Alpha2-adrenergic receptor agonists prevent emotional hyperthermia.

Emotionally significant stimuli, including potential threats from the external environment, trigger an increase in body temperature, a response known as emotional hyperthermia. Sympathetically-mediated brown adipose tissue (BAT) thermogenesis contributes substantially to this hyperthermic response. The systemic administration of α2-adrenergic agonists is known to inhibit both febrile and shivering responses. In the present study, we investigated whether systemic administration of clonidine, a α2-adrenoceptor agonist, attenuates the emotional hyperthermia evoked in conscious unrestrained rats suddenly confronted with a second (intruder) rat, itself confined to a small cage. Pre-implanted thermistors were used to measure BAT and body temperature in conscious, freely moving, male Sprague-Dawley rats. The rats were pre-treated with intraperitoneally administered vehicle (Ringer solution) or clonidine (1, 10 and 100 µg/kg). Clonidine, in a dose-dependent manner, reduced the intruder-elicited increases in BAT (log-dose linear regression F(1,16) = 9.52, R2 = 0.37, P < 0.01) and body temperature (F(1,16) = 6.48, R2 = 0.29, P < 0.05). We also investigated, in anesthetized rats, whether systemic clonidine administration inhibits BAT sympathetic nerve discharge evoked via activation of neurons in the lateral habenula (LHb) - a nucleus involved in the regulation of emotional hyperthermia. In anesthetized rats, clonidine abolished the BAT sympathetic nerve discharges elicited via bicuculline-mediated disinhibition of the LHb. These results suggest that activation of central α2-adrenergic receptors attenuates the process of emotional hyperthermia by reduction of BAT thermogenesis.

Activating dopamine D2 receptors reduces brown adipose tissue thermogenesis induced by psychological stress and by activation of the lateral habenula.

Emotional hyperthermia is the increase in body temperature that occurs as a response to an animal detecting a salient, survival-relevant stimulus. Brown adipose tissue (BAT) thermogenesis, controlled via its sympathetic innervation, contributes to this temperature increase. Here, we have used an intruder rat experimental model to determine whether quinpirole-mediated activation of dopamine D2 receptors attenuates emotional hyperthermia in conscious rats. In anesthetized rats, we determined whether systemic quinpirole reduces BAT nerve discharge induced by activation of the medullary raphé and the lateral habenula (LHb). We measured BAT and body temperature with chronically implanted thermistors in conscious, freely moving, individually housed, male rats (resident rats). Either vehicle or quinpirole was administered, intraperitoneally, to the resident rat 30 min before introduction of a caged intruder rat. Quinpirole, in a dose-dependent manner, reduced intruder-elicited increases in BAT and body temperature. Pre-treatment with the D2 antagonist spiperone, but not the selective D1 antagonist SCH-23390, prevented this quinpirole-elicited decrease. In anesthetized rats, quinpirole abolished BAT sympathetic nerve discharge elicited by bicuculline-mediated activation of the LHb, but not the medullary raphé. Thus, activation of dopamine D2 receptors reduces the BAT thermogenesis that contributes to emotional hyperthermia. We provide evidence that these dopamine D2 receptors are located in the thermogenic pathway between the LHb and the lower brainstem pre-sympathetic control centre in the medullary raphé.

Neurons in ventral tegmental area tonically inhibit sympathetic outflow to brown adipose tissue: possible mediation of thermogenic signals from lateral habenula.

The lateral habenula (LHb), a nucleus involved in the response to salient, especially adverse, environmental events, is implicated in brown adipose tissue (BAT) thermogenesis caused by these events. LHb-elicited thermogenesis involves a neural pathway to the lower brain stem sympathetic control center in the medullary raphé. There are no direct connections from the LHb to the medullary raphé. LHb-mediated behavioral responses involve inhibitory control over the dopamine neurons in the ventral tegmental area (VTA), mediated via an excitatory drive from the LHb to GABAergic neurons in the tail of the VTA. We hypothesized that inhibition of the VTA is also involved in LHb-mediated BAT thermogenesis. To test this hypothesis, inhibition of neurons in the VTA with muscimol increased BAT sympathetic nerve discharge by 22.0 ± 9.2 dBµV ( n = 24, P < 0.0001) and BAT temperature by 1.2 ± 0.1°C ( P < 0.001). This response was abolished by inhibition of the medullary raphé neurons with muscimol. BAT thermogenesis initiated with focal injections of bicuculline in the LHb was reversed by subsequent blockade of GABAA receptors in the VTA with bicuculline. These results suggest that, at least in anesthetized rats, neurons in the VTA tonically inhibit BAT thermogenesis via a link, presently unknown, to the medullary raphé. Removal of this VTA-initiated inhibition is an important mechanism whereby LHb neurons activate BAT thermogenesis.

