Regional cerebral, ocular and peripheral vascular effects of naloxone and morphine in unanesthetized rabbits

Novel Association between Opioid Use and Increased Risk of Retinal Vein Occlusion Using the National Institutes of Health All of Us Research Program

Purpose

To assess for risk factors for retinal vein occlusion (RVO) among participants in the National Institutes of Health All of Us database, particularly social risk factors that have not been well studied, including substance use.

Design

Retrospective, case-control study.

Participants

Data were extracted for 380 adult participants with branch retinal vein occlusion (BRVO), 311 adult participants with central retinal vein occlusion (CRVO), and 1520 controls sampled among 311 640 adult participants in the All of Us database.

Methods

Data were extracted regarding demographics, comorbidities, income, housing, insurance, and substance use. Opioid use was defined by relevant diagnosis and prescription codes, with prescription use > 30 days. Controls were sampled at a 4:1 control to case ratio from a pool of individuals aged > 18 years without a diagnosis of RVO and proportionally matched to the demographic distribution of the 2019 U.S. census. Multivariable logistic regression identified medical and social determinants significantly associated with BRVO or CRVO. Statistical significance was defined as P < 0.05.

Main Outcome Measure

Development of BRVO or CRVO based on diagnosis codes.

Results

Among patients with BRVO, the mean (standard deviation) age was 70.1 (10.5) years. The majority (53.7%) were female. Cases were diverse; 23.7% identified as Black, and 18.4% identified as Hispanic or Latino. Medical risk factors including glaucoma (odds ratio [OR], 3.29; 95% confidence interval [CI], 2.22-4.90; P < 0.001), hypertension (OR, 2.15; 95% CI, 1.49-3.11; P < 0.001), and diabetes mellitus (OR, 1.68; 95% CI, 1.18-2.38; P = 0.004) were re-demonstrated to be associated with BRVO. Black race (OR, 2.64; 95% CI, 1.22-6.05; P = 0.017) was found to be associated with increased risk of BRVO. Past marijuana use (OR, 0.68; 95% CI, 0.50-0.92; P = 0.013) was associated with decreased risk of BRVO; however, opioid use (OR, 1.98; 95% CI, 1.41-2.78; P < 0.001) was associated with a significantly increased risk of BRVO. Similar associations were found for CRVO.

Conclusions

Understanding RVO risk factors is important for primary prevention and improvement in visual outcomes. This study capitalizes on the diversity and scale of a novel nationwide database to elucidate a previously uncharacterized association between RVO and opioid use.

Significance of endogenous opioids in the maintenance of cerebral and spinal vascular CO2-sensitivity in deep hemorrhagic hypotension

High CO(2)-sensitivity, one of the major characteristics of the cerebrovascular bed, has been shown to be influenced by a variety of factors. There are no reports, however, on the involvement of the endogenous opioid peptides in the modulation of the CO(2)-sensitivity of the cerebral and spinal cord vessels, either in normotensive or, in hypotensive conditions. The effect of general opiate receptor blockade (1.0mg/kg naloxone, i.v.) on regional cerebrovascular CO(2)-sensitivity was studied with radiolabeled microspheres in 10 distinct brain and spinal cord regions of the anesthetized cat. The CO(2)-induced flow changes were investigated in normotensive, in moderately hypotensive (MAP=80 mmHg) and in deep hypotensive cats (MAP=40 mmHg). The systemic arterial pressure was lowered by hemorrhage. In the normotensive cats, opiate receptor blockade caused no changes in the vascular CO(2)-sensitivity in the investigated cerebral and spinal cord regions. In moderate hypotension, cerebral and spinal CO(2)-sensitivity was significantly reduced by the hemorrhage itself, but remained unaffected by the naloxone administration. In deep hemorrhagic hypotension, however, general opiate receptor blockade resulted not only in a further reduction of the already impaired CO(2)-sensitivity, but even in a reversal of the effect of CO(2) from flow increase to flow decrease. These results indicate that endogenous opioid peptides, which do not seem to influence cerebrovascular reactions in steady-state, normotensive conditions, may contribute significantly to the maintenance of the normal vasodilatory response of the cerebral and spinal cord vessels to CO(2) during hemorrhage-induced deep arterial hypotension.

