Caudal midline medulla mediates behaviourally-coupled but not baroreceptor-mediated vasodepression

Contribution of the serotonergic system to developmental brain abnormalities in autism spectrum disorder

This review highlights a key role of the serotonergic system in brain development and in distortions of normal brain development in early stages of fetal life resulting in cascades of abnormalities, including defects of neurogenesis, neuronal migration, neuronal growth, differentiation, and arborization, as well as defective neuronal circuit formation in the cortex, subcortical structures, brainstem, and cerebellum of autistic subjects. In autism, defects in regulation of neuronal growth are the most frequent and ubiquitous developmental changes associated with impaired neuron differentiation, smaller size, distorted shape, loss of spatial orientation, and distortion of cortex organization. Common developmental defects of the brain in autism include multiregional focal dysplastic changes contributing to local neuronal circuit distortion, epileptogenic activity, and epilepsy. There is a discrepancy between more than 500 reports demonstrating the contribution of the serotonergic system to autism's behavioral anomalies, highlighted by lack of studies of autistic subjects' brainstem raphe nuclei, the center of brain serotonergic innervation, and of the contribution of the serotonergic system to the diagnostic features of autism spectrum disorder (ASD). Discovery of severe fetal brainstem auditory system neuronal deficits and other anomalies leading to a spectrum of hearing deficits contributing to a cascade of behavioral alterations, including deficits of social and verbal communication in individuals with autism, is another argument to intensify postmortem studies of the type and topography of, and the severity of developmental defects in raphe nuclei and their contribution to abnormal brain development and to the broad spectrum of functional deficits and comorbid conditions in ASD.

Serotonin transporter inhibition during neonatal period induces sex-dependent effects on mitochondrial bioenergetics in the rat brainstem

The serotonin reuptake is mainly regulated by the serotonin transporters (SERTs), which are abundantly found in the raphe nuclei, located in the brainstem. Previous studies have shown that dysfunction in the SERT has been associated with several disorders, including depression and cardiovascular diseases. In this manuscript, we aimed to investigate how gender and the treatment with a serotonin selective reuptake inhibitor (SSRI) could affect mitochondrial bioenergetics and oxidative stress in the brainstem of male and female rats. Fluoxetine, our chosen SSRI, was used during the neonatal period (i.e., from postnatal Day 1 to postnatal Day 21-PND1 to PND21) in both male and female animals. Thereafter, experiments were conducted in adult rats (60 days old). Our results demonstrate that, during lactation, fluoxetine treatment modulates the mitochondrial bioenergetics in a sex-dependent manner, such as improving male mitochondrial function and female antioxidant capacity.

Effects of 12 Months Continuous Positive Airway Pressure on Sympathetic Activity Related Brainstem Function and Structure in Obstructive Sleep Apnea

Muscle sympathetic nerve activity (MSNA) is greatly elevated in patients with obstructive sleep apnea (OSA) during normoxic daytime wakefulness. Increased MSNA is a precursor to hypertension and elevated cardiovascular morbidity and mortality. However, the mechanisms underlying the high MSNA in OSA are not well understood. In this study we used concurrent microneurography and magnetic resonance imaging to explore MSNA-related brainstem activity changes and anatomical changes in 15 control and 15 OSA subjects before and after 6 and 12 months of continuous positive airway pressure (CPAP) treatment. We found that following 6 and 12 months of CPAP treatment, resting MSNA levels were significantly reduced in individuals with OSA. Furthermore, this MSNA reduction was associated with restoration of MSNA-related brainstem activity and structural changes in the medullary raphe, rostral ventrolateral medulla, dorsolateral pons, and ventral midbrain. This restoration occurred after 6 months of CPAP treatment and was maintained following 12 months CPAP. These findings show that continual CPAP treatment is an effective long-term treatment for elevated MSNA likely due to its effects on restoring brainstem structure and function.

