activated by designer drugs (DREADDs), behavioral pharmacology, electrophysiology, and immunoelectron microscopy in male and female mice to elucidate mechanisms in the vPAG that promote arousal.

RESULTS

Activation of locus coeruleus projections to the vPAG or vPAGDA neurons induced by DREADDs promoted arousal. Similarly, agonist stimulation of vPAG alpha1-adrenergic receptors (α1ARs) increased latency to fall asleep, whereas α1AR blockade had the opposite effect. α1AR stimulation drove vPAGDA activity in a glutamate-dependent, action potential-independent manner. Compared with other dopaminergic brain regions, α1ARs were enriched on astrocytes in the vPAG, and mimicking α1AR transmission specifically in vPAG astrocytes via Gq-DREADDS was sufficient to increase arousal. In general, the wake-promoting effects observed were not accompanied by hyperactivity.

CONCLUSIONS

These experiments revealed that vPAG α1ARs increase arousal, promote glutamatergic input onto vPAGDA neurons, and are abundantly expressed on astrocytes. Activation of locus coeruleus inputs, vPAG astrocytes, or vPAGDA neurons increase sleep latency but do not produce hyperactivity. Together, these results support an arousal circuit whereby noradrenergic transmission at astrocytic α1ARs activates wake-promoting vPAGDA neurons via glutamate transmission.

First evidence of neuronal connections between specific parts of the periaqueductal gray (PAG) and the rest of the brain in sheep: placing the sheep PAG in the circuit of emotion

The periaqueductal gray (PAG) is a mesencephalic brain structure organised in subdivisions with specific anatomical connections with the rest of the brain. These connections support the different PAG functions and especially its role in emotion. Mainly described in territorial and predatory mammals, examination of the PAG connections suggests an opposite role of the ventral and the dorsal/lateral PAG in passive and active coping style, respectively. In mammals, the organisation of PAG connections may reflect the coping style of each species. Based on this hypothesis, we investigated the anatomical connections of the PAG in sheep, a gregarious and prey species. Since emotional responses expressed by sheep are typical of active coping style, we focused our interest on the dorsal and lateral parts of the PAG. After injection of fluorogold and fluororuby, the most numerous connections occurred with the anterior cingulate gyrus, the anterior hypothalamic region, the ventromedial hypothalamic nucleus and the PAG itself. Our observations show that the sheep PAG belongs to the neuronal circuit of emotion and has specific parts as in other mammals. However, unlike other mammals, we observed very few connections between PAG and either the thalamic or the amygdalar nuclei. Interestingly, when comparing across species, the PAG connections of sheep were noticeably more like those previously described in other social species, rabbits and squirrel monkeys, than those in territorial species, rats or cats.

Efferents of anterior cingulate areas 24a and 24b and midcingulate areas 24a' and 24b' in the mouse

The anterior cingulate cortex (ACC), constituted by areas 25, 32, 24a and 24b in rodents, plays a major role in cognition, emotion and pain. In a previous study, we described the afferents of areas 24a and 24b and those of areas 24a' and 24b' of midcingulate cortex (MCC) in mice and highlighted some density differences among cingulate inputs (Fillinger et al., Brain Struct Funct 222:1509-1532, 2017). To complete this connectome, we analyzed here the efferents of ACC and MCC by injecting anterograde tracers in areas 24a/24b of ACC and 24a'/24b' of MCC. Our results reveal a common projections pattern from both ACC and MCC, targeting the cortical mantle (intracingulate, retrosplenial and parietal associative cortex), the non-cortical basal forebrain, (dorsal striatum, septum, claustrum, basolateral amygdala), the hypothalamus (anterior, lateral, posterior), the thalamus (anterior, laterodorsal, ventral, mediodorsal, midline and intralaminar nuclei), the brainstem (periaqueductal gray, superior colliculus, pontomesencephalic reticular formation, pontine nuclei, tegmental nuclei) and the spinal cord. In addition to an overall denser ACC projection pattern compared to MCC, our analysis revealed clear differences in the density and topography of efferents between ACC and MCC, as well as between dorsal (24b/24b') and ventral (24a/24a') areas, suggesting a common functionality of these two cingulate regions supplemented by specific roles of each area. These results provide a detailed analysis of the efferents of the mouse areas 24a/24b and 24a'/24b' and achieve the description of the cingulate connectome, which bring the anatomical basis necessary to address the roles of ACC and MCC in mice.

[Spinal serotonergic receptor is involved in descending inhibition of cardiac nociception by the lateral reticular nucleus in rats]

OBJECTIVE

To investigate the role of the lateral reticular nucleus (LRN) in descending inhibition of cardiac nociception in rats and the involvement of spinal serotonergic receptors in the descending inhibition.

METHODS

Male SD rats were randomly divided into 5 groups, namely bradykinin (BK) group, BK + glutamate group, BK + methysergide group, BK + glutamate + methysergide group, and BK + glutamate + vehicle group. The rats received glutamate microinjection in the LRN combined with the intrathecal injection of serotonergic receptor antagonist methysergide, and the changes in the cardiacsomatic motor reflex induced by intrapericardial BK injection were monitored by observing electromyogram (EMG) responses of the dorsal spinotrapezius muscle; c-Fos expression in the spinal dorsal horn was also tested.

RESULTS

Compared with BK group, intra-LRN glutamate administration produced a significant inhibitory effect on intrapericardial BK-induced EMG in a dose-dependent manner, and c-Fos expression was significantly decreased in the spinal dorsal horn (P<0.05). Compared with BK + glutamate group, intrathecal administration of methysergide significantly attenuated the inhibitory effect of chemical stimulation of the LRN on intrapericardial BK-induced EMG and increased c-Fos expression in the spinal dorsal horn (P<0.05). Intrathecal administration of the vehicle did not produce any effect on EMG or c-Fos expression (P>0.05).

CONCLUSION

The serotonergic receptors in the spinal cord are involved in LRN-mediated descending inhibition of cardiac nociception in rats.