Low-dose remifentanil increases regional cerebral blood flow and regional cerebral blood volume, but decreases regional mean transit time and regional cerebrovascular resistance in volunteers

We have used contrast media-enhanced perfusion magnetic resonance imaging MRI to measure regional cerebral blood flow (rCBF), regional cerebral blood volume (rCBV), regional mean transit time (rMTT) and regional cerebrovascular resistance (rCVR) in volunteers at baseline and during infusion of remifentanil (0.1 microgram kg-1 min-1). Remifentanil increased rCBF and rCBV in white and grey matter (striatal, thalamic, occipital, parietal, frontal) regions, with a parallel decrease in rMTT in those regions with the exception of occipital grey matter. rCVR was decreased in all regions studied. The relative increase in rCBF was greater than that in rCBV. Cerebral haemodynamics were increased significantly in areas less rich in mu-opioid receptors with a tendency towards more pronounced increases in rCBF and rCBV in pain-processing areas. Furthermore, interhemispheric differences in rCBF, rCBV and rMTT found prior to drug administration were almost eliminated during infusion of remifentanil. We conclude that, apart from direct and indirect cerebrovascular effects of remifentanil, these findings are consistent with cerebral excitement and/or disinhibition.

Functional magnetic resonance imaging of the acute effect of intravenous heroin administration on visual activation in long-term heroin addicts: results from a feasibility study

This preliminary report is the first demonstration of the acute effects of diacetylmorphine (heroin) administration on functional activation in the human brain using functional magnetic resonance imaging (fMRI). Four opiate addicts who received regular prescriptions for heroin, underwent fMRI using a visual activation paradigm before and after a dose of 30 mg heroin. All four showed a decrease after the heroin dose in the extent of significant activation. This method shows promise for sequential scanning to determine brain activity in response to different drugs and routes of drug administration.

Effects of morphine and naloxone on cerebral blood flow and metabolism in experimental subarachnoid hemorrhage

OBJECTIVE

Naloxone is reported to improve the clinical condition of patients with subarachnoid hemorrhage (SAH). If this effect is vascular determined is unknown, wherefore the influence of morphine and naloxone on cerebral blood flow (CBF) and metabolic rate of oxygen (CMRO2) after SAH was evaluated.

MATERIAL AND METHODS

Two groups of 8 rats each with SAH and 2 corresponding groups of controls were investigated. CBF was calculated by the intracarotid 133Xenon method and CMRO2 as the product of CBF and the difference between systemic arterial and cerebral venous oxygen content.

RESULTS

In controls morphine, 1 mg/kg administered intravenously, decreased CBF by 25% (P < 0.001) without changing the CBF/CMRO2 ratio. In animals with SAH CBF was decreased by 32% (P < 0.001) and CBF/CMRO2 ratio by 38% (P < 0.01). Naloxone, 40 micrograms/kg administered intravenously neither influenced CBF nor the CBF/CMRO2 ratio in the 2 groups.

CONCLUSION

The reported clinical effect of naloxone after SAH can, according to our results, not be explained by changing the relationship between CBF and metabolism.

Cerebral perfusion in early and late opiate withdrawal: a technetium-99m-HMPAO SPECT study

The purpose of this study was to determine if cerebral blood flow (CBF) alterations are associated with discontinuation of heroin in chronic heroin users, and whether these alterations are reversible during abstinence. Ten physically healthy opioid-dependent males, hospitalized on an inpatient drug rehabilitation unit, were studied. Each patient had an initial single photon emission computed tomographic (SPECT) scan with the radiotracer technetium-99m-d,l-hexamethylpropyleneamine oxime (99mTc-HMPAO) 1 week after opiate discontinuation and a repeat scan 2 weeks later. The initial scans in 9 of the 10 subjects demonstrated significant, often discrete, perfusion defects, especially in the frontal, parietal, and temporal cortices. Two weeks later, repeat brain perfusion SPECT scans showed improvement in all nine subjects who had abnormal scans. Comparisons of the first scan with the second scan showed an increase in cortical uptake on the repeat SPECT study. All subjects had normal computed tomographic or magnetic resonance imaging scans. The results of this preliminary study suggest that the chronic use of opiates, like chronic use of cocaine, results in perfusion abnormalities without corresponding abnormalities on imaging studies of cerebral anatomy and morphology. This study also documents that these perfusion defects are partially reversible with short-term abstinence.