Thermoregulation and the ultradian basic rest-activity cycle.

Daily life involves interactions with the external environment. In rats these apparently spontaneous interactions, often associated with the search for food, alternate with periods of rest in both the dark and light periods of the 24-hour day. Kleitman, in whose laboratory rapid eye movement sleep was discovered, referred to the temporal pattern as "the basic rest-activity cycle." The active periods of the basic rest-activity cycle occur approximately every 1-2 hours in an irregular stochastic pattern that has been described (perhaps unhelpfully) as ultradian rhythmicity. Both the spontaneous interactions and those evoked by salient, potentially threatening environmental events are accompanied by increases in brown adipose tissue (BAT) temperature of approximately 1°C. The heat produced in BAT contributes to associated increases in the temperature of the brain (approximately 0.8°C) and the body (approximately 0.6°C). These temperature changes require extension of the conventional "homeostasis" framework of temperature regulation. They may function to facilitate the cognitive processing that underlies the vital decision making necessary for safe and effective interaction with the external environment.

Clozapine, chlorpromazine and risperidone dose-dependently reduce emotional hyperthermia, a biological marker of salience.

RATIONALE: We recently introduced a new rat model of emotional hyperthermia in which a salient stimulus activates brown adipose tissue (BAT) thermogenesis and tail artery constriction. Antipsychotic drugs, both classical and second generation, act to reduce excessive assignment of salience to objects and events in the external environment. The close association between salient occurrences and increases in body temperature suggests that antipsychotic drugs may also reduce emotional hyperthermia. OBJECTIVES: We determined whether chlorpromazine, clozapine, and risperidone dose dependently reduce emotionally elicited increases in BAT thermogenesis, cutaneous vasoconstriction, and body temperature in rats. METHODS: Rats, chronically instrumented for measurement of BAT and body temperature and tail artery blood flow, singly housed, were confronted with an intruder rat (confined within a small wire-mesh cage) after systemic pre-treatment of the resident rat with vehicle or antipsychotic agent. BAT and body temperatures, tail blood flow, and behavioral activity were continuously measured. RESULTS: Clozapine (30 µg-2 mg/kg), chlorpromazine (0.1-5 mg/kg), and risperidone (6.25 µg-1 mg/kg) robustly and dose-relatedly reduced intruder-elicited BAT thermogenesis and tail artery vasoconstriction, with consequent dose-related reduction in emotional hyperthermia. CONCLUSIONS: Chlorpromazine, a first-generation antipsychotic, as well as clozapine and risperidone, second-generation agents, dose-dependently reduce emotional hyperthermia. Dopamine D2 receptor antagonist properties of chlorpromazine do not contribute to thermoregulatory effects. Interactions with monoamine receptors are important, and these monoamine receptor interactions may also contribute to the therapeutic effects of all three antipsychotics. Thermoregulatory actions of putative antipsychotic agents may constitute a biological marker of their therapeutic properties.

Lateral habenula regulation of emotional hyperthermia: mediation via the medullary raphé.

