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.     

Immunoreactivity for the NMDA NR1 subunit in bulbospinal catecholamine and serotonin neurons of rat ventral medulla

Bulbospinal neurons in the ventral medulla play important roles in the regulation of sympathetic outflow. Physiological evidence suggests that these neurons are activated by N-methyl-d-aspartate (NMDA) and non-NMDA subtypes of glutamate receptors. In this study, we examined bulbospinal neurons in the ventral medulla for the presence of immunoreactivity for the NMDA NR1 subunit, which is essential for NMDA receptor function. Rats received bilateral injections of cholera toxin B into the tenth thoracic spinal segment to label bulbospinal neurons. Triple immunofluorescent labeling was used to detect cholera toxin B with a blue fluorophore, NR1 with a red fluorophore, and either tyrosine hydroxylase or tryptophan hydroxylase with a green fluorophore. In the rostral ventrolateral medulla, NR1 occurred in all bulbospinal tyrosine hydroxylase–positive neurons and 96% of bulbospinal tyrosine hydroxylase–negative neurons, which were more common in sections containing the facial nucleus. In the raphe pallidus, the parapyramidal region, and the marginal layer, 98% of bulbospinal tryptophan hydroxylase-positive neurons contained NR1 immunoreactivity. NR1 was also present in all of the bulbospinal tryptophan hydroxylase-negative neurons, which comprised 20% of bulbospinal neurons in raphe pallidus and the parapyramidal region. These results show that virtually all bulbospinal tyrosine hydroxylase and non–tyrosine hydroxylase neurons in the rostral ventrolateral medulla and virtually all bulbospinal serotonin and non-serotonin neurons in raphe pallidus and the parapyramidal region express NR1, the obligatory subunit of the NMDA receptor. NMDA receptors on bulbospinal neurons in the rostral ventral medulla likely influence sympathoexcitation in normal and pathological conditions.     

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.     

Transneuronal labelling of neurons in rabbit brain after injection of Herpes simplex virus type 1 into the renal nerve

We mapped the distribution of virus-labelled neurons in the brain after injection of Herpes simplex virus type 1 (HSV1) into the rabbit renal nerve. Seven days after injection, labelled neurons were observed in four brain regions, the rostral ventrolateral medulla (47 ± 3% of neurons), the A5 area of the lower pons (38 ± 4%), the caudal raphe nuclei and the parapyramidal area of the medulla (13 ± 2%), and the paraventricular nucleus of the hypothalamus (1 ± 1%). In the rostral ventrolateral medulla approximately one half of the HSV1-labelled neurons also contained tyrosine hydroxylase, characterizing them as C1 neurons. In the A5 area virtually all HSV1-labelled neurons also contained tyrosine hydroxylase. In the raphe nuclei and the parapyramidal area 47% of HSV1-positive neurons contained serotonin. The distribution of labelled neurons was similar to that observed after injection of HSV1 into the adrenal gland.     

Cardiovascular responses elicited by chemical stimulation of the rostral ventrolateral medulla in conscious, unrestrained rats

The cardiovascular effects of microinjection of L-glutamic acid into the rostral ventrolateral medulla have been investigated in anaesthetized and in conscious, unrestrained rats. In conscious rats L-glutamic acid (0.36, 1.2 and 2.4 nmol) produced significant increases in blood pressure, generally accompanied by a bradycardia. A lower dose of L-glutamic acid (0.12 nmol) had no effect on blood pressure or heart rate. The cardiovascular responses elicited by L-glutamic acid (2.4 nmol) were completely abolished by intravenous infusion of the ganglion blocker, pentolinium. However, in urethane-anaesthetized rats, the pressor response (+27 +/- 2/+21 +/- 2 mmHg) to microinjection of L-glutamic acid (2.4 nmol) was markedly less than that seen in conscious rats (+64 +/- 6/+47 +/- 3 mmHg) and there was a tachycardia (+21 +/- 2 beats/min) rather than a bradycardia. These results corroborate previous studies, carried out in anaesthetized animals, indicating that stimulation of the rostral ventrolateral medulla provides direct and/or indirect excitatory drive to the preganglionic sympathetic neurons of the spinal cord to increase blood pressure, but demonstrate a marked difference in sensitivity in conscious vs urethane-anaesthetized animals.     

