The behavioral effects of D-amphetamine are correlated with its effects on cAMP in different brain regions

Genomic and Reverse Translational Analysis Discloses a Role for Small GTPase RhoA Signaling in the Pathogenesis of Schizophrenia: Rho-Kinase as a Novel Drug Target

Schizophrenia is one of the most serious psychiatric disorders and is characterized by reductions in both brain volume and spine density in the frontal cortex. RhoA belongs to the RAS homolog (Rho) family and plays critical roles in neuronal development and structural plasticity via Rho-kinase. RhoA activity is regulated by GTPase-activating proteins (GAPs) and guanine nucleotide exchange factors (GEFs). Several variants in GAPs and GEFs associated with RhoA have been reported to be significantly associated with schizophrenia. Moreover, several mouse models carrying schizophrenia-associated gene variants involved in RhoA/Rho-kinase signaling have been developed. In this review, we summarize clinical evidence showing that variants in genes regulating RhoA activity are associated with schizophrenia. In the last half of the review, we discuss preclinical evidence indicating that RhoA/Rho-kinase is a potential therapeutic target of schizophrenia. In particular, Rho-kinase inhibitors exhibit anti-psychotic-like effects not only in Arhgap10 S490P/NHEJ mice, but also in pharmacologic models of schizophrenia (methamphetamine- and MK-801-treated mice). Accordingly, we propose that Rho-kinase inhibitors may have antipsychotic effects and reduce cognitive deficits in schizophrenia despite the presence or absence of genetic variants in small GTPase signaling pathways.

Amphetamine activates Rho GTPase signaling to mediate dopamine transporter internalization and acute behavioral effects of amphetamine

Acute amphetamine (AMPH) exposure elevates extracellular dopamine through a variety of mechanisms that include inhibition of dopamine reuptake, depletion of vesicular stores, and facilitation of dopamine efflux across the plasma membrane. Recent work has shown that the DAT substrate AMPH, unlike cocaine and other nontransported blockers, can also stimulate endocytosis of the plasma membrane dopamine transporter (DAT). Here, we show that when AMPH enters the cytoplasm it rapidly stimulates DAT internalization through a dynamin-dependent, clathrin-independent process. This effect, which can be observed in transfected cells, cultured dopamine neurons, and midbrain slices, is mediated by activation of the small GTPase RhoA. Inhibition of RhoA activity with C3 exotoxin or a dominant-negative RhoA blocks AMPH-induced DAT internalization. These actions depend on AMPH entry into the cell and are blocked by the DAT inhibitor cocaine. AMPH also stimulates cAMP accumulation and PKA-dependent inactivation of RhoA, thus providing a mechanism whereby PKA- and RhoA-dependent signaling pathways can interact to regulate the timing and robustness of AMPH's effects on DAT internalization. Consistent with this model, the activation of D1/D5 receptors that couple to PKA in dopamine neurons antagonizes RhoA activation, DAT internalization, and hyperlocomotion observed in mice after AMPH treatment. These observations support the existence of an unanticipated intracellular target that mediates the effects of AMPH on RhoA and cAMP signaling and suggest new pathways to target to disrupt AMPH action.

(+)-UH 232, a partial agonist of the D3 dopamine receptors, attenuates cognitive effects of angiotensin IV and des-Phe(6)-angiotensin IV in rats

We have recently found that postsynaptic D3 dopamine (DA) receptors appear not to participate in the memory enhancing effects of the angiotensin AT4 receptor agonists angiotensin IV (Ang IV) and des-Phe(6)-Ang IV. In this study we evaluated role of the presynaptic DA D3 receptors in these effects. For that purpose effect of (+)-UH 232, a selective D3 DA receptors partial agonist preferring presynaptic sites, on the pro-cognitive action of intracerebroventricularly (icv) injected Ang IV and des-Phe(6)-Ang IV was examined. Male Wistar rats weighing 180-200 g were used. Both peptides given at the dose of 1 nmol facilitated recall of a passive avoidance (PA) behaviour, improved object recognition (OR), and increased apomorphine-induced stereotype behaviour. In the auxiliary tests performed to control for the unspecific influence of motor (open field, OF) and emotional ('plus' maze, PM) effects of our treatments on the results of the memory tests they had either no (OF) or negligible (PM) effects. Intraperitoneal pre-treatment of the animals with an ineffective on its own dose (1 mg/kg) of (+)-UH 232 abolished or markedly diminished effects of both peptides on PA and OR but did not influence enhancement of stereotypy caused by the peptides.