Abnormal cerebral metabolism in polydrug abusers during early withdrawal: a 31P MR spectroscopy study

Phosphorus magnetic resonance spectroscopy (31P MRS) at 1.5 T was performed on nine polysubstance abusing men. All nine patients met DSM-III-R criteria for concurrent cocaine and heroin dependence, were neurologically normal, were negative for the human immunodeficiency virus, and had normal clinical brain MRI scans. Patients were scanned 2-7 days after admission to a drug treatment unit. Eleven age-matched control subjects also were studied. The ISIS localized phosphorus spectra were obtained from a 5-cm thick axial brain slice and a 100-cc white matter volume. In the brain slice, the phosphorus metabolite signal expressed as a percentage of total phosphorus signal was 15% higher for phosphomonoesters, 10% lower for nucleotide triphosphates (beta-NTP), and 7% lower for total nucleotide phosphates in polydrug abusers compared with those in controls. Phosphodiesters, inorganic phosphate, phosphocreatine, total phosphorus, pH, and free magnesium concentration were unchanged. None of these parameters correlated with the methadone dose or the number of days abstinence. Single photon emission computed tomographic imaging of a subgroup of the patients revealed abnormal cerebral perfusion in 80% of the patients scanned. These data suggest that cerebral high energy phosphate and phospholipid metabolite changes result from long term drug abuse and/or withdrawal and that these changes can be detected and studied by 31P MRS.

Central nervous system control of intraocular pressure

Normal intraocular pressure (IOP) is the result of an equilibrium between aqueous humor (AH) production, AH outflow and episcleral venous pressure. Most available antiglaucoma agents produce their effects by interacting with autonomic mechanisms (beta-blockers, epinephrine or parasympathomimetics). In contrast, the role of the central nervous system (brain and nerves) in the regulation of IOP remains unclear in view of the complex haemodynamic, metabolic or hormonal changes which occur under experimental conditions. In this paper, we discuss a basic understanding of the anatomic and physiological relationships between central nervous system and IOP and describe how the brain can affect functions in ciliary body and trabeculum meshwork.

The influence of opioids on local cerebral glucose utilization in the newborn pig

Topical methionine enkephalin, leucine enkephalin, and dynorphin (10(-6)M) previously have been observed to produce prominent pial arteriolar dilation. Dilation to these opioids could be caused directly by opioids acting on vascular receptors, or indirectly, as a consequence of increased metabolism. Therefore, we examined this possibility by determining the influence of opioids on cerebral glucose utilization in piglets with closed cranial windows using the [14C]deoxyglucose method. Qualitatively, the autoradiographic images expressed as a change in relative optical density from vehicle were unchanged by these opioids. Quantitatively, the opioids similarly had no effect on cerebral glucose utilization (53 +/- 5, 70 +/- 8, 63 +/- 5, and 52 +/- 3, mumol.100 g-1.min-1 for vehicle, methionine enkephalin, leucine enkephalin, and dynorphin, respectively). In contrast, topical glutamate (10(-3) M) produced similar dilation but increased cerebral glucose utilization (41 +/- 3 vs 89 +/- 8 mumol.100 g-1.min-1 for vehicle and glutamate, respectively). Therefore, these opioids do not appear to produce vascular effects through a change in cerebral metabolic utilization of glucose.

Naloxone and TRH affect regional blood flows in the anesthetized rabbit

The cardiovascular effects of IV naloxone and a subsequent administration of TRH IV were studied in the rabbit. Naloxone caused a vasodilation in the myocardium and adrenal glands. Naloxone elicited an increment in cerebral blood flow in several regions which attenuated the cerebrovasodilating effect of TRH in a few regions. The blockade of endogenous opioids with naloxone did not modify the peripheral vasoconstricting effect of TRH or affect the vascular effects of TRH mediated by the peripheral sympathetic nerves. The results indicate that naloxone has a vasodilating effect in the myocardium and CNS in anesthetized rabbits. The major part of the cardiovascular effect of TRH is not dependent on mechanisms sensitive to naloxone.