Involvement of I(1)-imidazoline receptors in baroreceptor reflex in the caudal ventrolateral medulla of rats

There is ample evidence to show the existence of center I(1)-imidazoline receptors that are involved in the regulation of cardiovascular activities. The purpose of this study was to examine the possible role of I(1)-imidazoline receptors and alpha(2)-adrenoceptors within the caudal ventrolateral medulla (CVLM) in mediating the baroreceptor reflex in anesthetized rats. Unilateral microinjection of idazoxan (2 nmol in 50 nl), a mixed antagonist of I(1)-imidazoline receptors and alpha(2)-adrenoceptors, into the CVLM significantly (P<0.01) decreased blood pressure (BP), heart rate (HR), and the firing rate of presympathetic neurons in the rostral ventrolateral medulla (RVLM) by 21+/-6 mmHg, 25+/-5 beats per min and 3.5+/-0.9 spikes/s, respectively. Moreover, unilateral injection of idazoxan into the CVLM significantly (P<0.01) reduced the inhibitory responses of the ipsilateral RVLM presympathetic neurons evoked by stimulation of aortic nerve and elevation of BP, and partially inhibited the neuronal cardiac cycle-related rhythm. Depressor responses evoked by aortic nerve stimulation were significantly (P<0.01) attenuated 10 and 20 min after bilateral microinjection of idazoxan (2 nmol in 50 nl for each side) into the CVLM (-20+/-4 and -30+/-4 vs. -40+/-1 mmHg). However, injection of yohimbine (500 pmol in 50 nl), a selective alpha(2)-adrenoceptor antagonist, into the CVLM did not affect the resting cardiovascular activities and baroreceptor reflex. It is concluded that the CVLM I(1)-imidazoline receptors are involved in maintenance of tonic cardiovascular activities and transmission of the baroreceptor reflex.     

Projections from rabbit caudal medulla to C1 and A5 sympathetic premotor neurons, demonstrated with phaseolus leucoagglutinin and herpes simplex virus.

We combined Phaseolus vulgaris leucoagglutinin anterograde tracing and Herpes simplex virus transneuronal retrograde tracing to determine whether neurons in the vasodepressor region of the rabbit caudal ventrolateral medulla project to brainstem neurons containing the virus after its transneuronal transport from the adrenal medulla. Five days after adrenal injection of virus, 764 +/- 159 virus-positive neurons were found bilaterally in the brainstem: 61% in the C1 sympathoexcitatory region of the rostral ventrolateral medulla, 30% in the A5 region, 5% in the parapyramidal region, and 3% in the paraventricular nucleus of the hypothalamus. Many of the virus-positive neurons in the C1 and A5 areas also contained tyrosine hydroxylase and, in the parapyramidal area, many contained 5-hydroxytryptamine. After iontophoretic deposit of leucoagglutinin into the vasodepressor region of the caudal ventrolateral medulla, brain regions containing varicose processes labeled with leucoagglutinin included the regions containing virus-positive neurons. We examined the C1 and A5 regions following injections of both tracers in the same rabbits, leucoagglutinin into the caudal ventrolateral medulla and virus into the adrenal gland. Varicosities containing leucoagglutinin were seen in contiguity with perikarya and dendritic branches of neurons containing HSV1, in both the C1 and A5 regions. Studies also revealed labeled varicosities in contiguity with TH-containing C1 and A5 neurons. The projection from the caudal medulla to presumed sympathetic premotor neurons in the C1 area, including some C1 cells, represents a potential pathway whereby activity of neurons in the caudal medulla could reduce blood pressure by inhibiting sympathoexcitatory neurons in the rostral medulla.     

Midbrain central grey: regional haemodynamic control and excitatory amino acidergic mechanisms.