Repeated threat (without direct harm) alters metabolic capacity in select regions that drive defensive behavior

To understand the behavioral consequences of intermittent anticipatory stress resulting from threats without accompanying physiological challenges, we developed a semi-naturalistic rodent housing and foraging environment that can include threats that are unpredictable in timing. Behavior is automatically recorded while rats forage for food or water. Over three weeks, the threats have been shown to elicit risk assessment behaviors, increase defensive burying and increase adrenal gland weight. To identify brain regions activated by this manipulation, we measured cytochrome c oxidase (COX), which is tightly coupled to neural activity. Adolescent male Sprague-Dawley rats were randomly assigned to control (CT) or unpredictable threat/stress (ST) housing conditions consisting of two tub cages, one with food and another with water, separated by a tunnel. Over three weeks (P31-P52), the ST group received randomly timed (probability of 0.25), simultaneous presentations of ferret odor, an abrupt light, and sound at the center of the tunnel. The ST group had consistently fewer tunnel crossings than the CT group, but similar body weights. Group differences in COX activity were detected in regions implicated in the control of defensive burying. There was an increase in COX activity in the hypothalamic premammillary dorsal nucleus (PMD) and lateral septum (LS), whereas a decrease was observed in the periaqueductal gray (PAG) and CA3 region of the hippocampus. There were no significant differences in the anterior cingulate cortex, prefrontal cortex, striatum or motor cortex. The sites with changes in metabolic capacity are candidates for the sites of plasticity that may underlie the behavioral adaptations to intermittent threats.

Behavioral, metabolic and functional brain changes in a rat model of chronic neuropathic pain: a longitudinal MRI study

Peripheral neuropathy often manifests clinically with symptoms of mechanical and cold allodynia. However, the neuroplastic changes associated with peripheral neuropathic pain and the onset and progression of allodynic symptoms remain unclear. Here, we used a chronic neuropathic pain model (spared nerve injury; SNI) to examine functional and metabolic brain changes associated with the development and maintenance of mechanical and cold hypersensitivity, the latter which we assessed both behaviorally and during a novel acetone application paradigm using functional MRI (fMRI). Female Sprague-Dawley rats underwent SNI (n=7) or sham (n=5) surgery to the left hindpaw. Rats were anesthetized and scanned using a 7 T MRI scanner 1 week prior to (pre-injury) and 4 (early/subchronic) and 20 weeks (late/chronic) post-injury. Functional scans were acquired during acetone application to the left hindpaw. (1)H magnetic resonance spectroscopy was also performed to assess SNI-induced metabolic changes in the anterior cingulate cortex (ACC) pre- and 4 weeks post-injury. Mechanical and cold sensitivity, as well as anxiety-like behaviors, were assessed 2 weeks pre-injury, and 2, 5, 9, 14, and 19 weeks post-injury. Stimulus-evoked brain responses (acetone application to the left hindpaw) were analyzed across the pre- and post-injury time points. In response to acetone application during fMRI, SNI rats showed widespread and functionally diverse changes within pain-related brain regions including somatosensory and cingulate cortices and subcortically within the thalamus and the periaqueductal gray. These functional brain changes temporally coincided with early and sustained increases in both mechanical and cold sensitivity. SNI rats also showed increased glutamate within the ACC that correlated with behavioral measures of cold hypersensitivity. Together, our findings suggest that extensive functional reorganization within pain-related brain regions may underlie the development and chronification of allodynic-like behaviors.

Dendritic morphology of caudal periaqueductal gray projecting retinal ganglion cells in Mongolian gerbil (Meriones unguiculatus)

In this study we investigated the morphological features of the caudal periaqueductal gray (cPAG)-projecting retinal ganglion cells (RGCs) in Mongolian gerbils using retrograde labeling, in vitro intracellular injection, confocal microscopy and three-dimensional reconstruction approaches. cPAG-projecting RGCs exhibit small somata (10–17 µm) and irregular dendritic fields (201–298 µm). Sizes of somata and dendritic fields do not show obvious variation at different distance from the optic disk (eccentricity). Dendrites are moderately branched. Morphological analysis (n = 23) reveals that cPAG-projecting RGCs ramified in sublamina a and b in the inner plexiform layer. These cells exhibit different stratification patterns based on the thickness of dendritic bands in sublaminas a and b: majority of analyzed cells (16 out of 23) have two bands of arborizations share similar thickness. The rest of analyzed cells (7 out of 23) exhibit thinner band in sublamina a than in sublamina b. Together, the present study suggests that cPAG of Mongolian gerbil could receive direct retinal inputs from two types of bistratified RGCs. Furthermore, a small subset of melanopsin-expressing RGCs (total 41 in 6 animals) is shown to innervate the rostral PAG (rPAG). Functional characteristics of these non-visual center projecting RGCs remain to be determined.

Stimulation of the dorsal periaqueductal gray enhances spontaneous recovery of a conditioned taste aversion

Due to its relevance to clinical practice, extinction of learned fears has been a major focus of recent research. However, less is known about the means by which conditioned fears re-emerge (i.e., spontaneously recover) as time passes or contexts change following extinction. The periaqueductal gray represents the final common pathway mediating defensive reactions to fear and we have reported previously that the dorsolateral PAG (dlPAG) exhibits a small but reliable increase in neural activity (as measured by c-fos protein immunoreactivity) when spontaneous recovery (SR) of a conditioned taste aversion (CTA) is reduced. Here we extend these correlational studies to determine if inducing dlPAG c-fos expression through electrical brain stimulation could cause a reduction in SR of a CTA. Male Sprague-Dawley rats acquired a strong aversion to saccharin (conditioned stimulus; CS) and then underwent CTA extinction through multiple non-reinforced exposures to the CS. Following a 30-day latency period after asymptotic extinction was achieved; rats either received stimulation of the dorsal PAG (dPAG) or stimulation of closely adjacent structures. Sixty minutes following the stimulation, rats were again presented with the saccharin solution as we tested for SR of the CTA. The brain stimulation evoked c-fos expression around the tip of the electrodes. However, stimulation of the dPAG failed to reduce SR of the previously extinguished CTA. In fact, dPAG stimulation caused rats to significantly suppress their saccharin drinking (relative to controls) - indicating an enhanced SR. These data refute a cause-and-effect relationship between enhanced dPAG c-fos expression and a reduction in SR. However, they highlight a role for the dPAG in modulating SR of extinguished CTAs.