Striatal gene expression of RGS2 and RGS4 is specifically mediated by dopamine D1 and D2 receptors: clues for RGS2 and RGS4 functions

Of all partners involved in G-protein coupled receptor (GPCR) signalling, the regulator of G-protein signalling (RGS) proteins are the only ones showing fast gene expression changes after various stimuli. These expression changes can offer feedback regulation to GPCR signalling as RGS accelerate the return of G-proteins to their inactive form and exert regulatory functions on intracellular effectors. However, it is not yet known which RGS regulate which receptor transduction pathways in the brain. To start to answer this question, we studied the influence of specific agonists and antagonists of the dopamine D1 and D2 receptors on the gene expression of the five most abundant RGS in the striatum: RGS2, RGS4, RGS8, RGS9 and RGS10. Only changes in RGS2 and RGS4 mRNA levels were observed. The D1 agonist SKF82958 and D2 antagonist haloperidol caused an up-regulation of RGS2 (+ 38.0% and + 41.6%, respectively). The D1 antagonist SCH23390 and D2 agonist quinpirole caused a down-regulation of RGS2 (- 25.0% and - 35.0%) and an up-regulation of RGS4 (+ 57.2% and + 52.5%). D1 and D2 receptors exert opposite effects on RGS2 expression, as they do on cAMP levels, suggesting a cAMP-mediated transcription of RGS2. This was confirmed by the unique induction of RGS2 (+ 111.1%) observed in the periventricular zone of the striatum after intracerebroventricular injection of forskolin. RGS4 was up-regulated only when RGS2 was down-regulated. This suggests that both RGS exert distinct functions. Considering the coupling of D1 and D2 receptors to the intracellular effector adenylate cyclase 5 (AC5) through their respective Galpha subunits in the striatum, our data allow us to suggest that RGS2 regulates the D1/Galphaolf/AC5 pathway and RGS4 the D2/Galphao/AC5 pathway.

Behavioural activity of (S)-3,5-DHPG, a selective agonist of group I metabotropic glutamate receptors

The influence of intracerebroventricular (i.c.v.) injections of (S)-3,5-dihydroxyphenyl-glycine (S)-3,5-DHPG, a selective agonist of group I metabotropic glutamate receptors (mGluRs), on the activity of the central nervous system was examined in male rats. (S)-3,5-DHPG at doses of 25, 50 and 100 nmol significantly attenuated crossings of squares and rearings, but not bar approaches, in an 'open field' test and failed to change apomorphine-induced stereotypy. (S)-3,5-DHPG at the above doses, given immediately after the learning trial, significantly facilitated the consolidation process in a passive avoidance situation, but given before the learning trial and before the retention testing did not have any influence on acquisition and retrieval processes, respectively. Moreover, (S)-3,5-DHPG did not influence recognition memory evaluated in an object recognition test. These results may suggest that activation of group I mGluRs takes part in the consolidation process in affectively-motivated memory, but is probably not necessary for processing of recognition memory, and that (S)-3,5-DHPG memory facilitation seems to be independent of glutamatergic and dopaminergic interaction.

The 3-7 fragment of angiotensin II is probably responsible for its psychoactive properties

The abilities of angiotensin II-(3-7)-pentapeptide (A-II-(3-7), 1 nmol) and angiotensin II (A-II, 1 nmol) to influence rat's psychomotor and cognitive behaviours were compared. Both peptides, given intracerebroventricularly (i.c.v.), 15 min before the experiment, increased number of crossings, rearings and bar approaches in the open field. A-II-(3-7) as well as A-II, at the same doses and routes, significantly intensified stereotypy produced by apomorphine (1 mg/kg) and amphetamine (6.5 mg/kg), both given intraperitoneally. The 3-7 fragment of A-II and A-II in equimolar doses (1 nmol, i.c.v.) were similarly effective in improving learning of conditioned avoidance responses and recall of a passive avoidance behaviour. Taken together, these data and our previous findings indicate that, in rats, the 3-7 fragment of A-II is responsible for the psychoactive properties of angiotensins.

