Cardiorespiratory effects of L-glutamate microinjected into the rat ventral medulla.

We investigated the cardiorespiratory effects elicited by microinjections of L-glutamate (L-glu, 25 nmol, 200 nl) at various sites in the ventral medulla (VMS) of urethane-anesthetized rats. The results demonstrated that regions responsive to the drug are located along a column in the VMS extending from the VI cranial nerve to the first cervical nerve in the caudal medulla. Within this column three breathing patterns were elicited from four distinct areas. In the most rostral and caudal portion of this hypothetical column, the breathing patterns observed in response to L-glu were similar and characterized by increases in minute ventilation, tidal volume, inspiratory drive, respiratory frequency, mean arterial blood pressure (MAP) and heart rate (HR). In the regions located between the areas described above two different breathing patterns were obtained without significant changes in MAP or HR. These patterns were characterized by decreases and increases in the respiratory indices analyzed, with the exception of respiratory frequency, which decreased in both regions. These results suggest that within the VMS discrete areas may act as functional units modulating cardiorespiratory responses while in others these functions are spatially segregated.

Putative pathways involved in cardiovascular responses evoked from the caudal pressor area.

1. The caudal pressor area (CPA) is a recently identified site within the ventrolateral medulla which is involved in cardiovascular regulation. CPA chemical stimulation by L-glutamate produces an increase in arterial blood pressure (ABP) while its inhibition by GABA or glycine evokes marked hypotension. In the present study, we sought to determine the potential neural pathways underlying these responses. 2. In urethane-anesthetized, paralyzed, artificially ventilated rats, CPA inhibition by bilateral microinjection of the inhibitory amino acid glycine (Gly, 100 nmol 200 nl-1 site-1) produced an average decrease of -38 +/- 4.3 mmHg in ABP (N = 6). Ten min after bilateral microinjection of the broad-spectrum glutamate antagonist kynurenic acid (KYN, 2 nmol 200 nl-1 site-1) into the caudal ventrolateral medulla (CVLM) depressor responses to CPA inhibition were virtually abolished (-3 +/- 1.7 mmHg, P < 0.05). Similar microinjection of KYN into the rostral ventrolateral medulla (RVLM) or into the CPA itself did not modify depressor responses to CPA inhibition by glycine. 3. CPA stimulation by bilateral microinjection of the excitatory amino acid L-glutamate (L-glu, 50 nmol 200 nl-1 site-1) produced an increase in ABP (+43 +/- 5.4 mmHg, N = 6). Bilateral microinjection of the GABAA antagonist bicuculline methiodide (BIC, 200 pmol 200 nl-1 site-1) into the CVLM markedly reduced pressor responses to CPA stimulation (+6 +/- 2.7 mmHg, P < 0.05). Similar application of BIC into the RVLM or CPA did not modify pressor responses to CPA stimulation by glutamic acid.

Putative pathways involved in cardiovascular responses evoked from the caudal pressor area

1. The caudal pressor area (CPA) is a recently identified site within the ventrolateral medulla which is involved in cardiovascular regulation. CPA chemical stimulation by L-glutamate produces an increase in arterial blood pressure (ABP) while its inhibition by GABA or glycine evokes marked hypotension. In the present study, we sought to determine the potential neural pathways underlyng these responses. 2. In urethane-anesthetized, paralyzed, artificially ventilated rats, CPA inhibition by bilateral microinjection of the inhibitory amino acid glycine (Gly, 100 nmol 200 nl-1 site-1) produced an average decrease of -38 + or - 4.3 mmHg in ABP (n = 6). Ten min after bilateral microinjection of the broad-spectrum glutamate antagonist kynurenic acid (KYN, 2 nmol 200 nl-1 site-1) into the cauldal ventrolateral medulla (CVLM) depressor responses to CPA inhibition were virtually abolished (-3 + or - 1.7 mmHg, P<0.05). Similar microinjection of KYN into the rostral ventrolateral medulla (RVLM) or into the CPA itself did not modify depressor responses to CPA inhibiton by glycine. 3. CPA stimulation by bilateral microinjection of the excitatory amino acid L-glutamate (L-glu, 50 nmol 200 nl-1 site-1) produced an increase in ABP (+43 + or - 5.4 mmHg, N= 6). Bilateral microinjection of the GABA A antagonist bicuculline methiodide (BIC, 200 pmol 200 nl-1 site-1) into the CVLM markedly reduced pressor responses to CPA stimulation (+6 + or - 2.7 mmHg, P<0.05). Similar application of BIC into the RVLM or CPA did not modify pressor responses to CPA stimulation by glutamic acid