Effects of heroin and naloxone on cerebral blood flow in the conscious rat

The widespread, heterogeneous distribution of opiate receptors and their endogenous ligands in the nervous system are reflective of the variety of central and systemic effects seen after opiate administration. Most neurons respond to either systemic or local opiate application with a decrease in firing rate, although increased neuronal activity has also been reported in such regions as the caudate, amygdala, ventral tegmentum, and substantia nigra. While regional metabolic studies have consistently reported neuronal suppression, some portion of this might be secondary to systemic hypercapnia. Using a brief blood flow marker, we recently reported a heterogenous increase in activity in more than half of the brain regions examined. To extend that study, we report herein the results of a dose-response and antagonist challenge experiment. Rats received an acute injection of one of the following: heroin (0.1, 0.3 or 1.0 mg/kg), naloxone (1.0 mg/kg), a cocktail of heroin (0.3 mg/kg) plus naloxone or saline. One min after drug administration, 160 muCi/kg [1-14C] octanoate, a marker for cerebral blood flow, was delivered IV. Rats were sacrificed two min later, brains removed and prepared for autoradiography. Of the fifty-eight areas analyzed, heroin caused an increase in blood flow in the caudate, claustrocortex, laterodorsal thalamus and dentate gyrus. Decreases were found for the bed nucleus of the stria terminalis, preoptic area, basolateral nucleus of the amygdala, dorsomedial and paraventricular hypothalamus, entorhinal and cingulate cortices and dorsal raphe. Naloxone resulted in significant increases in the olfactory tubercle and paraventricular nucleus while decreases were seen in the cingulate and basolateral amygdala.

Opioids in cerebrospinal fluid in hypotensive newborn pigs

This study was designed to determine if opioids were detectable in cerebrospinal fluid (CSF) and if these concentrations were altered by hemorrhagic hypotension. This study was further designed to determine the effects of topically administered opioids on pial arteriolar diameter during normotension and hypotension. Closed cranial windows were used to determine pial arteriolar diameter. Periarachnoid cortical and cisterna magna CSF was collected from piglets during normotension and hypotension (systemic arterial pressure decreased from 63 +/- 1 to 33 +/- 1 mm Hg). Opioid profiles were assessed qualitatively by radioreceptor assay, and individual opioids were measured quantitatively by radioimmunoassay. Periarachnoid cortical and cisterna magna CSF methionine enkephalin-, leucine enkephalin-, dynorphin-, and beta-endorphin-like receptor active values all were increased by hypotension. When quantified by radioimmunoassay, periarachnoid cortical CSF values for methionine enkephalin-like immunoreactivity were 1,167 +/- 58 and 2,975 +/- 139 pg/ml for normotension and hypotension, respectively. Periarachnoid cortical CSF radioimmunoassay values for dynorphin-like immunoreactivity were 15 +/- 2 and 28 +/- 2 pg/ml for normotension and hypotension, respectively. When applied topically to the cortical surface, synthetic methionine enkephalin increased pial arteriolar diameter (134 +/- 4, 158 +/- 4, and 163 +/- 4 microns for control, 574 pg/ml [10(-10) M], and 5,740 pg/ml [10(-9) M], respectively). Similarly, topical synthetic leucine enkephalin and dynorphin elicited pial arteriolar dilation. However, beta-endorphin produced arteriolar constriction. Hypotension attenuated methionine and leucine enkephalin-induced dilation and reversed dynorphin-induced dilation to concentration-dependent constriction. beta-Endorphin-induced constriction was not changed by hypotension. Therefore, opioids could contribute to the control of the cerebral circulation during hypotension.

The effects of naloxone on cerebral blood flow and cerebral function during relative cerebral ischemia

CBF and somatosensory evoked potentials (SEPs) were measured in a model of moderate cerebral ischemia in anesthetized spontaneously hypertensive rats. The rats were bled to reduce SEP amplitudes to about 50% of prebleeding control. The consequent blood pressure fall reduced CBF to 77% of control as measured by the laser-Doppler technique. Naloxone (5 mg kg-1 i.v. plus 25 mg kg-1 h-1 i.v. for 30 min) caused a significant increase in SEP amplitudes, while CBF did not change significantly. In addition, the latency of the first SEP component decreased toward prebleeding values. Heart rate (HR) decreased, but MABP was held constant by a pressure-regulating reservoir. In unbled rats, naloxone (5 mg kg-1 i.v.) caused a transient small increase in MABP and SEP amplitudes and decrease in HR. These results indicate that sensory input is regulated by opioid systems. Increased opioid activity may inhibit ascending sensory pathways during relative cerebral ischemia and thereby depress SEP responses. Thus, naloxone can release this inhibition and enhances SEP independently of CBF during relative cerebral ischemia. Similar mechanisms might explain the apparently beneficial effects of naloxone in some stroke models.