Maps of cardiovascular and respiratory regions of rat ventral medulla: focus on the caudal medulla

The ventral medulla oblongata is critical for cardiorespiratory regulation. Here we review previous literature relating to sites within the ventral medulla that have been identified as having a 'cardiovascular' or 'respiratory' function. Together with the maps generated here, of sites from which cardiovascular and respiratory responses were evoked by glutamate microinjection, specific 'cardiovascular' regions have been defined and delineated. Commonly investigated regions, including the vasopressor rostral ventrolateral medulla (RVLM) and vasodepressor caudal ventrolateral medulla (CVLM), or areas only described by others, such as the medullary cerebral vasodilator area, are included for completeness. Emphasis is given to the caudal medulla, where three pressor regions, the caudal pressor area (CPA), the intermediate pressor area (IPA) and the medullo-cervical pressor area (MCPA), caudal to the vasodepressor CVLM were defined in the original data provided. The IPA is most responsive under pentobarbitone rather than urethane anaesthesia clearly delineating it from both the rostrally located CPA and the caudally located MCPA. The description of these multiple pressor areas appears to clarify the confusion that surrounds the identification of the 'CPA'. Also noted is a vasopressor region adjacent to the vasodepressor CVLM. Apart from the well described ventral respiratory column, a region medial to the pre-Bötzinger is described, from which increases in both phrenic nerve frequency and amplitude were evoked. Limitations associated with the technique of glutamate microinjection to define functionally specific regions are discussed. Particular effort has been made to define and delineate the regions with respect to ventrally located anatomical landmarks rather than the commonly used ventral surface or dorsal landmarks such as the obex or calamus scriptorius that may vary with the brain orientation or histological processing. This should ensure that a region can easily be defined by all investigators. Study of defined regions will help expedite the identification of the role of the multiple cell groups with diverse neurotransmitter complements that exist even within each of the regions described, in coordinating the delivery of oxygenated blood to the tissues.

Vulnerability to simple faints is predicted by regional differences in brain anatomy

Neurocardiogenic syncope (NCS, simple fainting) is a common and typically benign familial condition, which rarely may result in traumatic injury or hypoxic convulsions. NCS is associated with emotional triggers, anxiety states and stress. However, the etiology of NCS, as a psychophysiological process, is poorly understood. We therefore investigated the relationship between NCS and brain anatomy. We studied a non-clinical sample of eighteen individuals with histories characteristic of NCS, and nineteen matched controls who had never fainted. We recorded fainting frequency, resting heart rate variability measures and anxiety levels. Structural T1-weighted magnetic resonance images (MRI) were acquired at 1.5 T. Associations between brain morphometry (regional gray and white matter volumes) and NCS, resting physiology and anxiety were tested using voxel-based morphometry (VBM). Compared to controls, NCS participants had lower regional brain volume within medulla and midbrain (a priori regions of interest). Moreover, across NCS individuals, lower gray matter volume in contiguous regions of left caudate nucleus predicted enhanced parasympathetic cardiac tone, fainting frequency and anxiety levels. Our findings provide preliminary evidence for a hierarchical anatomical basis to NCS. First, differences in the volume of brainstem centers supporting cardiovascular homeostasis may relate to constitutional predisposition to NCS. Second, differences in the structural organization of the caudate nucleus in NCS individuals may relate to fainting frequency via interactions between emotional state and parasympathetic control of the heart. These observations highlight the application of VBM to the identification of neurovisceral mechanisms relevant to psychosomatic medicine and the neuroscience of emotion.

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

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

Hypotensive but not normotensive haemorrhage increases tryptophan hydroxylase-2 mRNA in caudal midline medulla

Severe blood loss triggers shock, a precipitous hypotension and bradycardia. The integrity of (i) neurons in the vasodepressor region of the caudal midline medulla and (ii) central 5-HT neurotransmission are critical for the expression of haemorrhagic shock. This study investigated whether progressive blood loss triggers altered synthesis of 5-HT in the vasodepressor region of the caudal midline medulla by measuring changes in relative expression levels of tryptophan hydroxylase 2 (TpH 2) mRNA, the rate-limiting enzyme in the synthesis of neuronal 5-HT. Hypotensive but not normotensive haemorrhage triggered a significant increase in TpH 2 mRNA in the vasodepressor region of the caudal midline medulla, identifying an important role for 5-HT-containing caudal midline medullary neurons in haemorrhagic shock.

