Cardiovascular responses to microinjections of glutamate into the nucleus tractus solitarii of unanesthetized supracollicular decerebrate rats

Blockade of endogenous insulin receptor signaling in the nucleus tractus solitarius potentiates exercise pressor reflex function in healthy male rats

Insulin not only regulates glucose and/or lipid metabolism but also modulates brain neural activity. The nucleus tractus solitarius (NTS) is a key central integration site for sensory input from working skeletal muscle and arterial baroreceptors during exercise. Stimulation of the skeletal muscle exercise pressor reflex (EPR), the responses of which are buffered by the arterial baroreflex, leads to compensatory increases in arterial pressure to supply blood to working muscle. Evidence suggests that insulin signaling decreases neuronal excitability in the brain, thus antagonizing insulin receptors (IRs) may increase neuronal excitability. However, the impact of brain insulin signaling on the EPR remains fully undetermined. We hypothesized that antagonism of NTS IRs increases EPR function in normal healthy rodents. In decerebrate rats, stimulation of the EPR via electrically induced muscle contractions increased peak mean arterial pressure (MAP) responses 30 min following NTS microinjections of an IR antagonist (GSK1838705, 100 μM; Pre: Δ16 ± 10 mmHg vs. 30 min: Δ23 ± 13 mmHg, n = 11, p = .004), a finding absent in sino-aortic baroreceptor denervated rats. Intrathecal injections of GSK1838705 did not influence peak MAP responses to mechano- or chemoreflex stimulation of the hindlimb muscle. Immunofluorescence triple overlap analysis following repetitive EPR stimulation increased c-Fos overlap with EPR-sensitive nuclei and IR-positive cells relative to sham operation (p < .001). The results suggest that IR blockade in the NTS potentiates the MAP response to EPR stimulation. In addition, insulin signaling in the NTS may buffer EPR stimulated increases in blood pressure via baroreflex-mediated mechanisms during exercise.

Characterization of the Insular Role in Cardiac Function through Intracranial Electrical Stimulation of the Human Insula

OBJECTIVE

The link between brain function and cardiovascular dynamics is an important issue yet to be elucidated completely. The insula is a neocortical brain area that is thought to have a cardiac chronotropic regulatory function, but its role in cardiac contractility is unknown. We aimed to analyze the variability in heart rate and cardiac contractility after functional activation of different insular regions through direct electrical stimulation (E-stim) in humans.

METHODS

This was an observational, prospective study, including patients admitted for stereo-electroencephalographic recording because of refractory epilepsy, in whom the insular cortex was implanted. Patients with anatomical or electrophysiological insular abnormalities and those in whom E-stim produced subjective symptoms were excluded. Variations in heart rate (HR), stroke volume (SV), and cardiac output (CO) were analyzed during insular E-stim and compared with control E-stim of non-eloquent brain regions and sham stimulations.

RESULTS

Ten patients were included, 5 implanted in the right insula (52 E-stim) and 5 in the left (37 E-stim). Demographic and clinical characteristics of both groups were similar. E-stim of both right and left insulas induced a significant decrease of the CO and HR, and an increase of the SV. E-stim of control electrodes and sham stimulations were not associated with variations in cardiac function. Blood pressure and respiratory rate remained unaltered.

INTERPRETATION

Our results suggest a direct chronotropic and inotropic cardiac depressor function of the right and left insulas. The evidence of an insular direct cardiac regulatory function might open a path in the prevention or treatment of heart failure, arrhythmias, and sudden unexpected death in epilepsy. ANN NEUROL 2021;89:1172-1180.

Treatment of muscle mechanoreflex dysfunction in hypertension: effects of L-arginine dialysis in the nucleus tractus solitarii

NEW FINDINGS

What is the central question of this study? Does increasing NO production within the nucleus tractus solitarii (NTS) affect mechanoreflex function in normotensive and hypertensive rats?What is the main finding and its importance? Dialysis of 1 μm l-arginine, an NO precursor, within the NTS significantly attenuated the pressor response to muscle stretch in normotensive and hypertensive rats. In contrast, 10 μm l-arginine had no effect in normotensive animals, while increasing and decreasing the pressor and tachycardic responses to stretch, respectively, in hypertensive rats. This suggests that increasing NO within the NTS using lower doses of l-arginine can partly normalize mechanoreflex overactivity in hypertensive rats, whereas the effects of larger doses are equivocal. The blood pressure response to exercise is exaggerated in hypertension. Recent evidence suggests that an overactive skeletal muscle mechanoreflex contributes significantly to this augmented circulatory responsiveness. Sensory information from the mechanoreflex is processed within the nucleus tractus solitarii (NTS) of the medulla oblongata. Normally, endogenously produced nitric oxide within the NTS attenuates the increase in mean arterial pressure (MAP) induced by mechanoreflex stimulation. Thus, it has been suggested that decreases in NO production in the NTS underlie the generation of mechanoreflex dysfunction in hypertension. Supporting this postulate, it has been shown that blocking NO production within the NTS of normotensive rats reproduces the exaggerated pressor response elicited by mechanoreflex activation in hypertensive animals. What is not known is whether increasing NO production within the NTS of hypertensive rats mitigates mechanoreflex overactivity. In this study, the mechanoreflex was selectively activated by passively stretching hindlimb muscle before and after the dialysis of 1 and 10 μm l-arginine (an NO precursor) within the NTS of decerebrate normotensive Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHRs). Stretch induced larger elevations in MAP in SHRs compared with WKY rats. In both groups, dialysis of 1 μm l-arginine significantly attenuated the pressor response to stretch. However, at the 10 μm dose, l-arginine had no effect on the MAP response to stretch in WKY rats, while it enhanced the response in SHRs. The data demonstrate that increasing NO availability within the NTS using lower doses of l-arginine partly normalizes mechanoreflex dysfunction in hypertension, whereas higher doses do not. The findings could prove valuable in the development of treatment options for mechanoreflex overactivity in this disease.

Cardiovascular responses elicited by a new endogenous angiotensin in the nucleus tractus solitarius of the rat

Cardiovascular effects of angiotensin-(1-12) [ANG-(1-12)] were studied in the medial nucleus of the tractus solitarius (mNTS) in anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (100 nl) of ANG-(1-12) (0.06 mM) into the mNTS elicited maximum decreases in mean arterial pressure (MAP; 34 ± 5.8 mmHg) and heart rate (HR; 39 ± 3.7 beats/min). Bilateral vagotomy abolished ANG-(1-12)-induced bradycardia. Efferent greater splanchnic nerve activity was decreased by microinjections of ANG-(1-12) into the mNTS. Blockade of ANG type 1 receptors (AT(1)Rs; using ZD-7155 or L-158,809), but not ANG type 2 receptors (AT(2)Rs; using PD-123319), significantly attenuated ANG-(1-12)-induced cardiovascular responses. Simultaneous inhibition of both angiotensin-converting enzyme (ACE; using captopril) and chymase (using chymostatin) completely blocked the effects of ANG-(1-12). Microinjections of A-779 [ANG-(1-7) antagonist] did not attenuate ANG-(1-12)-induced responses. Pressure ejection of ANG-(1-12) (0.06 mM, 2 nl) caused excitation of barosensitive mNTS neurons, which was blocked by prior application of the AT(1)R antagonist. ANG-(1-12)-induced excitation of mNTS neurons was also blocked by prior sequential applications of captopril and chymostatin. These results indicate that 1) microinjections of ANG-(1-12) into the mNTS elicited depressor and bradycardic responses by exciting barosensitive mNTS neurons; 2) the decreases in MAP and HR were mediated via sympathetic and vagus nerves, respectively; 3) AT(1)Rs, but not AT(2)Rs, mediated these actions of ANG-(1-12); 4) the responses were mediated via the conversion of ANG-(1-12) to ANG II and both ACE and chymase were involved in this conversion; and 5) ANG-(1-7) was not one of the metabolites of ANG-(1-12) in the mNTS.