The haemodynamic responses to electrical and chemical stimulation of the periaqueductal or central grey (CG) was investigated in urethane-anaesthetized rats. CG stimulation resulted in a characteristics pattern of mesenteric and renal vasoconstriction accompanied by modest hindquarter vasodilatation. This haemodynamic response was also accompanied by widening of the palpebral fissure, tachycardia and by twitching of the vibrissae. This constellation of physiological responses constitutes the 'defence reaction' and indicates that the CG area under investigation is involved in these phenomena. Both electrical and chemical (kainic acid) lesions of the pressor area of the rostral ventrolateral medulla (RVLM) attenuated the pressor responses to CG stimulation. Intrathecal administration of the excitatory amino acid receptor antagonist kynurenic acid (0.5 mumole/10 microliter) markedly reduced the pressor responses produced by stimulation of both the CG and the RVLM. These results provide additional evidence in support of the notion that neurons arising in the CG relay in the RVLM where they may, in turn, communicate with a descending excitatory amino-acidergic pathway involved in cardiovascular control.     

Transneuronal labeling of neurons in rabbit brain after injection of herpes simplex virus type 1 into the aortic depressor nerve.

Herpes simplex virus type 1 (HSV1) was injected into either the aortic depressor nerve or the vagus nerve in the rabbit. Four or 5 days after injection of virus, the rabbit brain was processed immunohistochemically to demonstrate viral antigen. After injection into the aortic nerve HSV1 positive cells were found principally ipsilaterally within the nucleus tractus solitarius, area postrema, caudal and rostral ventrolateral medulla oblongata, the spinal trigeminal complex, raphe nuclei, A5 area, locus coeruleus, parabrachial nucleus, periaqueductal gray, ventrolateral hypothalamic area, paraventricular nucleus, amygdala, bed nucleus of the stria terminalis and insular cortex. Double labeling studies indicated that approximately 85% of the virus-containing neurons in the ventrolateral medulla, and virtually all the HSV-positive neurons in the A5 area and locus coeruleus also contained tyrosine hydroxylase. In the raphe nuclei and parapyramidal region approximately 33% of virus-containing cells reacted positively with PH8 antibody, a marker for serotonin synthesis. After injection of HSV1 into the vagus nerve labeled cells were found in similar brain areas, with a more bilateral distribution. The HSV-positive neurons may be involved in the processing of baroreceptor-derived information.     

Midbrain central gray: regional haemodynamic control and excitatory amino acidergic mechanisms

The haemodynamic responses to electrical and chemical stimulation of the periaqueductal or central grey (CG) was investigated in urethane-anaesthetized rats. CG stimulation resulted in a characteristics pattern of mesenteric and renal vasoconstriction accompanied by modest hindquarter vasolidation. This haemodynamic response was also accompanied by widening of the palpebral fissure, tachycardia and by twitching of the vibrissae. This constellation of physiological responses constitutes the ‘defence reaction’ and indicates that the CG area under investigation is involved in these phenomena. Both electrical and chemical (kainic acid) lesions of the pressor area of the rostral ventrolateral medulla (RVLM) attenuated the pressor responses to CG stimulation. Intrathecal administration of the excitatory amino acid receptor antagonist kynurenic acid (0.5 μmole/10 μl) markedly reduced the pressor responses produced by stimulation of both the CG and the RVLM. These results provide additional evidence in support of the notion that neurons arising in the CG relay in the RVLM where they may, in turn, communicate with a descending excitatory amino-acidergic pathway involved in cardiovascular control.     

