Role of renal sympathetic nerve activity in hypertension induced by chronic nitric oxide inhibition

Nitric oxide levels are diminished in hypertensive patients, suggesting nitric oxide might have an important role to play in the development of hypertension. Chronic blockade of nitric oxide leads to hypertension that is sustained throughout the period of the blockade in baroreceptor-intact animals. It has been suggested that the sympathetic nervous system is involved in the chronic increase in blood pressure; however, the evidence is inconclusive. We measured renal sympathetic nerve activity and blood pressure via telemetry in rabbits over 7 days of nitric oxide blockade. Nitric oxide blockade via N(omega)-nitro-L-arginine methyl ester (L-NAME) in the drinking water (50 mg x kg(-1) x day(-1)) for 7 days caused a significant increase in arterial pressure (7 +/- 1 mmHg above control levels; P < 0.05). While the increase in blood pressure was associated with a decrease in heart rate (from 233 +/- 6 beats/min before the L-NAME to 202 +/- 6 beats/min on day 7), there was no change in renal sympathetic nerve activity (94 +/- 4 %baseline levels on day 2 and 96 +/- 5 %baseline levels on day 7 of L-NAME; baseline nerve activity levels were normalized to the maximum 2 s of nerve activity evoked by nasopharyngeal stimulation). The lack of change in renal sympathetic nerve activity during the L-NAME-induced hypertension indicates that the renal nerves do not mediate the increase in blood pressure in conscious rabbits.     

Interaction of nitric oxide and serotonin in aggressive behavior

Nitric oxide (NO) modulates many behavioral and neuroendocrine responses. Genetic or pharmacological inhibition of the synthetic enzyme that produces NO in neurons evokes elevated and sustained aggression in male mice. Recently, the excessive aggressive and impulsive traits of neuronal NO synthase knockout (nNOS-/-) mice were shown to be caused by reductions in serotonin (5-HT) turnover and deficient 5-HT1A and 5-HT1B receptor function in brain regions regulating emotion. The consistently high levels of aggression observed in nNOS-/- mice could be reversed by 5-HT precursors and by treatment with specific 5-HT1A and 5-HT1B receptor agonists. The expression of the aggressive phenotype of nNOS-/- knockout mice requires isolated housing prior to testing. The effects of social factors such as housing condition and maternal care can affect 5-HT and aggression, but the interaction among extrinsic factors, 5-HT, NO, and aggression remains unspecified. Taken together, NO appears to play an important role in normal brain 5-HT function and may have significant implications for the treatment of psychiatric disorders characterized by aggressive and impulsive behaviors.     

AT1 receptors mediate pressor responses induced by angiotensin II in the periaqueductal gray area of rats

Microinjection of angiotensin II (ANGII) (0.01 to 1 nmol) into the periaqueductal gray area (PAG) of anaesthetised rats caused dose-dependent increases in blood pressure. Preinjection (10 min before) of losartan (a selective non-peptide AT₁ receptor antagonist; 50 nmol) to the PAG reduced the pressor response to ANGII whereas PD123319 (a selective non-peptide AT₂ receptor antagonist; 50 nmol) did not affect the ANGII-induced hypertension. Thus, our data suggest that the activation of AT₁ but not AT₂ receptors mediates ANGII-induced blood pressure changes in the PAG area.     

Role of nitric oxide in the reperfusion induced injury in hyperthyroid rat hearts

