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Differences in extracellular dopamine concentrations in the nucleus accumbens during response-dependent and response-independent cocaine administration in the rat.

Abstract Studies indicate that nucleus accumbens (NAcc) dopamine neurotransmission is involved in the reinforcing and direct effects of cocaine. The present study was initiated to explore further the relationship of NAcc extracellular dopamine concentrations ([DA]e) and cocaine self-administration using a yoked littermate design. In the first experiment, one rat from each litter was trained to self-administer cocaine IV (SA; 0.33 mg/inf) under a fixed ratio 2 schedule, while a second rat received simultaneous infusions of cocaine yoked to the infusions of the SA (YC). NAcc [DA]e and cocaine concentrations ([COC]) were assessed during the test sessions using in vivo microdialysis combined with microbore HPLC procedures. [DA]e and [COC] were significantly elevated in the SA and YC groups during the self-administration session; however, [DA]e were greater in the SA group compared to the YC group in the first hour of the session, even though [COC] were not significantly different. On the following day, the rats previously allowed to self-administer cocaine were administered response-independent cocaine infusions yoked to the infusion pattern from the previous day. [DA]e were significantly elevated above baseline levels during the session but were significantly less than concentrations obtained when cocaine was self-administered by these subjects. [COC] during the sessions were not significantly different between the two days. Baseline [DA]e were not significantly different between the SA and YC groups or between Day 1 and Day 2. Furthermore, there was no significant difference in the in vitro probe recovery between one and two days following probe implantation. These results suggest that the context in which cocaine was administered significantly altered the neurochemical response to equivalent brain concentrations of cocaine. NAcc [DA]e was significantly increased when the delivery of cocaine infusions was contingent on the behavior of the rat, indicative of a role in the neural processes underlying cocaine reinforcement.

Rat brain neurotransmitter turnover rates altered during withdrawal from chronic cocaine administration.

This experiment utilized neurotransmitter turnover rates to assess the effects of withdrawal from chronic cocaine on the brain. A triad-littermate design was used to evaluate the importance of response dependency on the effects of withdrawal from chronic cocaine administration upon brain biogenic monoamine and amino acid putative neurotransmitter turnover rates. Each member of a triad was exposed to one of three conditions. Cocaine infusions (0.33 mg/inf) were used to engender and maintain lever pressing by one rat under an FR 2 schedule, while the second and third rats received simultaneous infusions of either cocaine or saline, respectively. After a minimum of 15 days exposure to the three treatment conditions and 24 h after the start of the last drug session, the triads were pulse labeled with [14C]glucose, [3H]tyrosine and [3H]tryptophan and killed 60 or 90 min later by total immersion in liquid nitrogen, The frozen brains were removed and dissected at -20 degrees C into 22 areas. The content and specific radioactivities for dopamine (DA), noradrenaline (NA), serotonin (5-HT), aspartate (Asp), glutamate (Glu), glycine (Gly) and gamma-aminobutyric acid (GABA) were determined in each brain region using high pressure liquid chromatography with electrochemical (biogenic monoamines) or fluorescence (amino acids) detection followed by liquid scintillation spectrometry. Turnover rates (TOR) were calculated and compared across treatment conditions. The significant decreases in TOR resulting from chronic cocaine exposure included 5-HT in the frontal cortex, DA in the cingulate cortex, entorhinal-subicular and motor-somatosensory cortices and NA in the inferior colliculus. Significant increases in TOR were also observed which included 5-HT in the preoptic-diagonal band region, DA in the hippocampus and NA in the pyriform and temporal-auditory cortices, the dentate gyrus and brainstem. GABA TOR was also increased in the preoptic-diagonal band region, dentate gyrus and brainstem of both groups receiving cocaine as was Glu TOR in the pyriform cortex and cerebellum. In addition, changes were seen in the rats under the ratio schedule of cocaine self-administration that were not seen in rats receiving yoked-cocaine infusions that included increased TOR of 5-HT in the pyriform cortex, NA in the caudate-putamen and GABA in the pyriform and motor-somatosensory cortices. Decreased 5-HT TO was seen in the motor-somatosensory cortex and dentate gyrus in the rats that had self-administered cocaine compared to the yoked-cocaine infused group. Perhaps the most interesting changes were those seen in the yoked-cocaine group that were reversed in the rats whose responding was maintained by cocaine.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of 6-OHDA lesions of the central medial nucleus accumbens on rat intravenous morphine self-administration

The function of dopaminergic innervations of the central medial nucleus accumbens in the processes maintaining intravenous morphine self-administration was assessed by lesioning with 6-OHDA and comparing drug intake with sham-vehicle treated littermates. Localized bilateral lesions of this structure resulted in significant increases in morphine intake shifting the dose-effect relationship to the right with twice the dose necessary to maintain prelesion rates of self-administration. Content of dopamine and dihydroxyphenylacetic acid was decreased in the nucleus accumbens after the lesion, but unchanged in the adjacent pyriform cortex and anterior caudate nucleus-putamen, while serotonin was significantly decreased in the pyriform cortex. High affinity uptake measurements also suggested nucleus accumbens dopaminergic and pyriform cortex serotonergic innervations to be affected by the lesion. The shift to the right in the dose effect relationship after the lesion suggests these neuronal systems to be excitatory to the processes mediating self-administration.