Responses of neurons containing VGLUT3/nNOS-cGMP in the rVLM to cardiac stimulation

Responses of glutamatergic neurons in the rostral ventrolateral medulla to stimulation of cardiac sympathetic afferents have not been defined. Nitric oxide influences neural function of glutamate. We evaluated the relationship between vesicular glutamate transporter 3, c-Fos and neuronal nitric oxide synthase/soluble guanylyl cyclase in the rostral ventrolateral medulla following cardiac stimulation. In anesthetized cats with barodenervation and vagotomy, epicardial application of bradykinin, but not its vehicle, caused pressor responses. More vesicular glutamate transporter 3-containing neurons colocalized with c-Fos or c-Fos and neuronal nitric oxide synthase in the rostral ventrolateral medulla in bradykinin-treated cats (n = 6; P < 0.05) than in control animals (n = 4). Colocalization of neuronal nitric oxide synthase, soluble guanylyl cyclase and c-Fos or vesicular glutamate transporter 3 was noted following bradykinin stimulation. Findings indicate activation of rostral ventrolateral medulla glutamatergic neurons by cardiac stimulation, which may be influenced by nitric oxide via cGMP.     

Cardiovascular effects of L-glutamate and tetrodotoxin microinjected into the rostral and caudal ventrolateral medulla in normotensive and spontaneously hypertensive rats

The purpose of this study was to compare the responsiveness of the rostral and caudal ventrolateral medulla in spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats to microinjection of L-glutamate, and to estimate tonic output of these areas by microinjecting the neurotoxin tetrodotoxin. Rats were anesthetized with 1.25 g/kg urethane s.c., implanted with arterial (femoral) and venous (femoral) catheters, artificially ventilated and paralyzed with gallamine triethiodide (10 mg/kg). Using a ventral approach to the brainstem, the mean arterial pressure and heart rate responses to microinjection (30 nl) of L-glutamate (1, 10 and 100 mM) and tetrodotoxin (10 microM) into the rostral and caudal ventrolateral medulla were compared in SH (n = 7) and WKY (n = 7) groups. Microinjection of L-glutamate into the rostral ventrolateral medulla produced equivalent increases in mean arterial pressure (maximum +33 +/- 3 and +36 +/- 6 mm Hg, SH and WKY groups respectively) and minimal changes in heart rate. Similar administration of L-glutamate into the caudal ventrolateral medulla caused decreases in mean arterial pressure and heart rate; changes in mean arterial pressure were significantly greater in the SH group than in the WKY group (-52.3 +/- 2.9 mm Hg for SH, -22.6 +/- 2.6 mm Hg for WKY). Bilateral microinjection of tetrodotoxin into the caudal ventrolateral medulla produced significantly larger increases of mean arterial pressure in WKY rats (+8 +/- 4 vs +46 +/- 8 mm Hg for SH vs WKY). These data indicate that SH rats may have a lower tonic activity of neurons in the caudal ventrolateral medulla, resulting in a lower restraining influence on sympathetic outflow in the SH rat.     

Responses of adrenal sympathetic preganglionic neurons to stimulation of cardiopulmonary receptors

The current study examined whether or not the activation of Bezold-Jarisch reflex with administration of phenylbiguanide (PBG, 100 microg/kg) into right atrium elicits differential responses in the two populations of adrenal sympathetic preganglionic neurons (SPNs) regulating the release of epinephrine (EPI ADR SPNs) and norepinephrine (NE ADR SPNs), respectively, from adrenal medullary chromaffin cells. Extracellular activity of 48 adrenal SPNs in the intermediolateral cell column (IML) were recorded in urethane/chloralose-anesthetized rats. Twenty-three EPI ADR SPNs and 25 NE ADR SPNs were antidromically activated by stimulation of left adrenal nerve and orthodromically activated by rostral ventrolateral medulla (RVLM) stimulation. At a mean arterial pressure (MAP) of 99. 6+/-2.8 mmHg, the mean spontaneous discharge rates of EPI ADR SPNs and NE ADR SPNs were 6.2+/-0.5 and 4.3+/-0.5 spikes/s, respectively. Intra-atrial PBG markedly inhibited 96% of EPI ADR SPNs (by 3.8+/-0. 4 spikes/s; n=22) and 76% of NE ADR SPNs (by 2.9+/-0.5 spikes/s; n=19) with hypotensive responses (DeltaMAP=33.2+/-5.3 and 26.4+/-5.0 mmHg, respectively). The remaining SPNs were weakly excited or unaffected. We conclude that both groups of SPNs regulating catecholamine release are primarily inhibited by stimulation of cardiopulmonary receptors and that these responses parallel the sympathoinhibitory and hypotensive components of the Bezold-Jarisch reflex.     