We recently reported that hyperthyroidism affects the heart response to ischemia/reperfusion. A significant tachycardia during reperfusion was associated with an increase in the oxidative stress of hearts from T₃-treated animals. In the present study we checked the possible role of nitric oxide (NO) in this major stress induced by the hyperthyroid state. We compared the functional recovery from ischemia/reperfusion of Langendorff preparations from euthyroid (E) and hyperthyroid (H, ten daily intraperitoneal injections of T₃, 10 μg/100 g body weight) rats, in the presence and in the absence of 0.2 mM N^ω-nitro-L-arginine (L-NNA). At the end of the ischemia/reperfusion protocol (10 min preischemic perfusion, 20 min global ischemia, 30 min reperfusion) lipid peroxidation, antioxidant capacity (C_A) and susceptibility to in vitro oxidative stress were determined on heart homogenates. The main effect of hyperthyroidism on the reperfusion functional response was confirmed to be a strong tachycardic response (154% recovery at 25 min reperfusion) accompanied by a low recovery in both left ventricular diastolic pressure (LVDP) and left ventricular dP/dt_max. This functional response was associated with a reduction in C_A and an increase in both lipid peroxidation and susceptibility to oxidative stress. Perfusion of hearts with L-NNA per se had small but significant negative chronotropic and positive inotropic effects on preischemic performance of euthyroid rat hearts only. More importantly, L-NNA perfusion completely blocked the reperfusion tachycardic response in the hyperthyroid rats. Concomitantly, myocardium oxidative state (lipid peroxidation, C_A and in vitro susceptibility to oxidative stress) of L-NNA perfused hearts was similar to that of E animals. These results suggest that the higher reperfusion-induced injury occurring in hyperthyroid animals is associated with overproduction of nitric oxide.     

Role of nitric oxide in the reperfusion induced injury in hyperthyroid rat hearts

We recently reported that hyperthyroidism affects the heart response to ischemia/reperfusion. A significant tachycardia during reperfusion was associated with an increase in the oxidative stress of hearts from T3-treated animals. In the present study we checked the possible role of nitric oxide (NO) in this major stress induced by the hyperthyroid state. We compared the functional recovery from ischemia/reperfusion of Langendorff preparations from euthyroid (E) and hyperthyroid (H, ten daily intraperitoneal injections of T3, 10 microg/100 g body weight) rats, in the presence and in the absence of 0.2 mM Nomega-nitro-L-arginine (L-NNA). At the end of the ischemia/reperfusion protocol (10 min preischemic perfusion, 20 min global ischemia, 30 min reperfusion) lipid peroxidation, antioxidant capacity (CA) and susceptibility to in vitro oxidative stress were determined on heart homogenates. The main effect of hyperthyroidism on the reperfusion functional response was confirmed to be a strong tachycardic response (154% recovery at 25 min reperfusion) accompanied by a low recovery in both left ventricular diastolic pressure (LVDP) and left ventricular dP/dtmax. This functional response was associated with a reduction in CA and an increase in both lipid peroxidation and susceptibility to oxidative stress. Perfusion of hearts with L-NNA per se had small but significant negative chronotropic and positive inotropic effects on preischemic performance of euthyroid rat hearts only. More importantly, L-NNA perfusion completely blocked the reperfusion tachycardic response in the hyperthyroid rats. Concomitantly, myocardium oxidative state (lipid peroxidation, CA and in vitro susceptibility to oxidative stress) of L-NNA perfused hearts was similar to that of E animals. These results suggest that the higher reperfusion-induced injury occurring in hyperthyroid animals is associated with overproduction of nitric oxide.     

Cytotoxicity induced by grape seed proanthocyanidins: Role of nitric oxide

Grape seed proanthocyanidin extract (GPSE) at high doses has been shown to exhibit cytotoxicity that is associated with increased apoptotic cell death. Nitric oxide (NO), being a regulator of apoptosis, can be increased in production by the administration of GSPE. In a chick cardiomyocyte study, we demonstrated that high-dose (500 μg/ml) GSPE produces a significantly high level of NO that contributes to increased apoptotic cell death detected by propidium iodide and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) staining. It is also associated with the depletion of intracellular glutathione (GSH), probably due to increased consumption by NO with the formation of S-nitrosoglutathione. Co-treatment with L-NAME, a NO synthase inhibitor, results in reduction of NO and apoptotic cell death. The decline in reduced GSH/oxidized GSH (GSSG) ratio is also reversed. N-Acetylcysteine, a thiol compound that reacts directly with NO, can reduce the increased NO generation and reverse the decreased GSH/GSSG ratio, thereby attenuating the cytotoxicity induced by high-dose GSPE. Taken together, these results suggest that endogenous NO synthase (NOS) activation and excessive NO production play a key role in the pathogenesis of high-dose GSPE-induced cytotoxicity.     