Inhibition of the NMDA receptor/Nitric Oxide pathway in the dorsolateral periaqueductal gray causes anxiolytic-like effects in rats submitted to the Vogel conflict test

Background

Several studies had demonstrated the involvement of the dorsolateral portion of periaqueductal grey matter (dlPAG) in defensive responses. This region contains a significant number of neurons containing the enzyme nitric oxide synthase (NOS) and previous studies showed that non-selective NOS inhibition or glutamate NMDA-receptor antagonism in the dlPAG caused anxiolytic-like effects in the elevated plus maze.

Methods

In the present study we verified if the NMDA/NO pathway in the dlPAG would also involve in the behavioral suppression observed in rats submitted to the Vogel conflict test. In addition, the involvement of this pathway was investigated by using a selective nNOS inhibitor, Nω-propyl-L-arginine (N-Propyl, 0.08 nmol/200 nL), a NO scavenger, carboxy-PTIO (c-PTIO, 2 nmol/200 nL) and a specific NMDA receptor antagonist, LY235959 (4 nmol/200 nL).

Results

Intra-dlPAG microinjection of these drugs increased the number of punished licks without changing the number of unpunished licks or nociceptive threshold, as measure by the tail flick test.

Conclusion

The results indicate that activation of NMDA receptors and increased production of NO in the dlPAG are involved in the anxiety behavior displayed by rats in the VCT.

Cardiovascular responses to microinjections of urocortins into the NTS: role of inotropic glutamate receptors

Urocortin 1 (Ucn1) and urocortin 3 (Ucn3) are new members of the corticotrophin-releasing factor (CRF) peptide family. Ucn1 is a ligand for both the CRF type 1 receptors (CRF(1)Rs) and the CRF type 2 receptors (CRF(2)Rs), whereas Ucn3 is a high-affinity ligand for the CRF(2)Rs. Recently, we reported that Ucn3 microinjections into the medial nucleus tractus solitarius (mNTS) elicit decreases in mean arterial pressure (MAP) and heart rate (HR) (Nakamura T, Kawabe K, Sapru HN. Am J Physiol Heart Circ Physiol 296: H325-H332, 2009). The presence of CRF(2)Rs on afferent terminals has been reported in the mNTS of the rat. It was hypothesized that activation of CRF(2)Rs on afferent terminals in the mNTS may release glutamate, which, in turn, may elicit decreases in MAP and HR via activation of ionotropic glutamate receptors (iGLURs). This hypothesis was tested in urethane-anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (100 nl) of Ucn1 (0.12 mM) into the mNTS elicited decreases in MAP and HR. The responses were partially blocked by microinjections of iGLUR antagonists into the mNTS. On the other hand, the decreases in MAP and HR elicited by microinjections of Ucn3 (0.06 mM) into the mNTS were completely blocked by microinjections of iGLUR antagonists into the mNTS. These results indicate that activation of CRF(2)Rs in the mNTS, by Ucn1 and Ucn3, releases glutamate, which, in turn, elicits decreases in MAP and HR via activation of iGLURs.

Mechanism of heart rate responses elicited by chemical stimulation of the hypothalamic paraventricular nucleus in the rat

This study was designed to examine the mechanism of heart rate (HR) responses elicited by the stimulation of hypothalamic paraventricular nucleus (PVN). Experiments were done in urethane-anesthetized, barodenervated, adult, male Wistar rats. Chemical stimulation of the PVN by unilateral microinjections of N-methyl-d-aspartic acid (NMDA) elicited increases in HR which were attenuated by bilateral vagotomy. PVN-induced tachycardia was also attenuated by the blockade of the spinal ionotropic glutamate receptors (iGLURs) which was accomplished by intrathecal injections at T9-T10 or direct application at T1-T4 of iGLUR antagonists. The blockade of spinal iGLURs combined with bilateral vagotomy completely blocked PVN-induced tachycardia. Blockade of GABA receptors in the medial nucleus tractus solitarius (mNTS) also attenuated the PVN-induced tachycardia. Complete blockade of PVN-induced tachycardia was also observed after the blockade of iGLURs in both the spinal cord and mNTS. Combination of the blockade of mNTS GABA receptors and spinal iGLURs also abolished PVN-induced tachycardia. PVN-induced tachycardia was not altered by the blockade of spinal vasopressin or oxytocin receptors at T1-T4. These results suggested that in barodenervated rats: 1) tachycardia elicited by the chemical stimulation of the PVN was mediated via both inhibition of vagal and activation of sympathetic outflows to the heart, 2) the vagal inhibition contributing to the PVN-induced tachycardia was mediated by the iGLURs and GABARs in the mNTS, 3) sympathetic activation contributing to the PVN-induced tachycardia was mediated via spinal iGLURs, and 4) spinal vasopressin and oxytocin receptors were not involved in the mediation of PVN-induced tachycardia.

Cardiovascular responses to microinjections of urocortin 3 into the nucleus tractus solitarius of the rat

Urocortin 3 (Ucn3) is a new member of the corticotropin-releasing factor (CRF) peptide family and is considered to be a specific and endogenous ligand for CRF type 2 receptors (CRF2Rs). The presence of CRF(2)Rs has been reported in the nucleus tractus solitarius (NTS) of the rat. It was hypothesized that the activation of CRF2Rs in the medial NTS (mNTS) may play a role in cardiovascular regulation. This hypothesis was tested in urethane-anesthetized, artificially ventilated, adult male Wistar rats. Microinjections (100 nl) of Ucn3 (0.03, 0.06, 0.12, and 0.25 mM) into the mNTS of anesthetized rats elicited decreases in mean arterial pressure (MAP: 5.0 +/- 1.0, 21.6 +/- 2.6, 20.0 +/- 2.8, and 12.7 +/- 3.4 mmHg, respectively) and heart rate (HR: 7.8 +/- 2.6, 46.2 +/- 9.3, 34.5 +/- 8.4, and 16.6 +/- 4.9 beats/min, respectively). Microinjections of artificial cerebrospinal fluid (100 nl) into the mNTS did not elicit cardiovascular responses. Maximum decreases in MAP and HR were elicited by 0.06 mM concentration of Ucn3. Cardiovascular responses to Ucn3 were similar in unanesthetized midcollicular decerebrate rats. A bilateral vagotomy completely abolished Ucn3-induced bradycardia. The decreases in MAP and HR elicited by Ucn3 (0.06 mM) were completely blocked by astressin (1 mM; nonselective CRFR antagonist) and K41498 (5 mM; selective CRF2R antagonist). Microinjections of Ucn3 (0.06 mM) into the mNTS decreased the efferent greater splanchnic nerve activity. After the blockade of CRF2Rs in the mNTS, a Ucn3-induced decrease in the efferent sympathetic nerve discharge was abolished. These results indicate that Ucn3 microinjections into the mNTS exerted excitatory effects on the mNTS neurons via CRF2Rs, leading to depressor and bradycardic responses.