The lateral habenula (LHb) has an important role in the behavioural response to salient, usually aversive, events. We previously demonstrated that activation of neurons in the LHb increases brown adipose tissue (BAT) thermogenesis and constricts the cutaneous vascular bed, indicating that the LHb contributes to the central control of sympathetic outflow to thermoregulatory effector organs. We have now investigated whether the LHb mediates BAT thermogenesis elicited by emotional stress, and whether the LHb modulates thermoregulatory sympathetic outflow via the rostral medullary raphé, a key integrative lower brainstem sympathetic control centre. In conscious animals, lesioning the LHb attenuated emotional BAT thermogenesis, suggesting that the LHb is part of the central circuitry mediating emotional hyperthermia. In anesthetized animals, inhibition of neurons in the rostral medullary raphé reversed BAT thermogenesis and cutaneous vasoconstriction elicited by activation of neurons in the LHb, indicating that the LHb-induced autonomic responses are mediated through activation of the rostral medullary raphé neurons. The latency to activate BAT sympathetic discharge from electrical stimulation of the LHb was substantially greater than the corresponding latency after stimulation of the medullary raphé, suggesting that the neuronal pathway connecting those two nuclei is quite indirect.

Locus coeruleus noradrenergic innervation of the amygdala facilitates alerting-induced constriction of the rat tail artery.

The amygdala, innervated by the noradrenergic locus coeruleus, processes salient environmental events. α2-adrenoceptor-stimulating drugs (clonidine-like agents) suppress the behavioral and physiological components of the response to salient events. Activation of sympathetic outflow to the cutaneous vascular bed is part of the physiological response to salience-mediated activation of the amygdala. We have determined whether acute systemic and intra-amygdala administration of clonidine, and chronic immunotoxin-mediated destruction of the noradrenergic innervation of the amygdala, impairs salience-related vasoconstrictor episodes in the tail artery of conscious freely moving Sprague-Dawley rats. After acute intraperitoneal injection of clonidine (10, 50, and 100 µg/kg), there was a dose-related decrease in the reduction in tail blood flow elicited by alerting stimuli, an effect prevented by prior administration of the α2-adrenergic blocking drug idazoxan (1 mg/kg ip or 75 nmol bilateral intra-amygdala). A dose-related decrease in alerting-induced tail artery vasoconstriction was also observed after bilateral intra-amygdala injection of clonidine (5, 10, and 20 nmol in 200 nl), an effect substantially prevented by prior bilateral intra-amygdala injection of idazoxan. Intra-amygdala injection of idazoxan by itself did not alter tail artery vasoconstriction elicited by alerting stimuli. Intra-amygdala injection of saporin coupled to antibodies to dopamine-ß-hydroxylase (immunotoxin) destroyed the noradrenergic innervation of the amygdala and the parent noradrenergic neurons in the locus coeruleus. The reduction in tail blood flow elicited by standardized alerting stimuli was substantially reduced in immunotoxin-treated rats. Thus, inhibiting the release of noradrenaline within the amygdala reduces activation of the sympathetic outflow to the vascular beds elicited by salient events.

SR59230A, a beta-3 adrenoceptor antagonist, inhibits ultradian brown adipose tissue thermogenesis and interrupts associated episodic brain and body heating.

Brown adipose tissue (BAT) thermogenesis occurs episodically in an ultradian manner approximately every 80-100 min during the waking phase of the circadian cycle, together with highly correlated increases in brain and body temperatures, suggesting that BAT thermogenesis contributes to brain and body temperature increases. We investigated this in conscious Sprague-Dawley rats by determining whether inhibition of BAT thermogenesis via blockade of beta-3 adrenoceptors with SR59230A interrupts ultradian episodic increases in brain and body temperatures and whether SR59230A acts on BAT itself or via sympathetic neural control of BAT. Interscapular BAT (iBAT), brain, and body temperatures, tail artery blood flow, and heart rate were measured in unrestrained rats. SR59230A (1, 5, or 10 mg/kg ip), but not vehicle, decreased iBAT, body, and brain temperatures in a dose-dependent fashion (log-linear regression P < 0.01, R(2) = 0.3, 0.4, and 0.4, respectively, n = 10). Ultradian increases in BAT, brain, and body temperature were interrupted by administration of SR59230A (10 mg/kg ip) compared with vehicle, resuming after 162 ± 24 min (means ± SE, n = 10). SR59230A (10 mg/kg ip) caused a transient bradycardia without any increase in tail artery blood flow. In anesthetized rats, SR59230A reduced cooling-induced increases in iBAT temperature without affecting cooling-induced increases in iBAT sympathetic nerve discharge. Inhibition of BAT thermogenesis by SR59230A, thus, reflects direct blockade of beta-3 adrenoceptors in BAT. Interruption of episodic ultradian increases in body and brain temperature by SR59230A suggests that BAT thermogenesis makes a substantial contribution to these increases.

