Oxygen fluctuations in the brain and periphery induced by intravenous fentanyl: effects of dose and drug experience.

Fentanyl is the leading contributor to drug overdose deaths in the US. Its potency, rapid onset of action, and lack of effective reversal treatment make the drug more lethal than other opioids. Although it is understood that fentanyl is dangerous at higher doses, literature surrounding fentanyl's physiological effects remains contradictory at lower doses. To explore this discrepancy, we designed a study incorporating electrochemical assessment of oxygen in the brain (nucleus accumbens; NAc) and subcutaneous (SC) space, multi-site thermorecording (brain, skin, muscle), and locomotor activity at varying doses of fentanyl (1.0, 3.0, 10, 30, 90 µg/kg) in rats. In the NAc, lower doses of fentanyl (3.0, 10 µg/kg) led to an increase in oxygen levels while higher doses (30, 90 µg/kg) led to a biphasic pattern, with initial dose-dependent decrease followed by increase. In the SC space, oxygen decreases started to appear at relatively lower doses (>3 µg/kg), had shorter onset latencies, and were stronger and prolonged. In the temperature experiment, lower doses of fentanyl (1.0, 3.0, 10 µg/kg) led to an increase in brain, skin, and muscle temperatures, while higher doses (30 and 90 µg/kg) resulted in a dose-dependent biphasic temperature change, with an increase followed by a prolonged decrease. We also compared oxygen and temperature responses induced by fentanyl over six consecutive days and found no evidence of tolerance in both parameters. In conclusion, we report that fentanyl's effects are highly dose-dependent, drawing attention to the importance of better characterization to adequately respond in emergent cases of fentanyl misuse.

Brain oxygen responses induced by opioids: focus on heroin, fentanyl, and their adulterants.

Opioids are important tools for pain management, but abuse can result in serious health complications. Of these complications, respiratory depression that leads to brain hypoxia is the most dangerous, resulting in coma and death. Although all opioids at large doses induce brain hypoxia, danger is magnified with synthetic opioids such as fentanyl and structurally similar analogs. These drugs are highly potent, act rapidly, and are often not effectively treated by naloxone, the standard of care for opioid-induced respiratory depression. The goal of this review paper is to present and discuss brain oxygen responses induced by opioids, focusing on heroin and fentanyl. In contrast to studying drug-induced changes in respiratory activity, we used chronically implanted oxygen sensors coupled with high-speed amperometry to directly evaluate physiological and drug-induced fluctuations in brain oxygen levels in awake, freely moving rats. First, we provide an overview of brain oxygen responses to physiological stimuli and discuss the mechanisms regulating oxygen entry into brain tissue. Next, we present data on brain oxygen responses induced by heroin and fentanyl and review underlying mechanisms. These data allowed us to compare the effects of these drugs on brain oxygen in terms of their potency, time-dependent response pattern, and potentially lethal effect at high doses. Then, we present the interactive effects of opioids during polysubstance use (alcohol, ketamine, xylazine) on brain oxygenation. Finally, we consider factors that affect the therapeutic potential of naloxone, focusing on dosage, timing of drug delivery, and contamination of opioids by other neuroactive drugs. The latter issue is considered chiefly with respect to xylazine, which strongly potentiates the hypoxic effects of heroin and fentanyl. Although this work was done in rats, the data are human relevant and will aid in addressing the alarming rise in lethality associated with opioid misuse.

Combined treatment with naloxone and the alpha2 adrenoceptor antagonist atipamezole reversed brain hypoxia induced by a fentanyl-xylazine mixture in a rat model.