Cold pressor test in the rat: medullary and spinal pathways and neurotransmitters

This study was designed to delineate the medullary and spinal pathways mediating the cardiovascular responses to cold pressor test (CPT) and to identify neurotransmitters in these pathways. Experiments were done in barodenervated, urethane-anesthetized, male Wistar rats. The CPT was performed by immersing the limbs and ventral half of the body of the rat in ice-cold water (0.5 degrees C) for 2 min. CPT elicited an immediate increase in mean arterial pressure (MAP), heart rate (HR), and greater splanchnic nerve activity (GSNA). Bilateral blockade of ionotropic glutamate receptors (iGLURs) in the rostral ventrolateral medullary pressor area (RVLM) significantly attenuated the CPT-induced responses. Bilateral blockade of gamma-aminobutyric acid (GABA) receptors, but not iGLURs, in the nucleus ambiguus (nAmb) significantly reduced the CPT-induced increases in HR, but not MAP. Blockade of spinal iGLURs caused a significant reduction in CPT-induced increases in MAP and GSNA, whereas the increases in HR were reduced to a lesser extent. Combination of the blockade of spinal iGLURs and bilateral vagotomy or intravenous atropine almost completely blocked CPT-induced tachycardia. Midcollicular decerebration significantly reduced CPT-induced increases in MAP and HR. These results indicated that: 1) CPT-induced increases in MAP, HR, and GSNA were mediated by activation of iGLURs in the RVLM and spinal cord, 2) activation of GABA receptors in the nAmb also contributed to the CPT-induced tachycardic responses, and 3) brain areas rostral to the brain stem also participated in the CPT-induced pressor and tachycardic responses.

Cardiovascular function of a glutamatergic projection from the hypothalamic paraventricular nucleus to the nucleus tractus solitarius in the rat

Experiments were done in urethane-anesthetized, barodenervated, male Wistar rats. Chemical stimulation of the hypothalamic paraventricular nucleus (PVN) by unilateral microinjections of N-methyl-D-aspartic acid (NMDA) elicited increases in mean arterial pressure (MAP) and greater splanchnic nerve activity (GSNA). The increases in the MAP and GSNA induced by chemical stimulation of the PVN were significantly exaggerated by bilateral microinjections of D(-)-2-amino-7-phosphono-heptanoic acid (D-AP7) and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydro-benzo[f]quinoxaline-7-sulfonamide disodium (NBQX) (ionotropic glutamate receptor antagonists) into the medial subnucleus of the nucleus tractus solitarius (mNTS). These results were confirmed by single unit recordings; i.e. excitation of mNTS barosensitive neurons caused by chemical stimulation of the ipsilateral PVN was blocked by application of D-AP7 and NBQX to these neurons. Bilateral microinjections of D-AP7 and NBQX into the mNTS elicited pressor responses which were significantly attenuated by inhibition of PVN neurons by bilateral microinjections of muscimol. Unilateral microinjections of fluorogold into the mNTS resulted in bilateral retrograde labeling of the PVN neurons. Unilateral microinjections of biotinylated dextran amine into the PVN resulted in anterograde labeling of axons and terminals in the mNTS bilaterally and the labeled terminals exhibited vesicular glutamate transporter-2 immunoreactivity. These results indicated that 1) a tonically active glutamatergic bilateral projection from the PVN to the mNTS exists; 2) bilateral blockade of ionotropic glutamate receptors in the mNTS exaggerates the increases in MAP and GSNA, but not heart rate, to the chemical stimulation of the PVN; and 3) this projection may serve as a restraint mechanism for excitatory cardiovascular effects of PVN stimulation.

Medullary pathways mediating the parasubthalamic nucleus depressor response

The parasubthalamic nucleus (PSTN) projects extensively to the nucleus of the solitary tract (NTS); however, the function of PSTN in cardiovascular regulation is unknown. Experiments were done in alpha-chloralose anesthetized, paralyzed, and artificially ventilated rats to investigate the effect of glutamate (10 nl, 0.25 M) activation of PSTN neurons on mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA). Glutamate stimulation of PSTN elicited depressor (-20.4 +/- 0.7 mmHg) and bradycardia (-26.0 +/- 1.0 beats/min) responses and decreases in RSNA (67 +/- 17%). Administration (intravenous) of atropine methyl bromide attenuated the bradycardia response (46%), but had no effect on the MAP response. Subsequent intravenous administration of hexamethonium bromide blocked both the remaining bradycardia and depressor responses. Bilateral microinjection of the synaptic blocker CoCl(2) into the caudal NTS region attenuated the PSTN depressor and bradycardia responses by 92% and 94%, respectively. Additionally, prior glutamate activation of neurons in the ipsilateral NTS did not alter the magnitude of the MAP response to stimulation of PSTN, but potentiated HR response by 35%. Finally, PSTN stimulation increased the magnitude of the reflex bradycardia to activation of arterial baroreceptors. These data indicate that activation of neurons in the PSTN elicits a decrease in MAP due to sympathoinhibition and a cardiac slowing that involves both vagal excitation and sympathoinhibition. In addition, these data suggest that the PSTN depressor effects on circulation are mediated in part through activation of NTS neurons involved in baroreflex function.

The expression of contextual fear conditioning involves activation of an NMDA receptor-nitric oxide pathway in the medial prefrontal cortex

The ventral portion of medial prefrontal cortex (vMPFC) is involved in contextual fear-conditioning expression in rats. In the present study, we investigated the role of local N-methyl-D-aspartic acid (NMDA) glutamate receptors and nitric oxide (NO) in vMPFC on the behavioral (freezing) and cardiovascular (increase of arterial pressure and heart rate) responses of rats exposed to a context fear conditioning. The results showed that both freezing and cardiovascular responses to contextual fear conditioning were reduced by bilateral administration of NMDA receptor antagonist LY235959 (4 nmol/200 nL) into the vMPFC before reexposition to conditioned chamber. Bilateral inhibition of neuronal NO synthase (nNOS) by local vMPFC administration of the Nω-propyl-L-arginine (N-propyl, 0.04 nmol/200 nL) or the NO scavenger carboxy-PTIO (1 nmol/200 nL) caused similar results, inhibiting the fear responses. We also investigated the effects of inhibiting glutamate- and NO-mediated neurotransmission in the vMPFC at the time of aversive context exposure on reexposure to the same context. It was observed that the 1st exposure results in a significant attenuation of the fear responses on reexposure in vehicle-treated animals, which was not modified by the drugs. The present results suggest that a vMPFC NMDA–NO pathway may play an important role on expression of contextual fear conditioning.

