Cardiovascular responses to microinjections of GABA or anesthetics into the rostral ventrolateral medulla of conscious and anesthetized rats

The rostral ventrolateral medulla (RVLM) contains neurons involved in tonic and reflex control of arterial pressure. We describe the effects of gamma-aminobutyric acid (GABA) and anesthetics injected into the RVLM of conscious and urethane (1.2 g/kg, iv) anesthetized Wistar rats (300-350 g). In conscious rats, bilateral microinjection of GABA (50 nmol/200 nl) induced a small but significant decrease in blood pressure (from 130 ± 3.6 to 110 ± 5.6 mmHg, N = 7). A similar response was observed with sodium pentobarbital microinjection (24 nmol/200 nl). However, in the same animals, the fall in blood pressure induced by GABA (from 121 ± 8.9 to 76 ± 8.8 mmHg, N = 7) or pentobarbital (from 118 ± 4.5 to 57 ± 11.3 mmHg, N = 6) was significantly increased after urethane anesthesia. In contrast, there was no difference between conscious (from 117 ± 4.1 to 92 ± 5.9 mmHg, N = 7) and anesthetized rats (from 123 ± 6.9 to 87 ± 8.7 mmHg, N = 7) when lidocaine (34 nmol/200 nl) was microinjected into the RVLM. The heart rate variations were not consistent and only eventually reached significance in conscious or anesthetized rats. The right position of pipettes was confirmed by histology and glutamate microinjection into the RVLM. These findings suggest that in conscious animals the RVLM, in association with the other sympathetic premotor neurons, is responsible for the maintenance of sympathetic vasomotor tone during bilateral RVLM inhibition. Activity of one or more of these premotor neurons outside the RVLM can compensate for the effects of RVLM inhibition. In addition, the effects of lidocaine suggest that fibers passing through the RVLM are involved in the maintenance of blood pressure in conscious animals during RVLM inhibition

Cardiovascular responses to microinjections of GABA or anesthetics into the rostral ventrolateral medulla of conscious and anesthetized rats.

The rostral ventrolateral medulla (RVLM) contains neurons involved in tonic and reflex control of arterial pressure. We describe the effects of gamma-aminobutyric acid (GABA) and anesthetics injected into the RVLM of conscious and urethane (1.2 g/kg, iv) anesthetized Wistar rats (300-350 g). In conscious rats, bilateral microinjection of GABA (50 nmol/200 nl) induced a small but significant decrease in blood pressure (from 130 +/- 3.6 to 110 +/- 5.6 mmHg, N = 7). A similar response was observed with sodium pentobarbital microinjection (24 nmol/200 nl). However, in the same animals, the fall in blood pressure induced by GABA (from 121 +/- 8.9 to 76 +/- 8.8 mmHg, N = 7) or pentobarbital (from 118 +/- 4.5 to 57 +/- 11.3 mmHg, N = 6) was significantly increased after urethane anesthesia. In contrast, there was no difference between conscious (from 117 +/- 4.1 to 92 +/- 5.9 mmHg, N = 7) and anesthetized rats (from 123 +/- 6.9 to 87 +/- 8.7 mmHg, N = 7) when lidocaine (34 nmol/200 nl) was microinjected into the RVLM. The heart rate variations were not consistent and only eventually reached significance in conscious or anesthetized rats. The right position of pipettes was confirmed by histology and glutamate microinjection into the RVLM. These findings suggest that in conscious animals the RVLM, in association with the other sympathetic premotor neurons, is responsible for the maintenance of sympathetic vasomotor tone during bilateral RVLM inhibition. Activity of one or more of these premotor neurons outside the RVLM can compensate for the effects of RVLM inhibition. In addition, the effects of lidocaine suggest that fibers passing through the RVLM are involved in the maintenance of blood pressure in conscious animals during RVLM inhibition.

Evidence for neuronal release of isotopically labelled glycine from the rat ventral medullary surface in vivo.

Spontaneous and stimulus-induced release of isotopically labelled glycine was studied in the superfused rat dorsal or ventral medullary surface in vivo. Superfusion of the ventral medullary surface of anesthetized (urethane, 1.2 g/kg, ip) male adult Wistar rats (250-350 g) with high K+ (40 mM) surrogate cerebrospinal fluid (CSF) produced an average increase of 45% over the spontaneous efflux of exogeneously applied glycine (N = 5, P < 0.01). In experiments in which the calcium of the CSF was replaced by an equimolar amount of magnesium, the increase in glycine efflux in response to high K+ was reduced to 15%, a value not statistically different from that observed in control experiments (N = 6). Veratridine stimulation evoked a large (80%) increase in glycine efflux (N = 5, P < 0.001), which was inhibited by tetrodotoxin. High potassium or veratridine failed to modify spontaneous release of glycine on the dorsal medullary surface. Results obtained in control experiments showed that neither high K+ nor veratridine is effective in modifying spontaneous efflux of [3H]-leucine or [3H]-inulin on the ventral or dorsal medullary surface. These data support the hypothesis that glycine is a neurotransmitter on the ventral medullary surface and that it may be part of neural pathways involved in cardiorespiratory regulation present in this region.

