Alessandra Silvagni

Effect of amphetamine, cocaine and depolarization by high potassium on extracellular dopamine in the nucleus accumbens shell of SHR rats. An in vivo microdyalisis study.

Spontaneously hypertensive rats (SHR) exhibit behavioural abnormalities (hyperactivity and hyper reactivity to stress) that resemble the behavioural abnormalities of human attention-deficit with hyperactivity disorder (ADHD). Because dopamine has been implicated in ADHD we studied by in vivo microdialysis the dopamine output in the nucleus accumbens (NAc) shell of 6 week-old (pre-hypertensive stage) SHR rats and in their normotensive age matched Wistar Kyoto controls (WKY). We observed that SHR rats had significant higher basal dialysate dopamine concentrations (about 20%) than WKY. Systemic administration of amphetamine (0.25 and 0.5 mg/kg s.c.), and methylphenidate (1 and 2 mg/kg i.p.) produced an higher increase in dialysate dopamine in the NAc shell of SHR rats as compared with WKY rats, although only after the administration of the lowest dose of amphetamine and methylphenidate this difference was found to be significant. In contrast when the microdialysis fiber was perfused by 30 or 60 mM K(+), a lower increase of dialysate dopamine was observed in SHR rats as compared with WKY rats. These apparently contradictory results can be explained by postulating that SHR rats have a higher tone of NAc shell dopamine transmission and synthesis associated with a lower storage capacity of vesicles in dopamine terminals of the same area.

Cumulative effect of norepinephrine and dopamine carrier blockade on extracellular dopamine increase in the nucleus accumbens shell, bed nucleus of stria terminalis and prefrontal cortex.

We investigated, by microdialysis in various brain areas, the possibility that dopamine could be captured by the norepinephrine transporter when the dopamine transporter is pharmacologically blocked. Administration of reboxetine, a selective blocker of the norepinephrine transporter, 20 min after the administration of GBR 12909, a selective blocker of the dopamine transporter, produced an increase of dopamine output in the nucleus accumbes shell (+408% above basal) greater than that obtained by GBR 12909 alone (+308% above basal). On the contrary, reboxetine did not increase further the dopamine output produced by GBR 12909 in the nucleus accumbens core or in the dorsal caudate, areas lacking a consistent noradrenergic innervations. A cumulative effect of dopamine and norepinephrine transporter blockade on the output of dopamine in dialysates was also observed in the bed nucleus of stria terminalis and in the prefrontal cortex. This study shows that dopamine extracellular concentration can be elevated by norepinephrine transporter blockade, even in areas where the dopamine transporter is predominant, when the latter is pharmacologically blocked. This phenomenon may have relevance in psychostimulant dependence as well as in antidepressant pharmacology.

Prenatal restraint stress differentially modifies basal and stimulated dopamine and noradrenaline release in the nucleus accumbens shell: an ‘in vivo’ microdialysis study in adolescent and young adult rats

Gestational stress [prenatal stress (PNS)] has been associated with low birth weight, preterm delivery, and higher vulnerability to psychiatric disorders such as schizophrenia, depression or attention deficit with hyperactivity disorder. The alteration of catecholamine transmission has been attributed a major role in the etiology of psychiatric disturbances. We investigated the effect of PNS on basal and stimulated dopamine and noradrenaline output in the nucleus accumbens of freely moving adolescent and young adult rats (30–35 and 60–70 postnatal days respectively) because of the importance of this area in drug dependence and possibly in psychiatric disorders that are treated with drugs that act on dopamine and noradrenaline transmission. Stimulation was obtained with intraperitoneal amphetamine (0.25 mg/kg) or subcutaneous nicotine (0.4 mg/kg). The results showed the following: (i) basal and amphetamine‐stimulated dopamine output in adolescent and adult PNS rats is higher than in controls; (ii) nicotine‐stimulated dopamine output is lower than in controls in adolescent but not in adult PNS rats; (iii) basal noradrenaline output is lower than in controls in adolescent but not in adult PNS rats; (iv) amphetamine‐stimulated noradrenaline output is higher than in controls in adult but not in adolescent PNS rats; (v) nicotine‐stimulated noradrenaline output in PNS rats is higher than in controls, although only in adults. These results show that PNS may produce a complex change in accumbal dopamine and noradrenaline transmission. We discuss the possibility that these changes might be correlated with the development of psychiatric disorders or with an increased vulnerability to drug addiction.

Prenatal restraint stress: an in vivo microdialysis study on catecholamine release in the rat prefrontal cortex

There is substantial evidence that prenatal exposure to adverse environmental conditions might lead to the psychiatric disorders that can appear in adolescence or in adulthood; vulnerability to drug addiction may increase as well. It is currently accepted that the alteration of catecholamine transmission in the prefrontal cortex plays a prominent role in the etiology of psychiatric disorders. We assessed basal and stimulated dopamine and noradrenaline extracellular concentration in the medial prefrontal cortex by means of microdialysis in awake male adolescent and young adult offspring of rats exposed to restraint stress in the last week of pregnancy. Catecholamine stimulation was obtained by amphetamine or nicotine. We observed that prenatal stress (PNS) did not change dopamine but decreased noradrenaline basal output in both adolescents and adults. Moreover, it decreased amphetamine stimulated dopamine output and increased amphetamine stimulated noradrenaline output. PNS decreased nicotine stimulated noradrenaline (but not dopamine output) in adults, though not in adolescents. These data show that PNS stress modifies prefrontal cortex catecholamine transmission in a complex and age dependent manner. Our results support the view that prenatal stress may be a contributing factor for the development of psychiatric disorders and that its effect may augment drug addiction vulnerability.