Preliminary findings on the relation between the personality trait of stress reaction and the central neural control of human vocalization

The objectives of this study were to examine whether the personality trait of stress reaction (SR), as assessed with the Multidimensional Personality Questionnaire-Brief Form (MPQ-BF), (1) influences prefrontal and limbic area activity during overt sentence reading and if (2) SR and associated individual differences in prefrontal and limbic activations correlate with sensorimotor cortical activity during overt sentence reading. Ten vocally healthy adults (22-57 years) participated in a functional MRI study using an event-related sparse sampling design to acquire brain activation data during sentence production tasks (covert, whispered, overt). The outcome measure was the blood oxygenation level-dependent signal change in prefrontal, limbic, and primary somatosensory (S1) and motor cortices (M1). Significant positive correlations were found between SR scores and S1, dorsolateral prefrontal cortex (both r =.73, p <.05), and periaqueductal gray (r =.88, p <.01) activity. M1 activity was positively correlated with SR (r =.64, p <.05) and negatively with social potency (r = -.70, p <.05). Our findings suggest that motor cortical control subserving voice and speech production varies with expression of selected personality traits. Future studies should investigate the functional significance of personality differences in the central neural control of vocalization.

Bilateral lesions of the medial frontal cortex disrupt recognition of social hierarchy during antiphonal communication in naked mole-rats (Heterocephalus glaber)

Generation of the motor patterns of emotional sounds in mammals occurs in the periaqueductal gray matter of the midbrain and is not directly controlled by the cortex. The medial frontal cortex indirectly controls vocalizations, based on the recognition of social context. We examined whether the medial frontal cortex was responsible for antiphonal vocalization, or turn-taking, in naked mole-rats. In normal turn-taking, naked mole-rats vocalize more frequently to dominant individuals than to subordinate ones. Bilateral lesions of the medial frontal cortex disrupted differentiation of call rates to the stimulus animals, which had varied social relationships to the subject. However, medial frontal cortex lesions did not affect either the acoustic properties of the vocalizations or the timing of the vocal exchanges. This suggests that the medial frontal cortex may be involved in social cognition or decision making during turn-taking, while other regions of the brain regulate when animals vocalize and the vocalizations themselves.

Temporal filtering of nociceptive information by dynamic activation of endogenous pain modulatory systems

Endogenous pain control mechanisms have long been known to produce analgesia during "flight or fight" situations and to contribute to cognitively driven pain modulation, such as placebo analgesia. Afferent nociceptive information can also directly activate supraspinal descending modulatory systems, suggesting that these mechanisms may participate in feedback loops that dynamically alter the processing of nociceptive information. The functional significance of these feedback loops, however, remains unclear. The phenomenon of offset analgesia -- disproportionately large decreases in pain ratings evoked by small decreases in stimulus intensity -- suggests that dynamic activation of endogenous pain inhibition may contribute to the temporal filtering of nociceptive information. The neural mechanisms that mediate this phenomenon remain currently unknown. Using functional magnetic resonance imaging, we show that several regions of the midbrain and brainstem are differentially activated during offset analgesia. These activations are consistent with the location of areas such as the periaqueductal gray (PAG), rostral ventral medulla, and locus ceruleus that have substantial roles in descending inhibition of pain. This transient analgesia contributes to the temporal filtering of nociceptive information by producing a perceptual amplification of the magnitude and duration of decreases in noxious stimulus intensity. Together with the involvement of PAG and associated brainstem mechanisms in cognitively generated analgesia, the present observations suggest that the fundamental role of endogenous pain modulatory mechanisms is to dynamically shape the processing of nociceptive signals to best fit with the ever-changing demands of the environment.

Inputs to serotonergic neurons revealed by conditional viral transneuronal tracing

Descending projections arising from brainstem serotonergic (5HT) neurons contribute to both facilitatory and inhibitory controls of spinal cord "pain" transmission neurons. Unclear, however, are the brainstem networks that influence the output of these 5HT neurons. To address this question, here we used a novel neuroanatomical tracing method in a transgenic line of mice in which Cre recombinase is selectively expressed in 5HT neurons (ePet-Cre mice). Specifically, we injected the conditional pseudorabies virus recombinant (BA2001) that can replicate only in Cre-expressing neurons. Because BA2001 transports exclusively in a retrograde manner, we were able to reveal a subset of the neurons and circuits that are located upstream of the Cre-expressing 5HT neurons. We show that diverse brainstem regions differentially target the 5HT neurons of the dorsal raphe (DR) and the nucleus raphe magnus of the rostroventral medulla (RVM). Among these are several catecholaminergic and cholinergic cell groups, the periaqueductal gray, several brainstem reticular nuclei, and the nucleus of the solitary tract. We conclude that a brainstem 5HT network integrates somatic and visceral inputs arising from various areas of the body. We also identified a circuit that arises from projection neurons of deep spinal cord laminae V-VIII and targets the 5HT neurons of the NRM, but not of the DR. This spinoreticular pathway constitutes an anatomical substrate through which a noxious stimulus can activate 5HT neurons of the NRM and in turn could trigger descending serotonergic antinociceptive controls.

