Differential effects of mu and kappa opioid analgesics on cerebral glucose utilization in the rat

Effects of isoflurane anesthesia and intravenous morphine self-administration on regional glucose metabolism ([18 F]FDG-PET) of male Sprague-Dawley rats

Although certain drugs of abuse are known to disrupt brain glucose metabolism (BGluM), the effects of opiates on BGluM are not well characterized. Moreover, preclinical positron emission tomography (PET) studies anesthetize animals during the scan, which limits clinical applications. We investigated the effects of (i) isoflurane anesthesia and (ii) intravenous morphine self-administration (MSA) on BGluM in rats. Jugular vein cannulated adult male Sprague-Dawley rats self-administered either saline (SSA) or morphine (0.5 mg/kg/infusion, 4 h/day for 12 days). All animals were scanned twice with [¹⁸ F]-fluoro-deoxy-glucose (FDG)-PET/CT at a baseline and at 2-day withdrawal from self-administration. After the IV injection of FDG, one batch of animals (n = 14) was anesthetized with isoflurane and the other batch (n = 16) was kept awake during the FDG uptake (45 min). After FDG uptake, all animals were anesthetized in order to perform a PET/CT scan (30 min). Isoflurane anesthesia, as compared to the awake condition, reduced BGluM in the olfactory, cortex, thalamus, and basal ganglia, while increasing BGluM in the midbrain, hypothalamus, hippocampus, and cerebellum. Morphine self-administered animals exhibited withdrawal signs (piloerection and increased defecation), drug seeking, and locomotor stimulation to morphine (0.5 mg/kg) during the 2 day withdrawal. The BGluM in the striatum was increased in the MSA group as compared to the SSA group; this effect was observed only in the isoflurane anesthesia, not the awake condition. These findings suggest that the choice of the FDG uptake condition may be important in preclinical PET studies and increased BGluM in the striatum may be associated with opiate seeking in withdrawal.

Time- and dose-dependent effects of corticotropin releasing factor on cerebral glucose metabolism in rats

The time course and the relation to dose of locomotor activity and of the regional cerebral metabolic rates for glucose (rCMRglc) were measured in freely moving Sprague-Dawley rats after intracerebroventricular administration of ovine corticotropin releasing factor (oCRF). Motor activity was determined using a familiar photocage cell. rCMRglc was measured, using the quantitative autoradiographic [(14)C]2-deoxyglucose procedure, in 73 brain regions at 10, 30, 90 and 180 min after administration of oCRF 10 microg and at 90 min after oCRF 0.1, 1 and 100 microg. oCRF 10 microg increased motor activity in a sustained fashion and increased rCMRglc with different time courses throughout brain regions. In cerebellar regions rCMRglc increases peaked at 90 min and were sustained up to 180 min. In non-cerebellar regions rCMRglc increases peaked at 90 min but declined thereafter. At lower doses (0.1 and 1 microg) oCRF increased rCMRglc in fewer brain regions (1 and 5 regions affected, average increases 1% and 7%) including cerebellar areas and brainstem sensory nuclei and decreased rCMRglc in medial prefrontal cortex. At the highest dose (100 microg) oCRF induced large and widespread rCMRglc increases in cerebellar, brainstem, hypothalamic, limbic and neocortical areas (40 brain regions affected, average increase 32%). The findings indicate that cerebellar areas and brainstem nuclei are highly sensitive to oCRF and may mediate oCRF autonomic and behavioral effects.

Systemic naloxone enhances cerebral blood flow in anesthetized morphine-dependent rats

Laser-Doppler flowmetry was used to study cerebral cortical blood flow responses to morphine and naloxone in morphine-naive and -dependent rats. The experiments were performed in spontaneously breathing anesthetized rats. Morphine (10 mg/kg, i.p.) administration reduced regional cerebral blood flow in control, sham-operated and morphine-dependent rats, but the depressant effect of morphine in morphine-dependent animals was less than that in control and sham-operated groups. While naloxone (0.5 mg/kg, s.c.) had no considerable effect on regional cerebral blood flow in control and sham-operated groups, it increased regional blood flow in morphine dependent ones. The depressant effect of morphine in all groups and the enhancing effect of naloxone in morphine-dependent animals were not seen after local application of lidocaine at the recording site. This study may provide a framework to study the cellular and molecular mechanisms responsible for coupling neuronal electrical activity with regional alterations in blood flow during precipitation of morphine withdrawal.

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.

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.