Atypical antipsychotics cause an acute increase in cutaneous hand blood flow in patients with schizophrenia and schizoaffective disorder.

OBJECTIVE: Clinical studies suggest resting thermoregulatory cutaneous vasomotor tone could be increased in schizophrenia, resulting in reduced hand blood flow. In animal models, atypical antipsychotics including clozapine potently inhibit sympathetic neural outflow to the thermoregulatory cutaneous vascular beds. We have now determined whether antipsychotic medication administration is associated with an acute increase in hand blood flow in patients with schizophrenia and schizoaffective disorder, and whether this increase correlates with clinical status. METHOD: Hand temperature was measured with an infrared camera in 12 patients with chronic schizophrenia or schizoaffective disorder 30 min prior to, then 30 and 60 min following medication. Clinical status was assessed via the Brief Psychiatric Rating Scale (BPRS). Results were compared using regression and repeated measures analysis of variance. RESULTS: A robust and significant increase in hand temperature (p < 0.001) was observed following antipsychotic administration. The mean increase after 60 min was 4.1 ± 2.4°C. This increase was significantly associated with colder hand temperature prior to medication (p < 0.05; suggestive of increased resting vasoconstriction) and with more severe psychiatric symptoms (p < 0.05). CONCLUSIONS: Atypical antipsychotics were associated with increased hand blood flow, consistent with inhibition of thermoregulatory sympathetic outflow to the cutaneous vascular bed in patients with schizophrenia and schizoaffective disorder. This increase correlated with symptom severity. Hand temperature increase following antipsychotic medication may therefore be a simple and informative physiological marker of disease activity and potential response in patients with schizophreniform disorders. Given that antipsychotics also inhibit sympathetic outflow to brown adipose tissue, which normally converts energy to heat, future studies should examine whether antipsychotic-induced hand temperature increase is associated with antipsychotic-induced weight gain.

Sympathetic cutaneous vasomotor alerting responses (SCVARs) are associated with hippocampal theta rhythm in non-moving conscious rats.

Alerting stimuli that cause sympathetic cutaneous vasomotor alerting responses ("SCVARs") in the thermoregulatory vascular bed of the rabbit ear pinna also induce theta (5-8 Hz) rhythm in the hippocampal EEG, a marker that the animal is engaged with the environment. The present study determines the relation between SCVARs in the thermoregulatory tail artery vascular bed and hippocampal EEG theta rhythm in Sprague-Dawley rats. A Doppler ultrasonic flow probe chronically implanted around the base of the tail artery was used to measure SCVARs. Unipolar electrodes were implanted in the hippocampus (CA1 region) to measure EEG. Six standard non-noxious brief alerting stimuli were administered during continuous recording of tail blood flow and EEG. The SCVAR index was calculated as the percentage fall from pre-alerting blood flow values. After stimuli the SCVAR index for the tail artery blood flow was 84+/-2% of the pre-alerting control. In contrast, the same stimuli caused little fall in blood flow within the superior mesenteric bed (SCVAR index=18+/-2%). The proportion of theta power in the total frequency range (0-20 Hz) increased significantly after alerting stimuli (46+/-2% vs. 29+/-1% before stimuli, p<0.05). Theta proportion began to increase approximately 0.5 s after the stimuli and preceded SCVARs by approximately 1 s. The SCVAR index was correlated with the magnitude of the increment in theta power. Our study demonstrates that alerting responses resulting in selective vasoconstriction of the tail vascular bed are associated with hippocampal theta rhythm in conscious rats.

Selective blockade of 5-HT2A receptors attenuates the increased temperature response in brown adipose tissue to restraint stress in rats.