The influence of bilateral electrical preganglionic sympathetic stimulation on intra- and extracranial blood flow

The effects of bilateral electrical stimulation (SS) of the cervical sympathetic chain on intra- and extra cerebral blood flows were studied with the labelled microsphere method in the rabbit. Control blood flow was determined before the SS was started. The stimulation frequency was 7 Hz, the impulse duration 2 ms, the intensity 7 V and the stimulation time varied between 1 to 5 minutes before the second blood flow determination. Arterial blood gas values and blood pressure were unaffected by the stimulation. Due to the SS there were blood flow decrements in the extracranial tissues between 60-96%. The blood flow in the eyes, the dura, pineal gland and choroid plexa was markedly reduced during the SS. No obvious effect was elicited by the SS in the regional or total cerebral blood flow. The stimulation to control blood flow ratio ranged between 0.92 +/- 0.08 to 1.13 +/- 0.09 in different parts of the brain. The conclusions are that SS elicits vasoconstriction in several extra- and intracranial nonneuronal tissues and in the eye. Cerebral blood flow is not influenced by the SS.

Dual effect of naloxone on blood platelet aggregation and cerebral blood flow in gerbils

The effect of naloxone on blood platelet aggregation and cerebral blood flow in gerbils was studied. Administration of naloxone in dose 1 mg/kg to intact gerbils resulted in a marked increase in platelet aggregability accompanied by 27% reduction in cerebral blood flow. Focal cerebral ischemic injury significantly enhanced platelet aggregatory response and treatment with naloxone was without any additional effect on platelet aggregation. Cerebral blood flow in ischemic hemisphere, however, increased following naloxone injection by 46%. In vitro naloxone in millimolar concentrations inhibited platelet aggregation in a dose-dependent way. Apparent decrease in fluorescence of platelet membranes tagged with fluorescence probe due to naloxone suggests conformational changes in platelet membrane as a primary mechanism for the antiaggregatory effect of naloxone in vitro.

Effects of TRH on cerebral and peripheral blood flows; role of submesencephalic brain stem centres

The localization of the origin of the cardiovascular effects elicited by thyrotropin-releasing hormone (TRH) was attempted in this study. The radioactively labelled microsphere method was employed for measurement of regional cerebral (rCBF) and peripheral blood flow in albino rabbits anaesthetized with urethane. The effect of 50 micrograms and 2 mg kg-1 TRH (administered i.v.) on rCBF and peripheral blood flow was evaluated in animals with the brain stem sectioned (BSS) at the level of pons-mesencephalon. The cerebral vasodilating effect of TRH was abolished or attenuated, while the peripheral vasoconstriction and increase in mean arterial blood pressure (MAP) was unaffected. Cordotomy at the CI level caused a marked fall in MAP and abolished the pressor response to TRH. In animals infused with angiotensin II, in order to normalize the decreased MAP after cordotomy, TRH caused a marked increase in rCBF. Administration of 50 ng and 5 micrograms TRH into the fourth ventricle caused a marked peripheral vasoconstriction and pressor response. The same amounts of TRH administered into the mesencephalic aqueduct caused a marked increase in rCBF and peripheral vasoconstriction. The results indicate that TRH elicits the pressor and peripheral vasoconstrictor responses from a submesencephalic brain stem region. The increase in rCBF caused by TRH is probably mediated by a somewhat higher submesencephalic level.

Effects of B-HT 920 in the eye and on regional blood flows in anaesthetized and conscious rabbits