Chronic escapable footshock causes a reduced response to morphine in rats as assessed by local cerebral metabolic rates

The 2-deoxy-D-[14C]glucose (2-DG) method was used to examine the effects of morphine sulfate (MS) on local cerebral metabolic rates for glucose (LCMRglu) in male F-344 rats required to turn a wheel manipulandum in order to escape from nociceptive footshock. This nociceptive stimulus was identical with that utilized in a previous 2-DG study from this laboratory [15] except that animals were exposed to 15 daily 30 min sessions of footshock prior to the 2-DG testing day rather than a single footshock exposure. This allows a direct comparison of the effects of morphine in chronic and acute pain. Unlike the acute footshock study, morphine in chronic footshock rats did not have a significant effect compared with chronic footshock alone in any of the 73 measured brain structures, including limbic and midline thalamic structures previously shown to be important in morphine-induced analgesia during acute pain [15]. Whereas 93% of measured cerebral structures showed decreases in LCMRglu following morphine administration in the acute footshock rats, morphine given to chronic footshock rats caused decreases in only 56% of the structures as compared with chronic footshock plus saline. It is hypothesized that these differential effects of morphine are due in part to a habituation to the chronic stressor such that chronic footshock rats are less stressed than acute footshock rats. Additionally, it is suggested that chronic exposure to pain produces a constant elevation of opioid peptides leading to opioid receptor downregulation and consequently morphine tolerance. These results demonstrate that, even in the presence of the same nociceptive stimulus, morphine can have widely disparate effects on brain metabolism if there are differences in the pain history of the animal.

Brain sites involved in mu-opioid receptor-mediated actions: a 2-deoxyglucose study

Brain regions that may be functionally involved in the neuropharmacological actions of mu-opioid agonists have been examined in conscious rats using the quantitative [14C]2-deoxyglucose autoradiographic technique. At 0.5 microgram and 1 microgram intracerebroventricularly the highly selective mu-opioid receptor agonist D-Ala2, MePhe4, Gly-ol5-enkephalin effected statistically significant increases as well as statistically significant decreases in regional glucose utilization: in limbic structures, such as hippocampal formation, medial amygdala and lateral septum, glucose utilization was most prominently increased after D-Ala2, MePhe4, Gly-ol5-enkephalin; glucose utilization was further increased in the lateral habenular nucleus, the hypothalamus, ventromedial nucleus and dorsal raphe; whereas decreases were found in the mamillary body and anterior thalamus. Glucose utilization in structures associated with somatosensory and nociceptive processing was increased in the central gray of the midbrain and decreased in the nucleus gelatinosus. Only increases in glucose utilization were produced by D-Ala2; MePhe4, Gly-ol5-enkephalin in brain regions involved in motor control, including the globus pallidus, the substantia nigra, pars reticulata, the nucleus ruber and the cerebellum, and brain regions involved in visual processing--the visual cortex and superior colliculus deep layer. It is concluded that this pattern of regional changes underlies the mu-opioid receptor-mediated antinociceptive-, epileptogenic-, memory- and mood-modulating actions of mu-opioid agonists.

The local cerebral metabolic effects of morphine in rats exposed to escapable footshock

The 2-deoxy-D-[1-14C]glucose (2-DG) method was used to examine the effects of morphine sulfate (MS) on local cerebral metabolic rates for glucose (LCMRglu) in male F-344 rats required to turn a wheel manipulandum in order to escape from nociceptive footshock. Four groups of rats were studied: control-saline, control-MS, footshock-saline and footshock-MS. All animals were administered MS (4 mg/kg, s.c.) or saline 7 days, 3 days and 10 min prior to the start of the 2-DG experiment. In agreement with its well-known effect on the emotional component of pain, MS administered to rats exposed to footshock caused a significant decrease in LCMRglu compared to footshock-saline rats in limbic structures such as the diagonal band of Broca, lateral septum, bed nucleus of the stria terminalis, horizontal limb of the diagonal band, habenular complex and medial amygdala. Additionally, two components of the midline thalamus with extensive connections with the limbic system, the paraventricular and paratenial thalamic nuclei, were similarly affected by morphine. Footshock caused an overall increase in cerebral metabolism as 52 of 73 measured structures demonstrated increases in activity compared to saline control; however, statistically significant effects in specific structures were limited. These results identify limbic and midline thalamic structures important in morphine-induced analgesia and indicate that footshock tends to have a generalized stimulatory effect on LCMRglu.