Evidence for neuronal release of isotopically labelled glycine from the rat ventral medullary surface in vivo

Spontaneous and stimulus-induced release of isotopically labelled glycine was studied in the superfused rat dorsal or ventral medullary surface in vivo. Superfusion of the ventral medullary surface of anesthetized (urethane, 1.2 g/kg, ip) male adult Wistar rats (250-350 g) with high K+ (40 mM) surrogate cerebrospinal fluid (CSF) produced an average increase of 45 per cent over the spontaneous efflux of exogenously applied glycine (N = 5, P<0.01). In experiments in which the calcium of the CSF was replaced by an equimolar amount of magnesium, the increase in glycine efflux in response to high K+ was reduced to 15 per cent, a value not statistically different from that observed in control experiments (N = 6). Veratridine stimulation evoked a large (80 per cent) increase in glycine efflux (N = 5, P<0.001), which was inhibited by tetrodotoxin. High potassium or veratridine failed to modify spontaneous release of glycine on the dorsal medullary surface. Results obtained in control experiments showed that neither high K+ nor veratridine is effective in modifying spontaneous efflux of [(3)H]-leucine or [(3)H]-inulin on the ventral or dorsal medullary surface. These data support the hypothesis that glycine is a neurotransmitter on the ventral medullary surface and that it may be part of neural pathways involved in cardiorespiratory regulation present in this region.

A fall in arterial blood pressure produced by inhibition of the caudalmost ventrolateral medulla: the caudal pressor area.

The caudal edge of the ventrolateral medulla was mapped to localize sites where microinjections of L-glutamate (L-glu) produce pressor responses in paralyzed and artificially ventilated urethane-anesthetized rats. Pressor responses ranging from 15 to 65 mmHg were obtained when L-Glu (0.25 M, 200 nl) was microinjected in the ventral medullary surface within an area localized between the rootlets of the XII and first cervical nerves, lateral to the pyramids and just medial to the spinal roots of the XI cranial nerve. This area has been called the caudal pressor area (CPA). Inhibition of the CPA by microinjection of GABA or glycine resulted in marked falls (15-45 mmHg) of arterial blood pressure (AP). Hypotension in response to CPA inhibition was also obtained in unanesthetized decerebrate animals. Cardiovascular responses to CPA stimulation or inhibition depend on the activity of neurons in the rostral ventrolateral medulla (RVLM). During hypotension provoked by RVLM inhibition, pressor responses to CPA stimulation were abolished. Conversely, pressor responses to RVLM stimulation were maintained during hypotension produced by inhibition of CPA. Pressor response to bilateral carotid occlusion were not reduced by CPA inhibition. We conclude that cells in the caudal most ventrolateral medulla exert a tonic pressor activity that contributes to maintenance of basal levels of the vasomotor tone and arterial blood pressure, its inhibition, however, does not prevent the pressor response to carotid occlusion.

Evidence that angiotensin-(1-7) plays a role in the central control of blood pressure at the ventro-lateral medulla acting through specific receptors.

In this study we determined which angiotensin receptors may mediate the cardiovascular effects elicited by angiotensin-(1-7) [Ang-(1-7)] in the rostral ventrolateral medulla (RVLM) and caudal pressor area (CPA) of the ventrolateral medulla (VLM) of anesthetized rats. Furthermore the role of endogenous angiotensins in these areas was also investigated. The pressor effect produced by unilateral microinjection of Ang-(1-7) into the RVLM or CPA was not modified by either the AT1 receptor antagonist, DuP 753 or by the AT2 receptor antagonist, CGP 42112A, but was completely blocked by the Ang-(1-7) selective antagonist, A-779. In contrast, the pressor effect produced by microinjection of angiotensin II (Ang II) was completely blocked by DuP 753 but was not changed by CGP 42112A or A-779. Bilateral microinjection of A-779 into the RVLM or CPA produced a significant fall in mean arterial pressure and heart rate. Microinjection of DuP 753 produced a pressor effect comparable to bilateral injection of vehicle. These results indicate that, although Ang II acts in the VLM through an AT1 receptor subtype, the cardiovascular effects produced by microinjection of Ang-(1-7) into the RVLM and CPA are mediated by a specific angiotensin receptor (AT5?). Furthermore, our data provide evidence that endogenous Ang-(1-7) participates at the VLM in the neural control of arterial blood pressure.

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

Cardiovascular effects produced by micro-injection of angiotensin-(1-7) on vasopressor and vasodepressor sites of the ventrolateral medulla.

In this study, we determined the cardiovascular effects produced by micro-injection of the heptapeptide Angiotensin-(1-7) [Ang-(1-7)] into the rat ventrolateral medulla (VLM). Micro-injection of Ang-(1-7) into the rostral VLM and the caudal pressor area of the VLM produced significant increases in arterial pressure, comparable to that observed with micro-injection of Ang II. The changes in arterial pressure were associated with more variable changes in heart rate (HR) (usually tachycardia). On the other hand, micro-injection of Ang-(1-7) into the caudal depressor area induced decreases in arterial pressure and HR. The results suggest that, besides Ang II, Ang-(1-7) is involved in the mediation of the cardiovascular actions of the renin-angiotensin system in the VLM.

