Concurrent pharmacological MRI and in situ microdialysis of cocaine reveal a complex relationship between the central hemodynamic response and local dopamine concentration.

The mechanisms underlying the signal changes observed with pharmacological magnetic resonance imaging (phMRI) remain to be fully elucidated. In this study, we obtained microdialysis samples in situ at 5-min intervals during phMRI experiments using a blood pool contrast agent to correlate relative cerebral blood volume (rCBV) changes with changes in dopamine and cocaine concentrations following acute cocaine challenge (0.5 mg/kg iv) in the rat over a duration of 30 min. Three brain areas were investigated: the dorsal striatum (n = 8), the medial prefrontal cortex (mPFC; n = 5), and the primary motor cortex (n = 8). In the striatum and mPFC groups, cocaine and dopamine temporal profiles were tightly correlated, peaking during the first 5-min period postinjection, then rapidly decreasing. However, the local rCBV changes were uncorrelated and exhibited broader temporal profiles than those of cocaine and dopamine, attaining maximal response 5-10 min later. This demonstrates that direct vasoactivity of dopamine is not the dominant component of the hemodynamic response in these regions. In the motor cortex group, microdialysis revealed no local change in dopamine in any of the animals, despite large local cocaine increase and strong rCBV response, indicating that the central hemodynamic response following acute iv cocaine challenge is not driven directly by local dopamine changes in the motor cortex. The combination of phMRI and in situ microdialysis promises to be of great value in elucidating the relationship between the phMRI response to psychoactive drugs and underlying neurochemical changes.

Reinforcing and locomotor stimulant effects of cocaine are absent in mGluR5 null mutant mice.

Both ionotropic and metabotropic glutamate receptors (mGluRs) are involved in the behavioral effects of pyschostimulants; however, the specific contributions of individual mGluR subtypes remain unknown. Here we show that mice lacking the mGluR5 gene do not self-administer cocaine, and show no increased locomotor activity following cocaine treatment, despite showing cocaine-induced increases in nucleus accumbens (NAcc) dopamine (DA) levels similar to wild-type (WT) mice. These results demonstrate a significant contribution of mGlu5 receptors to the behavioral effects of cocaine, and suggest that they may be involved in cocaine addiction.

Intrastriatal injection of D1 or D2 dopamine agonists affects glucose utilization in both the direct and indirect pathways of the rat basal ganglia.

Two distinct pathways are thought to connect the striatum to the basal ganglia output nuclei: a direct pathway, originating from neurons bearing dopamine, D(1) receptors and an indirect pathway, originating from neurons expressing D(2) receptors. It has been recently suggested, however, that dopamine receptor sub-types may co-localize and co-operate in the striatum. We sought to verify the functional segregation of the two pathways by measuring cerebral glucose utilization following intrastriatal injection of selective D(1) (SKF 38393), D(2) (quinpirole), or non-selective indirect (amphetamine) and direct (apomorphine) dopamine agonists, in freely-moving rats. All drugs -- regardless of receptor selectivity -- reduced glucose utilization in nuclei of both the direct and indirect pathways, thus lending further support to the existence of a functional co-operation of striatal D(1) and D(2) receptors.

Differential effects of cocaine on local cerebral glucose utilization in the mouse and in the rat.

The aim of the study was to determine whether cocaine causes a different pattern of functional changes in the rat as compared to the mouse. The [(14)C]2-deoxyglucose method, for measuring local cerebral metabolic rates for glucose, was carried out in Sprague-Dawley rats and in two strains of mice, C57BL/6 and DBA/2, following a single intravenous administration of cocaine. Cocaine, according to previous reports, increased glucose utilization in the nucleus accumbens of the rat, while the drug decreased metabolic rates in most of brain areas of both strains of mice. The post-hoc analysis, however, suggests that the pattern of metabolic changes differ in the two strains. In particular, the effect on the shell of the nucleus accumbens was present in the C57, but not in the DBA mice. As the C57 mice are more likely than DBA to initiate cocaine self-administration, the effect on the nucleus accumbens support the role of the mesolimbic pathway in mediating the motivational properties of psychostimulants.

Metabolic mapping of the effects of WIN 55212-2 intravenous administration in the rat.

The [14C]2-deoxyglucose method was applied to measure the effects of the synthetic cannabinoid agonist WIN 55212-2 intravenous administration on glucose utilization in the rat brain. Two doses of the drug, that have been reported previously to increase extracellular dopamine concentrations in the shell of the nucleus accumbens, were used (0.15-0.30 mg/kg). At the lower dose, WIN 55212-2 increased energy metabolism selectively in the accumbens shell. Conversely, the higher dose of the drug reduced glucose utilization in the hippocampal formation and ventromedial thalamic nucleus without affecting energy metabolism in the accumbens shell. These results may be useful to further understanding the addictive potentials of cannabinoid drugs.