Distribution of effects of the kappa-opioid agonist CI-977 on cerebral glucose utilization in rat brain

The effects of the kappa-opioid agonist CI-977 upon local cerebral glucose utilization have been examined in conscious, lightly restrained rats to gain insight into the potential adverse effects of this neuroprotective agent. Cerebral glucose utilization was assessed quantitatively in 45 anatomically discrete brain regions by means of [14C]2-deoxyglucose autoradiography. The i.v. administration of CI-977 (0.03-3 mg/kg) induced relatively homogeneous patterns of altered cerebral glucose utilization with moderate statistically significant reductions (approximately 25%) being observed in 29 brain regions, and a statistically significant increase (approximately 40%) in one brain region, the lateral habenular nucleus. Glucose use throughout the entire neocortex and inferior colliculus was particularly sensitive to reduction (approximately 35%) following CI-977 administration, although there was only a limited dose dependency to the response. Minimal alterations in glucose use were observed in 15 of the 45 brain regions, particularly in the lower brain stem (e.g. superior olives, cochlear nucleus and median raphe) and forebrain limbic regions (e.g. septal nucleus, nucleus accumbens and mediodorsal thalamus). These data demonstrate that CI-977 produces widespread, anatomically organized alterations in function-related glucose use which contrast those seen previously with the NMDA receptor antagonists, thereby suggesting that CI-977 may be intrinsically safer as an in vivo neuroprotective agent.

Electron microscopic identification of lamina I axon terminations in the nucleus submedius of the cat thalamus

Ascending lamina I axons were labeled with Phaseolus vulgaris leucoagglutinin and the synaptic connections of their terminals in nucleus submedius (Sm) were studied in the electron microscope. The terminals were large, contained rounded synaptic vesicles, and were involved in complex synaptic aggregations with pre- and postsynaptic dendrites. It was observed that clustered large boutons from a single axon could contact a single dendritic shaft. These observations support a sensory role for lamina I input to Sm.

Analgesia induced by morphine injected into the pallidum

Bilateral microinjections of morphine hydrochloride (10; 20; 30 micrograms/0.5 microliter/side) or saline were aimed at three different regions of the rat globus pallidus: dorsal, medial, ventral. Before and at various intervals after intrapallidal morphine (15; 30; 60; 90; 180 min), estimation of pain threshold was made by the hot plate procedure. Dose-dependent morphine analgesia was elicited from all three regions injected. Differences between the pallidal areas as to the intensity and duration of the drug's effect were noticed. Pretreatment with subcutaneous naloxone (1 mg/kg, s.c.) inhibited the morphine (20 micrograms) analgesia elicited from the medial and dorsal pallidum; it decreased and delayed the effect of morphine injected into the ventral pallidum. The results suggest that the three pallidal areas tested are involved to a different degree (medial/dorsal greater than ventral) in the morphine analgesia mediated by opiate receptors.

Pallidal substrate of morphine-induced locomotion

Bilateral microinjections of morphine hydrochloride (5.0; 7.5; 10.0 micrograms/0.5 microliters/side) or saline were infused into 3 different regions (dorsal, medial, ventral) of the rat globus pallidus, to examine their effects on locomotor activity. Locomotor activity of each rat was measured 45 min before and 90 min after saline or morphine pallidal microinjections. Morphine induced a dose-dependent increase in locomotion. This increase in locomotion was also significantly different between the 3 pallidal regions. Pretreatment with naloxone (1 mg/kg, sc) inhibited the morphine (7.5 micrograms) hyperlocomotion elicited from all three pallidal areas. The results suggest that the entire pallidum serves as substrate of morphine hyperlocomotion mediated by opiate receptors.

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.

The role of the olfactory tubercle in the effects of cocaine, morphine and brain-stimulation reward

Using the quantitative 2-[14C]deoxyglucose autoradiographic method, local rates of glucose utilization were measured in rats after the administration of morphine or cocaine in the presence or absence of rewarding brain stimulation to the medial forebrain bundle. In animals that did not receive brain stimulation, cocaine significantly increased glucose utilization in the olfactory tubercle, medial prefrontal cortex and substantia nigra pars reticulata, whereas morphine significantly increased glucose metabolism in the olfactory tubercle only. Stimulation itself increased metabolic rates in a number of sites, such as the olfactory tubercle, nucleus accumbens, medial prefrontal cortex, ventral tegmental area and others. However, in self-stimulating animals both morphine and cocaine caused further increases in activity in the olfactory tubercle. Since the olfactory tubercle was the only structure to cause a significant increase in metabolic rate following each treatment, the results implicate this limbic structure in the rewarding effects of morphine, cocaine and brain-stimulation reward.