Cellular action of cholecystokinin-8S-mediated excitatory effects in the rat periaqueductal gray

The peptide cholecystokinin (CCK) is one of the major neurotransmitters modulating satiety, nociception, and anxiety behavior. Although many behavioral studies showing anti-analgesic and anxiogenic actions of CCK have been reported, less is known about its cellular action in the central nervous system (CNS). Therefore, we examined the action of CCK in rat dorsolateral periaqueductal gray (PAG) neurons using slice preparations and whole-cell patch-clamp recordings. Application of CCK-8S produced an inward current accompanied by increased spontaneous synaptic activities. The CCK-8S-induced inward current (I(CCK)) was recovered after washout and reproduced by multiple exposures. Current-voltage plots revealed that I(CCK) reversed near the equilibrium potential for K(+) ions with a decreased membrane conductance. When several K(+) channel blockers were used, application of CdCl(2), TEA, or apamin significantly reduced I(CCK). I(CCK) was also significantly reduced by the CCK(2) receptor antagonist, L-365,260, while it was not affected by the CCK(1) receptor antagonist, L-364,718. Furthermore, we examined the effects of CCK-8S on miniature excitatory postsynaptic currents (mEPSCs) in order to determine the mechanism of CCK-mediated increase on synaptic activities. We found that CCK-8S increased the frequency of mEPSCs, but had no effect on mEPSC amplitude. This presynaptic effect persisted in the presence of CdCl(2) or Ca(2+)-free bath solution, but was completely abolished by pre-treatment with BAPTA-AM, thapsigargin or L-365,260. Taken together, our results indicate that CCK can excite PAG neurons at both pre- and postsynaptic loci via the activation of CCK(2) receptors. These effects may be important for the effects of CCK on behavior and autonomic function that are mediated via PAG neurons.     

Role of serotonin2 receptors in regulating aggressive behavior

Quipazine and pirenperone , the drugs interacting with serotonin2 -receptors, more readily displaced 3H-spiroperidol from its binding sites in the frontal cortex than in the striatum. Pirenperone (0,07-0,3 mg/kg), antagonist of serotonin2 -receptors, selectively decreased the intensity of apomorphine aggressiveness. The antiaggressive action of haloperidol (0,01-0,2 mg/kg) was in correlation with its antistereotypic activity. Long-term administration of naloxone (0,5; 15,0 mg/kg), together with apomorphine (0,5 mg/kg) reduced the number of head-twitches caused by quipazine (2,5 mg/kg). The administration of quipazine 48 hours after the last injection of naloxone and apomorphine caused spontaneous aggressiveness that did not differ from apomorphine aggressiveness. Intracerebroventricular injection of cholecystokinin tetrapeptide (CCK-4) markedly enhanced the foot-shock aggression. The same dose of CCK-4 also decreased the intensity of quipazine (2,5 mg/kg) head-twitches. Compared to haloperidol, pirenperone was a more selective antagonist of CCK-4. After long-term apomorphine treatment (0,5 mg/kg during 10 days, twice daily), the effect of CCK-4 on aggressive behaviour was markedly enhanced. It is possible that two subtypes of serotonin2 -receptors exist in the brain and have opposite action on the aggressive behaviour. CCK-4 may play the role of an endogenous modulator of sensitivity of serotonin2 -receptors involved in the control of aggressiveness.     