Modulation of spontaneous breathing via limbic/paralimbic-bulbar circuitry: an event-related fMRI study

It is well established that pacemaker neurons in the brainstem provide automatic control of breathing for metabolic homeostasis and survival. During waking spontaneous breathing, cognitive and emotional demands can modulate the intrinsic brainstem respiratory rhythm. However the neural circuitry mediating this modulation is unknown. Studies of supra-pontine influences on the control of breathing have implicated limbic/paralimbic-bulbar circuitry, but these studies have been limited to either invasive surgical electrophysiological methods or neuroimaging during substantial respiratory provocation. Here we probed the limbic/paralimbic-bulbar circuitry for respiratory-related neural activity during unlabored spontaneous breathing at rest as well as during a challenging cognitive task (sustained random number generation). Functional magnetic resonance imaging (fMRI) with simultaneous physiological monitoring (heart rate, respiratory rate, tidal volume, end-tidal CO(2)) was acquired in 14 healthy subjects during each condition. The cognitive task produced expected increases in breathing rate, while end-tidal CO(2) and heart rate did not significantly differ between conditions. The respiratory cycle served as the input function for breath-by-breath, event-related, voxel-wise, random-effects image analyses in SPM5. Main effects analyses (cognitive task+rest) demonstrated the first evidence of coordinated neural activity associated with spontaneous breathing within the medulla, pons, midbrain, amygdala, anterior cingulate and anterior insular cortices. Between-condition paired t-tests (cognitive task>rest) demonstrated modulation within this network localized to the dorsal anterior cingulate and pontine raphe magnus nucleus. We propose that the identified limbic/paralimbic-bulbar circuitry plays a significant role in cognitive and emotional modulation of spontaneous breathing.

An NK1 receptor antagonist microinjected into the periaqueductal gray blocks lateral hypothalamic-induced antinociception in rats

Substantial data are accumulating that implicate the lateral hypothalamus (LH) as part of the descending pain modulatory system. The LH modifies nociception in the spinal cord dorsal horn partly through connections with the periaqueductal gray (PAG), an area known to play a central role in brainstem modulation of nociception. Early work demonstrated a putative substance P connection between the LH and the PAG, but the connection is not fully defined. To determine whether LH-induced antinociception mediated by the PAG is neurokinin1 (NK1) receptor-dependent, we conducted behavioral experiments in which the cholinergic agonist carbachol (125 nmol) was microinjected into the LH of lightly anesthetized female Sprague-Dawley rats (250-350 g) and antinociception was obtained on the tail flick or foot withdrawal tests. Cobalt chloride (100 nM), which reversibly blocks synaptic activation, blocked LH-induced antinociception. In another set of experiments, the specific NK1 receptor antagonist L-703,606 (5 microg) was microinjected in the PAG following LH stimulation with carbachol abolished LH-induced antinociception as well. Microinjection of cobalt chloride or L-703,606 in the absence of LH stimulation had no effect. These behavioral experiments coupled with earlier work provide converging evidence to support the hypothesis that antinociception produced by activating neurons in the LH is mediated in part by the subsequent activation of neurons in the PAG by NK1 receptors.

Expression of TRPM8 in the distal cerebrospinal fluid-contacting neurons in the brain mesencephalon of rats

Background

It has been shown that distal cerebrospinal fluid-contacting neurons (dCSF-CNs) exist near the ventral midline of the midbrain aqueduct and also in the grey matter of the inferior third ventricle and the fourth ventricle floor in the superior segment of the pons. The dCSF-CNs communicate between the cerebrospinal fluid (CSF) and the brain parenchyma and may participate in the transduction and regulation of pain signals. The cold sensation receptor channel, TRPM8 is involved in analgesia for neuropathic pain, but whether the TRPM8 receptor exists on dCSF-CNs remains unknown. However, there is preliminary evidence that TRPM8 is expressed in dCSF-CNs and may participate in the transmission and regulation of sensory information between brain parenchyma and cerebrospinal fluid (CSF) in rats.

Methods

Retrograde tracing of the cholera toxin subunit B labeled with horseradish peroxidase (CB-HRP) injected into the lateral ventricle was used to identify dCSF-CNs. A double-labeled immunofluorescent technique and laser scanning confocal microscopy were used to identify the expression of TRPM8 in dCSF-CNs. Software Image-Pro Plus was used to count the number of neurons in three sections where CB-HRP positive neurons were located in the mesencephalon of six rats.

Results

The cell bodies of CB-HRP-positive dCSF-CNs were found in the brain parenchyma near the midline of the ventral Aq, also in the grey of the 3V, and the 4V floor in the superior segment of the pons. In the mesencephalon their processes extended into the CSF. TRPM8 labeled neurons were also found in the same area as were CB-HRP/TRPM8 double-labeled neurons. CB-HRP/TRPM8 double-labeled neurons were found in 42.9 ± 2.3% of neurons labeled by TRPM8, and all CB-HRP-labeled neurons were also labeled with TPRM8.

Conclusion

This study has demonstrated that the cold sensation receptor channel, TRPM8, is localised within the dCSF-CNs of the mesencephalon. TRPM8 acts as receptor of dCSF-CNs for sensation transmission and pain regulation.

The trajectory of sensory pathways from the lamina terminalis to the insular and cingulate cortex: a neuroanatomical framework for the generation of thirst

The pathways involved in the emotional aspects of thirst, the arousal and affect associated with the generation of thirst and the motivation to obtain satiation, have been studied but remain poorly understood. Rats were therefore injected with the neurotropic virus pseudorabies in either the insular or cingulate cortex. After 2 days of infection, pseudorabies-positive neurons were identified within the thalamus and lamina terminalis. In a separate group of rats, the retrograde tracer cholera toxin subunit b (CTb) was used in combination with either isotonic (0.15 M NaCl) or hypertonic (0.8 M NaCl) saline (1 ml/100 g body wt ip). Rats injected with CTb in the insular cortex and stimulated with hypertonic saline had increased numbers of Fos/CTb double-positive neurons in the paraventricular, rhomboid, and reuniens thalamic nuclei, whereas those rats injected with CTb in the cingulate cortex and challenged with hypertonic saline had increased numbers of Fos/CTb double-positive neurons in the medial part of the mediodorsal, interanteromedial, anteromedial, and ventrolateral part of the laterodorsal thalamic nuclei. Rats injected with CTb in the dorsal midline of the thalamus and challenged with hypertonic saline had increased numbers of Fos/CTb double-positive neurons within the organum vasculosum of the lamina terminalis (OVLT), median preoptic nucleus, and insular cortex but not the subfornical organ. A small proportion of the CTb-positive neurons in the OVLT were immunopositive for transient receptor potential vanilloid 1, a putative osmoresponsive membrane protein. These results identify functional thalamocortical pathways involved in relaying osmotic signals to the insular and cingulate cortex and may provide a neuroanatomical framework for the emotional aspects of thirst.