The role of prostaglandins and the hypothalamus in thermoregulation in the lizard, Phrynocephalus przewalskii (Agamidae)

Typically, small lizards rely heavily on behavioral thermoregulation rather than physiological mechanisms to control their rates of warming and cooling. We tested the hypothesis that prostaglandins participate in mediating the cardiovascular response to heating and cooling and temperature regulating neurons in the hypothalamus of the small lizard Phrynocephalus przewalskii. In vivo and in vitro treatments, heart rates (HRs) were all found to be higher during heating than during cooling, hysteresis was distinct below 30 and 26 degrees Celsius, respectively. In vivo, as administration of COX inhibitor, there were no differences in HR between heating and cooling at any body temperature and administration of agonist prostaglandins only produced a significant effect on HR below 25 degrees Celsius. Single-unit activity was recorded extracellularly in vitro with microelectrodes, found the firing rate of the continuous unit increased 23% when the temperature of the artificial cerebrospinal fluid dropped from 30-20 degrees Celsius. We conclude that prostaglandins appear to play only a limited role in modulating heart activity in Phrynocephalus przewalskii and suggest that cold-sensitive neurons in the preoptic and anterior hypothalamus (PO/AH) are involved in thermoregulatory control during heating or cooling.

Haemorrhage-evoked decompensation and recompensation mediated by distinct projections from rostral and caudal midline medulla in the rat

The haemodynamic response to blood loss consists of three phases: (i) an initial compensatory phase during which resting arterial pressure is maintained; (ii) a decompensatory phase characterized by a sudden, life-threatening hypotension and bradycardia; and (iii) if blood loss ceases, a recompensatory phase during which arterial pressure returns to normal. Previous research indicates that topographically distinct, rostral and caudal parts of the caudal midline medulla (CMM) contain neurons that differentially regulate the timing and magnitude of each of the three phases. Specifically, decompensation depends critically on the integrity of the rostral CMM; whereas compensation and recompensation depend upon the integrity of the caudal CMM. This study aimed to determine, using retrograde and anterograde tracing techniques, if the rostral and caudal CMM gave rise to different sets of projections to the major cardiovascular region of the ventrolateral medulla (VLM) and spinal cord. It was found that rostral and caudal CMM each have projections of varying density to the region containing bulbospinal (presympathetic) motor neurons in the rostral VLM and preganglionic sympathetic motor neurons in the intermediolateral cell column of the spinal cord. Via these projections vasomotor tone and hence arterial pressure can be regulated. More strikingly: (i) consistent with a role in mediating bradycardia during decompensation, the rostral CMM projects uniquely to VLM regions containing vagal cardiac motor neurons; and (ii) consistent with its role in mediating recompensation, the caudal CMM projects uniquely onto tyrosine hydroxylase-containing, caudal VLM (A1) neurons whose activity mediates vasopressin release, on which recompensation depends.

The hypotension evoked by visceral nociception is mediated by delta opioid receptors in the periaqueductal gray