Interaction between glutamatergic and nitrergic mechanisms mediating cardiovascular responses to l-glutamate injection in the diagonal band of Broca in anesthetized rats

In a previous study, we reported depressor and bradycardiac responses after L-glutamate (L-glu) microinjection into the diagonal band of Broca (dbB) in anesthetized rats. Here, we report the glutamatergic-receptor subtype mediating the cardiovascular effects evoked by L-glu injection into the dbB and the involvement of local nitric oxide (NO) mechanisms as well as peripheral effectors. Microinjections of 100 nL of L-glu (1, 27, 81, 130 or 200 nmol) into the dbB of urethane-anesthetized rats caused short-lasting depressor and bradycardiac responses. Responses were dose-related, with an ED(50) of approximately 81 nmol. This dose was used in later experiments. The cardiovascular responses to L-glu in the dbB were abolished by local pretreatment (100 nL) with the selective N-methyl-D-aspartic acid (NMDA) receptor antagonist LY235959 (4 nmol) but were not affected by pretreatment with the selective non-NMDA receptor antagonist NBQX (4 nmol). Responses to L-glu in the dbB were blocked by local pretreatment with the selective neuronal NO-synthase (nNOS) inhibitor N(omega)-propyl-L-arginine (NPLA, 0.04 nmol); the NO scavenger carboxy-PTIO (C-PTIO, 1 nmol) or the guanylate cyclase inhibitor ODQ (1 nmol). These results suggest that the microinjection of L-glu into the dbB of urethane-anesthetized rats causes dose-related depressor and bradycardiac responses through the NMDA receptor-NO-guanylate cyclase pathway.

Cardiovascular actions of adrenocorticotropin microinjections into the nucleus tractus solitarius of the rat

The presence of adrenocorticotropin (ACTH) containing cells and melanocortin (MC) receptors has been reported in the nucleus tractus solitarius (NTS) of the rat. The importance of the NTS in the regulation of cardiovascular function is also well established. Based on these reports, it was hypothesized that ACTH acting within the NTS may modulate the central regulation of cardiovascular function. To test this hypothesis, cardiovascular effects of ACTH in the NTS were investigated in intact urethane-anesthetized and unanesthetized decerebrate, artificially ventilated, adult male Wistar rats. Microinjections of ACTH (0, 0.5, 1, 2, and 4 mM) into the medial subnucleus of NTS (mNTS) elicited decreases in mean arterial pressure (MAP; 0+/-0, 24.4+/-3.5, 35.7+/-4.3, 44.5+/-5.8 and 53.7+/-5.6 mm Hg, respectively) and heart rate (HR; 0+/-0, 25.7+/-5.3, 35.5+/-6.4, 47.5+/-12.1 and 55.0+/-5.6 beats/min, respectively). The onset and duration of the responses to microinjections of ACTH (0.5-4 mM) were 5-10 s and 45-120 s, respectively. Control microinjections of artificial cerebrospinal fluid (aCSF) did not elicit any response. The volume of all microinjections was 100 nl. The concentrations of ACTH that elicited depressor and bradycardic responses when microinjected into the mNTS (e.g. 1 or 2 mM, 100 nl), did not elicit a response when injected i.v. (n=5) or i.c.v. (n=2) indicating that there was no leakage of the drug from the injection site in the mNTS. Microinjections of MC3/4 receptor antagonists (acetyl-[Nle(4), Asp(5), d-2-Nal(7), Lys(10)]-cyclo-alpha-MSH amide, fragments 4-10 (SHU9119) and agouti-related protein (83-132) amide) into the mNTS blocked the responses to ACTH. Microinjections of ACTH (2 mM) into the mNTS decreased efferent greater splanchnic nerve activity. Bilateral vagotomy significantly attenuated ACTH-induced bradycardia. These results indicated that: 1) microinjections of ACTH into the mNTS elicited depressor and bradycardic responses, 2) these responses were mediated via MC3/4 receptors, 3) the depressor effects were mediated via a decrease in the activity of the sympathetic nervous system, and 4) the bradycardic responses were vagally mediated.

Autonomic and respiratory responses to microinjection of L-glutamate into the commissural subnucleus of the NTS in the working heart-brainstem preparation of the rat

Changes in heart rate (HR), thoracic sympathetic nerve activity (tSNA) and frequency of phrenic nerve discharge (PND) in response to microinjection of L-glutamate before and after local microinjection of ionotropic or metabotropic glutamate receptors antagonists into the commissural subnucleus of the NTS (comNTS) were investigated. The experiments were performed in an in situ unanesthetized decerebrated working heart-brainstem preparation (WHBP), and the main findings were as follows: (a) microinjection of increasing concentrations of L-glutamate (5, 25, 50, 250 and 500 mM) into the comNTS produced bradycardia, increase in tSNA and reduction in the frequency of the PND in a concentration-dependent manner; (b) both bradycardia and increase in tSNA were almost abolished by kynurenic acid (KYN, 250 mM, a nonselective ionotropic glutamate receptor antagonist); (c) the reduction in the frequency of the PND was reversed to an increase in the frequency of the PND after KYN and this increase was blocked by the sequential microinjection of MCPG (100 mM, a nonselective metabotropic glutamate receptor antagonist); and (d) microinjection of increasing concentrations of trans-ACPD (0.5, 1.0, 2.5, 5.0 and 10 mM, a metabotropic glutamate receptor agonist), elicited bradycardia and increase in the frequency of the PND in a concentration-dependent manner, which were blocked by MCPG. Taken together, these data indicate that l-glutamate and its ionotropic receptors are involved in the sympathoexcitatory, bradycardic and reduction in the frequency of the PND responses whereas/although its metabotropic receptors are involved in the bradycardic and mainly in the increase in the frequency of the PND to microinjection of L-glutamate into the comNTS in the WHBP.

Involvement of NMDA receptors in the hypotensive response to the injection of l-glutamate into the lateral hypothalamus of unanesthetized rats

We report that microinjections of L-glutamate (L-glu) or N-methyl-D-aspartic acid (NMDA) in the lateral hypothalamus (LH) of unanesthetized rats caused a hypotensive response. Guide cannulas were stereotaxically placed in the LH 3 days before the experiments, under tribromoethanol anesthesia. One day before the experiments, the femoral artery was cannulated for pulsatile arterial pressure (PAP), mean arterial pressure (MAP) and heart rate (HR) measurements. In the first experiment, unanesthetized rats received microinjections of 2.5, 5.0 or 10.0 nmol/100 nL of L-glu in the LH. Dose-dependent hypotensive responses were observed, without significant concomitant changes in heart rate. In a second group of experiments, 5.0 nmol of L-glu was microinjected into the LH before and 10 min after pretreatment with glutamatergic antagonists. Pretreatments with the non-selective ionotropic glutamatergic-receptor antagonist kynurenic acid or the selective NMDA receptor antagonists AP-7 and LY235959 significantly reduced the hypotensive response to microinjection of L-glu in the LH. Pretreatment with the selective AMPA-receptor antagonist NBQX or with vehicle did not affect the hypotensive response. The present results suggest that the hypotensive response to the injection of L-glu into the LH is mediated by NMDA receptors.