Tail artery blood flow measured by chronically implanted Doppler ultrasonic probes in unrestrained conscious rats

We describe a surgical procedure for chronically implanting a Doppler ultrasonic probe around the tail artery of the rat to measure phasic flow velocity in the tail artery of the unrestrained conscious rat. The phasic tail flow signal is highly correlated with the simultaneously recorded superior mesenteric flow signal (range 0.70-0.89 in seven rats) during vasoconstriction induced by exposure to formaldehyde vapour. In response to two quick alerting taps on the cage, tail flow velocity fell from 20+/-2 to 7+/-1 cm/s (P<0.01) and mesenteric flow fell from 30+/-5 to 25+/-4 cm/s (P<0.05), with the fall in tail flow being significantly greater than the fall in mesenteric flow (P<0.05, n=7 rats). In anesthetized rats, the phasic tail flow signal was highly correlated with phasic arterial pressure (range 0.71-0.83 in seven rats). The ability to reliably measure phasic arterial tail flow in the conscious unrestrained rat should facilitate experimental studies of brain pathways regulating flow to this principally cutaneous vascular bed in different physiological situations.     

Descending projections of the posterior nucleus of the hypothalamus: Phaseolus vulgaris leucoagglutinin analysis in the rat

No previous report in any species has systematically examined the descending projections of the posterior nucleus of the hypothalamus (PH). The present report describes the descending projections of the PH in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin. PH fibers mainly descend to the brainstem through two routes: dorsally, within the central tegmental tract; and ventromedially, within the mammillo-tegmental tract and its caudal extension, ventral reticulo-tegmental tracts. PH fibers were found to distribute densely to several nuclei of the brainstem. They are (from rostral to caudal) 1) lateral/ventrolateral regions of the diencephalo-mesopontine periaqueductal gray (PAG); 2) the peripeduncular nucleus; 3) discrete nuclei of pontomesencephalic central gray (dorsal raphe nucleus, laterodorsal tegmental nucleus, and Barrington's nucleus); 4) the longitudinal extent of the central core of the mesencephalic through medullary reticular formation (RF); 5) the ventromedial medulla (nucleus gigantocellularis pars alpha, nucleus raphe magnus, and nucleus raphe pallidus); 6) the ventrolateral medulla (nucleus reticularis parvocellularis and the rostral ventrolateral medullary region); and 7) the inferior olivary nucleus. PH fibers originating from the caudal PH distribute much more heavily than those from the rostral PH to the lower brainstem. The PH has been linked to the control of several important functions, including respiration, cardiovascular activity, locomotion, antinociception, and arousal/wakefulness. It is likely that descending PH projections, particularly those to the PAG, the pontomesencephalic RF, Barrington's nucleus, and parts of the ventromedial and ventrolateral medulla, serve a role in a PH modulation of complex behaviors involving an integration of respiratory, visceromotor, and somatomotor activity. © 1996 Wiley-Liss, Inc.     

Dorsal raphe nucleus serotonergic neurons innervate the rostral ventrolateral medulla in rat

