Osvaldo Giorgi

Developmental and age-related changes in D1-dopamine receptors and dopamine content in the rat striatum

The relationship between the postnatal development of dopaminergic (DAergic) nerve endings and the maturation of D1 DA receptors in the rat striatum was analyzed by measuring the content of DA and dihydroxyphenylacetic acid (DOPAC), two biochemical markers of DAergic nerve terminal proliferation, and the ontogenetic changes in [3H]SCH 23390 binding sites. DA-stimulated adenylate cyclase (AC) activity was also measured in order to characterize the coupling of [3H]SCH 23390 binding sites to the responses mediated by the activation of D1 DA receptors. Striatal levels of DA and DOPAC, as well as the density and affinity of [3H]SCH 23390 binding sites and DA-stimulated AC activity were also measured in senescent rats. The striatal content of DA increased slowly after birth, reaching adult levels by postnatal day 60 and remaining constant through adulthood and senescence (up to 20 months of age). The density of [3H]SCH 23390 binding sites increased 14-fold from birth to postnatal day 35, when a peak value was reached, whereas a significant decrease was observed in the striatum of aged rats. In contrast, the affinity of D1 DA receptors for [3H]SCH 23390 remained unchanged from birth through senescence. The stimulation of cyclic AMP formation induced by 100 microM DA increased 4-fold from birth to postnatal day 14, when the maximal responsiveness to DA was observed and then returned to adult levels. No significant alterations were observed in the Km values during development, whereas the stimulatory effect of 100 microM DA on AC activity was significantly decreased in senescent rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Genetic selection and differential stress responses - The Roman lines/strains of rats

ETH, Institut fur Natztierwissenschaften, Schorenstrasse 16, CH-8603 Schwerzenbach, Switzerland Autonomous University of Barcelona, Medical Psychology Unit, E-08193 Bellaterra (Barcelona), Spain University of Santiago de Compostela, Department of Psychobiology, E-15705 Santiago de Compostela, Spain University of Cagliari, Department of Toxicology, Viale A. Diaz 182, I-09126 Cagliari, Italy University of Magdeburg, Anatomy Institute, Leipzigerstrasse 44, D-39120 Magdeburg, Germany IUPG, Clinical Psychopharmacology Unit, 100 avenue de Bel-Air, CH-1225 Chene-Bourg (GE), Switzerland NV Organon, RE 2211, P.O. Box 20, NL-5340 BH Oss, the Netherlands University of Oxford, Institute of Molecular Medicine, John Radcliffe Hospital, Headington, GB-Oxford OX3 9DU, England University of Groningen, Department of Animal Physiology, Kerklaan 30, P.O. Box 14, NL-9750 AA Haren, the Netherlands University of Delaware, Departments of Psychology and Biology, Newark, Delaware 19716 USA

Decrease in the function of the γ-aminobutyric acid-coupled chloride channel produced by the repeated administration of pentylenetetrazol to rats

Abstract: The acute administration of pentylenetetrazol (PTZ; 25–75 mg/kg i.p.) failed to modify the specific binding of t‐[35S]butylbicyclophosphorothionate ([35S]TBPS) to membrane preparations from the cerebral cortex of the rat. In contrast, the repeated administration of PTZ (30 mg/kg i.p., three times a week for 12 weeks) reduced by 26% the density of [33S]TBPS binding sites without modifying the dissociation constant. This effect was observed 3 days after the last PTZ administration. A parallel reduction of 7‐amino‐butyric acid (GABA)‐stimulated 36CI− uptake was measured in the cerebral cortex of PTZ‐treated rats 3 days after the last injection. The repeated administration of PTZ produced sensitization to the drug, or chemical kindling. In fact, no convulsions were observed in the first week of treatment, but all the animals became sensitized to PTZ by the 12th week. The results are consistent with the hypothesis that chronic treatment with PTZ at a subconvulsant dose causes a decrease in GABA‐coupled chloride channel activity that may be related to the chemical kindling produced by this compound.

Dissociation between mesocortical dopamine release and fear-related behaviours in two psychogenetically selected lines of rats that differ in coping strategies to aversive conditions.