GABAA receptors expression pattern in rat brain following low pressure distension of the stomach

It is known that gastric mechanoreceptor stimuli are widely integrated into neuronal circuits that involve visceral nuclei of hindbrain as well as several central brain areas. GABAergic neurons are widely represented in hindbrain nuclei controlling gastric motor functions, but limited information is available specifically about GABA(A)-responding neurons in brain visceral areas. The present investigation was designed to determine the central sensory neuronal pathways and their GABA(A)-alpha1 and -alpha3 receptor presenting neurons that respond to gastric mechanoreceptor stimulation within the entire rat brain. Low pressure gastric distension was used to deliver physiological mechanical stimuli in anesthetized rats, and different protocols of gastric distension were performed to mimic different stimulation patterns with and without sectioning vagal and/or splanchnic afferent nerves. Mapping of activated neurons was investigated using double colorimetric immunohistochemistry for GABA(A)-alpha1 or -alpha3 subunits and c-Fos. Following stomach distension, neurons expressing GABA(A) receptors with alpha1 or alpha3 subunits were detected. Low frequency gastric distension induced c-Fos expression in nucleus tractus solitarii (NTS) only, whereas in the high frequency gastric distension c-Fos positive nuclei were found in lateral reticular nucleus and in NTS in addition to some forebrain areas. In contrast, during the tonic-rapid gastric distension the neuronal activation was found in hindbrain, midbrain and forebrain areas. Moreover different protocols of gastric stimulation activated diverse patterns of neurons presenting GABA(A)-alpha1 or -alpha3 receptors within responding brain nuclei, which may indicate a probable functional significance of differential expression of GABA(A)-responding neurons. The same protocol of gastric distension performed in vagotomized rats has confirmed the primary role of the vagus in the response of activation of gastric brain areas, whereas neuronal input of splanchnic origins was shown to play an important role in modulating the mechanogastric response of brain areas.

Norepinephrinergic afferents and cytology of the macaque monkey midline, mediodorsal, and intralaminar thalamic nuclei

The midline and intralaminar thalamic nuclei (MITN), locus coeruleus (LC) and cingulate cortex contain nociceptive neurons. The MITN that project to cingulate cortex have a prominent innervation by norepinephrinergic axons primarily originating from the LC. The hypothesis explored in this study is that MITN neurons that project to cingulate cortex receive a disproportionately high LC input that may modulate nociceptive afferent flow into the forebrain. Ten cynomolgus monkeys were evaluated for dopamine-beta hydroxylase (DBH) immunohistochemistry, and nuclei with moderate or high DBH activity were analyzed for intermediate neurofilament proteins, calbindin (CB), and calretinin (CR). Sections of all but DBH were thionin counterstained to assure precise localization in the mediodorsal and MITN, and cytoarchitecture was analyzed with neuron-specific nuclear binding protein. Moderate-high levels of DBH-immunoreactive (ir) axons were generally associated with high densities of CB-ir and CR-ir neurons and low levels of neurofilament proteins. The paraventricular, superior centrolateral, limitans and central nuclei had relatively high and evenly distributed DBH, the magnocellular mediodorsal and paracentral nuclei had moderate DBH-ir, and other nuclei had an even and low level of activity. Some nuclei also have heterogeneities in DBH-ir that raised questions of functional segregation. The anterior multiformis part of the mediodorsal nucleus but not middle and caudal levels had high DBH activity. The posterior parafascicular nucleus (Pf) was heterogeneous with the lateral part having little DBH activity, while its medial division had most DBH-ir axons and its multiformis part had only a small number. These findings suggest that the LC may regulate nociceptive processing in the thalamus. The well established role of cingulate cortex in premotor functions and the projections of Pf and other MITN to the limbic striatum suggests a specific role in mediating motor outflow for the LC-innervated nuclei of the MITN.

Stimulation of lateral septum CRF2 receptors promotes anorexia and stress-like behaviors: functional homology to CRF1 receptors in basolateral amygdala

The corticotropin-releasing factor (CRF) system is the primary central mediator of stress-like states, coordinating behavioral, endocrine, and autonomic responses to stress. Although induction of anorexia is a well documented effect of CRF receptor agonist administration, the central sites and behavioral processes underlying this phenomenon are poorly understood. The present studies addressed this question by examining the neuroanatomical, behavioral, and pharmacological mechanisms mediating decreases in feeding produced by the CRF1/CRF2 receptor agonist urocortin. Separate groups of food-restricted male Sprague Dawley rats were given infusions of urocortin (0, 50, 125, 250 ng/0.5 microl) into the lateral septum (LS) and immediately afterward were rated on a wide array of behaviors (locomotion, rearing, grooming, stereotypies) including a microstructural analysis of ingestive behavior. Intra-LS urocortin infusion dose-dependently reduced feeding and drinking while concomitantly increasing grooming, stereotypies, and ethological plus traditional measures of anxiety-like responses in the elevated plus-maze. Urocortin infusion into neighboring sites (lateral ventricle, medial caudate) had no effects. Coinfusion into the LS of the mixed CRF1/CRF2 receptor antagonist D-Phe-CRF(12-41) (0, 100, 1000 ng/0.5 microl) or the novel selective CRF2 receptor antagonist Astressin2B (0, 500, 1000 ng/0.5 microl) blocked urocortin-induced effects, but the CRF1-selective antagonist NBI27914 (0, 500, 1000 ng/0.5 microl) had no effect, although it completely reversed the behavioral sequelae of CRF when infused into the basolateral amygdala. These results indicate that one of the modes through which the CRF system promotes anorexia is the recruitment of stress-like states after stimulation of CRF2 receptors within the LS.