This study tested the hypothesis that the ventrolateral column of the midbrain periaqueductal gray (vlPAG) region mediates the hypotension and bradycardia evoked by visceral nociception. To test this, the local anesthetic lidocaine (2%; 0.5 microl) was microinjected into the vlPAG of halothane-anesthetized rats bilaterally and visceral nociception was induced 2 min later by injecting 5% acetic acid (0.5 ml) intraperitoneally. Acetic acid injection caused an abrupt fall in arterial pressure (-12.2+/-2.1 mm Hg) and heart rate (-37+/-93 bpm) lasting approximately 15 min. Lidocaine injection into the vlPAG prevented the fall in arterial pressure and heart rate completely. Cobalt chloride (5 mM; 0.2 or 0.5 microl) injection into the vlPAG also prevented nociceptive hypotension but it did not affect the fall in heart rate significantly. Lidocaine pretreatment also inhibited the depressor response caused by intramuscular formalin (5%; 0.2 ml) administration, a model of deep somatic nociception, although it did not prevent the response completely. To determine if opioid receptors mediate the response, selective mu, delta or kappa opioid receptor antagonists were microinjected into the vlPAG 5 min before intraperitoneal (ip) acetic acid administration. Naltrindole, a delta receptor antagonist, inhibited the response significantly but mu and kappa antagonists were completely ineffective. Lidocaine and naltrindole had no effect when injected into the dorsolateral PAG and did not influence cardiovascular function when injected into the vlPAG of saline treated control animals. These data support the hypothesis that the vlPAG mediates the depressor response evoked by visceral nociception and indicate that delta opioid receptors participate in the response.

Integration of autonomic and local mechanisms in regulating cardiovascular responses to heating and cooling in a reptile (Crocodylus porosus)

Reptiles change heart rate and blood flow patterns in response to heating and cooling, thereby decreasing the behavioural cost of thermoregulation. We tested the hypothesis that locally produced vasoactive substances, nitric oxide and prostaglandins, mediate the cardiovascular response of reptiles to heat. Heart rate and blood pressure were measured in eight crocodiles (Crocodylus porosus) during heating and cooling and while sequentially inhibiting nitric-oxide synthase and cyclooxygenase enzymes. Heart rate and blood pressure were significantly higher during heating than during cooling in all treatments. Power spectral density of heart rate and blood pressure increased significantly during heating and cooling compared to the preceding period of thermal equilibrium. Spectral density of heart rate in the high frequency band (0.19-0.70 Hz) was significantly greater during cooling in the saline treatment compared to when nitric-oxide synthase and cyclooxygenase enzymes were inhibited. Cross spectral analysis showed that changes in blood pressure preceded heart rate changes at low frequencies (< 0.1 Hz) only. We conclude that the autonomic nervous system controls heart rate independently from blood pressure at higher frequencies while blood pressure changes determine heart rate at lower frequencies. Nitric oxide and prostaglandins do not control the characteristic heart rate hysteresis response to heat in C. porosus, although nitric oxide was important in buffering blood pressure against changes in heart rate during cooling, and inhibition caused a compensatory decrease in parasympathetic stimulation of the heart.

Differential development of 5-HT receptor and the serotonin transporter binding in the human infant medulla

Tissue receptor autoradiography with 3H-lysergic acid diethylamide (3H-LSD), 3H-8-hydroxy-2-[di-N-propylamine] tetralin (3H-8-OH-DPAT), and 125I-RTI-55 was used to map the distribution and developmental profile of 5-HT(1A-1D) and 5-HT2 receptors, 5-HT1A receptors, and the serotonin (5-HT) transporter (SERT), respectively, to nuclei with cardiorespiratory function in the human medulla from midgestation to maturity. The distribution pattern of the 5-HT markers was heterogeneous, with variable densities of binding of each observed both in nuclei with and without 5-HT cell bodies. The highest density of binding for each marker was observed in the raphé nuclei, the site of the highest density of 5-HT cell bodies. A significant reduction in 5-HT receptor binding measured with 3H-LSD was observed between midgestation and infancy, and between infancy and maturity in multiple nuclei, but no changes were observed across infancy. A significant increase in 5-HT1A receptor binding density was observed across infancy in the hypoglossal nucleus (regression slope coefficient = 0.008 +/- 0.002, P = 0.02), and a marginally significant increase was observed in the raphé obscurus (regression slope coefficient = 0.061 +/- 0.026 [mean +/- SEM], P = 0.05). No significant age-related changes in SERT binding were observed at any time. With the exception of the hypoglossal nucleus, where 5-HT1A receptor binding increases while SERT binding remains stable, the medullary 5-HT markers analyzed in the study are essentially "in place" at birth. This study provides important baseline data that serve as a foundation for future work in pediatric 5-HT brainstem disorders, including sudden infant death syndrome.