The mesocortical and mesolimbic dopaminergic (DAergic) pathways are activated by either aversive or rewarding stimuli. The functional tone of these DAergic neurons also increases during the execution of cognitive tasks. The present study was designed to examine the relationship between mesocortical and mesolimbic DAergic function and the expression of fear‐related behaviours as compared with attention‐ and cognition‐related mechanisms (e.g. coping strategies), in response to aversive conditions. To this aim, we used two psychogenetically selected rat lines, Roman high‐avoidance (RHA/Verh) and Roman low‐avoidance (RLA/Verh), which display drastically different emotion‐ and coping‐related behaviours in response to stressors: RLA/Verh rats are ‘reactive copers’ and more fearful than RHA/Verh rats, which are ‘proactive copers’. Brain dialysis experiments demonstrated that tail‐pinch (TP) and the anxiogenic compounds pentylenetetrazol (PTZ) and ZK 93426 increased DA output in the medial prefrontal cortex (PFCX) of RHA/Verh but not RLA/Verh, rats. In contrast, in the shell compartment of the nucleus accumbens (NAC shell), TP caused a small increase in DA output only in RLA/Verh rats, whereas PTZ and ZK 93426 had no significant effect on either line. RHA/Verh rats displayed more robust and longer lasting coping activity and less frequent freezing and self‐grooming episodes than did RLA/Verh rats after TP, PTZ or ZK 93426. This dissociation between fear‐related behaviour and cortical DAergic activation argues against the view that the latter may be involved in the control of fear‐like responses. We therefore propose that the activation of mesocortical DAergic projections by aversive stimuli underlies the cognitive mechanisms that are triggered in an attempt to gain control over the stressor.

3H-SCH 23390 binding sites in the rat substantia nigra: evidence for a presynaptic localization and innervation by dopamine

Chronic treatment with SCH 23390, a selective D-1 dopamine receptor antagonist, elicited a 32% increase in the density of 3H-SCH 23390 binding sites in nigral membrane preparations but failed to change the apparent KD of the ligand for its binding sites. Haloperidol, a D-2 dopamine receptor antagonist which blocks the dopamine-sensitive adenylate cyclase and (-) sulpiride, a selective D-2 dopamine receptor blocker, which does not block the dopamine-sensitive adenylate cyclase, failed to change both the Bmax and KD of 3H-SCH 23390 binding. Finally, the intrastriatal injection of kainic acid produced a marked decrease of both GAD activity and GABA content and 3H-SCH 23390 binding sites (65%) in the homolateral substantia nigra. The results show that in the rat substantia nigra most of the 3H-SCH 23390 binding sites have a presynaptic localization on the striato-nigral GABAergic afferent terminals and suggest that dopamine released from nigral dendrites exerts a tonic influence on these presynaptic D-1 dopamine receptors.

The psychogenetically selected Roman high-and low-avoidance rat lines : A model to study the individual vulnerability to drug addiction

The Roman high- (RHA) and low-avoidance (RLA) rat lines were selected for, respectively, rapid vs poor acquisition of two-way active avoidance in the shuttle-box. Here, we review experimental evidence indicating that, compared with their RLA counterparts, RHA rats display a robust sensation/novelty seeking profile, a marked preference and intake of natural or drug rewards, and more pronounced behavioral and neurochemical responses to the acute administration of morphine and psychostimulants. Moreover, we show that (i) the repeated administration of morphine and cocaine elicits behavioral sensitization in RHA, but not RLA, rats, (ii) in sensitized RHA rats, acute morphine and cocaine cause a larger increment in dopamine output in the core, and an attenuated dopaminergic response in the shell of the nucleus accumbens, as compared with RHA rats repeatedly treated with saline, and (iii) such neurochemical changes are not observed in the mesoaccumbens dopaminergic system of the sensitization-resistant RLA line. Behavioral sensitization plays a key role in several cardinal features of addiction, including drug craving, compulsive drug seeking and propensity to relapse following detoxification. Comparative studies in the Roman lines may therefore represent a valid approach to evaluate the contribution of the genotype on the neural substrates of drug sensitization and addiction.

Behavior of the Roman/Verh high- and low-avoidance rat lines in anxiety tests: relationship with defecation and self-grooming

The Swiss sublines of Roman high- and low-avoidance (RHA/Verh and RLA/Verh) rats have been selected and bred for rapid (RHA/Verh) vs. extremely poor (RLA/Verh) acquisition of two-way active avoidance. Behavioral and physiological measures of emotionality, or reactivity to stress, appear to be among the most prominent characteristics differentiating both rat lines. The present study shows that RLA/Verh rats are more sensitive, as compared to their RHA/Verh counterparts, to the conflict involved in the shock-induced suppression of drinking paradigm, as well as in a hyponeophagia test. RLA/Verh rats also showed higher defecation values which were significantly correlated with the main hyponeophagia test variables. Likewise, self-grooming was more frequent in RLA/Verh rats than in their RHA/Verh counterparts and showed significant correlations with conflict-related behaviors (i.e., latency to start eating and time spent eating) from the hyponeophagia test. These results give additional support to the contention that RLA/Verh rats present higher anxiety (emotionality) than their RHA/Verh counterparts.