Afferents of vocalization-controlling periaqueductal regions in the squirrel monkey

In order to determine the input of vocalization-controlling regions of the midbrain periaqueductal gray (PAG), wheat germ agglutinin-horseradish peroxidase was injected in six squirrel monkeys (Saimiri sciureus) at PAG sites yielding vocalization when injected with the glutamate agonist homocysteic acid. Brains were scanned for retrogradely labeled areas common to all six animals. The results show that the vocalization-eliciting sites receive a widespread input, with the heaviest projections coming from the surrounding PAG, dorsomedial and ventromedial hypothalamus, medial preoptic region, substantia nigra pars diffusa, zona incerta and reticular formation of the mesencephalon, pons, and medulla. The heaviest cortical input reaches the PAG from the mediofrontal cortex. Moderate to weak projections come from the insula, lateral prefrontal, and premotor cortex as well as the superior and middle temporal cortex. Subcortical moderate to weak projections reach the PAG from the central and medial amygdala, nucleus of the stria terminalis, septum, nucleus accumbens, lateral preoptic region, lateral and posterior hypothalamus, globus pallidus, pretectal area, deep layers of the superior colliculus, the pericentral inferior colliculus, mesencephalic trigeminal nucleus, locus coeruleus, substantia nigra pars compacta, dorsal and ventral raphe, vestibular nuclei, spinal trigeminal nucleus, solitary tract nucleus, and nucleus gracilis. The input of the periaqueductal vocalization-eliciting regions thus is dominated by limbic, motivation-controlling afferents; input, however, also comes from sensory, motor, arousal-controlling, and cognitive brain areas.

Polysynaptic pathways from the vestibular nuclei to the lateral mammillary nucleus of the rat: substrates for vestibular input to head direction cells

The activity of some neurons in the lateral mammillary nucleus (LMN) of the rat corresponds with the animal's current head direction (HD). HD cells have been studied extensively but the circuitry responsible for the generation and maintenance of the HD signal has not been established. The present study tested the hypothesis that a polysynaptic pathway connects the vestibular nuclei with the LMN via one or more relay nuclei. This circuitry could provide a substrate for the integration of sensory input necessary for HD cell activity. This hypothesis is based upon the prior demonstration that labyrinthectomy abolishes HD selectivity in thalamic neurons. Viral transneuronal tracing with pseudorabies virus (PRV) was used to test this hypothesis. We injected recombinants of PRV into the LMN and surrounding nuclei of adult male rats and defined the patterns of retrograde transneuronal infection at survival intervals of 60 and 72 h. Infected medial vestibular neurons (MVN) were only observed at the longest postinoculation interval in animals in which the injection site was localized largely to the LMN. Robust infection of the dorsal tegmental nucleus (DTN) and nucleus prepositus hypoglossi (PH) in these cases, but not in controls, at both survival intervals identified these nuclei as potential relays of vestibular input to the LMN. These data are consistent with the conclusion that vestibular information that contributes to the LMN HD cell activity is relayed to this caudal hypothalamic cell group via a polysynaptic brainstem circuit.

Opiate anti-nociception is attenuated following lesion of large dopamine neurons of the periaqueductal grey: critical role for D1 (not D2) dopamine receptors

The periaqueductal grey (PAG) area is involved in pain modulation as well as in opiate-induced anti-nociceptive effects. The PAG possess dopamine neurons, and it is likely that this dopaminergic network participates in anti-nociception. The objective was to further study the morphology of the PAG dopaminergic network, along with its role in nociception and opiate-induced analgesia in rats, following either dopamine depletion with the toxin 6-hydroxydopamine or local injection of dopaminergic antagonists. Nociceptive responses were studied through the tail-immersion (spinal reflex) and the hot-plate tests (integrated supraspinal response), establishing a cut-off time to further minimize animal suffering. Heroin and morphine were employed as opiates. Histological data indicated that the dopaminergic network of the PAG is composed of two types of neurons: small rounded cells, and large multipolar neurons. Following dopamine depletion of the PAG, large neurons (not small ones) were selectively affected by the toxin (61.9% dopamine cell loss, 80.7% reduction of in vitro dopaminergic peak), and opiate-induced analgesia in the hot-plate test (not the tail-immersion test) was reliably attenuated in lesioned rats (P < 0.01). After infusions of dopaminergic ligands into the PAG, D(1) (not D(2)) receptor antagonism attenuated opiate-induced analgesia in a dose-dependent manner in the hot-plate test. The present study provides evidence that large neurons of the dopaminergic network of the PAG participate in supraspinal (not spinal) nociceptive responses after opiates through the involvement of D(1) dopamine receptors. This dopaminergic system should be included as another network within the PAG involved in opiate-induced anti-nociception.

Differential modulation of nociceptive neural responses in medial and lateral pain pathways by peripheral electrical stimulation: a multichannel recording study

It is well accepted that peripheral electrical stimulation (PES) can produce an analgesic effect in patients with acute and chronic pain. However, the neural basis underlying stimulation-induced analgesia remains unclear. In the present study, we examined the pain-related neural activity modified by peripheral stimulation in rats. The stimulation frequency of pulses applied to needle electrodes in the hindlimb was 2 Hz alternating with 100 Hz, with 0.6 ms pulse width for 2 Hz and 0.2 ms for 100 Hz. The intensity of the stimulation was increased stepwise from 1 to 3 mA with each 1-mA step lasting for 10 min. The nociceptive neural and behavioral responses were examined immediately after the termination of stimulation. Using a multiple-channel recording technique, we simultaneously recorded the activity of many single neurons located in the primary somatosensory and anterior cingulate cortex (ACC), as well as the ventral posterior and medial dorsal thalamus in behaving rats. Our results showed that peripheral electrical stimulation significantly reduced the nociceptive responses in ventroposterior thalamus and somatosensory cortex, indicating an inhibition of nociceptive processing. In contrast, the analgesic stimulation produced a significant increase in mediodorsal thalamus while a less significant decrease in cingulate cortex, reflecting a complicated effect associated with combined antinociceptive activation and nociceptive suppression. These results support the idea that peripheral electrical stimulation can ultimately alter the pain perception by specifically inhibiting the nociceptive transmission in the sensory pathway while mobilizing the antinociceptive action in the affective pathway, thus to produce pain relief.