Mechanism of cardiovascular effects of nociceptin microinjected into the nucleus tractus solitarius of the rat

Microinjections (100 nl) of 0.15, 0.31, 0.62, and 1.25 mmol/l of nociceptin into the medial nucleus tractus solitarius (mNTS) elicited decreases in mean arterial pressure (11 +/- 1.8, 20 +/- 2.1, 21.5 +/- 3.1, and 15.5 +/- 1.9 mmHg, respectively) and heart rate (14 +/- 2.7, 29 +/- 5.5, 39 +/- 5.2, and 17.5 +/- 3.1 beats/min, respectively). Because maximal responses were elicited by microinjections of 0.62 mmol/l nociceptin, this concentration was used for other experiments. Repeated microinjections of nociceptin (0.62 mmol/l) into the mNTS, at 20-min intervals, did not elicit tachyphylaxis. Bradycardia induced by microinjections of nociceptin into the mNTS was abolished by bilateral vagotomy. The decreases in mean arterial pressure and heart rate elicited by nociceptin into the mNTS were blocked by prior microinjections of the specific ORL1-receptor antagonist [N-Phe(1)]-nociceptin-(1-13)-NH(2) (9 mmol/l). Microinjections of the ORL1-receptor antagonist alone did not elicit a response. Prior combined microinjections of GABA(A) and GABA(B) receptor antagonists (2 mmol/l gabazine and 100 mmol/l 2-hydroxysaclofen, respectively) into the mNTS blocked the responses to microinjections of nociceptin at the same site. Prior microinjections of ionotropic glutamate receptor antagonists (2 mmol/l NBQX and 5 mmol/l d-AP7) also blocked responses to nociceptin microinjections into the mNTS. These results were confirmed by direct neuronal recordings. It was concluded that 1) nociceptin inhibits GABAergic neurons in the mNTS, 2) GABAergic neurons may normally inhibit the release of glutamate from the terminals of peripheral afferents in the mNTS, and 3) inhibition of GABAergic neurons by nociceptin results in an increase in the release of glutamate in the mNTS, which in turn elicits depressor and bradycardic responses via activation of ionotropic glutamate receptors on secondary mNTS neurons.

Cardiovascular response to a group III mGluR agonist in NTS requires NMDA receptors

Previous studies have demonstrated that microinjection of the putative group III metabotropic glutamate receptor (mGluR) agonist, l(+)-2-amino-4-phosphonobutyric acid (L-AP4), into the nucleus tractus solitarius (NTS) produces depressor and sympathoinhibitory responses. These responses are significantly attenuated by a group III mGluR antagonist and may involve ionotropic glutamatergic transmission. Alternatively, a previous report in vitro suggests that preparations of L-AP4 may nonspecifically activate NMDA channels due to glycine contamination (Contractor A, Gereau RW, Green T, and Heinemann SF. Proc Natl Acad Sci USA 95: 8969-8974, 1998). Therefore, the present study tested whether responses to L-AP4 specifically require the N-methyl-D-aspartate (NMDA) receptor and whether they are due to actions at the glycine site on the NMDA channel. To test these possibilities in vivo, we performed unilateral microinjections of L-AP4, glycine, and selective antagonists into the NTS of urethane-anesthetized rats. L-AP4 (10 mM, 30 nl) produced sympathoinhibitory responses that were abolished by the NMDA receptor antagonist 2-amino-5-phosphonovaleric acid (AP-5, 10 mM) but were unaffected by the non-NMDA antagonist 6-nitro-7-sulfamobenzoquinoxaline-2,3-dione (NBQX, 2 mM). Microinjection of glycine (0.02-20 mM) failed to mimic sympathoinhibitory responses to L-AP4, even in the presence of the inhibitory glycine antagonist, strychnine (3 mM). Strychnine blocked pressor and sympathoexcitatory actions of glycine (20 mM) but failed to reveal a sympathoinhibitory component due to presumed activation of NMDA receptors. The results of these experiments suggest that responses to L-AP4 require NMDA receptors and are independent of non-NMDA receptors. Furthermore, although it is possible that glycine contamination or other nonspecific actions are responsible for the sympathoinhibitory actions of L-AP4, our data and data in the literature argue against this possibility. Thus we conclude that responses to L-AP4 in the NTS are mediated by an interaction between group III mGluRs and NMDA receptors. Finally, we also caution that nonselective actions of L-AP4 should be considered in future studies.

Depressor and bradycardic responses to microinjections of endomorphin-2 into the NTS are mediated via ionotropic glutamate receptors

The presence of endomorphin-like immunoreactivity has been reported in the nucleus tractus solitarius (NTS). It was hypothesized that endomorphins may play a role in cardiovascular regulation in the medial subnucleus of the NTS (mNTS). Endomorphin-2 (E-2, 0.1–4 mmol/l) was microinjected (100 nl) into the mNTS of urethane-anesthetized, artificially ventilated, adult male Wistar rats. E-2 (0.2 mmol/l) elicited decreases in mean arterial pressure (40 ± 3.5 mmHg) and heart rate (50 ± 7.0 beats/min). These responses were blocked by prior microinjections of naloxonazine (1 mmol/l) into the mNTS. Responses to microinjections of E-2 into the mNTS were abolished by prior combined microinjections of d-2-amino-7-phosphonoheptanoic acid (an NMDA receptor antagonist, 5 mmol/l) and 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[ f]quinoxaline-7-sulfonamide disodium (a non-NMDA receptor antagonist, 2 mmol/l) into the mNTS. These results were confirmed by extracellular neuronal recordings. Blockade of GABA receptors in the mNTS by prior combined microinjections of gabazine (a GABAA receptor antagonist, 2 mmol/l) and 2-hydroxysaclofen (a GABAB receptor antagonist, 100 mmol/l) also blocked the responses to E-2. It was concluded that 1) the depressor and bradycardic responses to microinjections of E-2 into the mNTS are mediated via μ1-opioid receptors as well as ionotropic glutamate receptors, 2) GABAergic neurons in the mNTS, which may inhibit the release of glutamate from nerve terminals, are inhibited by E-2 via μ1-opioid receptors, and 3) disinhibition caused by the inhibition of GABAergic neurons by E-2 may result in an increase in the glutamate release from nerve terminals, which, in turn, may elicit depressor and bradycardic responses.