Xylazine, a veterinary tranquillizer known by drug users as "Tranq", is being increasingly detected in people who overdose on opioid drugs, indicating enhanced health risk of fentanyl-xylazine mixtures. We recently found that xylazine potentiates fentanyl- and heroin-induced brain hypoxia and eliminates the rebound-like post-hypoxic oxygen increases. Here, we used oxygen sensors coupled with high-speed amperometry in rats of both sexes to explore the treatment potential of naloxone plus atipamezole, a selective α2-adrenoceptor antagonist, in reversing brain (nucleus accumbens) and periphery (subcutaneous space) hypoxia induced by a fentanyl-xylazine mixture. Pretreatment with naloxone (0.2 mg/kg, IV) fully blocked brain and peripheral hypoxia induced by fentanyl (20 µg/kg, IV), but only partially decreased hypoxia induced by a fentanyl-xylazine mixture. Pretreatment with atipamezole (0.25 mg/kg, IV) fully blocked the hypoxic effects of xylazine (1.0 mg/kg, IV), but not fentanyl. Pretreatment with atipamezole + naloxone was more potent than naloxone alone in blocking the hypoxic effects of the fentanyl-xylazine mixture. Both naloxone and naloxone + atipamezole, delivered at the peak of brain hypoxia (3 min post fentanyl-xylazine exposure), reversed the rapid initial brain hypoxia, but only naloxone + atipamezole decreased the prolonged weaker hypoxia. There were no sex differences in the effects of the different drugs and their combinations on brain and peripheral oxygen responses. Results indicate that combined treatment with naloxone and atipamezole is more effective than naloxone alone in reversing the hypoxic effects of fentanyl-xylazine mixtures. Naloxone + atipamezole treatment should be considered in preventing overdoses induced by fentanyl-xylazine mixtures in humans.

Xylazine effects on opioid-induced brain hypoxia.

RATIONALE: Xylazine has emerged in recent years as an adulterant in an increasing number of opioid-positive overdose deaths in the United States. Although its exact role in opioid-induced overdose deaths is largely unknown, xylazine is known to depress vital functions and cause hypotension, bradycardia, hypothermia, and respiratory depression. OBJECTIVES: In this study, we examined the brain-specific hypothermic and hypoxic effects of xylazine and its mixtures with fentanyl and heroin in freely moving rats. RESULTS: In the temperature experiment, we found that intravenous xylazine at low, human-relevant doses (0.33, 1.0, 3.0 mg/kg) dose-dependently decreases locomotor activity and induces modest but prolonged brain and body hypothermia. In the electrochemical experiment, we found that xylazine at the same doses dose-dependently decreases nucleus accumbens oxygenation. In contrast to relatively weak and prolonged decreases induced by xylazine, intravenous fentanyl (20 µg/kg) and heroin (600 µg/kg) induce stronger biphasic brain oxygen responses, with the initial rapid and strong decrease, resulting from respiratory depression, followed by a slower, more prolonged increase reflecting a post-hypoxic compensatory phase, with fentanyl acting much quicker than heroin. The xylazine-fentanyl mixture eliminated the hyperoxic phase of oxygen response and prolonged brain hypoxia, suggesting xylazine-induced attenuation of the brain's compensatory mechanisms to counteract brain hypoxia. The xylazine-heroin mixture strongly potentiated the initial oxygen decrease, and the pattern lacked the hyperoxic portion of the biphasic oxygen response, suggesting more robust and prolonged brain hypoxia. CONCLUSIONS: These findings suggest that xylazine exacerbates the life-threatening effects of opioids, proposing worsened brain hypoxia as the mechanism contributing to xylazine-positive opioid-overdose deaths.

The pattern of brain oxygen response induced by intravenous fentanyl limits the time window of therapeutic efficacy of naloxone.