Previous studies have demonstrated that 5-HT2A receptors may be involved in the central control of thermoregulation and of the cardiovascular system. Our aim was to test whether these receptors mediate thermogenic and tachycardiac responses induced by acute psychological stress. Three groups of adult male Hooded Wistar rats were instrumented with: (i) a thermistor in the interscapular area (for recording brown adipose tissue temperature) and an ultrasound Doppler probe (to record tail blood flow); (ii) temperature dataloggers to record core body temperature; (iii) ECG electrodes. On the day of the experiment, rats were subjected to a 30-min restraint stress preceded by s.c. injection of either vehicle or SR-46349B (a serotonin 2A receptor antagonist) at doses of 0.01, 0.1 and 1.0 mg/kg. The restraint stress caused a rise in brown adipose tissue temperature (from, mean +/- s.e.m., 36.6 +/- 0.2 to 38.0 +/- 0.2 degrees C), transient cutaneous vasoconstriction (tail blood flow decreased from 12 +/- 2 to 5 +/- 1 cm/s), increase in heart rate (from 303 +/- 15 to 453 +/- 15 bpm at the peak, then reduced to 393 +/- 12 bpm at the steady state), and defaecation (6 +/- 1 pellets per restraint session). The core body temperature was not affected by the restraint. Blockade of 5-HT2A receptors attenuated the increase in brown adipose tissue temperature and transient cutaneous vasoconstriction, but not tachycardia and defaecation elicited by restraint stress. These results indicate that psychological stress causes activation of 5-HT2A receptors in neural pathways that control thermogenesis in the brown adipose tissue and facilitate cutaneous vasoconstriction.

Fever response to intravenous prostaglandin E2 is mediated by the brain but does not require afferent vagal signaling.

PGE2 produced in the periphery triggers the early phase of the febrile response to infection and may contribute to later phases. It can be hypothesized that peripherally synthesized PGE2 transmits febrigenic signals to the brain via vagal afferent nerves. Before testing this hypothesis, we investigated whether the febrigenic effect of intravenously administered PGE2 is mediated by the brain and is not the result of a direct action of PGE2 on thermoeffectors. In anesthetized rats, intravenously injected PGE2 (100 microg/kg) caused an increase in sympathetic discharge to interscapular brown adipose tissue (iBAT), as well as increases in iBAT thermogenesis, end-expired CO2, and colonic temperature (Tc). All these effects were prevented by inhibition of neuronal function in the raphe region of the medulla oblongata using an intra-raphe microinjection of muscimol. We then asked whether the brain-mediated PGE2 fever requires vagal signaling and answered this question by conducting two independent studies in rats. In a study in anesthetized rats, acute bilateral cervical vagotomy did not affect the effects of intravenously injected PGE2 (100 microg/kg) on iBAT sympathetic discharge and Tc. In a study in conscious rats, administration of PGE2 (280 microg/kg) via an indwelling jugular catheter caused tail skin vasoconstriction, tended to increase oxygen consumption, and increased Tc; none of these responses was affected by total truncal subdiaphragmatic vagotomy performed 2 wk before the experiment. We conclude that the febrile response to circulating PGE2 is mediated by the brain, but that it does not require vagal afferent signaling.

Serotonin-synthesizing neurons in the rostral medullary raphé/parapyramidal region transneuronally labelled after injection of pseudorabies virus into the rat tail.

Serotonin-synthesizing raphé/parapyramidal neurons (5-HT neurons) may function as sympathetic premotor neurons regulating sympathetic outflow to the cutaneous vascular bed. In the present study a genetically engineered pseudorabies virus (PRV) expressing green fluorescent protein (GFP) was injected into the rat tail. After survival for 3-4 days the medulla oblongata was examined using double-label immunohistochemistry, with an antibody against GFP for the virus and an antibody against phenylalanine hydroxylase 8 (PH8) for 5-HT synthesis. Sections were examined using light microscopy, and conventional and confocal fluorescence microscopy. There were two subpopulations of PRV+ve neurons in the raphé/parapyramidal region: a more dorsally and laterally located subgroup of medium-sized and large neurons, mainly non-serotonergic, and a more ventrally located subgroup of small mainly serotonin-synthesizing neurons, including those just dorsal to the pyramids, those in raphé pallidus, and those in close relationship to the ventral surface in the parapyramidal-subependymal zone.

Activation of 5-HT1A receptors in rostral medullary raphé inhibits cutaneous vasoconstriction elicited by cold exposure in rabbits.