Stimulation of the dorsal raphe nucleus (DRN) alters arterial pressure, heart rate and cerebral blood flow, yet projections from the DRN to medullary autonomic nuclei have not been described. We examined whether serotonergic (5-HT) projections from the DRN terminate in the rostral ventrolateral medulla (RVL) and if so, whether the projection mediates cardiovascular responses to DRN stimulation. Studies were performed in adult male Sprague-Dawley rats. Horseradish peroxidase or choleratoxin B was injected unilaterally or bilaterally into the RVL. Levels of 5-HT, its precursors L-tryptophan and 5-hydroxytryptophan and the metabolite 5-hydroxyindole acetic acid were measured in the ventral medulla by HPLC three weeks following placement of electrolytic lesions in DRN. Serotonin transporter (3H-cyanoimipramine binding) was quantified by autoradiography in DRN-lesioned animals. Horseradish peroxidase or choleratoxin B injections into the medulla at the level of the RVL resulted in retrogradely labeled neurons bilaterally, with ipsilateral predominance, in the DRN. Labeled cells were preponderant in rostral ventrolateral portions of the DRN, but were also observed in the dorsal, lateral and interfascicular DRN subnuclei; fewer neurons were observed in caudal portions of the DRN. Three weeks following placement of electrolytic lesions in the DRN, the concentrations of 5-HT and 5-hydroxyindole acetic acid, but not L-tryptophan or 5-hydroxytryptophan, were reduced in the medulla by 45 and 48%, respectively, compared to sham-operated or unoperated controls. DRN lesions reduced binding to the 5-HT transporter in the RVL by approximately 30% compared to unlesioned controls. Unilateral lesions of the RVL reduced the evoked blood pressure response by 53+/-15%; bilateral RVL lesions reduced the response by 86+/-9%. The increase in cortical blood flow elicited by DRN stimulation was unchanged after unilateral or bilateral RVL lesions. These studies demonstrate that there is a descending serotonergic projection from the DRN to the RVL. This projection may mediate autonomic changes elicited by DRN stimulation.     

Evaluation of cardiovascular control by neurons in the dorsal medulla of rats

The contribution of sympathoexcitatory neurons in the dorsal medulla to the regulation of arterial pressure and the involvement of such neurons in integration of physiological responses or in the genesis of basal vasomotor tone are not well defined. In the present study discharge of neurons in the dorsal medulla of anesthetized rats was increased or decreased by microinjections of amino acids to examine effects on systemic arterial pressure, heart rate and blood flow and conductance of the renal and femoral vascular beds. Microinjections of excitatory D, L-homocysteic acid caused increases in arterial pressure of 18 +/- 2 mmHg, increases in heart rate ranging from 5-40 beats/min and renal vasoconstriction; the femoral bed constricted after some injections and dilated in response to others. Injections of the inhibitory amino acid glycine caused no consistent decreases in arterial pressure and heart rate and injections of the gamma-aminobutyric acid analog, muscimol were ineffective. These data demonstrate that neurons in the dorsal region of the rat medulla can contribute to regulation of arterial pressure and can integrate generalized differential changes in regional vascular resistance, but do not appear to be essential for the genesis of basal vasomotor tone.     

Pressor responses evoked by microinjections of L-glutamate into the caudal ventrolateral medulla of the rat

The caudal medulla of the rat was mapped for cardiovascular sensitive regions by recording changes in mean arterial pressure (MAP) and heart rate evoked by microinjections of L-glutamate (1 nmol/50 nl). Using this technique to selectively activate cell soma in the brainstem, a new pressor area in the caudal ventrolateral medulla has been identified. Several sites located approximately 1-1.5 mm posterior to the caudal medullary depressor zone were found where L-glutamate evoked pressor responses of 10-45 mm Hg. The most responsive area was located just dorsal to the lateral aspect of the lateral reticular nucleus at the level of the pyramidal decussation and the caudalmost pole of the inferior olives. Pressor responses at this site averaged 37 +/- 2 mm Hg. Changes in heart rate were inconsistent and both tachycardia and bradycardia were observed. Increases in arterial pressure elicited from the caudal pressor area (CPA) were abolished by ganglionic blockade. Pressor responses evoked from the CPA were also eliminated after functional inactivation of vasopressor neurons in the rostral ventrolateral medulla (RVM) was produced by microinjections of muscimol. Inhibition of CPA neurons by microinjections of GABA had no effect on MAP while GABA markedly reduced MAP when injected into the RVM. These studies demonstrate that a circumscribed region of the caudal ventrolateral medulla contains a population of 'vasopressor' neurons distinct from those located in the rostral medulla. No evidence was obtained to suggest that neural activity in the CPA contributes to the maintenance of arterial pressure. The precise functional role of the CPA in central cardiovascular regulation remains to be determined.     