A mapping study of cardiorespiratory responses to chemical stimulation of the midline medulla oblongata in ventilated and freely breathing rats

The aim of this study was to examine the cardiorespiratory effects of chemically stimulating neurons in the midline medulla oblongata (MM) of artificially ventilated and freely breathing anesthetized rats. Earlier studies reported that stimulation of the MM elicits increases or decreases in mean arterial pressure (MAP) and phrenic nerve activity, depending on the mode and site of stimulation, anesthetic, and species. In the first series of experiments, rats were anesthetized with urethane, artificially ventilated, paralyzed, and bilaterally vagotomized. The rostrocaudal extent of the MM was mapped by microinjections of DL-homocysteic acid or L-glutamate (both 100 mM, 100 nl), and, in line with previous studies, most injections produced only small responses in MAP, heart rate, and splanchnic sympathetic nerve activity. Increases in respiratory parameters were evoked in caudal regions. However, activation of a discrete region of the MM at the level of the caudal pole of the facial nucleus (CP7) consistently caused a dramatic reduction in phrenic nerve amplitude and/or frequency and, in six rats, produced a prolonged apnea. The second series of experiments was carried out on freely breathing pentobarbitone sodium-anesthetized rats, with a diaphragmatic electromyogram used to monitor respiratory activity. Respiratory activity could again be abolished at CP7 after microinjections of glutamate (100 mM, 50 nl); however, these responses were accompanied by large decreases in MAP and moderate reductions in heart rate. This depression of respiratory activity may be due to activation of propriobulbar inhibitory neurons that project to known respiratory centers in the brain stem.

Haemodynamic response to haemorrhage: distinct contributions of midbrain and forebrain structures

The haemodynamic response to a fixed volume haemorrhage passes through three distinct phases: a normotensive, compensatory phase; a hypotensive, decompensatory phase; and a post-haemorrhage, recompensatory phase. The role of the forebrain and midbrain in regulating the triphasic response to a 'fast' (1.5%/min) or 'slow' (0.75%/min) rate of blood withdrawal (30% haemorrhage) was evaluated by comparing, in unanaesthetised rats, the effects of pre-collicular (PCD) vs. pre-trigeminal decerebrations (PTD). It was found that pre-trigeminal decerebration attenuated the decompensatory (hypotensive) phase to either a fast or slow haemorrhage. In contrast, pre-collicular decerebration attenuated the compensatory and recompensatory phases of the response to a 'fast' (but not a slow) haemorrhage. These results suggest that the integrity of (i) forebrain structure(s) are critical for compensatory and recompensatory responses to 'rapid' blood loss; and (ii) midbrain structure(s) are critical for the decompensatory response to progressive blood loss irrespective of rate.

Sympathoinhibitory pathway from caudal midline medulla to RVLM is independent of baroreceptor reflex pathway

Glutamate stimulation of the caudal midline medulla (CMM) causes profound sympathoinhibition due to GABAergic inhibition of presympathetic neurons in the rostral ventrolateral medulla (RVLM). We investigated whether the sympathoinhibitory pathway from CMM to RVLM, like the central baroreceptor reflex pathway, includes a glutamatergic synapse in the caudal ventrolateral medulla (CVLM). In pentobarbital sodium-anesthetized rats, the RVLM on one side was inhibited by a muscimol microinjection. Then the response evoked by glutamate microinjections into the CMM or by baroreceptor stimulation was determined before and after 1) microinjection of the GABA receptor antagonist bicuculline into the RVLM on the other side or 2) microinjections of the glutamate receptor antagonist kynurenate bilaterally into the CVLM. Bicuculline in the RVLM greatly reduced both CMM- and baroreceptor-evoked sympathoinhibition. Compared with the effect of vehicle solution, kynurenate in the CVLM greatly reduced baroreceptor-evoked sympathoinhibition, whereas its effect on CMM-evoked sympathoinhibition was not different from that of the vehicle solution. These findings indicate that the output pathway from CMM sympathoinhibitory neurons, unlike the baroreceptor and other reflex sympathoinhibitory pathways, does not include a glutamatergic synapse in the CVLM.