The influence of unilateral ocular instillation of B-HT 920 (50 micrograms) on regional ocular, cerebral and peripheral blood flows was investigated with the labelled microsphere method in conscious and anaesthetized albino rabbits. In urethane-anaesthetized rabbits the intraocular pressure (IOP) fell during 1 hr following topical B-HT 920 whereas no changes in regional blood flows were observed. Only in conscious rabbits was a decrease in regional blood flows found. B-HT 920 caused a short-term reduction in choroidal blood flow by about 20%. Transient vasoconstrictor effects, due to systemic absorption, were also seen in some extraocular tissues. Concomitantly, B-HT 920 reduced the total cerebral blood flow (CBF) by 23%. In the grey matter and hypothalamic region the decrease in flow was about 20%, while in the hippocampal region, thalamic region, collicles and pons-mesencephalon it was about 10%. In experiments with direct blood flow determination from an opened vortex vein, there was no consistent change of uveal vascular resistance, while IOP and mean arterial pressure (MAP) fell dose-dependently following cumulative intravenously administered bolus doses (10 and 50 micrograms/kg) of B-HT 920. Unilateral loss of the mediated sympathetic tone seemed to increase the ocular responses to B-HT 920, unmasking a vasoconstrictor effect. Additional systemic pretreatment with the selective blocking agents rauwolscine and sulpiride suggests that B-HT 920 produces its ocular hypotensive effect, predominantly by acting on dopamine (DA2) receptors in the eye rather than on alpha 2-adrenoceptors, and its ocular vasoconstrictor effects via both receptor types.

Factors influencing the effects of intravenous naloxone on arterial pressure and heart rate after haemorrhage in conscious rabbits

The circulatory responses to different intravenous doses of naloxone were studied in conscious rabbits before and after haemorrhage, under different conditions including prior ganglion blockade. Unless there had been blood loss, naloxone elicited no pressor response, even in high dose. After bleeding so that arterial pressure fell to 40 mmHg, the dose-response relationship for naloxone had two components. Over a low-dose range (threshold 0.3 mg/kg) naloxone had a modest pressor effect but did not affect heart rate. Over a much higher dose range (threshold 0.6 mg/kg) naloxone caused a marked rise in arterial pressure and a profound bradycardia. The highest dose of naloxone examined (25 mg/kg) caused a rise in arterial pressure of 70 mmHg and a reduction in heart rate of 160 beats/min. The pressor and bradycardic effects of naloxone were the same whether post-haemorrhagic hypotension lasted 5, 10, 20 or 30 min. The responses to naloxone in low or high dose depended much more closely on the volume of blood removed than on the level to which arterial pressure fell. Even after non-hypotensive haemorrhage a high dose of naloxone had marked pressor and bradycardic effects. Ganglion blockade prior to haemorrhage abolished the pressor response to a low, but not to a high, dose of naloxone. It was concluded that prolonged and severe hypotension are not necessary to 'prime' the cardiovascular system to respond to naloxone after haemorrhage. In a high dose its pressor effects appear to be mediated post-ganglionically, but in a low dose it may act within the central nervous system.

Effect of low intravenous doses of TRH, acid-TRH and cyclo(His-Pro) on cerebral and peripheral blood flows

Local cerebral and peripheral blood flow in conscious and anaesthetized rabbits were investigated with the microsphere method, before and after the i.v. administration of 25 or 50 micrograms kg-1 thyrotropin-releasing hormone (TRH). Before the experiment, the cervical sympathetic chain was sectioned on one side in order to evaluate the possible effect of the sympathetic nerves on cranial and extracranial blood flows. Blood flow was also determined in anaesthetized rabbits before and after the administration of the TRH metabolites cyclo(His-Pro) and acid-TRH and after subsequent administration of 50 micrograms kg-1 TRH. TRH caused an increase in mean arterial blood pressure (MAP) of about 1 to 2 kPa whereas cyclo(His-Pro) and acid-TRH had no effect on MAP. In the anaesthetized animal an increase in total cerebral blood flow (CBFtot), from 71 +/- 7 to 107 +/- 12 g min-1 100 g-1 (P less than 0.05) was observed on the sympathetic intact side after 25 micrograms kg-1 TRH and a further increase to 130 +/- 9 g min-1 100g-1 (P less than 0.01) after 50 micrograms kg-1 TRH. A similar effect was observed on the sympathotomized side. An effect on CBF in the conscious animal was not detected. The control CBFtot (104 +/- 8 g min-1 100g-1) was higher in these animals than in the anaesthetized animals (P less than 0.02). Neither cyclo(His-Pro) nor acid-TRH mimicked the effect of TRH on CBF. In several peripheral tissues, e.g. skin, pancreas and gastric mucosa, a reduction in blood flow was noted after the administration of TRH in both anaesthetized and conscious rabbits. It was concluded that TRH can induce cerebral vasodilatation in animals with a depressed CBF, whereas the vasoconstrictor effect of TRH in peripheral organs is not markedly affected by the state of consciousness.