Opioids induce respiratory depression resulting in coma or even death during overdose. Naloxone, an opioid antagonist, is the gold standard reversal agent for opioid intoxication, but this treatment is often less successful for fentanyl. While low dosing is thought to be a factor limiting naloxone's efficacy, the timing between fentanyl exposure and initiation of naloxone treatment may be another important factor. Here, we used oxygen sensors coupled with amperometry to examine the pattern of oxygen responses in the brain and periphery induced by intravenous fentanyl in freely moving rats. At both doses (20 and 60 µg/kg), fentanyl induced a biphasic brain oxygen response-a rapid, strong, and relatively transient decrease (8-12 min) followed by a weaker and prolonged increase. In contrast, fentanyl induced stronger and more prolonged monophasic oxygen decreases in the periphery. When administered before fentanyl, intravenous naloxone (0.2 mg/kg) fully blocked the hypoxic effects of moderate-dose fentanyl in both the brain and periphery. However, when injected 10 min after fentanyl, when most of hypoxia had already ceased, naloxone had minimal effect on central and peripheral oxygen levels, but at a higher dose, it strongly attenuated hypoxic effects in the periphery with only a transient brain oxygen increase associated with behavioral awakening. Therefore, due to the rapid, strong but transient nature of fentanyl-induced brain hypoxia, the time window when naloxone can attenuate this effect is relatively short. This timing limitation is critical, making naloxone most effective when used quickly and less effective when used during the post-hypoxic comatose state after brain hypoxia has already ceased and harm for neural cells already done.

Basic metabolic and vascular effects of ketamine and its interaction with fentanyl.

Ketamine is a short-acting general anesthetic with hallucinogenic, analgesic, and amnestic properties. In addition to its anesthetic use, ketamine is commonly abused in rave settings. While safe when used by medical professionals, uncontrolled recreational use of ketamine is dangerous, especially when mixed with other sedative drugs, including alcohol, benzodiazepines, and opioid drugs. Since synergistic antinociceptive interactions between opioids and ketamine were demonstrated in both preclinical and clinical studies, such an interaction could exist for the hypoxic effects of opioid drugs. Here, we focused on the basic physiological effects of ketamine as a recreational drug and its possible interactions with fentanyl-a highly potent opioid that induces strong respiratory depression and robust brain hypoxia. By using multi-site thermorecording in freely-moving rats, we showed that intravenous ketamine at a range of human relevant doses (3, 9, 27 mg/kg) dose-dependently increases locomotor activity and brain temperature, as assessed in the nucleus accumbens (NAc). By determining temperature differentials between the brain, temporal muscle, and skin, we showed that the brain hyperthermic effect of ketamine results from increased intracerebral heat production, an index of metabolic neural activation, and decreased heat loss due to peripheral vasoconstriction. By using oxygen sensors coupled with high-speed amperometry we showed that ketamine at the same doses increases NAc oxygen levels. Finally, co-administration of ketamine with intravenous fentanyl results in modest enhancement of fentanyl-induced brain hypoxia also enhancing the post-hypoxic oxygen increase. Therefore, in contrast to fentanyl, ketamine increases brain oxygenation but potentiates brain hypoxia induced by fentanyl.

Dantrolene sodium fails to reverse robust brain hyperthermia induced by MDMA and methamphetamine in rats.

RATIONALE: Hyperthermia induced by psychomotor stimulants may cause leakage of the blood-brain barrier, vasogenic edema, and lethality in extreme cases. Current treatments such as whole-body cooling are only symptomatic and a clear need to develop pharmacological interventions exists. Dantrolene sodium, a peripheral muscle relaxant used in the treatment of malignant hyperthermia, has been proposed as potentially effective to treat MDMA-hyperthermia in emergency rooms. However, debate around its efficacy for this indication persists. OBJECTIVES: To investigate dantrolene as a treatment for illicit hyperthermia induced by psychomotor stimulant drugs, we examined how Ryanodex®, a concentrated formulation of dantrolene sodium produced by Eagle Pharmaceuticals, influences 3,4-methylenedioxymethamphetamine (MDMA)- and methamphetamine (METH)-induced hyperthermia in awake freely moving rats. We injected rats with moderate doses of MDMA (9 mg/kg) and METH (9 mg/kg) and administered Ryanodex® intravenously (6 mg/kg) after the development of robust hyperthermia (>2.5 °C) mimicking clinical acute intoxication. We conducted simultaneous temperature recordings in the brain, temporal muscle, and skin to determine the basic mechanisms underlying temperature responses. To assess the efficacy of dantrolene in attenuating severe hyperthermia, we administered MDMA to rats maintained in a warm ambient environment (29 °C), conditions which produce robust brain and body hyperthermia (>40 °C) and lethality. RESULTS: Dantrolene failed to attenuate MDMA- and METH-induced hyperthermia, though locomotor activity was significantly reduced. All animals maintained at warm ambient temperatures that received dantrolene during severe drug-induced hyperthermia died within or soon after the recording session. CONCLUSIONS: Our results suggest that dantrolene sodium formulations are not mechanistically suited to treat MDMA- and METH-induced hyperthermia.

