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

Brain Hyperglycemia Induced by Heroin: Association with Metabolic Neural Activation

Glucose enters the brain extracellular space from arterial blood, and its proper delivery is essential for metabolic activity of brain cells. By using enzyme-based biosensors coupled with high-speed amperometry in freely moving rats, we previously showed that glucose levels in the nucleus accumbens (NAc) display high variability, increasing rapidly following exposure to various arousing stimuli. In this study, the same technology was used to assess NAc glucose fluctuations induced by intravenous heroin. Heroin passively injected at a low dose optimal for maintaining self-administration behavior (100 μg/kg) induces a rapid but moderate glucose rise (∼150-200 μM or ∼15-25% over resting baseline). When the heroin dose was doubled and tripled, the increase became progressively larger in magnitude and longer in duration. Heroin-induced glucose increases also occurred in other brain structures (medial thalamus, lateral striatum, hippocampus), suggesting that brain hyperglycemia is a whole-brain phenomenon but changes were notably distinct in each structure. While local vasodilation appears to be the possible mechanism underlying the rapid rise in extracellular glucose levels, the driving factor for this vasodilation (central vs peripheral) remains to be clarified. The heroin-induced NAc glucose increases positively correlated with increases in intracerebral heat production determined in separate experiments using multisite temperature recordings (NAc, temporal muscle and skin). However, glucose levels rise very rapidly, preceding much slower increases in brain heat production, a measure of metabolic activation associated with glucose consumption.

The effect of morphine on regional cerebral blood flow measured by 99mTc-ECD SPECT in dogs

To gain insights into the working mechanism of morphine, regional cerebral blood flow (rCBF) patterns after morphine administration were assessed in dogs. In a randomized cross-over experimental study, rCBF was estimated with ^99mTc-Ethylcysteinate Dimer single photon emission computed tomography in 8 dogs at baseline, at 30 minutes and at 120 minutes after a single bolus of morphine. Perfusion indices (PI) in the frontal, parietal, temporal and occipital cortex and in the subcortical and cerebellar region were calculated. PI was significantly decreased 30 min after morphine compared to baseline in the right frontal cortex. The left parietal cortex and subcortical region showed a significantly increased PI 30 min after morphine compared to baseline. No significant differences were noted for the other regions or at other time points. In conclusion, a single bolus of morphine generated a changing rCBF pattern at different time points.

Interaction of adenosine and naloxone on regional cerebral blood flow in morphine-dependent rats

The present research aimed at investigating the opioid-adenosine interaction on regional cerebral blood flow (rCBF). Therefore rCBF in the sensory cortex of morphine-naive and -dependent rats was measured using the laser-Doppler flowmetry technique. The results showed that adenosine (10(-5), 10(-4), 10(-3) M) significantly increased rCBF in morphine-dependent rats (MDR) (P < 0.01). This effect was inhibited by theophylline (5 x 10(-5) M). Also systemic naloxone (0.5, 1.5 and 3 mg/kg, s.c.) significantly increased rCBF in MDR and it was accompanied by elevated blood pressure and heart rate. Local adenosine (10(-4) M) significantly augmented naloxone (0.5 mg/kg)-induced increase in rCBF of MDR but had no significant effect on naloxone's (1.5 and 3 mg/kg) increasing effect on rCBF. Theophylline also has no effect on naloxone increasing effect on rCBF. These data suggest that adenosine receptors responsiveness increase in sensory cortex of MDR. Naloxone also highly increased rCBF of MDR that probably not interfere with adenosine receptors. Also, it seems that adenosine acts as a modulator in rCBF regulation of morphine-dependent and morphine withdrawal rats.

Brain hyperthermia as physiological and pathological phenomena

Although brain metabolism consumes high amounts of energy and is accompanied by intense heat production, brain temperature is usually considered a stable, tightly "regulated" homeostatic parameter. Current research, however, revealed relatively large and rapid brain temperature fluctuations (3-4 degrees C) in animals during various normal, physiological, and behavioral activities at stable ambient temperatures. This review discusses these data and demonstrates that physiological brain hyperthermia has an intra-brain origin, resulting from enhanced neural metabolism and increased intra-brain heat production. Therefore, brain temperature is an important physiological parameter that both reflects alterations in metabolic neural activity and affects various neural functions. This work also shows that brain hyperthermia may be induced by various drugs of abuse that cause metabolic brain activation and impair heat dissipation. While individual drugs (i.e., heroin, cocaine, methamphetamine, MDMA) have specific, dose-dependent effects on brain and body temperatures, these effects are strongly modulated by an individual's activity state and environmental conditions, and change dramatically during the development of drug self-administration. Thus, brain thermorecording may provide new information on the central effects of various addictive drugs, drug-activity-environment interactions in mediating drugs' adverse effects, and alterations in metabolic neural activity associated with the development of drug-seeking and drug-taking behavior. While ambient temperatures and impairment of heat dissipation may also affect brain temperature, these environmental conditions strongly potentiate thermal effects of psychomotor stimulant drugs, resulting in pathological brain overheating. Since hyperthermia exacerbates drug-induced toxicity and is destructive to neural cells and brain functions, use of these drugs under activated conditions that restrict heat loss may pose a significant health risk, resulting in both acute life-threatening complications and chronic destructive CNS changes.