Parallel strain-dependent effect of amphetamine on locomotor activity and dopamine release in the nucleus accumbens: an in vivo study in mice.

Vulnerability to develop drug abuse could be related to differential sensitivity to some central effects of such drugs. Several results point to mesoaccumbens dopamine release elicited by psychostimulants as the rate-limiting factor of their reinforcing, hence addictive, effects and to locomotor stimulation as an indirect index of such a response. In this paper, we report parallel differences in sensitivity to amphetamine-induced locomotor stimulation and mesoaccumbens dopamine release in two inbred strains of mice characterized by differential susceptibility to develop drug self-administration. Thus, mice of the C57BL/6 strain responded with a simultaneous increase of locomotor activity and mesoaccumbens dopamine release measured by intracerebral microdialysis to amphetamine challenge. On the contrary, mice of the DBA/2 strain did not present either response. No strain differences in mesoaccumbens dopamine outflow or 3,4-dihydroxyphenylacetic acid concentration were found in basal conditions or following saline challenges. However, mice of the C57BL/6 strain were characterized by higher levels of accumbal homovanillic acid in basal conditions, in line with the results obtained in rats rendered more sensitive to the locomotor effects of psychostimulants by repeated administration. Finally, in both strains amphetamine decreased accumbal levels of the two metabolites. These results suggest that genotype modulates the locomotor effects of amphetamine through sensitivity of the mesoaccumbens system to amphetamine-stimulated dopamine release. Moreover, they provide a basis to test the hypothesis of mesoaccumbens dopamine involvement in individual susceptibility to the addictive effects of drugs by quantitative trait loci analysis in recombinant inbred strains.

Mapping of functional changes associated with administration of substances of abuse in the rat.

The [14C]2-deoxyglucose method allows us to investigate whether a drug affects cerebral functional activity in selected brain regions or along discrete neural pathways. Employment of the method in laboratory animals has shown that substances of abuse produce a wide variety of cerebral functional changes which depend upon a number of different variables including the kind of substance, the dose, the modality of administration, and the context. In spite of such a variability, different substances of abuse, when injected in a rat at doses, or with modalities effective in inducing self-administration, have in common the property of selectively modifying glucose utilization of the nucleus accumbens, in particular in the shell portion. It is likely that the change reflects a functional change of a neural circuitry which mediates motivational properties associated with addiction.

Time-related effects of stress on cholinergic sensitivity.

The effect of the administration of the muscarinic cholinergic agonist oxotremorine on locomotor activity was investigated in DBA/2 mice subjected to chronic restraint stress of different durations (120 min daily for 10, 14 or 18 days). Oxotremorine induced a depressant effect on locomotion, which was reduced after 10 and 14 days of restraint, but not after a 18-day restraint stress. Acetylcholine (ACh) content was significantly reduced in prefrontal cortex after 10 and 14 days of stress but returned to control values after 18 days of restraint. No changes in ACh content were observed in nucleus accumbens and striatum. These results are discussed in terms of possible changes in muscarinic receptor sensitivity.

Strain-dependent effects of dopamine agonists on acetylcholine release in the hippocampus: an in vivo study in mice.

The effects of selective D1 or D2 dopamine receptor agonists and the indirect dopamine agonist cocaine on hippocampal acetylcholine release in mice of the C57BL/6 and DBA/2 inbred strains were investigated using intracerebral microdialysis. The D1 SKF 38393 (10, 20, 30 mg/kg, i.p.), the D2 agonist LY 171555 (0.5, 1, 2 mg/kg, i.p.) and cocaine (5, 10, 15 mg/kg, i.p.) all increased, dose-dependently, acetylcholine release in the hippocampus of C57BL/6 mice. Both the D1 agonist and cocaine did not produce any significant effect in DBA/2 mice. In the latter strain, however, LY 171555 produced a decrease in acetylcholine release that was evident after 60 min from injection of the doses of 0.5 and 1 mg/kg, but not at the dose of 2 mg/kg. The effects observed in C57BL/6 mice as well as those produced by low doses of LY 171555 in the DBA/2 strain were consistent with previous results obtained in rats. The present results indicate major strain-dependent differences in the effects of dopamine agonists on hippocampal acetylcholine release in mice. Moreover, they suggest a complex genotype-related neural organization of dopamine-acetylcholine interactions in the mesolimbic system. Finally, the strain differences in the effects of the dopamine agonists on hippocampal acetylcholine release parallel previously reported strain differences in the effects of these substances on memory consolidation.

Galanin microinjected into the medial septum inhibits scopolamine-induced acetylcholine overflow in the rat ventral hippocampus.