Basic physiological effects of ketamine-xylazine mixture as a general anesthetic preparation for rodent surgeries.

Among the numerous general anesthetics utilized in rodent surgical procedures, the co-administration of ketamine and xylazine is the current standard for induction and maintenance of surgical planes of anesthesia and pain control. In contrast to classical GABAergic anesthetics, which act to inhibit CNS activity, inducing muscle relaxation, sedation, hypothermia, and brain hypoxia, ketamine and xylazine act through different mechanisms to induce similar effects while also providing potent analgesia. By using three-point thermorecording in freely moving rats, we show that the ketamine-xylazine mixture induces modest brain hyperthermia, resulting from increased intra-cerebral heat production due to metabolic brain activation and increased heat loss due to skin vasodilation. The first effect derives from ketamine, which alone increases brain and body temperatures due to brain metabolic activation and skin vasoconstriction. The second effect derives from xylazine, which increases heat loss due to potent skin vasodilation. By using oxygen sensors coupled with amperometry, we show that the ketamine-xylazine mixture modestly decreases brain oxygen levels that results from relatively weak respiratory depression. This tonic pharmacological effect was preceded by a strong but transient oxygen increase that may result from a stressful injection or unknown, possibly peripheral action of this drug combination. This pattern of physiological effects elicited by the ketamine-xylazine mixture differs from the effects of other general anesthetic drugs, particularly barbiturates.

Rapid fluctuations in brain oxygenation during glucose-drinking behavior in trained rats.

Proper inflow of oxygen into brain tissue is essential for maintaining normal neural functions. Although oxygen levels in the brain's extracellular space depend upon a balance between its delivery from arterial blood and its metabolic consumption, the use of high-speed electrochemical detection revealed rapid increases in brain oxygen levels elicited by various salient sensory stimuli. These stimuli also increase intrabrain heat production, an index of metabolic neural activation, but these changes are slower and more prolonged than changes in oxygen levels. Therefore, under physiological conditions, the oxygen inflow into brain tissue exceeds its loss due to consumption, thus preventing any metabolic deficit. Here, we used oxygen sensors coupled with amperometry to examine the pattern of real-time oxygen fluctuations in the nucleus accumbens during glucose-drinking behavior in trained rats. Following the exposure to a glucose-containing cup, oxygen levels rapidly increased, peaked when the rat initiated drinking, and relatively decreased during consumption. Similar oxygen changes but more episodic drinking occurred when Stevia, a calorie-free sweet substance, was substituted for glucose. When water was substituted for glucose, rats tested the water but refused to consume all of it. Although the basic pattern of oxygen changes during this water test was similar to that with glucose drinking, the increases were larger. Finally, oxygen increases were significantly larger when rats were exposed to concealed glucose and made multiple unsuccessful attempts to obtain and consume it. Based on these data, we discuss the mechanisms underlying behavior-related brain oxygen fluctuations and their functional significance.NEW & NOTEWORTHY Oxygen sensors coupled with high-speed amperometry were used to examine brain oxygen fluctuations during glucose-drinking behavior in trained rats. Oxygen levels rapidly increased following presentation of a glucose-contained cup, peaking at the initiation of glucose drinking, and relatively decreasing during drinking. Oxygen increases were larger when rats were exposed to concealed glucose and made multiple attempts to obtain it. We discuss the mechanisms underlying behavior-related brain oxygen fluctuations and their functional significance.

