Changes in the tail surface temperature of the rat following injection of 5-hydroxytryptamine into the ventrolateral medulla

Structural and functional characteristics of the special regulatory devices in the peripheral pulmonary circulation in rabbits

The present study intended to describe in detail the several blood vessels harboring special regulatory devices in rabbit's pulmonary tissue using light and electron microscopy and immuno-histochemistry. Numerous throttle arteries were recorded within the adventitia of the segmental and sub-segmental bronchi and within pulmonary pleura. These arteries showed characteristic narrow or obliterated lumens and some of them bear longitudinal muscular intimal bolsters. For the first time, TEM revealed some structural modifications of the vascular endothelial cells of these arteries indicating that they become more activated to perform some additional functions. Arteriovenous anastomoses (AVAs) including direct shunt vessels and glomus organs were also recognized. Direct arteriovenous shunts appeared as small connecting devices communicating between small arteries and small veins while glomus organs consisted of the tortuous glomus vessels and the related afferent and efferent vessels. Several arteries and veins showing unique unusual structural characteristics were also described. For the first time, serotonin (5-HT) was strongly expressed in the vascular endothelium and muscle fibers of throttle arteries, in glomus cells of the glomus vessels, and in vascular endothelium of some veins and venules of special structure. The exact role of 5-HT is still unknown and further investigations are required to determine the types and distribution of 5-HT receptors present in these vascular devices. We concluded that these special vascular devices can play a critical role in controlling blood flow and pressure in the peripheral pulmonary circulation; however, the exact physiological mechanisms by which they work or are controlled remain unknown providing a ripe area for further investigation.

Thermoregulatory vasomotor tone of the rat tail and paws in thermoneutral conditions and its impact on a behavioral model of acute pain

The tail and paws in rodents are heat exchangers involved in the maintenance of core body temperature (T(core)). They are also the most widely used target organs to study acute or chronic "models" of pain. We describe the fluctuations of vasomotor tone in the tail and paws in conditions of thermal neutrality and the constraints of these physiological processes on the responses to thermal nociceptive stimuli, commonly used as an index of pain. Skin temperatures were recorded with a calibrated thermal camera to monitor changes of vasomotor tone in the tail and paws of awake and anesthetized rats. In thermoneutral conditions, the sympathetic tone fluctuated at a rate of two to seven cycles/h. Increased mean arterial blood pressure (MAP; ∼46 mmHg) was followed by increased heart rate (HR; ∼45 beats/min) within 30 s, vasoconstriction of extremities (3.5-7°C range) within 3-5 min, and increased T(core) (∼0.7°C) within 6 min. Decreased MAP was followed by opposite events. There was a high correlation between HR and T(core) recorded 5-6 min later. The reaction time of the animal's response to a radiant thermal stimulus-heat ramp (6°C/s, 20 mm(2) spot) generated by a CO2 laser-directed to the tail depends on these variations. Consequently, the fluctuations in tail and paw temperature thus represent a serious confound for thermal nociceptive tests, particularly when they are conducted at thermal neutrality.

5-hydroxytryptamine (5-HT) reduces total peripheral resistance during chronic infusion: direct arterial mesenteric relaxation is not involved

Serotonin (5-hydroxytryptamine; 5-HT) delivered over 1 week results in a sustained fall in blood pressure in the sham and deoxycorticosterone acetate (DOCA)-salt rat. We hypothesized 5-HT lowers blood pressure through direct receptor-mediated vascular relaxation. In vivo, 5-HT reduced mean arterial pressure (MAP), increased heart rate, stroke volume, cardiac index, and reduced total peripheral resistance during a 1 week infusion of 5-HT (25 µg/kg/min) in the normotensive Sprague Dawley rat. The mesenteric vasculature was chosen as an ideal candidate for the site of 5-HT receptor mediated vascular relaxation given the high percentage of cardiac output the site receives. Real-time RT-PCR demonstrated that mRNA transcripts for the 5-HT_2B, 5-HT_1B, and 5-HT₇ receptors are present in sham and DOCA-salt superior mesenteric arteries. Immunohistochemistry and Western blot validated the presence of the 5-HT_2B, 5- HT_1B and 5-HT₇ receptor protein in sham and DOCA-salt superior mesenteric artery. Isometric contractile force was measured in endothelium-intact superior mesenteric artery and mesenteric resistance arteries in which the contractile 5- HT_2A receptor was antagonized. Maximum concentrations of BW-723C86 (5- HT_2B agonist), CP 93129 (5-HT_1B agonist) or LP-44 (5-HT₇ agonist) did not relax the superior mesenteric artery from DOCA-salt rats vs. vehicle. Additionally, 5-HT (10^–9 M to 10^–5 M) did not cause relaxation in either contracted mesenteric resistance arteries or superior mesenteric arteries from normotensive Sprague- Dawley rats. Thus, although 5-HT receptors known to mediate vascular relaxation are present in the superior mesenteric artery, they are not functional, and are therefore not likely involved in a 5-HT-induced fall in total peripheral resistance and MAP.