Galanin-like immunoreactive terminals hyperinnervate the basal forebrain cholinergic neurons in Alzheimer's disease. To investigate the hypothesis that galanin acts directly on basal forebrain cell bodies, in vivo microdialysis studies were conducted in awake rats which analyzed the actions of galanin on acetylcholine release. Microinjection of galanin into the cholinergic cell body region of the medial septum-diagonal band (MS-DBB) inhibited acetylcholine release in the ventral hippocampus. These results are consistent with an interpretation that galanin terminals synapsing on cholinergic cell bodies of the basal forebrain may serve to inhibit the release of acetylcholine in the terminal fields of the cholinergic neurons.

Strain-dependent effects of cocaine on memory storage improvement induced by post-training physostigmine.

Post-training administration of the inhibitor of cholinesterase enzymes, physostigmine, dose-dependently (0.025-0.4 mg/kg) improved retention of an inhibitory avoidance response in C57BL/6 (C57) as well as in DBA/2 (DBA) mice, the latter being more responsive than C57 mice. The effects on retention performance induced by physostigmine in C57 and DBA mice appeared to be due to an effect on memory consolidation. In fact, they were observed when drugs were given at short, but not long, periods of time after training, which is when the memory trace is susceptible to modulation. Moreover, these effects are not to be ascribed to a rewarding or non-specific action of the drugs on retention performance, as the latencies during the retention test of those mice that had not received a footshock during the training were not affected by the post-training drug administration. Post-training administration of cocaine (1-5 mg/kg) dose-dependently improved retention of an inhibitory avoidance response in C57 mice, while impairing it in the DBA strain, thus confirming previous results (Puglisi-Allegra et al. 1994b). Pretreatment with cocaine at ineffective doses as well as at an effective one potentiated the effects of an ineffective as well as of an effective dose of physostigmine in C57 mice, while it antagonized the effects of the inhibitor of cholinesterase enzymes on memory consolidation in DBA mice. The present results indicate that the indirect DA receptor agonist cocaine affects physostigmine action on memory consolidation in an opposite manner in the two inbred strains, pointing to genotype-dependent interaction between cholinergic and dopaminergic activity in memory consolidation.

A comparison of the behavioral effects of minaprine, amphetamine and stress.

Different types of clinically effective antidepressants prevent the behavioral effects of experimental stress, and some of these treatments affect mesolimbic dopamine (DA) functioning. Animal studies have demonstrated that repeated psychostimulant administration and repeated or chronic stressful experiences also affect mesolimbic DA functioning. These results could suggest homologies among stress, psychostimulants and antidepressants. The present experiments show that either repeated stress (120 min restraint daily for 10 consecutive days) or subchronic treatment with the antidepressant minaprine (5 mg/kg daily for 10 consecutive days) significantly reduced the inhibitory effect of 120 min of restraint on climbing, a behavioral response dependent on mesolimbic DA functioning. However, the antidepressant did not induce the altered sensitivity of presynaptic DA receptors promoted by repeated stress. Chronic stressful experience (13 days of food restriction) and repeated amphetamine (2.5 mg/kg daily for 10 consecutive days) were as effective as subchronic minaprine in reducing immobility in the Porsolt's swimming test. However, whilst both stress and amphetamine enhanced struggling, minaprine promoted swimming. Finally, chronically stressed mice and mice pretreated with amphetamine showed enhanced sensitivity to amphetamine-induced locomotion, whilst this effect was absent in animals pretreated with the antidepressant. These results indicate that although chronic and repeated stress as well as amphetamine have some antidepressant-like behavioral effects, their mode of action could be different from that of clinically active substances.

Sardinian alcohol-preferring rats: a genetic animal model of anxiety.

The present study was designed to assess the anxiety profile of the selectively bred alcohol-preferring sP and alcohol-nonpreferring sNP rats. Rats were offered either water (ethanol-naive rats) or a free choice of 10% (v/v) ethanol and water (ethanol-experienced rats) for 14 consecutive days prior to the test. Spontaneous exploration of an elevated plus maze was used as a behavioral measure of anxiety. Ethanol-naive sP rats spent less time in and made fewer entries into the open arms of the maze than ethanol-naive sNP rats. These results suggest a higher innate degree of anxiety in sP than in sNP rats. Moreover, time spent in and number of entries into the open arms of the maze were higher in ethanol-experienced than in ethanol-naive sP rats. This finding suggests that ethanol consumed voluntarily produces anxiolytic effects in sP rats. The results of the present study are discussed in terms of (a) anxiety as a genetic trait related to ethanol-preference in sP rats and (b) self-medication of anxiety as a possible factor promoting voluntary ethanol consumption in sP rats.

Alterations in striatal acetylcholine overflow by cocaine, morphine, and MK-801: relationship to locomotor output.