Functional role of peripheral vasoconstriction: not only thermoregulation but much more.

Peripheral vasoconstriction is a centrally mediated physiological effect known to play an important role in regulating body temperature by adjusting heat exchange with the external environment. However, peripheral vasoconstriction as a component of sympathetic activation also occurs following exposure to various salient stimuli and during motivated behavior at stable environmental temperatures. This review aims to consider available evidence suggesting a significant contribution of this peripheral effect to physiological increases in both brain temperature and entry of oxygen and glucose into the brain's extracellular space. While these effects are triggered by neuronal activation, constriction of blood vessels in the skin and most internal organs results in redistribution of blood from the peripheral to central domains, thus dilating cerebral vessels, increasing global cerebral blood flow, and enhancing the intra-brain entry of oxygen and glucose from arterial blood. This powerful influence appears to determine the long duration of physiological increases in both brain temperature and brain levels of glucose and oxygen and their basic similarity across different brain structures. This work underscores the tight interrelationship between the brain and periphery and a significant contribution of cardiovascular effects in providing the enhanced inflow of oxygen and glucose into brain tissue to prevent metabolic deficit during functional neural activation.

Effects of alcohol on brain oxygenation and brain hypoxia induced by intravenous heroin.

Alcohol is the most commonly used psychoactive drug, often taken in conjunction with opioid drugs. Since both alcohol and opioids can induce CNS depression, it is often assumed that alcohol potentiates the known hypoxic effects of opioid drugs. To address this supposition, we used oxygen sensors to examine the effects of alcohol on brain oxygenation and hypoxic responses induced by intravenous heroin in awake, freely moving rats. To eliminate robust sensory effects of alcohol following its oral or intraperitoneal delivery, alcohol was administered directly into the stomach via chronically implanted intragastric catheters at human relevant doses. Alcohol delivered at a 0.5 g/kg dose did not affect brain oxygen levels, except for a weak transient increase during drug delivery. This phasic oxygen increase was stronger at a 2.0 g/kg alcohol dose and followed by a weaker tonic increase. Since alcohol absorption from intragastric delivery is much slower and more prolonged than with intraperitoneal or intravenous injections, the rapid rise of brain oxygen levels suggests that alcohol has a direct action on sensory afferents in the stomach well before the drug physically reaches brain tissue via circulation. Despite slow tonic increases in brain oxygen, alcohol at the 2.0 g/kg dose strongly potentiates heroin-induced oxygen responses, increasing both the magnitude and duration of oxygen decrease. Therefore, under the influence of alcohol, the use of opioid drugs becomes much more dangerous, increasing brain hypoxia and enhancing the probability of serious health complications, including coma and death.

Relationships between oxygen changes in the brain and periphery following physiological activation and the actions of heroin and cocaine.

Using two-sensor electrochemical recordings in freely moving rats, we examined the relationship between physiological and drug-induced oxygen fluctuations in the brain and periphery. Animals chronically implanted with oxygen sensors in the nucleus accumbens (NAc) and subcutaneous (SC) space were subjected to several mildly arousing stimuli (sound, tail-pinch and social interaction) and intravenous injections of cocaine and heroin. Arousing stimuli induced rapid increases in NAc oxygen levels followed by and correlated with oxygen decreases in the SC space. Therefore, cerebral vasodilation that increases cerebral blood flow and oxygen entry into brain tissue results from both direct neuronal activation and peripheral vasoconstriction, which redistributes arterial blood from periphery to the brain. The latter factor could also explain a similar pattern of oxygen responses found in the substantia nigra reticulata, suggesting hyperoxia as a global phenomenon with minor structural differences during early time intervals following the stimulus onset. While arousing stimuli and cocaine induced similar oxygen responses in the brain and SC space, heroin induced a biphasic down-up brain oxygen fluctuation associated with a monophasic oxygen decrease in the SC space. Oxygen decreases occurred more rapidly and stronger in the SC space, reflecting a drop in blood oxygen levels due to respiratory depression.