Somatic and visceral afferents to the 'vasodepressor region' of the caudal midline medulla in the rat

Previous research has found that the integrity of a restricted region of the caudal midline medulla (including caudal portions of nucleus raphé obscurus and nucleus raphé pallidus) was critical for vasodepression (hypotension, bradycardia, decreased cardiac contractility) evoked either by haemorrhage or deep pain. In this anatomical tracing study we found that the vasodepressor part of the caudal midline medulla (CMM) receives inputs arising from spinal cord, spinal trigeminal nucleus (SpV) and nucleus of the solitary tract (NTS). Specifically: (i) a spinal-CMM projection arises from neurons of the deep dorsal horn, medial ventral horn and lamina X at all spinal segmental levels, with approximately 60% of the projection originating from the upper cervical spinal cord (C1-C4); (ii) a SpV-CMM projection arises primarily from neurons at the transition between subnucleus caudalis and subnucleus interpolaris; (iii) a NTS-CMM projection arises primarily from neurons in ventrolateral and medial subnuclei. In combination, the specific spinal, SpV and NTS regions which project to the CMM receive the complete range of somatic and visceral afferents known to trigger vasodepression. The role(s) of each specific projection is discussed.

Haemorrhage-evoked compensation and decompensation are mediated by distinct caudal midline medullary regions in the urethane-anaesthetised rat

Previous research using microinjections of excitatory amino acids suggested that the caudal midline medulla (including nucleus raphe obscurus and nucleus raphe pallidus) contained a mixed population of sympathoexcitatory and sympathoinhibitory neurones. The results of this study indicate that different anaesthetic regimes (urethane versus halothane) determine whether sympathoexcitatory (urethane only) or sympathoinhibitory (halothane only) responses are evoked by stimulation within distinct caudal midline medullary regions. In addition, anaesthetic regimes also affect the caudal midline medullary-mediated response to haemorrhage. Specifically, under conditions of urethane anaesthesia, inactivation (lignocaine) of the midline medullary region immediately caudal to the obex, prematurely triggered and dramatically potentiated the hypotension and bradycardia evoked by 15% haemorrhage; whereas under halothane anaesthesia, inactivation of the same region had no effect. In contrast, under urethane anaesthesia, inactivation of the midline medullary region immediately rostral to the obex, delayed the onset of the hypotension and bradycardia to 15% haemorrhage; inactivation of the same region under halothane anaesthesia blocked haemorrhage-evoked hypotension and bradycardia. Our findings indicate that topographically distinct parts of the caudal midline medulla contain neurones (i) that differentially regulate the timing and magnitude of the compensatory (normotensive) versus decompensatory (hypotensive) phases of the response to haemorrhage; and (ii) whose activity is altered by urethane versus halothane anaesthesia.

Delta- and kappa-opioid receptors in the caudal midline medulla mediate haemorrhage-evoked hypotension

In mammals blood loss can trigger, shock, an abrupt, life-threatening hypotension and bradycardia. In the halothane-anaesthetised rat this response is blocked by inactivation of a discrete, vasodepressor area in the caudal midline medulla (CMM). Haemorrhagic shock is blocked also by systemic or ventricular injections of the opioid antagonist, naloxone. This study investigated, in the halothane anaesthetised rat, the contribution of delta-, kappa- and mu-opioid receptors in the CMM vasodepressor region to haemorrhage-evoked shock (i.e. hypotension and bradycardia) and its recovery. It was found that microinjections into the CMM of the delta-opioid receptor antagonist, naltrindole delayed and attenuated the hypotension and bradycardia evoked by haemorrhage, but did not promote recompensation. In contrast, CMM microinjections of the kappa-opioid receptor antagonist, nor-binaltorphamine, although it did not alter haemorrhage-evoked hypotension and bradycardia, did lead to a rapid restoration of AP, but not HR. CMM microinjections of the mu-opioid receptor antagonist, CTAP had no effect on haemorrhage-evoked shock or recompensation. These data indicate that delta- and kappa- (but not mu-) opioid receptor-mediated events within the CMM contribute to the hypotension and bradycardia evoked by haemorrhage and the effectiveness of naloxone in reversing shock.