Alpha 1 and alpha 2-adrenergic receptor blockade influences angiotensin II facilitation of avoidance behavior and stereotypy in rats

Pretreatment of rats with prazosin (PRA), an alpha 1-adrenergic receptor blocker, abolished the increased rate of learning of conditioned avoidance responses stimulated by intracerebroventricular angiotensin II (AII) administration. Yohimbine (YOH), an alpha 2-receptor blocker, reversed the effect of AII. PRA did not affect, and YOH abolished, the improvement of recall of a passive avoidance behavior caused by AII. The stereotypies produced by apomorphine (APO) and amphetamine (AMP) were enhanced by AII. PRA changed neither stereotypy, but it abolished the AII effect in both cases. YOH did not alter APO stereotypy and abolished the enhancement of that behavior caused by AII. YOH increased AMP stereotypy and had an additive effect with AII. No significant changes of exploratory motor activity were caused by PRA, YOH, or their combination, with AII. These findings indicate that functioning alpha 1- and alpha 2-adrenergic receptors are necessary for the facilitation of learning by AII, while only alpha 2-receptors appear to be involved in AII improvement of recall. The central dopaminergic system may in part be responsible for the modulation by PRA and YOH of the effects of AII on learning and recall.

Some behavioural effects of captopril in rats

1. Effects of inhibition of angiotensin converting enzyme (ACE, EC 3.4.15.1) in brain on psychomotor, exploratory, stereotyped and cognitive behaviour in rats were investigated. To inhibit brain ACE captopril (D-3-mercaptopropanoyl-L-proline) was given orally (p.o., 50 mg/kg) or intracerebroventricularly (i.c.v., 5 micrograms/rat). 3. Captopril given p.o. but not i.c.v. significantly enhanced stereotypy, overall number of conditioned avoidance responses, and decreased blood pressure. 4. No statistically significant influence of captopril given by either route on the number of crossings, rearings and bar approaches in the open field, performance of passive avoidance and number of correct choices as well as the speed of running for food in the T-maze was observed. 5. In conclusion, a small decrease of the activity of nigrostriatal dopaminergic system caused by the decrease of AII and/or increase of bradykinin, substance P, enkephalins and neurotensin in brain resulting from ACE inhibition is postulated.

Angiotensin II-(3-8)-hexapeptide affects motor activity, performance of passive avoidance and a conditioned avoidance response in rats

Angiotensin II-(3-8)-hexapeptide, at the dose of 1 nmol given intracerebroventricularly, only slightly less than angiotensin II (the same dose and route) stimulated exploratory locomotor behaviour in an open field and electromagnetic motimeter. Both peptides considerably enhanced stereotyped behaviour produced by apomorphine and amphetamine. Angiotensin II-(3-8)-hexapeptide improved recall in a passive avoidance situation as well as angiotensin II. The 3-8 C-terminus of angiotensin II enhanced acquisition of active avoidance nearly as effectively as the complete peptide. The results indicate that the effectiveness of equimolar doses of angiotensin II-(3-8)-hexapeptide and angiotensin II in improving processes related to learning and memory in rats is almost identical and thus must be independent of specific angiotensin receptors in brain to which the hexapeptide binds with about 1000 times lower affinity than angiotensin II. The stimulation of stereotypy, a dopamine-controlled behaviour, by the peptides points to the possibility of dopaminergic mediation of their psychotropic effects.

Effect of angiotensin II and saralasin on motor activity and the passive avoidance behavior of rats

One nmole of angiotensin II (ANG II) or saralasin, given intracerebroventricularly, failed to alter the motor activity of rats in open field. A combined injection of both peptides caused a significant decrease of the number of crossings and rearings. In the electromagnetic motimeter horizontal activity of animals was changed by neither of the peptides while the vertical activity was increased by ANG II. Again, a combined injection of saralasin and ANG II inhibited both horizontal and vertical activity. Stereotypies evoked by both apomorphine (2 mg/kg) and amphetamine (6.5 mg/kg), given intraperitoneally, were markedly intensified by ANG II and saralasin. A five-fold increase of the re-entry latencies in the passive avoidance situation was observed after pre-test administration of ANG II or saralasin but not the two in combination. These results suggest that ANG II and saralasin may improve processes related to learning and memory through an unspecific mechanism involving central dopamine systems.