The Critical Role of Peripheral Targets in Triggering Rapid Neural Effects of Intravenous Cocaine.

Direct interaction of cocaine with centrally located monoamine transporters is the primary mechanism underlying its reinforcing properties. It is also often assumed that this drug action is responsible for all the physiological and behavioral effects of this drug. The goal of this review is to challenge this basic mechanism and demonstrate the importance of peripheral actions of cocaine in inducing its initial, rapid neural effects. The use of high-resolution electrophysiological, neurochemical and physiological techniques revealed that the effects of intravenous cocaine at behaviorally relevant doses are exceptionally rapid and transient correlating with strong, quick, and transient increases in blood cocaine levels. Some of these effects are mimicked by cocaine-methiodide, a cocaine analog that cannot cross the blood-brain barrier and they are resistant to dopamine (DA) receptor blockade. Therefore, it appears that rapid neural effects of cocaine result from its direct interaction with receptive sites on afferents of sensory nerves densely innervating blood vessels. This interaction creates a rapid neural signal to the CNS that results in generalized neural activation and subsequent changes in different physiological parameters. This drug's action appears to be independent from cocaine's action on central neurons, which requires a definite time to occur and induce neural and physiological effects with longer latencies and durations. The co-existence in the same drug on two timely distinct actions with their subsequent interaction in the CNS could explain consistent changes in physiological and behavioral effects of cocaine following their repeated use, playing a role in the development of drug-seeking and drug-taking behavior.

Cocaine added to heroin fails to affect heroin-induced brain hypoxia.

Heroin and cocaine are both highly addictive drugs that cause unique physiological and behavioral effects. These drugs are often co-administered and cocaine has been found in ~20% of cases of opioid overdose death. Respiratory depression followed by brain hypoxia is the most dangerous effect of high-dose opioids that could result in coma and even death. Conversely, cocaine at optimal self-administering doses increases brain oxygen levels. Considering these differences, it is unclear what pattern of oxygen changes will occur when these drugs are co-administered. Here, we used high-speed amperometry with oxygen sensors to examine changes in oxygen concentrations in the nucleus accumbens (NAc) induced by intravenous (iv) cocaine, heroin, and their mixtures in freely-moving rats. Cocaine delivered at a range of doses, both below (0.25 mg/kg) and within the optimal range of self-administration (0.5 and 1.0 mg/kg) modestly increased NAc oxygen levels. In contrast, heroin increased oxygen levels at a low reinforcing dose (0.05 mg/kg), but induced a biphasic down-up change at higher reinforcing doses (0.1 and 0.2 mg/kg), and caused a strong monophasic oxygen decrease during overdose (0.6 mg/kg). When combined at moderate doses, cocaine (0.25, 0.5 mg/kg) slightly increased and prolonged oxygen increases induced by heroin alone (0.5 and 0.1 mg/kg), but oxygen decreases were identical when cocaine (1 mg/kg) was combined with heroin at large doses (0.2 and 0.6 mg/kg). Therefore, health dangers of speedball may result from de-compensation of vital functions due to diminished intra-brain oxygen inflow induced by high-dose heroin coupled with enhanced oxygen use induced by cocaine.

In a Rat Model of Opioid Maintenance, the G Protein-Biased Mu Opioid Receptor Agonist TRV130 Decreases Relapse to Oxycodone Seeking and Taking and Prevents Oxycodone-Induced Brain Hypoxia.