GABAergic projection from the ventral tegmental area and substantia nigra to the periaqueductal gray region and the dorsal raphe nucleus

Previous studies have shown that neurons in the ventral tegmental area (VTA) and substantia nigra (SN) project to the ventrolateral periaqueductal gray (PAGvl) and dorsal raphe nucleus (DR). Research has also shown that stimulation of neurons in the VTA/SN elicits cardiovascular depressor responses that are mediated by a projection to the PAGvl/DR. Anatomic and physiological experiments were done in the present study to determine the neurochemical identity of the VTA/SN projection to the PAGvl/DR. Experiments were done to characterize the origin and chemical nature of this projection by combining cholera toxin B tracing with immunofluorescence for the 67K isoform of glutamic acid decarboxylase (GAD) and tyrosine hydroxylase. The PAGvl/DR region was found to receive a substantial input from neurons in the VTA, SN, and deep mesencephalic nucleus. The DR was preferentially innervated by neurons in the VTA, whereas the PAGvl was preferentially innervated by neurons in the SN. A proportion of neurons in the VTA and the reticular portion of the SN found to project to the PAGvl/DR were GAD positive. In addition, experiments were done in urethane-anesthetized rats to determine whether injections of a gamma-aminobutyric acid (GABA) antagonist in the region of the PAGvl/DR attenuated the cardiovascular depressor responses produced by glutamate stimulation of the VTA/SN. Injections of the GABA-blocking agent picrotoxin (2.5 nmol, 500 nl) into the PAGvl/DR eliminated the cardiovascular responses from stimulation of the VTA/SN (0.01 M, 50 nl). The results of the present investigation provide evidence for a GABAergic projection from the VTA/SN to the PAGvl/DR. This projection may be an important regulator of the PAGvl/DR, an area of the midbrain involved in the production of behavioral and physiological responses to pain and stress.

The distributions and signaling directions of the cerebrospinal fluid contacting neurons in the parenchyma of a rat brain

Many studies have been made on the distributions of CSF contacting neurons (CSF-CNs) in the parenchyma of the brain with horseradish peroxidase (HRP) or autoradiographics. A significant amount of data has shown that both HRP and autoradiographical substances could pass freely through the spaces of ependyma into the parenchyma of the brain. It is therefore possible that the results were not exact. We found that CB-HRP was a dependable tracer to CSF-CNs and studied the distributions and the signaling directions of cerebrospinal fluid contacting neurons (CSF-CNs) in the parenchyma of the brain with the cholera toxin subunit B with horseradish peroxidase (CB-HRP) tracing combined with transmission electron microscopy. The results were as follows: (1) CSF contacting tanycytes existed not only in the wall of the third ventricle (3V), but also in the walls of the lateral ventricle (LV), the fourth ventricle (4V) and the central canal (CC) of the spinal cord. (2) Some CSF contacting glia cells were observed in the lateral septal nucleus (LS). (3)The distal CSF-CNs in the parenchyma were found in LS, the anterodorsal thalamic nucleus (AD), the supramammillary nucleus (SuM), the dorsal raphe nucleus (DR), the floor of 4V and the lateral superior olive (LSO), but they were mainly found in DR and divided into groups A and B. (4) Axon terminals labeled by CB-HRP were found in the cavity of the brain ventricle. (5) The synaptic relationships between the neurons were labeled by CB-HRP in DR and no-labeled by CB-HRP in the parenchyma. Both synapses Gray I and II were found. It was significant that the presynaptic elements were formed by the neurons no-labeled CB-HRP and the postsynaptic elements labeled CB-HRP. Our results suggested firstly that the signaling directions of CSF-CNs in DR were only from the parenchyma to CSF.

Neonatal estrogen decreases neural density of the septum-midbrain central gray connection underlying the lordosis-inhibiting system in female rats

Neurons in the lateral septum (LS) with projecting axons to the midbrain central gray (MCG) exert an inhibitory influence on lordosis. The number of such neurons is greater in female than in male rats. In this experiment, effects of neonatal estrogen on the density of the LS-MCG connections and on lordosis behavior were examined in female rats. On postnatal day 4 (day 0 = day of birth), females were injected subcutaneously with 50 or 100 mug estradiol benzoate (EB) or oil. On postnatal day 60, females and control males were gonadectomized. Behavioral tests were carried out after the implantation of silicone tubes containing estradiol. Lordotic activities in both males and EB-treated females were lower than in oil-treated females. After completing the behavioral tests, the animals were injected with Fluoro-Gold (FG), a retrograde tracer, into the right-side MCG and the number of FG-labeled neurons in the LS was measured. In all groups, the right-side LS ipsilateral to the FG injection had more FG-labeled neurons than the left-side LS. The number of FG-labeled neurons in the LS of oil-treated females was larger than that of males on both right and left sides. In the females treated with 100 mug EB (EB100), the number of FG-labeled neurons was comparable with that of males and lower than that of oil-treated females. The number of FG-labeled neurons in the EB50 females was also lower than that in oil-treated females, but tended to be larger than that observed in the EB100 group. These results indicate that neonatal estrogen decreases both lordotic activity and the density of the LS-MCG neural connections in female rats.

The neurocircuitry and receptor subtypes mediating anxiolytic-like effects of neuropeptide Y

This review aims to give a brief overview of NPY receptor distribution and physiology in the brain and summarizes series of studies, test by test and region by region, aimed at identification receptor subtypes and neuronal circuitry mediating anxiolytic-like effects of NPY. We conclude that from four known NPY receptor subtypes in the rat (Y(1), Y(2), Y(4), Y(5)), only the NPY Y(1) receptor can be linked to anxiety-regulation with certainty in the forebrain, and that NPY Y(2) receptor may have a role in the pons. Microinjection studies with NPY and NPY receptor antagonists support the hypothesis that the amygdala, the dorsal periaqueductal gray matter, dorsocaudal lateral septum and locus coeruleus form a neuroanatomical substrate that mediates anxiolytic-like effects of NPY. The release of NPY in these areas is likely phasic, as NPY receptor antagonists are silent on their own. However, constant NPY-ergic tone seems to exist in the dorsal periaqueductal gray, the only brain region where NPY Y(1) receptor antagonists had anxiogenic-like effects. We conclude that endogenous NPY has an important role in reducing anxiety and serves as a physiological stabilizer of neural activity in circuits involved in the regulation of arousal and anxiety.