Clonidine attenuates increased brain glucose metabolism during naloxone-precipitated morphine withdrawal

The effect of two doses of clonidine on regional cerebral metabolic rates for glucose were measured during morphine withdrawal in rats. In the first study, 0 or 200 micrograms/kg clonidine was administered to rats subjected to naloxone-precipitated morphine withdrawal (naloxone, 0.5 mg/kg, s.c.), and to non-dependent control rats. In a second study of similar design, 0 or 20 micrograms/kg clonidine were administered. Withdrawal signs in rats subjected to naloxone-precipitated morphine withdrawal and receiving 0, 20 or 200 micrograms/kg clonidine were also assessed. Naloxone-precipitated morphine withdrawal stimulated regional cerebral metabolic rates for glucose (59 of 83 regions in study no. 1; 73 of 83 regions in study no. 2). At 200 micrograms/kg, clonidine attenuated this effect (33 of 59 regions). Although 200 micrograms/kg clonidine directly suppressed regional cerebral metabolic rates for glucose in many regions (significant main effect of clonidine), it attenuated the naloxone-precipitated morphine withdrawal effect specifically in the lateral septal nucleus, medial habenula, subiculum and gracile nucleus (significant interactions between clonidine and morphine withdrawal). The 20 micrograms/kg dose of clonidine had no statistically significant effect. In behavioral experiments, both doses of clonidine diminished withdrawal in that there was no diarrhea, fewer wet-dog shakes and less abnormal posturing. However, locomotion, grooming and jumping were increased by clonidine. Most of these effects were statistically significant only with the 200 micrograms/kg dose. The results of these studies show that clonidine reduces morphine withdrawal-induced increases in regional cerebral metabolic rates for glucose in many brain regions, irrespective of the distribution of alpha 2-adrenoceptors. Although clonidine has been thought to ameliorate morphine withdrawal by actions primarily at the locus coeruleus and central amygdala, it may play a major role in other regions as well.

Morphine-induced alterations of local cerebral glucose utilization in the basal ganglia of rats

The actions of various doses of morphine on the local cerebral glucose utilization (LCGU) were studied by means of the autoradiographic [14C]2-deoxyglucose technique. Morphine (1-15 mg/kg i.p.) decreased LCGU in most areas of the basal ganglia (caudate nucleus, globus pallidus, nucleus accumbens), but not in the substantia nigra pars compacta. LCGU was also decreased in limbic nuclei, such as septum, hippocampus and amygdala, and in most thalamic areas. In most cortical regions, a decrease was found as well. Findings in some efferent nuclei seemed of particular interest, namely in the substantia nigra pars reticulata, anteroventral and lateral nucleus of the thalamus and the subthalamic nucleus, where decreases in LCGU were found after administration of 7.5 mg/kg or sometimes lower doses, but not after 15 mg/kg of morphine. The decreases seem to reflect a general depressory effect of morphine on neuronal activity which is known from electrophysiological studies. Part of these effects might be, in addition, due to an activation of dopaminergic neurons, since dopamine mainly acts as an inhibitory neurotransmitter. This dopaminergic activation leads to characteristic behavioral effects after lower doses of morphine. The largest dose used (15 mg/kg) produces muscular rigidity, probably by a direct action on the striatum. This effect antagonizes and masks the dopaminomimetic effects. The results suggest that it also antagonizes the functional alterations in some efferent nuclei of the basal ganglia manifest after lower doses of morphine. Local injections of morphine (15 micrograms) led to decreases of LCGU in the various parts of the striatum, but to increases in lateral and anteroventral thalamus.(ABSTRACT TRUNCATED AT 250 WORDS)

Involvement of nigrotecto-reticulospinal pathways in the iminodipropionitrile (IDPN) model of spasmodic dyskinesias: a 2-deoxy-D-[1-14C]glucose study in the rat