Role of the midbrain periaqueductal gray matter in conditioned reflexes

Unit activity in the midbrain periaqueductal gray matter (PGM) during an instrumental placing reflex, its extinction, differentiation, and conditioned inhibition, was studied in chronic experiments on cats. Spike responses 1–2 sec in duration in 69 (36.7%) of 182 neurons preceded by 400–800 msec the beginning of conditioned-reflex and voluntary intertrial movements. These advanced responses appeared 200 msec before the corresponding advance responses of motor cortical neurons. Fifty-eight neurons (30.9%) responded directly to acoustic stimulation with a latent period of 10–50 msec for 2–6 sec, 19 neurons (10.1%) generated double responses, linked with both the acoustic stimulus and subsequent conditioned-reflex movement, and 42 neurons (22.3%) did not respond to acoustic stimulation, although individual neurons of this group changed the level of their spontaneous activity in response to repeated conditioned stimulation, and this change was maintained for some tens of minutes. Extinction, differentiation, and conditioned inhibition all abolished conditioned-reflex movements, but each type of internal inhibition was accompanied by its own characteristic changes in the firing pattern of PGM neurons. Functional independence of neurons of the first and second groups was demonstrated during extinction and recovery of the conditioned-reflex. The results indicate the important role of PGM not only in the mechanism of the conditioned reflex, but also in the development of its internal inhibition.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 403–419, May–June, 1984.     

Pre- and postsynaptic inhibition mediated by GABA(B) receptors in rat ventrolateral periaqueductal gray neurons

The present study examined the actions of a GABA(B)-receptor agonist, baclofen, on synaptic transmission in rat ventrolateral periaqueductal gray (PAG) neurons of brainstem slices by using whole-cell voltage-clamp recordings. Baclofen (10 microM) induced a slow outward current (peak amplitude: 30.1+/-3.1pA, n=13) at -70mV, which persisted in the presence of tetrodotoxin (0.5 microM) and was diminished in the presence of postsynaptic intracellular K(+)-channel blockers (Cs(+) and TEA) and GDP-beta-S, indicating a direct postsynaptic depression mediated by K(+) channels and G proteins. Baclofen (10 microM) also decreased the frequency of both glutamatergic spontaneous EPSC (by 36+/-7%, n=11) and GABAergic spontaneous IPSC (by 37+/-12%, n=6) without changes in their amplitudes, indicating its presynaptic inhibitions. Taken together, the activation of postsynaptic GABA(B) receptors inhibits ventrolateral PAG neurons directly. At the same time, activating presynaptic GABA(B) receptors on glutamatergic and GABAergic nerve terminals inhibits glutamate and GABA release, respectively. The overall effects might influence an output of ventrolateral PAG neurons that build up the descending pain control system to the spinal dorsal horn.     

Lack of interaction between NMDA and cholecystokinin-2 receptor-mediated neurotransmission in the dorsolateral periaqueductal gray in the regulation of rat defensive behaviors

Several neurotransmitters, including GABA, serotonin, glutamate, and cholecystokinin, modulate defensive behaviors in the dorsolateral periaqueductal gray (dlPAG). Although both glutamate and cholecystokinin have been shown to facilitate these behaviors, a possible interaction between them remains to be examined. The present study investigates whether activation or antagonism of N-methyl-D-aspartic acid (NMDA) glutamate and cholecystokinin 2 (CCK(2)) receptors located in the dlPAG would interact in animals tested in the elevated T-maze. The effect of the NMDA (50 pmol) was evaluated in rats pretreated with the CCK(2) receptor antagonist LY225910 (0.05 nmol). In addition, the effect of the CCK(2) receptor agonist CCK-4 (0.08 nmol) was evaluated in rats pretreated with the NMDA receptor antagonist AP-7 (1.0 nmol). Intra-dlPAG injection of NMDA increased risk assessment and inhibitory avoidance behaviors. This NMDA anxiogenic-like effect was unaltered by the pretreatment with LY225910. Similarly, the shortening of escape latencies induced by CCK-4 was unaffected by AP-7. No drug changed the general exploratory activity as assessed in the open-field. These results, showing that the activation of dlPAG NMDA or CCK(2) receptors facilitate anxiety- and fear-related behaviors, further implicate glutamate and cholecystokinin-mediated neurotransmission in this midbrain area on modulation of defensive behaviors. However, the regulatory action of these two excitatory neurotransmitters seems to be exerted through independent mechanisms.     