In both conscious and anesthetized rabbits, we determined whether microinjection of a 5-hydroxytryptamine (5-HT) 1A receptor agonist 8-hydroxy-2-(di-n-propylaminio) tetralin (8-OH-DPAT) into the medullary raphé/parapyramidal region inhibits thermoregulatory vasoconstriction and whether microinjection of a 5-HT1A receptor antagonist (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride) (WAY-100635) into the raphé reverses the cutaneous vasomotor changes induced by intravenous administration of 8-OH-DPAT. In conscious rabbits with measuring ear pinna blood flow, after microinjection of 8-OH-DPAT (3-5 nmol in 300-500 nl) into the raphé, transferring the animal from a warm cage (25-28 degrees C) to a cold cage (5-10 degrees C) did not reduce the ear pinna flow (from 57 +/- 7 cm/s to 59 +/- 3 cm/s, P > 0.05, n = 5), unlike Ringer-treated animals. Microinjection of WAY-100635 (5 nmol in 500 nl) into the raphé reversed ear pinna flow changes induced by intravenous administration of 8-OH-DPAT (0.1 mg/kg, i.v.). In anesthetized rabbits with measuring postganglionic ear pinna sympathetic nerve activity, microinjection of 8-OH-DPAT (1-2 nmol in 100-200 nl) into the raphé reduced resting ear pinna sympathetic nerve activity to 14 +/- 4% of pre-injection level (P < 0.01, n = 12) and attenuated increases in ear pinna sympathetic nerve activity normally elicited by cooling the animal's trunk. WAY-100635 (2 nmol into 200 nl) into the raphé reversed inhibition of ear pinna sympathetic nerve activity elicited by 8-OH-DPAT (0.1 mg/kg, i.v.). The activation of 5-HT1A receptors expressed on the medullary raphé neurons results in reversal of cold-elicited cutaneous vasoconstriction possibly through inhibition of sympathetic premotor neurons that innervate sympathetic preganglionic neurons controlling cutaneous vasomotion.

Thermogenesis in brown adipose tissue: increase by 5-HT2A receptor activation and decrease by 5-HT1A receptor activation in conscious rats.

Body temperature is decreased by 5-hydroxytryptamine 1A (5-HT1A) agonists and increased by 5-HT2A agonists. The present study determined whether changes in interscapular brown adipose tissue (iBAT) thermogenesis contribute to these effects in conscious unrestrained animals. Male Sprague-Dawley rats were pre-instrumented for measurement of iBAT and core temperature and tail artery blood flow one week before experiments. In the first series of experiments, rats were transferred from warm (25-28 degrees C) to cold (5-10 degrees C) environments. This increased iBAT temperature (+1.3 +/- 0.2 degrees C, P<0.01, n = 7) and reduced tail artery flow. Injection of the 5-HT1A agonist, 8-OH-DPAT (8-hydroxy-2-(di-n-propylamino)tetralin, 0.5 mg/kg, s.c.) reversed the increase in iBAT thermogenesis (-1.5 +/- 0.4 degrees C, P<0.01, n = 6), and decreased core temperature (-1.5 +/- 0.4 degrees C, P<0.01, n = 6). Pre-treatment with WAY-100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-N-(2-pyridinyl)cyclohexanecarboxamide trihydrochloride), a 5-HT1A antagonist, prevented effects of 8-OH-DPAT. In the second series of experiments, injection of a 5-HT2A agonist, DOI (R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride, 0.1 mg/kg, s.c.) increased both iBAT (+1.9 +/- 0.1 degrees C, P<0.01, n = 7) and core temperatures (+1.4+/-0.2 degrees C, P<0.01, n=7), and decreased tail artery blood flow. Subsequent injection of SR 46349B (trans-4-((3Z)3-[(2-dimethylaminoethyl)oxyimino]-3-(2-fluorophenyl) propen-1-yl)-phenol, hemifumarate, 0.5 mg/kg, s.c.), a 5-HT2A antagonist, reduced all these changes. Results indicate that activation of 5-HT1A receptors reduces sympathetic outflow to BAT and that activation of 5-HT2A receptors increases this outflow. Changes in core temperature mediated by brain/spinal pathways regulated by 5-HT1A and 5-HT2A receptors reflect coordinated changes in BAT-mediated heat production as well as changes in heat dissipation via the thermoregulatory cutaneous vascular beds.