Serotonin and blood pressure regulation

5-Hydroxytryptamine (5-HT; serotonin) was discovered more than 60 years ago as a substance isolated from blood. The neural effects of 5-HT have been well investigated and understood, thanks in part to the pharmacological tools available to dissect the serotonergic system and the development of the frequently prescribed selective serotonin-reuptake inhibitors. By contrast, our understanding of the role of 5-HT in the control and modification of blood pressure pales in comparison. Here we focus on the role of 5-HT in systemic blood pressure control. This review provides an in-depth study of the function and pharmacology of 5-HT in those tissues that can modify blood pressure (blood, vasculature, heart, adrenal gland, kidney, brain), with a focus on the autonomic nervous system that includes mechanisms of action and pharmacology of 5-HT within each system. We compare the change in blood pressure produced in different species by short- and long-term administration of 5-HT or selective serotonin receptor agonists. To further our understanding of the mechanisms through which 5-HT modifies blood pressure, we also describe the blood pressure effects of commonly used drugs that modify the actions of 5-HT. The pharmacology and physiological actions of 5-HT in modifying blood pressure are important, given its involvement in circulatory shock, orthostatic hypotension, serotonin syndrome and hypertension.

Reticulospinal neurons inactivated by warming of the preoptic area and anterior hypothalamus of rabbits

To identify the premotor neurons for vasoconstrictors of the skin, activities of reticulospinal neurons in the rostroventral medulla, the ear sympathetic nerve (ESNA) and the renal sympathetic nerve (RSNA) were recorded in anesthetized and immobilized Japanese White or New Zealand White rabbits. Two groups of neurons were identified according to their responses to thermal stimulation of the preoptic area and the anterior hypothalamus (POAH) and to electrical stimulation of baroreceptor afferents, the aortic nerve (AN). Neurons (Type I neurons, n = 21) whose activity was inhibited by warm stimulation of the POAH but not inhibited by the AN stimulation were located in sites medial to the rostral ventrolateral medulla (RVLM). The other neurons (Type II neurons, n = 20) whose activity was not inhibited by warm stimulation of the POAH but inhibited by the AN stimulation were located in the RVLM. Because the time course of the inhibitory response of Type I neurons to warm stimulation of the POAH was very similar to that of the inhibitory response of the ESNA and activities of these neurons and the ESNA were not inhibited by the stimulation of the AN, it was suggested the Type I neurons might participate in regulation of activity of the vasoconstrictors of the ear skin. The Type II neurons are considered to be the barosensitive RVLM neurons that regulate systemic arterial pressure by controlling the activity of visceral or muscular sympathetic vasoconstrictors or cardiac sympathetic fibers.