Carotid baroreflex in the rat: role of glutamate receptors in the medial subnucleus of the solitary tract

Experiments were done in urethane-anesthetized adult male Wistar rats to investigate the role of glutamate receptors in the medial subnucleus of the solitary tract (mNTS) in mediating the carotid sinus baroreflex responses. The carotid sinus on one side was isolated from the general circulation and perfused with a warm perfusion fluid (37 degrees C; pH 7.4) saturated with 100% oxygen. The carotid sinus was then connected to an apparatus that permitted application of pressure increments (20-100 mm Hg) to stimulate specifically baroreceptors. The mNTS ipsilateral to the isolated carotid sinus was identified by microinjections (100 nL) of L-glutamate (5 mM). The stereotaxic coordinates for mNTS were: 0.5-0.6 mm rostral to the calamus scriptorius, 0.5-0.6 mm lateral to the midline, and 0.5-0.6 mm deep from the dorsal medullary surface. Microinjections of either D(-)-2-amino-7-phosphono-heptanoic acid, which is an N-methyl-D-aspartic acid (NMDA) receptor antagonist (5 mM) or 2,3-dioxo-6-nitro-1,2,3,4-tetrahydro-benzo[f]quinoxaline-7-sulfonamide disodium (a non-NMDA receptor antagonist; 2 mM) significantly attenuated the depressor responses elicited by carotid baroreceptor stimulation. Simultaneous blockade of NMDA and non-NMDA receptors in the ipsilateral mNTS completely abolished the depressor responses to carotid baroceptor stimulation. Microinjections of either (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; 50 mM) or (RS)-alpha-cyclopropyl-4-phosphono-phenyl-glycine (CPPG; 80 mM) did not alter baroreflex responses. AIDA blocked group I and III while CPPG blocked all three groups of metabotropic glutamate receptors (mGLURs). These results suggest that ionotropic glutamate receptors, but not mGLURs, in the mNTS mediate the reflex depressor responses to carotid baroreceptor stimulation in the rat.

Stimulation of Specific Regions of the Parabrachial Nucleus Elicits Ingestive Oromotor Behaviors in Conscious Rats

The "waist" area and external subnuclei of the parabrachial nucleus (PBN) have been implicated in the processing of gustatory information, yet their behavioral roles are not clearly defined. In the current study, areas within and surrounding the PBN were stimulated while oromotor behaviors were monitored in conscious rats. Electrical and chemical (100 mM glutamate) stimulation of the waist area increased ingestive oromotor behaviors over baseline (p<.01). Stimulation of external PBN subnuclei and areas medial and ventral to the PBN failed to cause a behavioral change. These data support the hypothesis that the waist area of the PBN constitutes part of the neural substrate involved in eliciting oromotor behaviors in response to taste input.

Cocaine- and amphetamine-regulated transcript peptide attenuates phenylephrine-induced bradycardia in anesthetized rats

The present study was undertaken to investigate the origin of cocaine- and amphetamine-regulated transcript (CART) peptide immunoreactive (irCART) fibers observed in the nucleus of the solitary tract (NTS) and assess the role of CART peptide on phenylephrine (PE)-induced baroreflex. Immunohistochemical and retrograde tract-tracing studies showed that some of the irCART fibers observed in the NTS may have their cell bodies in the nodose ganglia. In urethane-anesthetized rats, intracisternal or bilateral intra-NTS microinjection of the CART peptide fragment 55-102 (0.1-3 nmol), referred to herein as CARTp, consistently and dose dependently attenuated PE-induced bradycardia. CARTp, in the doses used here, caused no significant changes of resting blood pressure or heart rate. Bilateral intra-NTS injections of CART antibody (1:500) potentiated PE-induced bradycardia. Injections of saline, normal rabbit serum, or concomitant injection of CARTp and CART antiserum into the NTS caused no significant changes of PE-induced baroreflex. The result suggests that endogenously released CARTp from primary afferents or exogenously administered CARTp modulates PE-induced baroreflex.

Cardiovascular responses to chemical stimulation of the lateral tegmental field and adjacent medullary reticular formation in the rat

Relatively few studies have been done to characterize cardiovascular responses to the chemical stimulation of sites located in the medullary lateral tegmental field (LTF) and most of them have been carried out in anesthetized animals. Our experiments were carried out in decerebrated, artificially ventilated, adult male Wistar rats. In the LTF, two types of cardiovascular responses were elicited. One type consisted of pressor responses accompanied by bradycardia. Such responses were elicited from a region 0.4 mm caudal to 0.8 mm rostral to the calamus scriptorius (CS); maximum responses were elicited from a site 0.6 mm rostral to the CS, 1.2 mm lateral to the midline and 1.2 mm deep from the dorsal medullary surface. Another type consisted of pressor responses without any change in heart rate; such responses were elicited from a region 1-1.6 mm rostral to the CS. Nucleus ambiguus (nAmb) and dorsal motor nucleus of the vagus (nDMX) and the reticular formation surrounding these areas were the main sites from which bradycardia (accompanied by either no or small changes in BP) was elicited. In the nAmb, maximum bradycardia was elicited from a site 0.6 mm rostral to the CS, 1.8 mm lateral to the midline and 2.4 mm deep from the dorsal medullary surface. In the nDMX, most prominent bradycardic responses were elicited at 0-0.6 mm rostral to the CS, and 0.6 mm lateral to the midline and 1 mm deep from the dorsal medullary surface. Cardiovascular effects elicited from sites in other well-known areas, such as the rostral ventrolateral medullary pressor area (RVLM) and caudal ventrolateral medullary depressor area (CVLM), and the nucleus tractus solitarius (nTS) were also included for comparison of different responses. These results are expected to prove useful in studies in which the microinjection technique is used to characterize cardiovascular responses.

Evidence for glutamatergic mechanisms in the vagal sensory pathway initiating cardiorespiratory reflexes in the shorthorn sculpin Myoxocephalus scorpius

Glutamate is a major neurotransmitter of chemoreceptor and baroreceptor afferent pathways in mammals and therefore plays a central role in the development of cardiorespiratory reflexes. In fish, the gills are the major sites of these receptors, and, consequently, the terminal field (sensory area) of their afferents (glossopharyngus and vagus) in the medulla must be an important site for the integration of chemoreceptor and baroreceptor signals. This investigation explored whether fish have glutamatergic mechanisms in the vagal sensory area (Xs) that could be involved in the generation of cardiorespiratory reflexes. The locations of the vagal sensory and motor (Xm) areas in the medulla were established by the orthograde and retrograde axonal transport of the neural tract tracer Fast Blue following its injection into the ganglion nodosum. Glutamate was then microinjected into identified sites within the Xs in an attempt to mimic chemoreceptor- and baroreceptor-induced reflexes commonly observed in fish. By necessity, the brain injections were performed on anaesthetised animals that were fixed by 'eye bars' in a recirculating water system. Blood pressure and heart rate were measured using an arterial cannula positioned in the afferent branchial artery of the 3rd gill arch, and ventilation was measured by impedance probes sutured onto the operculum. Unilateral injection of glutamate (40-100 nl, 10 mmol l(-1)) into the Xs caused marked cardiorespiratory changes. Injection (0.1-0.3 mm deep) in different rostrocaudal, medial-lateral positions induced a bradycardia, either increased or decreased blood pressure, ventilation frequency and amplitude and, sometimes, an initial apnea. Often these responses occurred simultaneously in various different combinations but, occasionally, they appeared singly, suggesting specific projections into the Xs for each cardiorespiratory variable and local determination of the modality of the response. Response patterns related to chemoreceptor reflex activation were predominantly located rostral of obex, whereas patterns related to baroreceptor reflex activation were more caudal, around obex. The glutamate-induced bradycardia was N-methyl-D-aspartate (NMDA) receptor dependent and atropine sensitive. Taken together, our data provide evidence that glutamate is a putative player in the central integration of chemoreceptor and baroreceptor information in fish.