Neural responses to intravenous serotonin revealed by functional magnetic resonance imaging

We examined the sequence of neural responses to the hypotension, bradycardia, and apnea evoked by intravenous administration of 5-hydroxytryptamine (serotonin). Functional magnetic resonance imaging signal changes were assessed in nine isoflurane-anesthetized cats during baseline and after a bolus intravenous low dose (10 microg/kg) or high dose (20-30 microg/kg) of 5-hydroxytryptamine. In all cats, high-dose challenges elicited rapid-onset, transient signal declines in the intermediate portion of the solitary tract nucleus, caudal midline and caudal and rostral ventrolateral medulla, and fastigial nucleus of the cerebellum. Slightly delayed phasic declines appeared in the dentate and interpositus nuclei and dorsolateral pons. Late-developing responses also emerged in the solitary tract nucleus, parapyramidal region, periaqueductal gray, spinal trigeminal nucleus, inferior olivary nucleus, cerebellar vermis, and fastigial nucleus. Amygdala and hypothalamic sites showed delayed and prolonged signal increases. Intravenous serotonin infusion recruits cerebellar, amygdala, and hypothalamic sites in addition to classic brain stem cardiopulmonary areas and exhibits site-specific temporal patterns.

Evidence that hemorrhagic hypotension is mediated by the ventrolateral periaqueductal gray region

Severe hemorrhage lowers arterial pressure by suppressing sympathetic activity. This study tested the hypothesis that the decompensatory phase of hemorrhage is mediated by the ventrolateral periaqueductal gray (vlPAG), a region importantly involved in the autonomic and behavioral responses to stress and trauma. Neuronal activity in the vlPAG was inhibited with either lidocaine or cobalt chloride 5 min before hemorrhage (2.5 ml/100 g body wt) was initiated in conscious, unrestrained rats. Bilateral injection of lidocaine (0.5 microl of a 2% or 1 microl of a 5% solution) into the caudal vlPAG delayed the onset and reduced the magnitude of the hypotension produced by hemorrhage significantly. In contrast, inactivation of the dorsolateral PAG with lidocaine was ineffective. Cobalt chloride (5 mM; 0.5 microl), which inhibits synaptic transmission but not axonal conductance, also attenuated hemorrhagic hypotension significantly. Microinjection of lidocaine or cobalt chloride into the vlPAG of normotensive, nonhemorrhaged rats did not influence cardiovascular function. These data indicate that the vlPAG plays an important role in the response to hemorrhage.

The effect of adrenergic compounds on neurogenic dural vasodilatation

The pharmacology of neurogenic trigeminovascular vasodilator responses in the dura mater is of interest for understanding the pathophysiology of migraine and to develop new therapies for this disabling common condition. Aminergic mechanisms have been implicated in migraine through direct study of amines in patients, and by inference from the pharmacology of many effective anti-migraine compounds, particularly preventative agents. This study used intravital microscopy to assess the role of aminergic transmission in neurogenic dural vasodilatation (NDV) by measuring directly the diameter of dural arteries in sodium pentobarbitone anaesthetised rats. Electrical stimulation of a closed cranial window produces, by local depolarisation of nerves, dural vessel dilation that is monitored continuously on-line using video-microscopy and a video dimension analyser. This dural vasodilatation was not affected by pre-treatment with an alpha1-adrenoceptor agonist (phenylephrine, 1 and 5 microg/kg), or antagonist (corynanthine, 1 and 2 mg/kg), nor by an alpha2-adrenoceptor agonist (UK14,304, 5 microg/kg) or antagonist (yohimbine, 1 and 3 mg/kg). Similarly, we saw no effect of beta-adrenoceptor blockade (propranolol, 1 and 3 mg/kg). The lack of an inhibitory effect of UK14,304 the model of neurogenic dural vasodilation contrasts with its effect in neurogenic dural plasma protein extravasation model. The lack of inhibition of beta-adrenoceptor antagonists in the neurogenic vasodilatation model contrasts with their usefulness as migraine prophylactics, and suggests that their mechanism of action in migraine is unlikely to be through sensory trigeminal fibre terminals at the neurovascular junction. Moreover, the data indicate that the adrenergic system does not play a significant role in neurogenic dural vasodilation.