The activity of cholinergic interneurons in the striatum appears to be modulated by a variety of different systems including dopamine, opiate, and glutamate. The purpose of this study was to characterize the effects of drugs known to act on these three systems (i.e., cocaine, morphine, and MK-801) on striatal ACh overflow with microdialysis procedures, and to determine if alterations in ACh function induced by these agents are related to changes in locomotor activity. Cocaine was found to increase striatal ACh following intraperitoneal injections of 20 and 40 mg/kg, but not 10 mg/kg. The increases in locomotor activity induced by cocaine appeared to be dose dependent, while the effects on striatal ACh were not. Injections of 0.1 mg/kg MK-801 (a non-competitive NMDA receptor antagonist) produced dramatic increases in locomotor activity while decreasing striatal ACh overflow. A lower dose (0.03 mg/kg) of MK-801 failed to alter locomotor activity or striatal ACh. Morphine produced an apparent dose-dependent elevation in striatal ACh while only the lowest dose (5 mg/kg) increased locomotor activity. There appears to be no relationship between alterations in striatal ACh and locomotor output following systemic administration of these psychoactive agents.

Electrical kindling is associated with increases in amygdala acetylcholine levels: an in vivo microdialysis study.

Using brain microdialysis, acetylcholine (Ach) levels were assessed in the nucleus amygdaloideus lateralis of electrically kindled rats using a cholinesterase inhibitor in the perfusion fluid to prevent Ach breakdown. During kindling development, when the animals displayed afterdischarges lasting more than 30 s but no seizures, a significant increase in Ach levels (+66%) was observed after electrical stimulation. Ach levels were also elevated after a stage 5 kindled seizure (+48%), with no additional increment compared to rats experiencing only afterdischarges. Chronic diazepam administration prevented kindling development and the increment in Ach overflow; however, diazepam itself (following acute administration) tended to decrease Ach levels in the amygdala. These findings are convergent with pharmacological data suggesting that changes in the cholinergic function may be important especially during amygdaloid kindling development.

Increases in striatal acetylcholine by SKF-38393 are mediated through D1 dopamine receptors in striatum and not the frontal cortex.

It has been proposed by some that the D1 receptor effects are mediated through striatal actions while others have suggested that they are determined indirectly through the frontal cortex. The experiments reported here represent a further attempt to resolve this controversy. It was found that focal applications of the inactive and active enantiomers of SKF-38393 (a D1 dopamine receptor agonist) to the rat striatum via reverse dialysis increased extracellular acetylcholine (ACh) in a stereoselective manner. Infusions of SKF-38393 into the frontal cortex, on the other hand, were ineffective in altering striatal ACh. Furthermore, partial hemisections caudal to the frontal cortex did not alter the ability of systemically administrated SKF-38393 to increase striatal ACh. Taken together, these results suggest that at least some of the effects of D1 receptor agonists on striatal cholinergic function are mediated through actions in the striatum and not the frontal cortex.

A pharmacological discrimination of two behavioral forms of the paradise fish (Macropodus opercularis).

Intracranial injections of 5 micrograms/fish (equivalent to 2 nmol/g) of the dopamine agonist Apomorphine into the paradise fish brain considerably decrease the occurrence of escape behavior with a parallel increase of time spent in swimming. All other elements of the behavioral repertoire are unaffected. The simultaneous administration of 0.5 micrograms/fish (equivalent to 0.15 nmol/g) of the dopamine antagonist Pimozide abolishes this effect. In a second experiment a dose dependent decrease in escape behavior was obtained with a parallel increase of swimming. On the bases of these pharmacological data it is concluded that the escape and the swimming reactions are truly independent units but their regulation might be closely related.

Repeated stressful experiences differently affect limbic dopamine release during and following stress.

The effects of repeated restraint stress exposures (daily 60 min, for 6 days) on extracellular dopamine in the nucleus accumbens, during and after the stress experience, have been investigated in rats by in vivo microdialysis. On the first day, restraint increased dopamine release during the first 40 min followed by a return to basal levels (50-60 min later). As soon as restraint ceased and the rats were set free, there was another increase in dopamine release lasting 40 min. On the second and third day, restraint produced only a slight increase in dopamine release, while no significant changes were evident from the fourth to the sixth day. By contrast, from the second to the sixth day the increase in dopamine release observed once rats were freed, was unchanged in comparison to the first day. The present results show that the activation of the mesolimbic dopaminergic system induced by aversive stimuli adapts to repeated experiences differently from that produced by pleasurable events, suggesting that aversive and rewarding experiences involve different neural systems.

Stress activation of limbic and cortical dopamine release is prevented by ICS 205-930 but not by diazepam.

Systemic administration of the 5-HT3 receptor antagonist ICS 205-930, but not of the benzodiazepine diazepam, was able to prevent the stimulation of dopamine release in the nucleus accumbens and prefrontal cortex induced by restraint stress. These findings suggest that stress is not simply co-extensive with anxiety and that 5-HT3 receptors could regulate the dopaminergic response to stress.