Cardiorespiratory alterations produced by pharmacological agents applied to the ventrolateral medulla (intermediate area) of the cat. Effect of exercise condition on glycine-induced inhibition.

1. To study the action of the intermediate area (IA), coextensive with the rostral ventrolateral medulla, on the neurophysiological mechanisms involved in the regulation of respiration, in terms of inspiratory drive and respiratory timing, cats were submitted to topical application of sodium pentobarbital (30 mg/ml), leptazol (200 mg/ml), glutamate (50 mg/ml) and glycine (100 and 50 mg/ml) to the IA. The effects of electrically induced exercise on the ventilatory response and oxygen uptake (VO2) obtained by topical application of glycine (50 mg/ml) to the IA were also studied. 2. Leptazol reduced minute ventilation (VE) and inspiratory drive (VT/TI) and changed the timing mechanism. Glutamate only increased tidal volume (VT), VE and VT/TI. Arterial blood pressure (AP) increased and heart rate (HR) did not change with either drug. 3. Sodium pentobarbital reduced VT and changed the timing mechanism. Glycine only reduced VE, VT and VT/TI. AP decreased and HR did not change with either drug. 4. The depressor effects of glycine on respiratory pattern, VO2 and CO2 production (VCO2) tended to be attenuated by exercise. 5. The fall in AP due to glycine application did not differ between resting and exercise conditions. 6. Our results indicate that at least two different nervous structures are involved in the IA: one responsible for the respiratory drive and sensitive to glycine and glutamate, and the other responsible for the regulation of the timing mechanism and sensitive to sodium pentobarbital and leptazol.

Cardiorespiratory alterations produced by pharmacological agents applied to the ventrolateral medulla (intermediate area) of the cat: effect of exercise condition on glycine-induced inhibition

1. To study the action of the intermediate area (IA), coextensive with the rostral ventrolateral medulla, on the neurophysiological mechanisms involved in the regulation of respiration, in terms of inspiratory drive and respiratory timing, cats were submitted to topical application of sodium pentobarbital (30 mg/ml), leptazol (200 mg/ml), glutamate (50 mg/ml) and glycine (100 and 50 mg/ml) to the IA. The effects of electrically induced exercise on the ventilatory response and oxygen uptake (VO2) obtained by topical application of glycine (50 mg/ml) to the IA were also studied. 2. Leptazol reduced minute ventilation (VE) and inspiratory drive (VT/TI) and changed the timing mechanism. Glutamate only increased tidal volume (VT), VE and VT/TI. Arterial blood pressure (AP) increased and heart rate (HR) did not change with either drug. 3. Sodium pentobarbital reduced VT and changed the timing mechanism. Glycine only reduced VE, VT and VT/TI. AP decreased and HR did not change with either drug. 4. The depressor effects of glycine on respiratory pattern, VO2 and CO2 production (VCO2) tended to be attenuated by exercise. 5. The fall in AP due to glycine application did not differ between resting and exercise conditions. 6. Our results indicate that at least two different nervous structures are involved in the IA: one responsible for the respiratory drive and sensitive to glycine and glutamate, and the other responsible for the regulation of the timing mechanism and sensitive to sodium pentobarbital and leptazol

Role of the rostral ventrolateral medulla in the pressor response to stimulation of the nucleus raphe obscurus.