ETA endothelin receptors are involved in the ouabain-induced haemodynamic effects in the periaqueductal gray area of rats

The aim of the present study was to asses the effects on blood pressure and vascular resistances elicited by microinjections of ouabain (OUA) within the periaqueductal gray area (PAG). We also tested whether peripheral vascular responses caused by exogenous intra-PAG ouabain involve activation of the PAG-endothelin system.In normotensive Sprague-Dawley rats the basal mean arterial blood pressure (MABP) was 114 +/- 3 mmHg. This was significantly increased by OUA (3 micro g, 122 +/- 2 mmHg, p < 0.05; and 6 micro g, 139 +/- 3 mmHg, p < 0.01) microinjected into the PAG area. Increases in MABP were associated with increases in total peripheral resistances (TPR), organ vascular resistances, and with reduced blood flow of almost all the organs tested: kidneys, skeletal muscle, skin, stomach, spleen, testes and intestine. Cardiac output did not change.Changes in the above vascular parameters induced by OUA were significantly (p < 0.01) reduced by intra-PAG microinjections of FR139317 (a selective ETA receptor antagonist, 5 nmol), SB209670 (a non-selective ETA/ETB receptor antagonist, 3 nmol), but not by BQ 788 (a selective ETB receptor antagonist, 5 nmol).In conclusion, OUA into the PAG area of normotensive rats caused significant changes in peripheral vascular parameters that are reduced by ETA receptor antagonists. These results indicate that PAG-ET-1 system via an action on ETA receptors is involved in the OUA effects.     

Downregulation of hypoxic vasoconstriction by chronic hypoxia in rabbits: effects of nitric oxide

Hypoxic pulmonary vasoconstriction (HPV) matches lung perfusion to ventilation for optimizing pulmonary gas exchange. Chronic alveolar hypoxia results in vascular remodeling and pulmonary hypertension. Previous studies have reported conflicting results of the effect of chronic alveolar hypoxia on pulmonary vasoreactivity and the contribution of nitric oxide (NO), which may be related to species and strain differences as well as to the duration of chronic hypoxia. Therefore, we investigated the impact of chronic hypoxia on HPV in rabbits, with a focus on lung NO synthesis. After exposure of the animals to normobaric hypoxia (10% O(2)) for 1 day to 10 wk, vascular reactivity was investigated in ex vivo perfused normoxic ventilated lungs. Chronic hypoxia induced right heart hypertrophy and increased normoxic vascular tone within weeks. The vasoconstrictor response to an acute hypoxic challenge was strongly downregulated within 5 days, whereas the vasoconstrictor response to the thromboxane mimetic U-46619 was maintained. The rapid downregulation of HPV was apparently not linked to changes in the lung vascular NO system, detectable in the exhaled gas and by pharmacological blockage of NO synthesis. Treatment of the animals with long-term inhaled NO reduced right heart hypertrophy and partially maintained the reactivity to acute hypoxia, without any impact on the endogenous NO system being noted. We conclude that chronic hypoxia causes rapid downregulation of acute HPV as a specific event, preceding the development of major pulmonary hypertension and being independent of the lung vascular NO system. Long-term NO inhalation partially maintains the strength of the hypoxic vasoconstrictor response.     