BACKGROUND: Maintenance treatment with opioid agonists (buprenorphine, methadone) is effective for opioid addiction but does not eliminate opioid use in all patients. We modeled maintenance treatment in rats that self-administered the prescription opioid oxycodone. The maintenance medication was either buprenorphine or the G protein-biased mu opioid receptor agonist TRV130. We then tested prevention of oxycodone seeking and taking during abstinence using a modified context-induced reinstatement procedure, a rat relapse model. METHODS: We trained rats to self-administer oxycodone (6 hours/day, 14 days) in context A; infusions were paired with discrete tone-light cues. We then implanted osmotic pumps containing buprenorphine or TRV130 (0, 3, 6, or 9 mg/kg/day) and performed 3 consecutive tests: lever pressing reinforced by oxycodone-associated discrete cues in nondrug context B (extinction responding), context-induced reinstatement of oxycodone seeking in context A, and reacquisition of oxycodone self-administration in context A. We also tested whether TRV130 maintenance would protect against acute oxycodone-induced decreases in nucleus accumbens oxygen levels. RESULTS: In male rats, buprenorphine and TRV130 decreased extinction responding and reacquisition of oxycodone self-administration but had a weaker (nonsignificant) effect on context-induced reinstatement. In female rats, buprenorphine decreased responding in all 3 tests, while TRV130 decreased only extinction responding. In both sexes, TRV130 prevented acute brain hypoxia induced by moderate doses of oxycodone. CONCLUSIONS: TRV130 decreased oxycodone seeking and taking during abstinence in a partly sex-specific manner and prevented acute oxycodone-induced brain hypoxia. We propose that G protein-biased mu opioid receptor agonists, currently in development as analgesics, should be considered as relapse prevention maintenance treatment for opioid addiction.

The Role of Peripheral Opioid Receptors in Triggering Heroin-induced Brain Hypoxia.

While it is known that opioid receptors (ORs) are densely expressed in both the brain and periphery, it is widely accepted that hypoxic effects of opioids result solely from their direct action in the CNS. To examine the role of peripheral ORs in triggering brain hypoxia, we used oxygen sensors in freely moving rats to examine how naloxone-HCl and naloxone-methiodide, the latter which is commonly believed to be peripherally restricted, affect brain oxygen responses induced by intravenous heroin at low, human-relevant doses. Similar to naloxone-HCl, naloxone-methiodide at a relatively low dose (2 mg/kg) fully blocked heroin-induced decreases in brain oxygen levels. As measured by mass spectrometry, naloxone-methiodide was found to be ~40-fold less permeable than naloxone-HCl across the blood-brain barrier, thus acting as a selective blocker of peripheral ORs. Despite this selectivity, a low but detectable amount of naloxone was found in brain tissue after naloxone-methiodide administration, potentially influencing our results. Therefore, we examined the effects of naloxone-methiodide at a very low dose (0.2 mg/kg; at which naloxone was undetectable in brain tissue) and found that this drug still powerfully attenuates heroin-induced brain oxygen responses. These data demonstrate the role of peripheral ORs in triggering heroin-induced respiratory depression and subsequent brain hypoxia.

6-Monoacetylmorphine (6-MAM), Not Morphine, Is Responsible for the Rapid Neural Effects Induced by Intravenous Heroin.

Heroin rapidly enters the CNS but is quickly metabolized into 6-monoacetylmorphine (6-MAM) and then morphine. Although morphine is often thought to mediate heroin's neural effects, pharmacokinetic data question this view. To further understand the effects of heroin and its metabolites, oxygen sensors were used to examine changes in nucleus accumbens (NAc) oxygen levels. Heroin, 6-MAM, and morphine were all administered intravenously at two human-relevant doses (0.25 µmol/kg and 0.98 µmol/kg) in freely moving rats. Intravenous heroin induced a biphasic change in NAc oxygen, with a decrease resulting from respiratory depression and an increase resulting from cerebral vasodilation. 6-MAM caused similar but more rapid and slightly weaker effects  than heroin. The stronger response to heroin can be primarily attributed to heroin's permeability and metabolism resulting in more 6-MAM in the brain. Morphine only induced weak increases in NAc oxygen. Therefore, it appears that 6-MAM is the major contributor to acute neural effects induced by iv heroin.

Leakage of the blood-brain barrier followed by vasogenic edema as the ultimate cause of death induced by acute methamphetamine overdose.