1. Electrical stimulation of the nucleus raphe obscurus (NRO) in urethane-anesthetized rats increases arterial blood pressure (BP) between 20 and 95 mmHg (mean, 61.14 +/- 6.57; N = 30). 2. Unilateral electrolytic destruction of the rostral ventrolateral medulla (RVLM) did not reduce BP or heart rate (HR) but significantly reduced the pressor response to NRO stimulation (control, delta 76.0 +/- 5.4 mmHg; after lesion, delta 26.0 +/- 13.9 mmHg; P < 0.01, N = 5). 3. Bilateral destruction of the RVLM reduced basal BP (control, 104.1 +/- 11.4 mmHg; after lesion, 58.0 +/- 5.7 mmHg; P < 0.01) and the pressor response to NRO stimulation (control, delta 71.6 +/- 7.3; after lesion, delta 12.5 +/- 3.8 mmHg; P < 0.01, N = 6). 4. When topically applied to or microinjected into the RVLM, pentobarbital sodium (200 nl/1 microliters, 10 nmol) decreased BP, HR and the pressor response to NRO stimulation (control, delta 56.2 +/- 6.7 mmHg; after pentobarbital, delta 11.2 +/- 3.1 mmHg; P < 0.01, N = 13). Similar effects were obtained when glycine (200 nl, 50 nmol) was microinjected into RVLM (control, delta 40.5 +/- 5.9 mmHg; after glycine, delta 18.1 +/- 4.9 mmHg; P < 0.01, N = 6). 5. We conclude that RVLM is essential for the pressor response to NRO stimulation.

Role of the rostral ventrolateral medulla in the pressor response to stimulation of the nucleus raphe obscurus

1. Electrical stimulation of the nucleus raphe obscurus (NRO) in urethane-anesthetized rats increases arterial blood pressure (BP) between 20 and 95 mmHg (mean, 61.14 +/- 6.57; N = 30). 2. Unilateral electrolytic destruction of the rostral ventrolateral medulla (RVLM) did not reduce BP or heart rate (HR) but significantly reduced the pressor response to NRO stimulation (control, delta 76.0 +/- 5.4 mmHg; after lesion, delta 26.0 +/- 13.9 mmHg; P < 0.01, N = 5). 3. Bilateral destruction of the RVLM reduced basal BP (control, 104.1 +/- 11.4 mmHg; after lesion, 58.0 +/- 5.7 mmHg; P < 0.01) and the pressor response to NRO stimulation (control, delta 71.6 +/- 7.3; after lesion, delta 12.5 +/- 3.8 mmHg; P < 0.01, N = 6). 4. When topically applied to or microinjected into the RVLM, pentobarbital sodium (200 nl/1 microliters, 10 nmol) decreased BP, HR and the pressor response to NRO stimulation (control, delta 56.2 +/- 6.7 mmHg; after pentobarbital, delta 11.2 +/- 3.1 mmHg; P < 0.01, N = 13). Similar effects were obtained when glycine (200 nl, 50 nmol) was microinjected into RVLM (control, delta 40.5 +/- 5.9 mmHg; after glycine, delta 18.1 +/- 4.9 mmHg; P < 0.01, N = 6). 5. We conclude that RVLM is essential for the pressor response to NRO stimulation

Breathing pattern during pharmacological activation and blockade of the intermediate area of the ventrolateral surface of the cat medulla

The present study analyzes the respiratory pattern of chloralose- (50-60 mg/kg,iv) anesthetized cats treated with Nembutal (NE) (30 mg/ml), glycine (GL) (200 mg/ml) or leptazol (LE) (200 mg/ml) topically applied to the intermediate area of the ventrolateral surface of the medulla oblongata in a volume of 20 micronl. Application of NE and GL produced a decrease in ventilation (-24%) and tidal volume (-25%) suggesting that the intermediate area facilitates respiratory drive and inhibits the inspiratory off-switch mechanism. These results are consistent with the view that intermediate area is necessary for the central chemosensitivity to CO2. The topical application of LE produced an increase in inspiration time (12.5%), expiration time (20.8%) and tidal volume (7%). The increased tidal volume caused by LE is compatible with it action as a GL antagonist