Experimental Investigations on Dopamine Transmission Can Provide Clues on the Mechanism of the Therapeutic Effect of Amphetamine and Methylphenidate in ADHD

The aim of this review is to compare the experimental evidence obtained from in vitro studies on the effect of amphetamine and methylphenidate on dopamine transmission with the results obtained in animal models of attention deficit hyperactivity disorder (ADHD). This comparison can extend the knowledge on the mechanism of action of the drugs used in the therapy of ADHD and provide insight into the etiology of ADHD. In particular, we considered the results obtained from in vitro methods, such as synaptosomes, cells in culture, and slices and from in vivo animal models of ADHD, such as spontaneous hypertensive rats (SHR) and the Naples high-excitability (NHE) rat lines. The different experimental approaches produce consonant results and suggest that in SHR rats, in contrast to Wistar Kyoto rats (WKY), amphetamine and depolarization by high K+ might release different pools of dopamine-containing vesicles. The pool depleted by amphetamine might represent dopamine that is stored in large dense core vesicles, whereas dopamine released by high K+ might be contained in small synaptic vesicles (SSV). The sustained dopamine transmission observed in the nucleus accumbens of SHR but not WKY rats can be supported by an elevated synthesis and release, which also might explain the stronger effect of methylphenidate on dopamine release in SHR but not in WKY rats. This hypothesis might enlighten the common therapeutic effect of these drugs, although their action takes place at different levels in catecholaminergic transmission.

Increase of Dialysate Dopamine in the Bed Nucleus of Stria Terminalis by Clozapine and Related Neuroleptics

Neuroleptics are known to stimulate dopamine release in neostriatal terminal areas. In the present study, we have investigated by brain microdialysis in freely moving rats the effect of typical and atypical neuroleptics on dopamine transmission in the bed nucleus of stria terminalis, a dopamine terminal area belonging to the limbic system and recently assigned the so-called extended amygdala. Mean basal dialysate dopamine values were 14.3 f moles/20 μl sample. Dopamine output in dialysates was increased dose-dependently by clozapine (max + 158%, 298%, and 461% of basal at 5, 10, and 20 mg/kg IP, respectively), risperidone (max + 115% and 221% of basal at 1 and 3 mg/kg IP, respectively), olanzapine (max + 138% and 235% of basal at 3 and 6 mg/kg IP, respectively), BIMG 80 (max + 64% and 164% of basal at 3 and 5 mg/kg IP, respectively), amperozide (max + 110% and 194% of basal at 3 and 6 mg/kg IP, respectively). The selective dopamine D4 antagonist L-745,870 increased dialysate dopamine but at rather high doses and not as effectively as clozapine (max + 32%, 89%, and 130% of basal at 2.7, 5.4, and 10.8 mg/kg IP, respectively). The typical neuroleptic haloperidol (0.1 and 0.5 mg/kg SC) and the selective D2 antagonist raclopride (0.14, 0.56, and 2.1 mg/kg SC), the serotonergic 5-HT2 antagonist ritanserin (0.5 and 1.5 mg/kg IP), and the adrenergic α1 antagonist prazosin (0.91 and 2.73 mg/kg IP) did not affect dialysate dopamine in the bed nucleus of stria terminalis. Saline (1 ml/kg SC or 3 ml/kg IP) did not modify dialysate dopamine. Therefore, atypical neuroleptics share the ability of stimulating dopamine transmission in the bed nucleus of stria terminalis, but this property is not mimicked by any of the drug tested that selectively act on individual receptors among those that are affected by atypical neuroleptics. These observations raise the possibility that the property of increasing dopamine transmission in the bed nucleus of stria terminalis is the result of combined blockade of dopamine, serotonin, and noradrenaline receptors and that might be predictive of an atypical neuroleptic profile.

Stimulation of Dopamine Release in the Bed Nucleus of Stria Terminalis: A Trait of Atypical Antipsychotics?

It has been hypothesized that schizophrenia involves an alteration of dopamine (DA) transmission in specific brain areas. This hypothesis is based in part on the observation that the therapeutic potency of neuroleptics correlates with their affinity for DA D2 receptor. Atypical neuroleptics (e.g., clozapine) differ from classical neuroleptics through having fewer side extrapyramidal effects and higher efficacy on negative symptoms of schizophrenia.1 Atypical neuroleptics enhance DA output in dialysates preferentially in the prefrontal cortex rather than in the striatum or in the n. accumbens, which is the case with classical neuroleptics.2 Some of the areas densely innervated by DA neurons such as the bed nucleus of stria terminalis (BNST) and the central amygdala have been assigned to the so-called extended amygdala. As no information exists on the effect of neuroleptics on DA transmission in these areas, we investigated the effect of typical and atypical neuroleptics on DA transmission in the BNST with concentric microdialysis probes in freely moving rats.3