Fenfluramine-induced hypothermia is associated with cutaneous dilation in conscious rats

The antiobesity agent, fenfluramine, produces hypothermia in rodents by an, as yet, uncharacterized mechanism. The present study was conducted in conscious rats to determine if fenfluramine-induced hypothermia was associated with cutaneous dilation. In animals maintained at 16 degrees C, core body temperature (T(CORE)) was measured telemetrically, and tail surface temperature was monitored with thermocouples fixed to the tail (T(TAIL)). D-Fenfluramine (10 mg/kg ip) produced a rapid increase in T(TAIL) of 7.7+/-0.4 degrees C (P<.001) and a decline in T(CORE) of 4+/-0.3 degrees C (P<.001). Two findings indicate that the increase in T(TAIL) was due to the withdrawal of a sympathetic vasoconstrictor tone. First, pretreatment with the ganglionic blocker, pentolinium, prevented fenfluramine-induced changes in T(TAIL). Second, when sympathetic tone to the tail was physiologically withdrawn by increasing the environmental temperature to 28 degrees C, fenfluramine treatment produced no increase in T(TAIL). Moreover, the effects of fenfluramine on T(TAIL) and T(CORE) depended on the uptake of fenfluramine into serotonergic neurons because these effects were markedly attenuated by pretreatment with the selective serotonin re-uptake inhibitor, fluoxetine. The hypothermic effect of fenfluramine occurred despite the fact that total body oxygen consumption increased by 20%. The results suggest that heat loss due to the dilation of the cutaneous circulation contributes to fenfluramine-induced hypothermia.

Rhythmic activities of the sympatho-excitatory neurons in the medulla of rabbits: neurons controlling cutaneous vasomotion

Spontaneous activities of the reticulospinal neurons in the reticular formation of the rostroventral medulla, of the ear sympathetic nerve (ESNA) and of the renal sympathetic nerve (RSNA) were analyzed with regard to cardiac cycle- and respiration-related rhythm in the anesthetized, vagotomized and immobilized rabbits. A reticulospinal neuron that was concurrently excited with increase in the ESNA and/or reduction of the blood flow of the ear skin by electrical stimulation of the dorsomedial hypothalamus was tentatively named as a cutaneous sympatho-excitatory neuron (Cu neuron). More than half of the Cu neurons (13/22) had a respiration-related rhythmic activity as well as the ESNA. Activity of most of the Cu neurons (19/22) was not modulated with the frequency of the heartbeat and the ESNA had little or no cardiac cycle-related activity. Simultaneous recording shows that the degree of modulation (relative power of the power spectrum of the post event time histogram at the frequency of the respiration) of activity of the Cu neurons correlated with that of the ESNA. On the other hand, most (13/18) of the barosensitive sympatho-excitatory reticulospinal neurons in the rostral ventrolateral medulla (RVLM neurons) had both cardiac cycle- and respiration-related activity as well as the RSNA had. The Cu neurons were located at the medial sites to the location of the RVLM neurons. These results further showed that the Cu neurons controlled the cutaneous vasoconstrictor fibers and that the sympatho-excitatory neurons were located at the different sites in the ventral medulla according to their function.

Role of the medullary raphé in thermoregulatory vasomotor control in rats

To investigate the involvement of the medullary raphé in thermoregulatory vasomotor control, we chemically manipulated raphé neuronal activity while monitoring the tail vasomotor response to preoptic warming. For comparison, neuronal activity in the rostral ventrolateral medulla (RVLM) was manipulated in similar experiments. Injections of D,L-homocysteic acid (DLH; 0.5 mM, 0.3 microl) into a restricted region of the ventral medullary raphé suppressed the tail vasodilatation normally elicited by warming the preoptic area to 42 degrees C. DLH injection into the RVLM also suppressed the vasodilatation elicited by preoptic warming. Injection of bicuculline (0.5 mM, 0.3 microl) into the same raphé region suppressed the vasodilatation elicited by preoptic warming. Bicuculline injection into the RVLM did not suppress tail vasodilatation. These results suggest that neurones in both the medullary raphé and the RVLM are vasoconstrictor to the tail, but only those in the raphé receive inhibitory input from the preoptic area. That input might be direct and/or indirect (e.g. via the periaqueductal grey matter).