Role of nitric oxide on atrial natriuretic peptide release induced by angiotensin II in superfused rat atrial tissue

The present study investigated the role of nitric oxide (NO) on atrial natriuretic peptide (ANP) release stimulated by angiotensin II (Ang II) (10(-7) M) in superfused sliced rat atrial tissue. The use of N(G)-nitro-L-arginine methyl ester (L-NAME) at 10(-4) M, an inhibitor of nitric oxide synthase did not modify basal ANP release. In presence of Ang II (10(-7) M), we observed that L-NAME enhanced ANP secretion induced by Ang II. Furthermore, cGMP levels increased significantly in the presence of Ang II and was attenuated by L-NAME. On the other hand, the perfusion of 8 bromo-cGMP (10(-5) M) with Ang II reduced the effect of this octapeptide on ANP secretion. Secondly, we evaluated the effect of authentic NO on ANP release and observed that perfusion of NO reduced significantly the effect of Ang II on ANP release. We propose that the effect of Ang II on ANP secretion was modulated by NO likely via cGMP pathway.     

一氧化氮在铁诱导的大鼠心肌损伤中的作用

采用Langendorff灌流大鼠心脏和酶解分离的心肌细胞为实验模型,研究铁负荷下心肌损伤情况以及一氧化氮（NO）在铁诱导的心肌损伤中的地位。结果显示:（1）心肌铁负荷（Fe-HQ）可使分离心肌细胞舒张期细胞长度缩短、收缩幅度和速度降低,离体灌流心脏左室发展压（LVDP）、±dp/dtmax、冠脉流量呈现双相变化;冠脉流出液中乳酸脱氢酶（LDH）、肌酸激酶（CK）释放量和心肌丙二醛（MDA）增高。（2）NO的前体L-精氨酸（L-argi-nine,L-Arg）引起心肌细胞舒张期细胞长度缩短、收缩幅度降低。离体灌流心脏LVDP、冠脉流量、和±dp/dtmax增高,用K-H液复灌后可恢复正常。（3）L-Arg预处理,再行Fe-HQ灌流,与单纯的L-Arg或Fe-HQ组相比,心肌细胞舒张期细胞长度、收缩幅度和速度减小;离体灌流心脏LVDP、±dp/dtmax、心率和冠脉流量明显下降,冠脉流出液中LDH、CK增加。（4）Nω-硝基-L-精氨酸甲酯（L-NAME）和Fe-HQ合并灌流后,与单纯Fe-HQ组相比,心肌细胞舒张期细胞长度、收缩幅度和速度增加。L-NAME可阻断Fe-HQ引起的LVDP、左室舒张末压（LVEDP）和±dp/dtmax降低,冠脉流出液中LDH、CK增高。（5）用Triton X-100短暂处理以去除冠脉内皮后,与保留冠脉内皮的心肌相比,Fe-HQ引起的LVDP和±dp/dtmax的一过性增高现象被抑制,但     

Glutamate release in midbrain periaqueductal gray by activation of skeletal muscle receptors and arterial baroreceptors

We have previously reported that both skeletal muscle receptor and arterial baroreceptor afferent inputs activate neurons in the dorsolateral (DL) and lateral regions of the midbrain periaqueductal gray (PAG). In this study, we determined whether the excitatory amino acid glutamate (Glu) is released to mediate the increased activity in these regions. Static contraction of the triceps surae muscle for 4 min was evoked by electrical stimulation of the L7 and S1 ventral roots in cats. Activation of arterial baroreceptor was induced by intravenous injection of phenylephrine. The endogenous release of Glu from the PAG was recovered with the use of a microdialysis probe. Glu concentration was measured by the HPLC method. Muscle contraction increased mean arterial pressure (MAP) from 98 +/- 10 to 149 +/- 12 mmHg (P < 0.05) and increased Glu release in the DL and lateral regions of the middle PAG from 0.39 +/- 0.10 to 0.73 +/- 0.12 microM (87%, P < 0.05) in intact cats. After sinoaortic denervation and vagotomy were performed, contraction increased MAP from 95 +/- 12 to 158 +/- 15 mmHg, and Glu from 0.34 +/- 0.08 to 0.54 +/- 0.10 microM (59%, P < 0.05). The increases in arterial pressure and Glu were abolished by muscle paralysis. Phenylephrine increased MAP from 100 +/- 13 to 162 +/- 22 mmHg and increased Glu from 0.36 +/- 0.10 to 0.59 +/- 0.18 microM (64%, P < 0.05) in intact animals. Denervation abolished this Glu increase. Summation of the changes in Glu evoked by muscle receptor and arterial baroreceptor afferent inputs was greater than the increase in Glu produced when both reflexes were activated simultaneously in intact state (123% vs. 87%). These data demonstrate that activation of skeletal muscle receptors evokes release of Glu in the DL and lateral regions of the middle PAG, and convergence of afferent inputs from muscle receptors and arterial baroreceptors in these regions inhibits the release of Glu. These results suggest that the PAG is a neural integrating site for the interaction between the exercise pressor reflex and the arterial baroreceptor reflex.     