Lack of an effect of 6-hydroxydopamine lesions of the nucleus accumbens on intravenous morphine self-administration

The neurotoxin, 6-hydroxydopamine (6-OHDA), has been used to selectively destroy dopamine containing neurons in discrete brain regions. Lesions of the nucleus accumbens with this neurotoxin decrease or eliminate cocaine and amphetamine self-administration and either increase or do not affect opiate self-administration in rats with unrestricted access to food and water. This study reports the effects of 6-OHDA lesions of the nucleus accumbens on responding maintained by food, water or morphine (3.3 mg/infusion). Six male rats with continuous access to three response levers were trained on a concurrent chained, fixed-ratio 1, fixed-ratio 9 schedule of reinforcer presentation. After stable patterns of responding were maintained by the three reinforcers, dose-effect curves for morphine were determined by substituting other doses of morphine or vehicle for 24-hour periods. Bilateral sham vehicle or 6-OHDA lesions of the nucleus accumbens were then completed and the effects of the lesion on food, water and morphine intake determined. Dose-effect evaluations were repeated after the lesion. The 6-OHDA lesions did not significantly affect responding maintained by food, water or morphine. The absence of an effect is most likely not the result of an insensitive baseline since other neurotoxin lesions produce long-term and selective decrements in morphine self-administration without affecting food and water responding. Like so many other manipulations, the magnitude of the effect that a neurotoxin lesion can exert on behavior may depend on the specific procedures that are used to maintain responding.

Brain neurotransmitter turnover correlated with morphine-seeking behavior of rats.

Neurochemical substrates of intravenous opiate self-administration were investigated in rats using littermate controls for vehicle and passive morphine infusion. The rates of turnover of the putative neurotransmitters, dopamine, norepinephrine, serotonin, gamma-aminobutyric acid, aspartate and glutamate were concurrently measured in eleven brain regions of rats intravenously self-administering morphine and yoked-morphine or yoked-vehicle infused littermates. The passive infusion of morphine resulted in significant changes in the rates of turnover of the biogenic monoamine and amino acid neurotransmitters in six brain regions with the caudate nucleus-putamen-globus pallidus showing the most changes. The contingent infusion of morphine resulted in changes in utilization rates that were generally greater in both magnitude and number than the effects of the drug itself. Twenty-nine significant changes were observed in the self-administering group with most changes occurring in limbic structures. The neurotransmitter turnover rate changes resulting from contingent administration suggest that the drug administration environment is an important factor that should be considered in studies of interactions between drugs and neuronal systems.

Ventral pallidal extracellular fluid levels of dopamine, serotonin, gamma amino butyric acid, and glutamate during cocaine self-administration in rats.

Abstract. Rationale :Dopamine innervation of the nucleus accumbens is thought to have a major role in the biological processes underlying cocaine self-administration. Recent data suggest that dopamine innervation of the ventral pallidum (VP) may also play an important role. Objectives: This experiment was initiated to assess extracellular fluid levels of dopamine (DA), serotonin (5-HT), gamma-aminobutyric acid (GABA), and glutamate (Glu) in the VP of rats self-administering cocaine using in vivo microdialysis. Methods: Rats were implanted with intravenous jugular catheters and a microdialysis probe guide cannula into the VP and trained to self-administer (SA) three different doses of cocaine during each daily session. Other rats (yoked rats) were surgically prepared in identical fashion and received vehicle infusions during microdialysis sessions when the SA rat to whom they were yoked produced cocaine infusions. When stable baselines of self-administration were obtained, microdialysates were collected during two consecutive daily self-administration sessions. Neurotransmitter levels were measured using HPLC with electrochemical (DA and 5-HT) or fluorescence detection (GABA and Glu). Results: In SA rats, extracellular fluid levels of DA [DA]e and 5-HT [5-HT]e were elevated throughout the session and levels of Glu [Glu]e showed small increases at a few isolated time points during the session. The increases in [DA]e and [5-HT]e were dose-dependent. Extracellular fluid levels of GABA [GABA]e were unchanged, as were levels of all four neurotransmitters in the yoked rats. Conclusions: These data support a potential role for DA and 5-HT innervations of the VP in intravenous cocaine self-administration.