Cardiovascular responses to activation of metabotropic glutamate receptors in the nTS of the rat

Although several agonists and antagonists for different subtypes of metabotropic glutamate receptors (mGLURs) have become available in recent years, detailed information regarding their selectivity is not complete in the in vivo animal models. The purpose of the present investigation was to study the cardiovascular effects of microinjections of some of these mGLUR agonists and antagonists into the nucleus tractus solitarius (nTS). Microinjections (100 nl) of EC(50) concentrations of 3,5-DHPG (0.005 mM; mGLUR(1) agonist), APDC (17.3 mM; mGLUR(2/3) agonist), PPG (11.7 mM; mGLUR(8) agonist) and L-AP(4) (1 mM; mGLUR(4) agonist) into the nucleus tractus solitarius of urethane-anesthetized male Wistar rats elicited depressor and bradycardic responses which may be mediated by pre- and/or postsynaptic mechanisms. The blocking effect of mGLUR antagonists used here was not specific for any one type of glutamate receptors (GLURs). For example, AIDA (50 mM; mGLUR(1) antagonist) blocked the effects of EC(50) concentrations of 3,5-DHPG, and PPG. LY341495 (135 mM; mGLUR(2/3) antagonist) blocked all of the mGLURs and ionotropic GLURs. EGLU, APICA and MCCG (250 mM each; mGLUR(2/3) antagonists) blocked the effects of APDC, NMDA and AMPA. CPPG (80 mM) and MSOP (125 mM), mGLUR(4) antagonists, blocked the effects of 3,5-DHPG, PPG and L-AP(4.) D-AP7 (NMDA receptor antagonist) and NBQX (a non-NMDA receptor antagonist) did not alter the responses of any of the mGLUR agonists. The data presented may be useful in assessing the role of metabotropic and ionotropic GLURs in mediating different cardiovascular reflexes.

Inhibition of neurons in commissural nucleus of solitary tract reduces sympathetic nerve activity in SHR

Neurons in the commissural nucleus of the solitary tract (commNTS) play an important role in certain cardiovascular responses dependent on sympathetic vasoconstrictor activation, including the arterial chemoreceptor reflex. Electrolytic lesions of the commNTS elicit a fall in arterial pressure (AP) in spontaneously hypertensive rats (SHR). To determine whether the latter result 1) arose from elimination of commNTS neuronal activity rather than en passant axons and 2) was accompanied by a reduction in sympathetic nerve activity, we evaluated the effect of inhibition of neurons in the commNTS on basal splanchnic sympathetic nerve activity (SNA), AP, and heart rate (HR) in SHR, Wistar-Kyoto (WKY), and Sprague-Dawley (SD) rats. In chloralose-anesthetized, paralyzed, and artificially ventilated SHR, microinjection of GABA into the commNTS markedly decreased splanchnic SNA, AP, and HR. The reductions in SNA and AP following similar microinjections in WKY and SD rats were significantly less than those in SHR. Our findings suggest that tonically active neurons in the commNTS contribute to the maintenance of SNA and the hypertension in SHR. The level of tonic discharge of these commNTS neurons in normotensive WKY and SD rats may be lower than in SHR.

Role of the medulla oblongata in hypertension

Brain pathways controlling arterial pressure are distributed throughout the neuraxis and are organized in topographically selective networks. In this brief review, we will focus on the medulla oblongata. The nucleus tractus solitarius (NTS) is the primary site of cardiorespiratory reflex integration. It is well accepted that lesions or other perturbations in the NTS can result in elevations of arterial pressure (AP), with many of the associated features so commonly found in humans. However, recent studies have shown 2 distinct subpopulations of neurons within the NTS that can influence AP in opposite ways. Commissural NTS neurons located on the midline may contribute to maintenance of hypertension in spontaneously hypertensive rats (SHR), because small lesions in this area result in a very significant reduction in AP. Also involved in this blood pressure regulation network are 2 distinct regions of the ventrolateral medulla: caudal (CVLM) and rostral (RVLM). Neurons in CVLM are thought to receive baroreceptor input and to relay rostrally to control the activity of the RVLM. Projections from CVLM to RVLM are inhibitory, and a lack of their activity may contribute to development of hypertension. The RVLM is critical to the tonic and reflexive regulation of AP. In different experimental models of hypertension, RVLM neurons receive significantly more excitatory inputs. This results in enhanced sympathetic neuronal activity, which is essential for the development and maintenance of the hypertension.

Cardiovascular responses to microinjections of nicotine into the caudal ventrolateral medulla of the rat

This study focuses on the role of nicotinic receptors located in the caudal ventrolateral medullary depressor area (CVLM) in regulating/modulating cardiovascular function. Blood pressure and heart rate were monitored by standard techniques in urethane-anesthetized, artificially ventilated, adult male Wistar rats. Multi-barreled glass-micropipettes (tip size 20-40 microm) were used to make microinjections (100 nl) into the CVLM. Concentrations of nicotine ranging from 250 micromto 10 mM were microinjected unilaterally into the CVLM. The maximum depressor and bradycardic responses were elicited by a 1 mM concentration of nicotine. Sequential microinjections of mecamylamine (1 mM), an antagonist for nicotinic receptors containing alpha3beta4 subunits, then alpha-bungarotoxin (1 microm), an antagonist for nicotinic receptors containing alpha-7 subunits, were made into the CVLM. Microinjecting a combination of a nicotinic receptor blocker and toxin resulted in the complete blockade of the cardiovascular responses induced by nicotine (1 mM, 100 nl). These results indicate that: (1) nicotinic receptors are present in the CVLM; (2) activation of these receptors results in depressor and bradycardic responses; (3) for a complete blockade of nicotine-induced cardiovascular responses, it is necessary to use a combination of mecamylamine and alpha-bungarotoxin; (4) since mecamylamine and alpha-bungarotoxin are known to block nicotinic receptors containing alpha3beta4 and alpha-7 subunits, respectively, two different subtypes of nicotinic receptors (one which contains a combination of alpha3beta4 subunits, and one which contains alpha-7 subunits) must be present in the CVLM; and (5) it is not clear whether these two subtypes of nicotinic receptor are located on the same or different populations of CVLM-neurons.