Ovarian hormone secretory response to gonadotropins and nitric oxide following chronic nitric oxide deficiency in the rat

Ovarian hormone secretion is regulated by gonadotropins, and it has been demonstrated that this response is modulated by nitric oxide (NO). The focus of this study was to determine the effect of chronic NO deficiency on the secretion of ovarian steroids. Female rats were given N-nitro-L-arginine (L-NNA; 0.6 g/L) in their drinking water, and vaginal smears were obtained daily. By 4 wk of treatment, all the rats were in constant estrus or proestrus. At 6-8 wk the animals were killed; the ovaries were removed and incubated in the presence of eCG (1 IU/ml) and hCG (1 IU/ml) and/or S-nitroso-L-acetyl penicillamine (an NO donor, S-NAP; 0.1 mM) for 4 h. Medium was collected at 30-min intervals, and estradiol, progesterone, and androstenedione were measured. Ovaries from proestrous rats served as controls. Ovaries from L-NNA-treated animals had a greater basal and gonadotropin-stimulated release of estradiol but not of androstenedione or progesterone in comparison to ovaries from untreated controls. S-NAP decreased the gonadotropin-stimulated estradiol, progesterone, and androstenedione in ovaries from NO-deficient rats. Steroid secretion in controls was not responsive to S-NAP. We conclude that chronic NO inhibition produces constant estrus due to increased estradiol production and that NO acts to inhibit estradiol and androstenedione production.     

Systemic morphine-induced release of serotonin in the rostroventral medulla is not mimicked by morphine microinjection into the periaqueductal gray

We used in vivo microdialysis in awake rats to test the hypothesis that intravenous morphine increases serotonin (5-HT) release within the rostral ventromedial medulla (RVM). We also injected morphine into various sites along the rostrocaudal extent of the periaqueductal gray (PAG), and examined the extent of its diffusion to the RVM. Intravenous morphine (3.0 mg/kg) produced thermal antinociception and increased RVM dialysate 5-HT, 5-hydroxyindole acetic acid (5-HIAA), and homovanillic acid (HVA) in a naloxone-reversible manner. As neither PAG microinjection of morphine (5 micro g/0.5 micro L) nor RVM administration of fentanyl or d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO) increased RVM 5-HT, we were unable to determine the precise site of action of morphine. Surprisingly, peak morphine levels in the RVM were higher after microinjection into the caudal PAG as compared to either intravenous injection or microinjection into more rostral sites within the PAG. Naloxone-precipitated withdrawal in morphine-tolerant rats not only increased extracellular 5-HT in the RVM, but also dopamine (DA) and HVA. We conclude that substantial amounts of morphine diffuse from the PAG to the RVM, and speculate that opioid receptor interactions at multiple brain sites mediate the analgesic effects of PAG morphine. Further studies will be required to elucidate the contribution of 5-HT and DA release in the RVM to opioid analgesia and opioid withdrawal.