Biogenic monoamine turnover in discrete rat brain regions is correlated with conditioned emotional response and its conditioning history

The content and turnover of dopamine, norepinephrine and 5-hydroxytryptamine (serotonin), and the content of their respective major metabolites were evaluated in 19 discrete brain areas of rats exposed to conditioned emotional response (CER), and in control groups which received either equivalent yoked shock (shock only) or compound stimulus presentation (tone only). On test day, CER animals suppressed responding and exhibited forms of emotional behavior after presentation of the conditioned stimulus (CS); while shock only and tone only control groups, and CER animals which received an acute dose of diazepam prior to testing, did not suppress. Few changes were observed in content of the biogenic amines or their metabolites, suggesting that the behavioral manipulations were acting within normal physiological limits. On the other hand, numerous changes were observed in the utilization of the 3 biogenic monoamines, which were correlated with the conditioning-anxiety (comparisons of CER vs shock only) and the shock history (comparison of shock only vs tone only). These observations are consistent with putative neural pathways in the frontal cortex, septum, nucleus accumbens, amygdala, striatum, hippocampus and brain stem (which utilize specific monoamines), and with discrete brain areas which have been implicated in classical conditioning and CER-related phenomena. These observations suggest roles for biogenic monoamines in mediating or responding to the classical conditioning and emotional components of the paradigm.

Involvement of cholinergic neuronal systems in intravenous cocaine self-administration.

Recent studies suggest the participation of cholinergic neurons in the brain processes underlying reinforcement. The involvement of cholinergic neurons in cocaine self-administration has been recently demonstrated in studies using muscarinic and nicotinic agonists and antagonists, microdialysis, assessment of choline acetyltransferase activity and acetylcholine (ACh) turnover rates. The present experiment was initiated to identify subsets of cholinergic neurons involved in the brain processes that underlie cocaine self-administration by lesioning discrete populations with a selective neurotoxin. Rats were trained to self-administer cocaine and the cholinergic neurotoxin 192-IgG-saporin or vehicle was then bilaterally administered into the posterior nucleus accumbens (NAcc)-ventral pallidum (VP). The 192-IgG-saporin induced lesions resulted in a pattern of drug-intake consistent with either a shift in the dose intake relationship to the left or downward compared to sham-treated controls. A second experiment used a self-administration threshold procedure that demonstrated this lesion shifted the dose intake relationship to the left compared to the sham-vehicle treated rats. The magnitude and extent of the lesion was assessed by measuring the expression of p75 (the target for 192-IgG-saporin) and choline acetyltransferase (ChAT) in the NAcc, VP, caudate nucleus-putamen (CP) and vertical limb of the medial septal nucleus-diagonal band (MS-DB) of these rats using real time reverse transcriptase-polymerase chain reaction. Significant reductions in gene expression for p75 (a selective marker for basal forebrain cholinergic neurons) and ChAT were seen in the MS-DB and VP while only small decreases were seen in the NAcc and CP of the 192-IgG-saporin treated rats. These data indicate that the overall influence of cholinergic neurons in the MS-DB and VP are inhibitory to the processes underlying cocaine self-administration and suggest that agonists directed toward subclasses of cholinergic receptors may have efficacy as pharmacotherapeutic adjuncts for the treatment of cocaine abuse.

Use of a single compartment LCEC cell in the determinations of biogenic amine content and turnover.

Content and specific radioactivity of the biogenic monoamines and content of their precursors and metabolites were simultaneously determined in CNS tissue extracts with a high pressure liquid chromatography system (HPLC). The content of dopamine, norepinephrine, serotonin. 5-hydroxyindoleacetic acid, homovanillic acid, 3,4-dihydroxyphenylacetic acid and 4-hydroxy-3-methoxyphenylethyleneglycol and the turnover rate of dopamine, norepinephrine and serotonin were measured in discrete rat brain regions using a one compartment electrochemical detector cell coupled to a C18-reverse phase HPLC column. The small fluid volume dead space of the cell allows the direct and precise collection of individual peaks for determining specific radioactivities. This method is especially suitable for central nervous system tissue samples from 8-20 mg wet weight and the sensitivity of the system in its routine configuration is approximately 2 pmol. This method for determinations of turnover is appropriate for investigations of animals in sensitive behavioral paradigms.U

Brain neurotransmitter turnover rates during rat intravenous cocaine self-administration

The turnover rates of dopamine, norepinephrine, serotonin, aspartate, glutamate and GABA were measured in 27 brain regions of rats self-administering cocaine and in yoked cocaine- and yoked vehicle-infused controls using radioactive pulse-labeling procedures to identify brain neuronal systems underlying self-administration. Changes in the activity of heretofore unrecognized dopamine, norepinephrine, serotonin, glutamate and GABA innervations of the forebrain specific to cocaine self-administration were found. This included innervations of the nucleus accumbens, ventral pallidum, lateral hypothalamus and the anterior and posterior cingulate, entorhinal-subicular and visual cortices. Turnover rates also were calculated using metabolite/neurotransmitter ratios which were inconsistent with the pulse-label technologies indicating that ratio procedures are not accurate measures of neurotransmitter utilization. Results with the pulse-label technique provide evidence of the involvement of neuronal systems in cocaine self-administration not previously known, some of which may have a broader role in brain reinforcement processes for natural reinforcers (i.e. food, water, etc.) since drugs of abuse are thought to produce reinforcing effects by modulating activity in these endogenous systems.