beta-Endorphin and enkephalins stimulate duodenal mucosal alkaline secretion in the rat in vivo

Secretion of HCO3- by duodenum just distal to the Brunner's glands area and devoid of pancreatic HCO3- was titrated in situ in anesthetized rats. Secretion increased significantly after intravenous injection of small amounts (10-20 ng/kg) of the opioid peptides beta-endorphin, methionine-enkephalin, and leucine-enkephalin. Maximum (approximately twofold) stimulation by beta-endorphin and leucine-enkephalin occurred at 20 ng/kg. Morphine (50 micrograms/kg) caused a similar stimulation and the mu-selective opiate antagonist naloxone prevented the stimulation by beta-endorphin and morphine. The synthetic analogue [D-Ala2,D-Leu5]-enkephalin (500 ng/kg), which is an agonist primarily at delta-opiate receptors, had no effect, further suggesting that the stimulation of duodenal HCO3- secretion is mediated by mu-receptors. Naloxone alone did not affect basal HCO3- secretion but reduced the duration of the rise in secretion in response to a 5-min exposure to luminal acid (pH 2.00). Endogenous opioid peptides may thus have a role in the humoral or neural control, or both, of duodenal surface epithelial HCO3- secretion and mucosal protection.

Effects of enkephalins, morphine, and naloxone on pial arteries during perivascular microapplication

The effects of the opiate receptor agonists, enkephalins and morphine, and the antagonist, naloxone, on cerebrovascular resistance vessels was investigated in situ by employing perivascular microapplication. Feline pial arteries with a resting diameter of 66-294 micron were tested. Vascular diameter was measured using television image splitting. Concentration-response curves revealed no change of diameter when Leu-enkephalin, D-Ala2-Leu-enkephalinamide, D-Ala2-Met-enkephalinamide, and morphine were applied in concentrations of 10(-11)-10(-5) M. Considering the concentrations of enkephalins that have been found in natural cerebrospinal fluid or that can be expected in the vicinity of enkephalinergic synapses, the data obtained with the lower concentrations indicate that enkephalins are probably not important for the regulation of pial arterial resistance. At 10(-4) M only the dilation (4.3%) elicited by D-Ala2-Leu-enkephalinamide was statistically significant (p less than 0.01). All four agonists at 10(-3) M induced significant dilatations varying between 5.4 and 13.6%. Naloxone exerted no vascular effect per se at 10(-5) and 10(-4) M but a dilatation of 15.3% at 10(-3) M. The latter can be explained by a partial agonist action. During simultaneous administration, naloxone (10(-4) M) reduced the dilatations induced by 10(-4) and 10(-3) M D-Ala2-Leu-enkephalinamide dose dependently. This indicates that mu- and delta-opioid receptors, probably located at the vascular smooth muscle cell, were involved in the mediation of the dilatation induced by the highest concentrations of the compounds.

Effects of raised intracranial pressure on regional cerebral blood flow: a comparison of effects of naloxone and TRH on the microcirculation in partial cerebral ischaemia

The effects on regional cerebral blood flow (rCBF) of raised intracranial pressure (ICP) and of naloxone and thyrotropin releasing hormone (TRH) during this condition were studied in anaesthetized rabbits. The ICP was elevated until a central ischaemic response was observed. The regional blood flow was determined with the microsphere technique before and during elevation of the ICP (ICPe) and after drug treatment. Total CBF was reduced by about 70% during ICPe while the uveal blood flow increased slightly and some other peripheral tissue blood flows remained unaffected. The administration of TRH caused an increase in mean arterial blood pressure (MAP) from 11.9 +/- 0.6 to 14.6 +/- 0.7 kPa and a normalization of the rCBF. In some peripheral tissues, e.g. gastric mucosa and spleen, TRH reduced the blood flow by 53% and 76%, respectively. In blood pressure stabilized animals no effect on rCBF was seen after TRH. Naloxone had no consistent effect on MAP or local blood flow. It was concluded that in the range of cerebral perfusion pressure studied there was a passive relationship between cerebral blood flow and perfusion pressure. The lack of effect of naloxone and the marked effect of TRH during cerebral ischaemia are consistent with a mechanism of action of TRH not related to a 'physiological' antagonism of opioids.