The discharge of a subset of serotonergic raphe magnus cells is influenced by baroreceptor input

In order to determine whether serotonergic cells in the medullary raphe magnus (RM) receive baroreceptor input, cells were tested for their responses to descending aortic occlusion, aortic nerve stimulation, or systemic phenylephrine administration in the lightly anesthetized rat. Serotonergic cells were identified physiologically by a quantitative analysis of their slow and steady discharge. Greater than 40% of the serotonergic RM cells tested responded to brief occlusion of the descending aorta at the level of the coeliac arteries, a stimulus that elevated blood pressure by about 30 mmHg. Similarly, about 40% of the serotonergic RM cells responded to stimulation of the aortic nerve, a nerve that contains primarily baroreceptor afferents from the aortic arch. Greater than 70% of RM serotonergic cells responded to phenylephrine administration which elevated blood pressure by an average of 50 mmHg. Serotonergic cell responses to all methods of baroreceptor activation were small in magnitude and were largely restricted in time to the stimulus duration. The results indicate that a subset of serotonergic cells in RM are influenced by baroreceptor activity.

Different cardiovascular neuron groups in the ventral reticular formation of the rostral medulla in rabbits: single neurone studies

To examine whether the cardiovascular neurons of the ventral medulla consist of functionally different kinds of neurons, single neuronal activity of the ventral medulla, activity of the renal sympathetic nerves (RSNA), blood flow of the ear (EarBF) and arterial pressure (AP) were recorded in urethane-anesthetized, vagotomized and immobilized rabbits during electrical stimulation of the aortic nerve (AN, baroreceptor afferent fibers) and electrical stimulation of the dorsomedial hypothalamus (DMH) that reduced EarBF but less affected on AP and RSNA. The dorsolateral funiculus of the second cervical cord was stimulated to evoke antidromic spikes of medullary neurons. Two kinds of reticulo-spinal neurons were identified. Activities of one kind of neurons were facilitated by stimulation of DMH (latency 48.6+/-27.6 ms, n=11) but they did not respond to stimulation of the AN. Therefore, it was presumed that these neurons controlled vasomotion of the ear through the vasoconstrictor neurons in the spinal cord but did not participate in regulation of systemic AP. Activities of the other neurons were inhibited by stimulation of the AN (latency 47.8+/-8 4 ms, n=16) but they did not respond to the DMH stimulation. These neurons were identical to those reported previously as the RVLM neurons, and they contributed to regulate systemic AP but might not participate in control of cutaneous vascular movement. The former neurons were located medially to the latter in the reticular formation of the rostral ventral medulla. These results provided evidence at the single neuronal level that the cardiovascular neurons in the ventral medulla were consisted of functionally different sympatho-excitatory neurons and they were located at the different sites in the rostral ventral medulla.

The lumbar preganglionic sympathetic supply to rat tail and hindpaw

The segmental origins of the preganglionic sympathetic neurons which control the circulation of the rat tail were investigated and compared with those that supply the hindpaw. The left ventral roots of the first, second and sometimes the third lumbar segments were stimulated with 2-min pulse trains in 10 male Sprague-Dawley rats, which were anaesthetised with urethane (1-1.5 g/kg i.v.) and paralysed with pancuronium. Rats were warmed to increase cutaneous blood flow, and vasoconstrictor responses were detected by thermocouples as stimulus-locked falls in skin temperature. Stimulating the L2 ventral root always vasoconstricted the tail: in 50% of the rats, it also vasoconstricted the ipsilateral hindpaw, while in 50%, it did not. Stimulating the L1 ventral root always vasoconstricted both tail and hindpaw. Stimulating the L3 ventral root caused no measurable effect. Electrodermal potentials were recorded from the ipsilateral hindpaw pad in 7 rats: these accompanied hindpaw vasoconstrictor responses but were not seen otherwise. We conclude that at least in 50% of rats, the preganglionic neurons supplying hindpaw and tail skin are not clearly segregated by spinal segment. Sweat glands and blood vessels in the hindpaw are supplied by the same segments.