Methamphetamine (METH) is a potent CNS stimulant that is widely used as a recreational drug. Due to its ability to increase bodily heat production and diminish heat loss due to peripheral vasoconstriction, METH is able to increase brain and body temperature. The hyperthermic effects of METH are potentiated when the drug is used under conditions of psycho-physiological activation and in warm ambient temperatures. In this short review, we present and discuss our data on the effects of METH on brain temperature and a number of neural parameters that characterize permeability of the blood-brain barrier (albumin immunoreactivity), glial activity (GFAP immunoreactivity), brain tissue water content, and structural abnormalities of brain cells. We demonstrate that the extent of these neural alterations strongly depends on METH-induced brain temperature elevation and they all dramatically increase following exposure to METH in warm (29°C) vs. standard (23°C) ambient temperatures. Based on these data we consider possible pathophysiological mechanisms underlying acute METH toxicity, suggesting the critical role of drug-induced brain hyperthermia, temperature-dependent leakage of the blood-brain barrier (BBB), and the development of vasogenic edema that could finally result in decompensation of vital functions and death.

Interactions of benzodiazepines with heroin: Respiratory depression, temperature effects, and behavior.

Benzodiazepines are important therapeutic drugs, but they are often abused and co-abused with opioids. Clinical evidence suggests that benzodiazepines can inhibit respiration, and when combined with the respiratory-depressive effects of opioids, may increase likelihood of death. In this study we used oxygen sensors coupled with high-speed amperometry and multi-site thermorecording to examine how intravenous (iv) midazolam, a potent benzodiazepine, modulates the brain hypoxic and temperature effects of iv heroin in freely-moving rats. Oxygen levels and brain temperature were assessed with high temporal resolution in the nucleus accumbens (NAc), an important structure in the motivational-reinforcement circuit. When administered alone, midazolam (2 mg/kg) modestly decreased NAc temperature but had no evident effects on oxygen levels in this structure. In contrast, heroin (0.4 mg/kg) induced a strong decrease in NAc oxygen that was followed by a weaker, rebound-like oxygen increase. Midazolam pretreatment did not affect heroin-induced brain hypoxia but potentiated the initial hypothermia induced by heroin. However, co-administration of these drugs potentiated the heroin-induced oxygen decrease and enhanced heroin-induced brain hypothermia. Co-administration of heroin and midazolam also resulted in enhanced locomotor inhibition and loss of motor control. This effect caused some rats to collapse, resulting in nose and mouth occlusion, which caused a secondary hypoxic phase. These results could have important implications for human drug users, as the combined use of benzodiazepines with potent opioids not only results in sustained brain hypoxia but creates conditions of loss of motor control which could result in asphyxia and death. This article is part of the Special Issue entitled 'New Vistas in Opioid Pharmacology'.

Respiratory depression and brain hypoxia induced by opioid drugs: Morphine, oxycodone, heroin, and fentanyl.

Opioid drugs are important tools to alleviate pain of different origins, but they have strong addictive potential and their abuse at higher doses often results in serious health complications. Respiratory depression that leads to brain hypoxia is perhaps the most dangerous symptom of acute intoxication with opioids, and it could result in lethality. The development of substrate-specific sensors coupled with amperometry made it possible to directly evaluate physiological and drug-induced fluctuations in brain oxygen levels in awake, freely-moving rats. The goal of this review paper is to consider changes in brain oxygen levels induced by several opioid drugs (heroin, fentanyl, oxycodone, morphine). While some of these drugs are widely used in clinical practice, they all are abused, often at doses exceeding the clinical range and often resulting in serious health complications. First, we consider some basic knowledge regarding brain oxygen, its physiological fluctuations, and mechanisms involved in regulating its entry into brain tissue. Then, we present and discuss data on brain oxygen changes induced by each opioid drug within a wide range of doses, from low, behaviorally relevant, to high, likely to be self-administered by drug users. These data allowed us to compare the effects of these drugs on brain oxygen in terms of their potency, time-course, and their potential danger when used at high doses via rapid-onset administration routes. While most data discussed in this work were obtained in rats, we believe that these data have clear human relevance in addressing the alarming rise in lethality associated with the opioid abuse.