Neural mechanisms in the cardiovascular responses to acute central hypovolaemia

1. The haemodynamic response to acute central hypovolaemia consists of two phases. During phase I, arterial pressure is well maintained in the face of falling cardiac output (CO) by baroreceptor-mediated reflex vasoconstriction and cardio-acceleration. Phase II commences once CO has fallen to a critical level of 50-60% of its resting value, equivalent to loss of approximately 30% of blood volume. 2. During phase II, sympathetic vasoconstrictor and cardiac drive fall abruptly and cardiac vagal drive increases. In humans, this response is invariably associated with fainting and has been termed vasovagal syncope. 3. In both experimental animals and in humans, the responses to acute central hypovolaemia are greatly affected by anaesthetic agents, in that the compensatory responses during phase I (e.g. halothane) or their failure during phase II (e.g. alfentanil) are blunted or abolished. 4. Therefore, our present knowledge of the neurochemical basis of the response to hypovolaemia depends chiefly on the results of experiments in conscious animals. Use of techniques for simulating haemorrhage has greatly enhanced this research effort, by allowing the effects of multiple treatments on the response to acute central hypovolaemia to be tested in the same animal. 5. The results of such experiments indicate that phase II of the response to hypovolaemia is triggered, at least in part, by a signal from cardiac vagal afferents. There is also strong evidence that phase II depends on brainstem delta-opioid receptor and nitrergic mechanisms and can potentially be modulated by circulating or neuronally released adrenocorticotropic hormone, brainstem serotonergic pathways operating through 5-HT1A receptors and opioids acting through mu- and kappa-opioid receptors in the brainstem. 6. Phase II also appears to require input from supramedullary brain centres. Future studies should determine how these neurotransmitter systems interact and their precise neuroanatomical arrangements.

Medullary serotonergic network deficiency in the sudden infant death syndrome: review of a 15-year study of a single dataset

The sudden infant death syndrome (SIDS) is the leading cause of postneonatal infant mortality in the United States today, despite a dramatic 38% decrease in incidence due to a national risk reduction campaign advocating the supine sleep position. Our research in SIDS brainstems, beginning in 1985 and involving a single, large dataset, has become increasingly focused upon a specific neurotransmitter (serotonin) and specific territories (ventral medulla and regions of the medullary reticular formation that contain secrotonergic neurons). Based on this research, we propose that SIDS, or a subset of SIDS, is due to a developmental abnormality in a medullary network composed of (at least in part) rhombic lip-derived, serotonergic neurons, including in the caudal raphé and arcuate nucleus (putative human homologue of the cat respiratory chemosensitive fields); and this abnormality results in a failure of protective responses to life-threatening stressors (e.g. asphyxia, hypoxia, hypercapnia) during sleep as the infant passes through a critical period in homeostatic control. We call this the medullary serotonergic network deficiency hypothesis. We review the triple-risk model for SIDS, the development of the dataset using tissue autoradiography for analyzing neurotransmitter receptor binding; age-dependent baseline neurochemical findings in the human brainstem during early life; the evidence for serotonergic, rhombic lip, and ventral medullary deficits in at least some SIDS victim; possible mechanisms of sudden infant death related to these deficits; and potential causes of the deficits in the medullary serotonergic network in SIDS victims. We conclude with a summary of future directions in SIDS brainstem research.