Receptor subtypes mediating depressor responses to microinjections of nicotine into medial NTS of the rat

Microinjections (50 nl) of nicotine (0.01-10 microM) into the nucleus of the solitary tract (NTS) of adult, urethan-anesthetized, artificially ventilated, male Wistar rats, elicited decreases in blood pressure and heart rate. Prior microinjections of alpha-bungarotoxin (alpha-BT) and alpha-conotoxin ImI (specific toxins for nicotinic receptors containing alpha7 subunits) elicited a 20-38% reduction in nicotine responses. Similarly, prior microinjections of hexamethonium, mecamylamine, and alpha-conotoxin AuIB (specific blockers or toxin for nicotinic receptors containing alpha3beta4 subunits) elicited a 47-79% reduction in nicotine responses. Nicotine responses were completely blocked by prior sequential microinjections of alpha-BT and mecamylamine into the NTS. Complete blockade of excitatory amino acid receptors (EAARs) in the NTS did not attenuate the responses to nicotine. It was concluded that 1) the predominant type of nicotinic receptor in the NTS contains alpha3beta4 subunits, 2) a smaller proportion contains alpha7 subunits, 3) the presynaptic nicotinic receptors in the NTS do not contribute to nicotine-induced responses, and 4) EAARs in the NTS are not involved in mediating responses to nicotine.

Different patterns of respiratory and cardiovascular responses elicited by chemical stimulation of dorsal medulla in the rat

Respiratory and cardiovascular responses to microinjections (10 nl) of L-glutamate (10 mM) into the dorsal medulla were studied in spontaneously breathing urethane-anesthetized, adult male Wistar rats. A total of 10 patterns of respiratory and cardiovascular responses were observed: (1) hypotension alone; (2) hypotension and bradycardia; (3) hypotension and apnea; (4) hypotension, bradycardia, and apnea; (5) apnea alone; (6) hypotension and fast and shallow breathing; (7) hypotension, bradycardia, and fast and shallow breathing; (8) fast and shallow breathing alone; (9) sighs; and (10) increase in BP and HR accompanied with fast and shallow breathing. The sites from which a combination of hypotension, bradycardia, and apnea was elicited, occupied a region in the medial subnucleus of nucleus tractus solitarius (nTS), the reticular formation just ventral to it, and the dorsal motor nucleus of vagus. The sites from which hypotension alone or a combination of hypotension and apnea were elicited occupied the margins of the medial subnucleus of nTS. The sites from which apnea alone was elicited were located in the ventrolateral part of nTS and the reticular formation just ventral to it. In the commissural subnucleus of nTS, the responses comparable to those elicited by peripheral chemoreceptor stimulation (i.e., increase in BP, HR, and respiratory rate) were located in a midline region just caudal to the calamus scriptorius, the sites from which sighs were elicited were located slightly lateral and deeper, the sites from which fast and shallow breathing were elicited were located in the dorsal portion, slightly lateral to the midline. These results are expected to prove useful in studies in which microinjection technique is used to identify transmitters/receptors involved in mediating respiratory and cardiovascular reflex responses.

Cardiovascular responses to microinjections of excitatory amino acids into the area postrema of the rat

Although, area postrema (AP) as been implicated in the regulation of cardiovascular function, there is no consensus regarding the type of responses elicited by stimulation of this brain structure. Microinjections (50 nl) of smaller concentrations of excitatory amino acid receptor agonists (e.g., NMDA, KA and trans-ACPD, 10 microM each) into the AP elicited pressor and tachycardic responses in unanesthetized decerebrate as well as urethane-anesthetized rats. Microinjections of higher concentrations (e.g., 50 microM NMDA) of excitatory amino acids (EAAs) into the AP elicited an initial pressor and tachycardic response which was followed by a depressor and bradycardic response; when high concentrations of NMDA were microinjected into the AP, enough concentration may have reached the nucleus tractus solitarius (nTS) to elicit depressor and bradycardic responses. Alternatively, high concentrations of NMDA may excite known projections from AP to the nTS.

Phrenic nerve responses to chemical stimulation of the subregions of ventral medullary respiratory neuronal group in the rat

Phrenic nerve (PN) responses to unilateral microinjections of L-glutamate (L-Glu, 5 mM) or N-methyl-D-aspartic acid (NMDA, 1 mM) into different subregions of ventral respiratory neuronal group (VRG) were studied in urethane-anesthetized, immobilized, and artificially ventilated, adult male Wistar rats. A 50-nl volume of microinjection was used in all the subregions of VRG except in Pre-Bötzinger complex (Pre-BötC) where a 20-nl volume was used. Unilateral microinjections of L-Glu or NMDA into the Bötzinger complex (BötC) and caudal VRG (cVRG), caused a transient cessation of phrenic nerve (PN) activity. Expiratory neurons, abundant in BötC and cVRG, were excited by stimulation of cardiopulmonary receptors while their responses to carotid chemoreceptor stimulation were variable. Microinjections of L-Glu or NMDA into the Pre-BötC caused an increase in the PN background discharge (this response was unique to Pre-BötC) superimposed on which was an increase in the PN burst frequency. Microinjections of L-Glu or NMDA into the rostral VRG (rVRG) caused an increase in the frequency and amplitude of PN bursts. Inspiratory neurons, abundant in Pre-BötC and rVRG, were excited and inhibited by activation of carotid chemoreceptors and cardiopulmonary receptors, respectively. The coordinates for the location of different subregions of VRG were as follows (reference points are listed in parentheses). BötC: 1.6-2.6 mm rostral (calamus scriptorius), 1.7-2.7 mm lateral (midline), and 2.3-2.8 mm deep (dorsal surface of medulla); Pre-BötC: 1.4-1.6 mm rostral, 1. 8-2.5 mm lateral, and 2.3-2.8 mm deep; rVRG: 0.4-1.4 mm rostral, 1. 6-2.5 mm lateral, and 2.3-2.8 mm deep; and cVRG: 0.5 mm caudal to 0. 5 mm rostral, 1.0-2.2 mm lateral, and 2.1-2.6 mm deep. A detailed map of the subregions of VRG, functionally identified by L-Glu and NMDA-microinjections, has been presented. These data are likely to prove useful in future studies on respiratory reflex mechanisms.

GABA receptors in the phrenic nucleus of the rat

The phrenic nucleus was identified by microinjections of N-methyl-D-aspartic acid in urethan-anesthetized adult male Wistar rats. Microinjections of GABAA and GABAB receptor agonists (muscimol and baclofen, respectively) at the same site decreased the phrenic nerve burst amplitude. Microinjections of GABAA and GABAB receptor antagonists (bicuculline and 2-hydroxysaclofen, respectively) blocked as well as reversed the effects of their respective agonists. These results were confirmed by recording extracellular action potentials from single phrenic neurons. Micropressure applications of muscimol and baclofen decreased the activity of single neurons in the phrenic nucleus; this effect was blocked as well as reversed by micropressure applications of bicuculline and 2-hydroxysaclofen, respectively. These results demonstrated the presence of GABA receptors on the neurons in the phrenic nucleus and suggested that their activation results in the decrease of the phrenic nerve burst amplitude. The importance of these results in the identification of neural circuits mediating inhibition of phrenic neurons is discussed.