Neurons in the caudal ventrolateral medulla mediate the arterial baroreceptor reflex by inhibiting barosensitive reticulospinal neurons in the rostral ventrolateral medulla in rabbits.

Participation of the caudal ventrolateral medulla in the arterial baroreceptor reflex was examined in urethane-anesthetized, vagotomized and immobilized rabbits whose aortic nerve was cut bilaterally. The extent of the caudal ventrolateral medulla was mapped by decreases in the renal sympathetic nerve activity and arterial pressure following a local microinjection of a neuroexcitatory amino acid, sodium glutamate (0.075-1.5 nmol). It extended between the levels 1.3 mm rostral and 3.0 mm caudal to the obex. An injection of sodium glutamate into the caudal ventrolateral medulla also diminished spontaneous activity of barosensitive reticulospinal neurons in the rostral ventrolateral medulla. In the 'split medulla preparation' in which the medulla was split along the midsagittal plane to disrupt fiber connections associating both sides, a neurotoxic agent, kainic acid, was injected unilaterally into the rostral ventrolateral medulla. This treatment markedly attenuated responses of renal sympathetic nerve activity and arterial pressure induced by a sodium glutamate injection into the ipsilateral caudal ventrolateral medulla, whereas responses to an injection into the contralateral caudal ventrolateral medulla were totally preserved. In four separate experiments, three to five injections of kainic acid were made unilaterally to cover the whole extent of the caudal ventrolateral medulla. The sympathoinhibitory and depressor responses to stimulation of the ipsilateral aortic nerve were then totally abolished. Simultaneously, the cardiac cycle-related rhythmic fluctuation of renal sympathetic nerve activity, which represented activity of the carotid sinus baroreceptor reflex, was attenuated to the noise level. These results, together with our previous electrophysiological demonstration of barosensitive caudal ventrolateral medulla neurons with axonal projections to the rostral ventrolateral medulla, strongly support the hypothesis that neurons in the caudal ventrolateral medulla mediate the arterial baroreceptor-vasomotor reflex through inhibition of barosensitive reticulospinal neurons in the rostral ventrolateral medulla.     

Projections of the dorsal raphe nucleus to the brainstem: PHA-L analysis in the rat

Early studies that used older tracing techniques reported exceedingly few projections from the dorsal raphe nucleus (DR) to the brainstem. The present report examined DR projections to the brainstem by use of the anterograde anatomical tracer Phaseolus vulgaris leucoagglutinin (PHA-L). DR fibers were found to terminate relatively substantially in several structures of the midbrain, pons, and medulla. The following pontine and midbrain nuclei receive moderate to dense projections from the DR: pontomesencephalic central gray, mesencephalic reticular formation, pedunculopontine tegmental nucleus, medial and lateral parabrachial nuclei, nucleus pontis oralis, nucleus pontis caudalis, locus coeruleus, laterodorsal tegmental nucleus, and raphe nuclei, including the central linear nucleus, median raphe nucleus, and raphe pontis. The following nuclei of the medulla receive moderately dense projections from the DR: nucleus gigantocellularis, nucleus raphe magnus, nucleus raphe obscurus, facial nucleus, nucleus gigantocellularis-pars alpha, and the rostral ventrolateral medullary area. DR fibers project lightly to nucleus cuneiformis, nucleus prepositus hypoglossi, nucleus paragigantocellularis, nucleus reticularis ventralis, and hypoglossal nucleus. Some differences were observed in projections from rostral and caudal parts of the DR. The major difference was that fibers from the rostral DR distribute more widely and heavily than do those from the caudal DR to structures of the medulla, including raphe magnus and obscurus, nucleus gigantocellularis-pars alpha, nucleus paragigantocellularis, facial nucleus, and the rostral ventrolateral medullary area. A role for the dorsal raphe nucleus in several brainstem controlled functions is discussed, including REM sleep and its events, nociception, and sensory motor control. © Wiley-Liss, Inc.