Brain and body hyperthermia associated with heroin self-administration in rats

Intravenous heroin self-administration in trained rats was accompanied by robust brain hyperthermia (+2.0-2.5 degrees C); parallel changes were found in the dorsal and ventral striatum, mediodorsal thalamus, and deep temporal muscle. Temperature began to increase at variable latency after a signal of drug availability, increased reliably (approximately 0.4 degrees C) before the first lever press for heroin, increased further (approximately 1.2 degrees C) after the first heroin injection, and rose more slowly after the second and third injections to stabilize at an elevated plateau (39-40 degrees C) for the remainder of the session. Brain and body temperature declined slowly when drug self-administration was terminated; naloxone precipitated a much more rapid decrease to baseline levels. Changes in temperature were similar across repeated daily sessions, except for the increase associated with the first self-administration of each session, which had progressively shorter latency and greater acceleration. Despite consistent biphasic fluctuations in movement activity associated with heroin self-administrations (gradual increase preceding the lever press, followed by an abrupt hypodynamia after drug infusion), mean brain temperature was very stable at an elevated plateau. Only mean muscle temperature showed evidence of biphasic fluctuations (+/-0.2 degrees C) that were time locked to and correlated with lever pressing and associated movements. Drug- and behavior-related changes in brain temperature thus appear to reflect some form of neuronal activation, and, because temperature is a factor capable of affecting numerous neural functions, it may be an important variable in the control of behavior by drugs of abuse.

Is there tonic activity in the endogenous opioid systems? A c-Fos study in the rat central nervous system after intravenous injection of naloxone or naloxone-methiodide

This study examined the possibility that a tonic activity in the endogenous opioid systems (EO systems) exists in animals under normal conditions. In a first set of experiments, concurrent changes in behavioral responses and in the numbers of c-Fos-like immunoreactive (Fos-LI) neurons in 58 structures of the brain and lumbosacral spinal cord were analyzed in rats after systemic administration of the opioid antagonist naloxone (NAL; 2 mg/kg). Possible roles of the EO systems were inferred from changes in the numbers of Fos-LI neurons between normal rats that received either NAL or the same volume of saline. Free-floating sections were processed immunohistochemically for c-Fos protein using standard avidin-biotin complex methods. After NAL, the numbers of Fos-LI neurons were significantly increased in the area postrema; in the caudal, intermediate, and rostral parts of the nucleus tractus solitarii; in the rostral ventrolateral medulla; in the Kölliker-Fuse nucleus; in the supramammillary nucleus; and in the central nucleus of the amygdala. In a second set of experiments examining changes in c-Fos expression in the latter structures, similar increases were found after NAL but not after an equimolar dose of NAL-methiodide, a preferential, peripherally acting opioid receptor antagonist. Therefore, Fos-LI was likely triggered after blockade of central opioid receptors, but not peripheral opioid receptors, releasing neurons from EO system-mediated inhibition. The results of this study suggest the existence of a tonic activity of the EO systems exerted on a restricted number of brain regions in normal rats. This tonic activity of the EO systems may control part of the neural networks involved in cardiorespiratory functions and in emotional and learning processes.

Alteration of local cerebral glucose utilization following intravenous administration of heroin in Fischer 344 rats

The 2-deoxyglucose method was used to study the effects of acute administration of small intravenous doses of heroin on rates of glucose utilization in rat brain to identify small brain regions that may be involved in the acute behavioral effects of heroin. In contrast to previous studies which have used relatively large doses, the doses of heroin used in this study have been shown to be self-administered [Martin, T.J., Dworkin, S.I. and Smith, J.E., Alkylation of mu-opioid receptors by beta-funaltrexamine in vivo: comparison of the effects on in situ binding and heroin self-administration in rats., J. Pharmacol. Exp. Ther., 272 (1995) 1135-1140.]. Administration of 18 microg/kg of heroin resulted in higher rates of glucose utilization in the medial olfactory tubercle, anterior nucleus accumbens and dorsolateral caudate while having no other effects on limbic structures compared to saline-treated animals. Conversely, the rate of glucose utilization was lower than control in the habenula, dorsal raphe, and central gray following adminstration of 18 microg/kg of heroin. Administration of two higher doses (60 and 100 microg/kg) resulted in lower rates of glucose utilization in the thalamus, habenula, inferior colliculus, dorsal raphe and central gray compared to saline. The higher rates of glucose utilization in the limbic areas were specific for the lowest dose of heroin, whereas the effect of lowering the rate of glucose utilization compared to control in the thalamus and inferior colliculus were an increasing function of dose. In the habenula and dorsal raphe, however, the dose-effect function was inverted. These data indicate that the alterations of glucose utilization in rat brain by heroin are site-specific and the systems involved as well as the nature of the alteration differs for individual doses of heroin.

Endogenous opiates: 1991

This paper is the fourteenth installment of our annual review of research concerning the opiate system. It includes papers published during 1991 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal and renal function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.