Inhibition of medullary raphé/parapyramidal neurons prevents cutaneous vasoconstriction elicited by alerting stimuli and by cold exposure in conscious rabbits.

In conscious rabbits, microinjection of muscimol into the medullary raphé/parapyramidal region decreased fluctuation (coefficient variation) of resting ear blood flow (from 62 +/- 8 to 25 +/- 4%, P < 0.01, n = 8). The muscimol injection also prevented falls in ear blood flow that normally occur in response to alerting stimuli and to cold exposure. Thus, raphé/parapyramidal neurons constitute an important brainstem center for mediating cutaneous vasoconstriction initiated by alerting stimuli and by cold exposure.

Activation of 5-HT1A receptors in the medullary raphe reduces cardiovascular changes elicited by acute psychological and inflammatory stresses in rabbits.

The present strategy for the prevention of excessive sympathetic neural traffic to the heart relies on the use of beta-blockers, drugs that act at the heart end of the brain-heart axis. In the present study, we attempted to suppress cardiac sympathetic nerve activity by affecting the relevant cardiomotoneurons in the brain using the selective serotonin-1A (5-HT(1A)) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). In conscious, unrestrained rabbits, instrumented for recordings of heart rate, arterial pressure, or cardiac output, we provoked increases in cardiac sympathetic activity by psychological (loud sound, pinprick, and air jet) or inflammatory (0.5 microg/kg iv lipopolysaccharide) stresses. Pinprick and air-jet stresses elicited transient increases in heart rate (+50 +/- 7 and +38 +/- 4 beats/min, respectively) and in mean arterial pressure (+16 +/- 2 and +15 +/- 3 mmHg, respectively). Lipopolysaccharide injection caused sustained increases in heart rate (from 210 +/- 3 to 268 +/- 10 beats/min) and in arterial pressure (from 74 +/- 3 to 92 +/- 4 mmHg). Systemically administered 8-OH-DPAT (0.004-0.1 mg/kg) substantially attenuated these responses in a dose-dependent manner. Drug effects were prevented by a selective 5-HT(1A) receptor antagonist, WAY-100635 (0.1 mg/kg iv). Similarly to systemic administration, microinjection of 8-OH-DPAT (500 nl of 10 mM solution) into the medullary raphe-parapyramidal region caused antitachycardic effects during stressful stimulation and during lipopolysaccharide-elicited tachycardia. This is the first demonstration that activation of 5-HT(1A) receptors in the medullary raphe-parapyramidal area causes suppression of neurally mediated cardiovascular changes during acute psychological and immune stresses.

Activation of slowly conducting medullary raphe-spinal neurons, including serotonergic neurons, increases cutaneous sympathetic vasomotor discharge in rabbit.

Neurons in the rostral medullary raphe/parapyramidal region regulate cutaneous sympathetic nerve discharge. Using focal electrical stimulation at different dorsoventral raphe/parapyramidal sites in anesthetized rabbits, we have now demonstrated that increases in ear pinna cutaneous sympathetic nerve discharge can be elicited only from sites within 1 mm of the ventral surface of the medulla. By comparing the latency to sympathetic discharge following stimulation at the ventral raphe site with the corresponding latency following stimulation of the spinal cord [third thoracic (T3) dorsolateral funiculus] we determined that the axonal conduction velocity of raphe-spinal neurons exciting ear pinna sympathetic vasomotor nerves is 0.8 +/- 0.1 m/s (n = 6, range 0.6-1.1 m/s). Applications of the 5-hydroxytryptamine (HT)(2A) antagonist trans-4-((3Z)3-[(2-dimethylaminoethyl)oxyimino]-3-(2-fluorophenyl)propen-1-yl)-phenol, hemifumarate (SR-46349B, 80 microg/kg in 0.8 ml) to the cerebrospinal fluid above thoracic spinal cord (T1-T7), but not the lumbar spinal cord (L2-L4), reduced raphe-evoked increases in ear pinna sympathetic vasomotor discharge from 43 +/- 9 to 16 +/- 6% (P < 0.01, n = 8). Subsequent application of the excitatory amino acid (EAA) antagonist kynurenic acid (25 micromol in 0.5 ml) substantially reduced the remaining evoked discharge (22 +/- 8 to 6 +/- 6%, P < 0.05, n = 5). Our conduction velocity data demonstrate that only slowly conducting raphe-spinal axons, in the unmyelinated range, contribute to sympathetic cutaneous vasomotor discharge evoked by electrical stimulation of the medullary raphe/parapyramidal region. Our pharmacological data provide evidence that raphe-spinal neurons using 5-HT as a neurotransmitter contribute to excitation of sympathetic preganglionic neurons regulating cutaneous vasomotor discharge. Raphe-spinal neurons using an EAA, perhaps glutamate, make a substantial contribution to the ear sympathetic nerve discharge evoked by raphe stimulation.