Neural pathways underlying vocal control

Vocalization is a complex behaviour pattern, consisting of essentially three components: laryngeal activity, respiratory movements and supralaryngeal (articulatory) activity. The motoneurones controlling this behaviour are located in various nuclei in the pons (trigeminal motor nucleus), medulla (facial nucleus, nucl. ambiguus, hypoglossal nucleus) and ventral horn of the spinal cord (cervical, thoracic and lumbar region). Coordination of the different motoneurone pools is carried out by an extensive network comprising the ventrolateral parabrachial area, lateral pontine reticular formation, anterolateral and caudal medullary reticular formation, and the nucl. retroambiguus. This network has a direct access to the phonatory motoneurone pools and receives proprioceptive input from laryngeal, pulmonary and oral mechanoreceptors via the solitary tract nucleus and principal as well as spinal trigeminal nuclei. The motor-coordinating network needs a facilitatory input from the periaqueductal grey of the midbrain and laterally bordering tegmentum in order to be able to produce vocalizations. Voluntary control of vocalization, in contrast to completely innate vocal reactions, such as pain shrieking, needs the intactness of the forebrain. Voluntary control over the initiation and suppression of vocal utterances is carried out by the mediofrontal cortex (including anterior cingulate gyrus and supplementary as well as pre-supplementary motor area). Voluntary control over the acoustic structure of vocalizations is carried out by the motor cortex via pyramidal/corticobulbar as well as extrapyramidal pathways. The most important extrapyramidal pathway seems to be the connection motor cortex-putamen-substantia nigra-parvocellular reticular formation-phonatory motoneurones. The motor cortex depends upon a number of inputs for fulfilling its task. It needs a cerebellar input via the ventrolateral thalamus for allowing a smooth transition between consecutive vocal elements. It needs a proprioceptive input from the phonatory organs via nucl. ventralis posterior medialis thalami, somatosensory cortex and inferior parietal cortex. It needs an input from the ventral premotor and prefrontal cortex, including Broca's area, for motor planning of longer purposeful utterances. And it needs an input from the supplementary and pre-supplementary motor area which give rise to the motor commands executed by the motor cortex.

Projections of the paratrigeminal nucleus to the ambiguus, rostroventrolateral and lateral reticular nuclei, and the solitary tract

The paratrigerminal nucleus (Pa5), a constituent of the spinal interstitial system, was linked to the pressor effect caused by bradykinin injected in the dorsal lateral medulla of the rat. The nucleus receives primary afferent sensory fibers contained in branches of the trigeminal, glossopharyngeal and vagus nerves. In this investigation connections of the paratrigeminal nucleus to other medullary structures were studied with the use of retrograde and anterograde neuronal tracers. Fluorescent light microscopy analyses of medullary sections of rats injected with the retrograde transport tracer Fluoro-gold in the nucleus of the solitary tract (NTS) or in the pressor area of the rostral ventrolateral medulla (RVLM) revealed labeled neuronal cell bodies in the ipsi- and contralateral Pa5. FluoroGold microinjections in the caudal ventrolateral medulla (CVLM) did not produce fluorescent labeling of Pa5 neurons. Microinjection of the anterograde transport neuronal tracer biocytin in the Pa5 produced bilateral labeling of the solitary tract (sol). rostroventrolateral reticular nucleus (RVL), ambiguus nucleus (Amb), lateral reticular nucleus (LRt) and ipsilateral parabrachial nuclei, but not the contralateral Pa5. Confocal laser microscopy showed fluorescence labeling of fibers and presumptive terminal varicosities in the NTS, RVL, Amb and LRt. The present findings showing the paratrigeminal nucleus interposed between sensory afferent and stuctures associated to cardiovascular and respiratory functions, suggest that the structure may act as a medullary relay nucleus for sensory stimuli directly connecting primary afferents to structures mediating cardiovascular and respiratory reflexes.

Afferents to the central nucleus of the amygdala and functional subdivisions of the periaqueductal gray: neuroanatomical substrates for affective behavior

Evidence suggests the periaqueductal gray (PAG) is involved in the integration of behavioral and autonomic components of affective behavior. Our laboratory has shown that electrical stimulation of the ventrolateral periaqueductal gray (vl PAG) versus the dorsolateral periaqueductal gray (dl PAG), in the rabbit, elicits two distinct behavioral/cardiorespiratory response patterns. Furthermore, evidence suggests that the amygdaloid central nucleus (ACe) may influence cardiovascular activity during emotional states. The purpose of this study was to delineate the topography and determine the origin of forebrain projections to the PAG and the ACe, as well as commonalties and differences in the pattern of afferents. Examination of common afferents may lend insights into their function as components of a forebrain system regulating autonomic activity during emotional states. Separate retrograde tracers were injected into functional subdivisions of the PAG and the ACe in rabbits. PAG injections led to neuronal labeling in numerous cortical regions including the ipsilateral medial prefrontal and insular cortices. Additionally, bilateral labeling was observed in several hypothalamic nuclei including the paraventricular nucleus, the dorsomedial nucleus and the ventromedial nucleus as well as the region lateral to the descending column of the fornix. Sparse labeling was also seen in various basal forebrain regions, thalamic nuclei and amygdaloid nuclei. Many of these regions were also labeled following injections in the ACe. Although double-labeled cells were never observed, afferents to the ACe were often proximal to PAG afferents. Implications of these findings are discussed in terms of two functionally distinct behavioral/cardiovascular response patterns.