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

Cardiorespiratory effects induced by acetazolamide on the ventromedullary surface of the cat.

1. Inhibition of carbonic anhydrase by acetazolamide in alpha-chloralose-anaesthetized cats, in a region of the brain stem co-extensive with the glycine-sensitive area, intermediate chemosensitive area, and probably C1 catecholaminergic neurones produces hypotension, bradycardia and depression of the central respiratory drive. 2. These responses are concentration dependent, and can still be observed when the enzyme substrate (CO2) is elevated. Therefore, in both the hypercapnic and the normocapnic condition, similar responses in arterial blood pressure, heart rate and respiratory rate are observed when acetazolamide is topically applied to the glycine-sensitive area. 3. To investigate further the contribution of peripheral baro-, chemo- and cardiopulmonary receptors to these responses, acetazolamide was topically applied to the glycine-sensitive area under three different conditions: intact gallamine-paralysed (5 mg kg-1 h-1) and artificially ventilated (A), sinoaortic denervated (B), and sinoaortic denervated plus bilaterally vagotomized cats (C). Under all conditions, similar responses were observed. The fall in arterial blood pressure was 75 +/- 11 (A), 90 +/- 13 (B), and 75 +/- 9 mmHg (C). Changes in heart rate during acetazolamide application were -23 +/- 6, -20 +/- 8, and -26 +/- 6 beats min-1, respectively. The decreases in respiratory rate were 9 +/- 2 (A), 11 +/- 2 (B), and 11 +/- 2 breaths min-1 (C). 4. The data indicate that the responses to topical application of acetazolamide are mainly due to its central action at the glycine-sensitive area and are not influenced by peripheral baroreceptor and chemoreceptor inputs.

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

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 microliters. Application of NE and GL produced a decrease in ventilation (approximately 24%) and tidal volume (approximately 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 its action as a GL antagonist.

Blood pressure effects of leptazol applied to the ventral surface of the brain stem of cats.

In anaesthetized cats leptazol (200 mg/ml) and sodium pentobarbitone (30 mg/ml) were applied topically to an area of the exposed ventral surface of the medulla oblongata, which lies between the rootlets of the twelfth cranial and first cervical nerve. The drugs were applied either bilaterally by means of paired Perspex rings or unilaterally by means of a single Perspex ring. Their effects on arterial blood pressure, heart rate and respiration were examined during two stages of anaesthesia, during 'surgical anaesthesia' produced by an intravenous injection of chloralose at 60 mg/kg, and during deeper anaesthesia attained by two additional intravenous injections of chloralose at 30 mg/kg. Both the bilateral and unilateral application of leptazol produced a fall in arterial blood pressure during surgical anaesthesia, but a rise during deepened anaesthesia. After a preceding topical application of sodium pentobarbitone the fall became attenuated or abolished, whereas the rise became potentiated. Sodium pentobarbitone itself affected blood pressure as well as respiration when applied bilaterally. It then produced pronounced tachypnoea independent of the depth of anaesthesia and a fall in arterial blood pressure during deepened anaesthesia. Its unilateral application did not affect respiration, nor did it affect usually arterial blood pressure, although during deepened anaesthesia it occasionally produced a fall in blood pressure. The area from which the pressor response to leptazol was obtained lay 7-11 mm caudal to the lower border of the trapezoid bodies, i.e. about 2 mm more caudally than the 'nicotine-sensitive area' from which a depressor response to leptazol is evoked. Thus the two areas, though not identical, overlap. The result obtained with sodium pentobarbitone suggest that the area for the pressor response to leptazol plays a role in maintaining vasomotor tone during deepened anaesthesia and exerts a strong inhibitory effect on the respiratory rate during both surgical and deepened anaesthesia.

Signs of synchronized sleep elicited by local application of leptazol to a circumscribed area of the ventral surface of medulla oblongata in the cat.

Topical application of some drugs to the ventral surface of medulla oblongata has been found to cause changes in blood pressure and respiration and release of vasopressin. In the present investigation, electrophysiological changes induced in the electrocorticogram of cats by drugs applied to this area of the central nervous system were studied. In eight animals kept anesthetized with sodium pentobarbiturate, leptazol (200 mg/ml) was applied bilaterally (20 microliter on each side) to a small area of the rostral portion of medulla oblongata while blood pressure, respiration, the electrocorticogram from the sigmoid gyri, the nictitating membrane and the pupil width were monitored. In some preparations sodium pentobarbiturate was also applied to the same area after leptazol. Within 30 s of leptazol application to the surface, active spindling started in the sigmoid gyri, simultaneously with myosis, relaxation of the nictitating membrane and inhibition of the limb retraction in response to noxious stimulation. All these changes characterize the state of synchronized sleep. Topical application of pentobarbital to the same area suppressed spindling and reversed the other signs of synchronized sleep. The possible intervention of some known hodological systems or of the sleeping factors described by some authors in the genesis of the above phenomena is discussed.