Spinal 5-HT2A receptors regulate cutaneous sympathetic vasomotor outflow in rabbits and rats; relevance for cutaneous vasoconstriction elicited by MDMA (3,4-methylenedioxymethamphetamine, "Ecstasy") and its reversal by clozapine.

We determined whether spinal 5-hydroxytryptamine 2A (5-HT2A) receptors contribute to resting cutaneous sympathetic vasomotor activity, and to increases in activity elicited by electrical stimulation of the medullary raphe/parapyramidal region, and whether these receptors are involved in the cutaneous vasoconstricting action of systemically administered MDMA (3,4-methylenedioxymethamphetamine, "Ecstasy") and its reversal by clozapine. Experiments were conducted in urethane-anesthetized rabbits and rats. Administration of the 5-HT2A antagonist, trans-4-((3Z)3-[(2-Dimethylaminoethyl)oxyimino]-3-(2-fluorophenyl)propen-1-yl)-phenol, hemifumarate (SR 46349B, 0.1 mg/kg, i.v.) inhibited resting ear pinna sympathetic vasomotor nerve discharge and reduced the extent to which raphe/parapyramidal electrical stimulation caused ear pinna (rabbit) and tail (rat) artery blood flow to fall. Clozapine (0.125-0.5 mg/kg, i.v.) also reduced the fall in ear pinna blood flow elicited by raphe/parapyramidal stimulation. In rabbits, after inactivation of raphe/parapyramidal function by local microinjection of muscimol (1 nmol in 100 nl), the 5-HT2A agonist R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI, 50 microg/kg, i.v.) increased ear pinna sympathetic nerve activity from 3+/-2% to 129+/-5% of pre-muscimol levels (P<0.01, n=6), and this increase was abolished by section of the ipsilateral cervical sympathetic nerve trunk. MDMA (2 mg/kg, i.v.) after muscimol decreased ear pinna blood flow from 33+/-10 to 2+/-1 cm/s (P<0.01, n=5) and increased ear pinna sympathetic nerve activity from 8+/-4% to 120+/-41% of pre-muscimol levels (P<0.01, n=6). The MDMA-elicited increase in nerve activity was abolished by SR 46349B. Data suggest that spinal 5-HT2A receptors contribute to sympathetically induced cutaneous vasoconstriction regulated by raphe/parapyramidal neurons in the brainstem, and that these receptors contribute to the cutaneous vasoconstricting action of MDMA and its reversal by clozapine.

Inhibition of rostral medullary raphé neurons prevents cold-induced activity in sympathetic nerves to rat tail and rabbit ear arteries.

Sympathetically-mediated vasoconstriction of cutaneous vessels is critical for thermoregulation in the cold. We determined whether cold-induced sympathetic discharge depends on activity of neurons in the rostral medullary raphé. In urethane-anesthetized rats and rabbits, cooling the trunk skin by a water jacket reproducibly increased cutaneous sympathetic discharge recorded in the tail (rats) and the ear pinna (rabbits). When neurons in the rostral medullary raphé were inhibited by microinjection of glycine (30-100 nmol in 60-200 nl in rats), or muscimol (1 nmol in 100 nl in rabbits), cutaneous sympathetic activity was silenced and no longer responded to cooling (7+/-3 and 5+/-2% of pre-injection increase in rats and rabbits, respectively, P < 0.01). Our data demonstrate that activity of rostral medullary raphé neurons is important for the CNS mediation of cold-induced increases in sympathetic cutaneous vasomotor nerves.