Behavioral effects of angiotensin II and angiotensin II-(4-8)-pentapeptide in rats

One nM of angiotensin II (AII) or angiotensin II-(4-8)-pentapeptide [AII(4-8)] given intracerebroventricularly did not affect locomotor and exploratory behavior of rats in open field. AII significantly increased and AII(4-8) did not affect vertical activity of animals in electromagnetic motimeter. Neither of the peptides influenced horizontal activity in the motimeter. Both peptides intensified stereotypy produced by apomorphine and amphetamine. AII significantly improved, while AII(4-8) did not affect, consolidation of memory of the correct way to food in T-maze. Similarly, AII increased and AII(4-8) did not change the rate of acquisition of conditioned avoidance responses in a shuttle-box. Of the two examined peptides only AII significantly improved retrieval of memory of the passive avoidance behavior. The results show that AII(4-8) influences central dopaminergic system but, unlike its parent peptide AII, has no apparent effect on memory.

Neurotensin blocks certain amphetamine-induced behaviours

Bilateral injections of either neurotensin (NT; 0.3, 1 or 5 micrograms in 1 microliter artificial CSF) or haloperidol (HA; 2.5 or 5 micrograms in 1 microliter 0.3% tartaric acid) into nucleus accumbens of rats markedly diminished the forward locomotion and rearing induced by d-amphetamine (AM; 2 mg per kg, IP). Neither NT nor HA affected the insomnia or sniffing component of AM arousal. Isovolumetric intra-accumbens injections of artificial CSF or the endogenous decapeptide, luteinizing hormone-releasing hormone (LHRH; 3 micrograms), did not affect AM behaviours. Since intra-accumbens injections of NT (1 microgram) or HA (2.5 micrograms) neither altered forward locomotion or rearing observed in untreated rats placed in an open field nor a variety of reflex activities, the observed effects of NT and HA in AM-treated rats were probably not due to impaired motor function per se. In contrast, NT does not produce neuroleptic-like effects when injected into nucleus caudatus; HA (5 micrograms) blocked stereotyped sniffing, licking, biting and head bobbing observed after AM (5 mg per kg, IP), but NT (3 or 5 micrograms) did not. Since NT and dopamine are present in substantial quantities in the nucleus accumbens, NT may act in the nucleus accumbens to modulate dopaminergic function.

Caffeine potentiation of apomorphine discrimination

Rats were trained to discriminate between the stimulus properties of intraperitoneal 0.16 mg/kg apomorphine and saline in a two-lever, food-motivated operant task. Apomorphine, at doses different than the training dose, produced a dose-response relationship, whereas, caffeine (7.5-30 mg/kg) produced saline-like responding. However, co-administered of 15 mg/kg caffeine with 0.01, 0.02 or 0.04 mg/kg apomorphine potentiated the discriminative stimulus properties of these low apomorphine doses. This potentiation was antagonized by pretreatment with 0.25 mg/kg haloperidol. The results are consistent with the idea that caffeine, by virtue of being a phosphodiesterase inhibitor, may increase post-synaptic cyclic-AMP and this, in turn, may supersensitize the dopamine receptors and result in the potentiation of the apomorphine-induced dopaminergic responses.

Orbital or medial frontal cortical lesions have different effects on tail pressure-elicited oral behaviors in rats

Three groups of rats were tested with daily tail pressure (TP) tests until reliable and stable baseline eating, gnawing or licking was observed. One group then received bilateral aspiration of the medial frontal cortex, a second group received orbital frontal cortical lesions, and a third group received control lesions of the motor cortex. Daily TP tests were continued postoperatively. There was no disruption of TP-elicited oral behaviors after medial frontal or motro cortex lesions. In contrast, orbital frontal lesions abolished TP behaviors on the first day postoperatively, and there was a slow recovery of TP until day 5 when the elicited behaviors were about 80% of preoperative levels. The time course of recovery of TP-elicited oral behavior closely paralleled the recovery of elective eating after orbital frontal lesions, both in the group given TP and in another group given orbital lesions but not TP. These data demonstrate a marked difference between the medial and orbital divisions of the prefrontal cortex in the mediation of stress-induced oral behavior, and we discuss our data in terms of the possible role of dopamine terminals in these regions.

Regulation of dopamine synthesis in striatal synaptosomes during depolarization

Incubation of striatal synaptosomes under depolarizing conditions (veratridine or KCl) increased the release of dopamine (DA) and produced a small increase in DA synthesis. This was not accompanied by an activation of either soluble or membrane-bound tyrosine hydroxylase, but was associated with a large depletion of synaptosomal DA. Thus, depolarization may increase DA synthesis by reducing the inhibition of tyrosine hydroxylase by end-product.