CNS cell groups projecting to sympathetic outflow of tail artery: neural circuits involved in heat loss in the rat

In the rat, approximately 20% of total body heat-loss occurs by sympathetically mediated increases in blood flow through an elaborate system of arteriovenous anastomoses in the skin of its tail. In this study, the CNS cell groups that regulate this sympathetic outflow were identified by the viral transneuronal labeling method. Pseudorabies virus was injected into the wall of the ventral tail artery in rats that had their cauda equina transected to eliminate the somatic innervation of the tail. After 4-7 days survival, the pattern of CNS transneuronal labeling was studied. Sympathetic preganglionic neurons in the T11-L2 (mainly L1) levels of the intermediolateral cell column (IML) were labeled by 4 days. After 5 days, sympathetic pre-motor neurons (i.e., supraspinal neurons that project to the IML) were identified near the ventral medullary surface; some of these contained serotonin immunoreactivity. Additional groups of the sympathetic premotor areas were labeled by 6 days post-injection, including the rostral ventrolateral medulla (C1 adrenergic neurons), rostral ventromedial medulla, caudal raphe nuclei (serotonin neurons in the raphe pallidus and magnus nuclei), A5 noradrenergic cell group, lateral hypothalamic area and paraventricular hypothalamic area (oxytocin-immunoreactive neurons). Seven days after the PRV injections, additional cell groups in the telencephalon (viz., bed nucleus of the stria terminalis, medial and lateral preoptic areas and medial preoptic nucleus), diencephalon (viz., subincertal nucleus, zona incerta as well as dorsal, dorsomedial, parafascicular, posterior and ventromedial hypothalamic nuclei) and midbrain (viz., periaqueductal gray matter, precommissural nucleus, Edinger-Westphal nucleus and ventral tegmental area) were labeled. The discussion is focused on the CNS cell groups involved in the control of body temperature and fever.

The influence of the ventrolateral medulla on thermoregulatory circulations in the rat

The rostral ventrolateral medulla (RVLM) was stimulated electrically and chemically (0.1-0.2 microliter, 0.4 M DL-homocysteic acid) in urethane-anaesthetised rats. Changes in the vasomotor tone of the cutaneous circulation of the tail, front and hind feet, the nose and the fur covered areas of the back, proximal hind leg and neck were assessed indirectly by measurement of skin temperatures. Electrical stimulation of the RVLM at 25 Hz (2-5 min) produced sustained decreases in skin temperatures and differed from the effect induced by stimulation at 100 Hz or chemical stimulation, in that the concomitant increases in blood pressure and respiration, as well as the accompanying bradycardia or tachycardia, were minimal and within 10-15% of basal control levels. In the hyperthermic animal changes in the skin temperatures of the tail and feet were observed on stimulation of sites lateral to the rostral third of the inferior olive (IO) and which extended through the nuc. paragigantocellularis lateralis to an area medial to the caudal half of the facial nucleus (FN). There was some degree of topographical organisation with the front and hind feet areas overlying the more extensive tail area in a rostro-caudal orientation. The representation of the other areas of the body was more limited. Neck and nose temperatures changed on stimulation of a small area of the RVLM adjacent to the caudal pole of FN, while the skin of the back and upper hind limb responded to stimulation of a limited area lateral to the rostral pole of IO. The unequal representation of the cutaneous surface probably reflects the degree of vascularisation and the importance of the tail and plantar foot surfaces in thermoregulatory responses.

The periaqueductal gray-rostral medulla connection in the defence reaction: efferent pathways and descending control mechanisms

Neuronal systems controlling cardiovascular components of emotional responses must have the capacity to generate different patterns of response and must also be able to modify those patterns in response to changes in environmental circumstances. Using the cardiovascular "defence" response as a model, evidence is presented to show that sympathetic premotor neurons of the rostral ventrolateral medulla (RVLM) possess such properties. Neurones in the RVLM act as relays in the descending efferent pathway to the sympathetic outflows from the dorsal periaqueductal gray matter (dPAG) which integrates the characteristic "defensive" pattern of cardiovascular response that accompanies activation of the midbrain aversive system. Activity in this pathway can be modulated, at the level of the RVLM, by a descending pathway which originates in the ventrolateral PAG. It is suggested that both the dorsolateral and the ventrolateral control systems in the PAG become activated during periods of physical or emotional stress, particularly those which involve sustained motor activity. Activity in the dorsal system initiates cardiovascular components of aversive/defensive behaviour whilst the ventrolateral system plays an important role in initiating the recuperative phase of behaviour characterised by sympathoinhibition, muscular relaxation and immobility which follows a stressful encounter.