Pentylenetetrazol-induced kindling in rats: effect of GABA function inhibitors

The repeated administration of subconvulsant doses of pentylenetetrazol (PTZ) produced a progressive sensitization to the effects of this compound (i.e., chemical kindling) in the rat. A very similar time-course for PTZ-induced kindling was observed using two different treatment schedules: 1) one injection every day (30 mg/kg, IP), and 2) one injection (30 mg/kg, IP) every second day. When these treatment schedules were used for eight consecutive weeks, more than 80% of the rats displayed convulsions by the end of treatment. In contrast, only 20% of the rats were sensitized if PTZ was administered twice daily at the dose of 15 mg/kg, IP. The increased sensitivity to the convulsant effect of PTZ was still present one year after completion of the chronic treatment. Moreover, rats kindled with PTZ showed an enhanced susceptibility to convulsions induced by different inhibitors of central GABAergic function, such as the chloride channel blocker picrotoxin, the benzodiazepine receptor ligands FG 7142 and Ro 15-4513, and the inhibitor of GABA synthesis isoniazid. In contrast, the sensitivity to the convulsant action of the glycine receptor antagonist strychnine was unchanged by repeated PTZ administration. It is suggested that kindling produced by PTZ may be associated with a persistent reduction in the inhibitory function of the GABAergic system in the brain.

The Roman High- and Low-Avoidance Rat Lines Differ in the Acquisition, Maintenance, Extinction, and Reinstatement of Intravenous Cocaine Self-Administration

The selective breeding of Roman high- (RHA) and low-avoidance (RLA) rats for, respectively, rapid vs extremely poor acquisition of avoidant behavior in a shuttlebox has produced two phenotypes that differ in temperament traits, in mesocortical/mesolimbic dopamine system function, and in the behavioral and neurochemical responses to the acute and repeated administration of psychostimulants and opiates. The phenotypic traits of the RHA line predict higher susceptibility, compared with RLA rats, to the reinforcing properties of addictive substances like cocaine. The present study was designed to compare the acquisition, maintenance, reinstatement of drug-seeking after long-term extinction, and reacquisition of intravenous cocaine self-administration (SA) behavior in the Roman lines. Compared with RLA rats, the rates of responding during cocaine SA acquisition were higher, extinction from cocaine SA was prolonged, and drug-induced reinstatement of cocaine-seeking behavior was more robust in RHA rats. Moreover, only RHA rats reacquired extinguished lever-pressing activity when a low reinforcing dose of cocaine was available. These findings are consistent with the view that subjects with genetically determined high responsiveness to the acute and chronic (ie, sensitizing) effects of psychostimulants, such as RHA rats, also display a higher propensity to self -administer cocaine. Further comparative studies in the Roman lines, using SA paradigms that distinguish mere drug-taking from the compulsive and uncontrolled drug use that characterizes addiction in humans, may eventually help to characterize the relationships among genotype, temperament traits, and neurobiological mechanisms involved in the individual vulnerability to cocaine addiction.

6-Hydroxydopamine-induced degeneration of nigral dopamine neurons: Differential effect on nigral and striatal D-1 dopamine receptors

Dopamine-sensitive adenylate cyclase and 3H-SCH 23390 binding parameters were measured in the rat substantia nigra and striatum 15 days after the injection of 6-hydroxydopamine into the medial forebrain bundle. The activity of nigral dopamine-sensitive adenylate cyclase and the binding of 3H-SCH 23390 to rat nigral D-1 dopamine receptors were markedly decreased after the lesion. On the contrary, 6-hydroxydopamine-induced degeneration of the nigrostriatal dopamine pathway enhanced both adenylate cyclase activity and the density of 3H-SCH 23390 binding sites in striatal membrane preparations. The changes in 3H-SCH 23390 binding found in both nigral and striatal membrane preparations were associated with changes in the total number of binding sites with no modifications in their apparent affinity. The results indicate that: within the substantia nigra a fraction (30%) of D-1 dopamine receptors coupled to the adenylate cyclase is located on cell bodies and/or dendrites of dopaminergic neurons; striatal D-1 dopamine receptors are tonically innervated by nigrostriatal afferent fibers.