Chronic administration of iminodipropionitrile (IDPN) to rats causes a persistent behavioral syndrome characterized by lateral and vertical head twitches, random circling, and increased tactile and acoustic startle responses. In order to identify brain areas which are affected in rats manifesting this syndrome, we used the autoradiographic 2-deoxy-D-[1-14C]glucose ([14C]DG) method to map cerebral glucose utilization in IDPN-treated rats. One day after the development of the dyskinetic syndrome, there were significant decreases in local glucose utilization in the substantia nigra pars reticulata (SNr) and compacta (SNc), the dorsal raphe, the superficial and deep layers of the superior colliculus, the inferior colliculi, the interpeduncular nucleus, the medial and dorsolateral geniculate nuclei, and the superior and lateral vestibular nuclei. There were also significant decreases in layer 2 of the cingulate cortex and in the temporal and occipital cortices. In contrast, there were no changes in the motor cortex, the caudate-putamen, the nucleus accumbens, or the median raphe. These findings suggest that deleterious effects of IDPN on the nigrotectal pathways which affect head and neck movements and circling behaviors via the brainstem reticulospinal tracts may play an important role in the IDPN-induced persistent spasmodic dyskinetic syndrome in rats.

Limbic brain structures are important sites of kappa-opioid receptor-mediated actions in the rat: a [14C]-2-deoxyglucose study

The [1-14C]-2-deoxyglucose technique was employed to evaluate the regional pattern of alterations in glucose utilization in the rat brain, pituitary and spinal cord induced by the selective kappa-opioid agonist U-50,488H (trans-3,4-dichloro-N-methyl-N[2-(1-pyrolidinyl) cyclohexyl]-benzeneacetamide). Within the dose range used (0.5-5 mg/kg), U-50,488H produced a dose-dependent attenuation of nociceptive thresholds and a place aversion in the place conditioning test, allowing for a correlation of the regional pattern of changes in glucose utilization with certain behavioral responses. The regional changes in glucose utilization induced by U-50,488H in the brain were most pronounced in components of the limbic forebrain circuit such as the anterior thalamic nuclei, mammillary body, frontal cortex, lateral septal nucleus, nucleus accumbens and lateral habenular nucleus as well as in the brainstem tegmental nuclei and the dorsal and median raphe nucleus (components of the limbic midbrain area). Glucose utilization was decreased in the frontal cortex and increased in the other regions. An increase in glucose utilization also was observed in the central gray pons. Increases in glucose utilization in the pituitary were restricted to the intermediate lobe. In the lumbar part of the spinal cord, glucose metabolism was enhanced in the region around the central canal and in the ventral horn. The changes in glucose metabolism observed in these structures suggest that the aversive (dysphoric) effects of U-50,488H may be due to the altered activity of the limbic structures of the forebrain and midbrain which have been implicated in emotional and affective processes. The increased activity in the intermediate lobe of the pituitary, furthermore, might reflect a stress component in the effects of this drug. Since the dorsal raphe nucleus and the region of the central gray pons have been implicated in both analgesia and pain processes a supraspinal site of antinociceptive action of U-50,488H, in addition to a spinal site of action, must be considered.

Naloxone fails to alter local cerebral glucose utilization in the rat

The autoradiographic, 2-deoxy-D-[1-14C]glucose ([14C]DG) method was used to map the effects of intravenous (IV) naloxone (1.0, 10.0, and 20.0 mg/kg) on local cerebral glucose utilization (LCGU), an index of local brain function. Naloxone injected 5 min before [14C]DG did not alter LCGU in any of the fifty-six brain regions examined. Our findings suggest that acute naloxone at these doses does not significantly affect cerebral metabolism.

Effect of heroin-conditioned auditory stimuli on cerebral functional activity in rats

Cerebral functional activity was measured as changes in distribution of the free fatty acid [1-14C]octanoate in autoradiograms obtained from rats during brief presentation of a tone previously paired to infusions of heroin or saline. Rats were trained in groups of three consisting of one heroin self-administering animal and two animals receiving yoked infusions of heroin or saline. Behavioral experiments in separate groups of rats demonstrated that these training parameters imparts secondary reinforcing properties to the tone for animals self-administering heroin while the tone remains behaviorally neutral in yoked-infusion animals. The optical densities of thirty-seven brain regions were normalized to a relative index for comparisons between groups. Previous pairing of the tone to heroin infusions irrespective of behavior (yoked-heroin vs. yoked-saline groups) produced functional activity changes in fifteen brain areas. In addition, nineteen regional differences in octanoate labeling density were evident when comparison was made between animals previously trained to self-administer heroin to those receiving yoked-heroin infusions, while twelve differences were noted when comparisons were made between the yoked vehicle and self administration group. These functional activity changes are presumed related to the secondary reinforcing capacity of the tone acquired by association with heroin, and may identify neural substrates involved in auditory signalled conditioning of positive reinforcement to opiates.