Involvement of dopamine receptors in the antipsychotic profile of (-) eticlopride

Sex-related differences in intravenous ketamine effects on dissociative stereotypy and antinociception in male and female rats

Ketamine, a multimodal dissociative anesthetic drug, is widely used to treat various conditions including acute pain and treatment-resistant depression. We previously reported that subanesthetic doses of intravenous (i.v.) ketamine produced transient dissociative stereotypy and antinociception in male rats. However, sex-related differences in the effects of i.v. ketamine on these measures are not well characterized. Adult male and female Sprague-Dawley rats (10 weeks old) received an i.v. bolus saline or ketamine (2 and 5 mg/kg), and dissociative stereotypy (head weaving, ataxia, and circling) and natural behaviors (horizontal activity, rearing, and grooming) were quantified over a 10-min period. Ten minutes after the behavioral observation, antinociception was measured using a tail flick test. The i.v. ketamine administration increased head weaving, ataxia, circling, and horizontal activity while decreasing rearing and grooming behaviors in male and female rats. Following 5 mg/kg ketamine administration, ataxia was greater in female rats, while head weaving was greater in male rats. Among the female rats, head weaving was greater in the low estrogen group (diestrus phase) as compared to the high estrogen group (proestrus/estrus phase). Ketamine doses (2 and 5 mg/kg) produced antinociception in male and female rats, and female rats were more sensitive to the antinociceptive effects of 2 mg/kg ketamine. The current findings suggest that i.v. ketamine administration, a clinically relevant route of administration, may produce sex-related differences in dissociative behaviors and analgesia between males and females.

Role of cannabis and endocannabinoids in the genesis of schizophrenia

RATIONALE

Cannabis abuse and endocannabinoids are associated to schizophrenia.

OBJECTIVES

It is important to discern the association between schizophrenia and exogenous Cannabis sativa, on one hand, and the endogenous cannabinoid system, on the other hand.

RESULTS

On one hand, there is substantial evidence that cannabis abuse is a risk factor for psychosis in genetically predisposed people, may lead to a worse outcome of the disease, or it can affect normal brain development during adolescence, increasing the risk for schizophrenia in adulthood. Regarding genetic predisposition, alterations affecting the cannabinoid CNR1 gene could be related to schizophrenia. On the other hand, the endogenous cannabinoid system is altered in schizophrenia (i.e., increased density of cannabinoid CB1 receptor binding in corticolimbic regions, enhanced cerebrospinal fluid anandamide levels), and dysregulation of this system can interact with neurotransmitter systems in such a way that a "cannabinoid hypothesis" can be integrated in the neurobiological hypotheses of schizophrenia. Finally, there is also evidence that some genetic alterations of the CNR1 gene can act as a protectant factor against schizophrenia or can induce a better pharmacological response to atypical antipsychotics.

CONCLUSIONS

Cannabis abuse is a risk factor for psychosis in predisposed people, it can affect neurodevelopment during adolescence leading to schizophrenia, and a dysregulation of the endocannabinoid system can participate in schizophrenia. It is also worth noting that some specific cannabinoid alterations can act as neuroprotectant for schizophrenia or can be a psychopharmacogenetic rather than a vulnerability factor.

[(3)H]4-(Dimethylamino)-N-[4-(4-(2-methoxyphenyl)piperazin- 1-yl)butyl]benzamide, a selective radioligand for dopamine D(3) receptors. I. In vitro characterization

4-(Dimethylamino)-N-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)benzamide (WC-10), a N-phenyl piperazine analog, has been shown to have high affinity and selectivity for dopamine D(3) receptors versus dopamine D(2) receptors (Chu et al. [2005] Bioorg Med Chem 13:77-87). In this study, WC-10 was radiolabeled with tritium (specific activity = 80 Ci/mmol) and [(3)H]WC-10 binding to genetically cloned dopamine D(2L) and D(3) receptors was evaluated in vitro. [(3)H]WC-10 binds with a 66-fold higher affinity to human HEK D(3) than HEK D(2L) receptors, with a dissociation constant (K(d)) of 1.2 nM at HEK D(3) receptors. However, [(3)H]WC-10 binds to rat Sf9 rD(3) receptors with a K(d) of 3.9 nM, a value that is 3-fold lower than binding to human HEK D(3) receptors and 40-fold value higher than binding to rat Sf9 rD(2L) receptors. The K(d) values obtained from saturation binding experiments were consistent with the results determined from kinetic (k(on) and k(off)) studies. The pharmacologic profiles of a series of dopaminergic drugs for inhibiting the binding of [(3)H]WC-10 to D(3) receptors was in agreement with previously reported data. In vitro autoradiography studies of rat and monkey brains show that [(3)H]WC-10 labeled D(3) sites in the striatal region.

Antipsychotic-like properties of 5-alpha-reductase inhibitors

Recent evidence indicates that neuroactive steroids may participate in the pathogenesis of schizophrenia spectrum disorders, yet the mechanisms of this involvement are elusive. As 5-alpha-reductase (5AR) is the rate-limiting enzyme of one of the two major metabolic pathways in brain steroidogenesis, we investigated the effects of its blockade in several rat models of psychotic-like behavior. The 5AR inhibitor finasteride (FIN, 60 or 100 mg/kg, intraperitoneal, i.p.) dose- and time-dependently antagonized prepulse inhibition (PPI) deficits induced by apomorphine (APO, 0.25 mg/kg, subcutaneous, s.c.) and d-amphetamine (AMPH, 5 mg/kg, s.c.), in a manner analogous to haloperidol (HAL, 0.1 mg/kg, i.p.) and clozapine (CLO, 5 mg/kg, i.p.). Similar results were observed with the other 5AR inhibitors dutasteride (DUT, 40 or 80 mg/kg, i.p.) and SKF 105111 (30 mg/kg, i.p.). FIN (60 or 100 mg/kg, i.p.) also reduced hyperlocomotion induced by AMPH (1 or 3 mg/kg, s.c.) and attenuated stereotyped behaviors induced by APO (0.25 mg/kg, s.c.). Nevertheless, FIN (100 mg/kg, i.p.) did not reverse the PPI disruption induced by the N-methyl-d-aspartate receptor antagonist dizocilpine (0.1 mg/kg, s.c.). FIN (60-300 mg/kg, i.p.) induced no catalepsy in either the bar test or the paw test. Our results suggest that 5AR inhibitors elicit antipsychotic-like effects in animals and may be proposed as a putative novel target in the management of psychotic disorders.

A review of the discovery, pharmacological characterization, and behavioral effects of the dopamine D2-like receptor antagonist eticlopride

Eticlopride is a substituted benzamide analog with high affinity and selectivity for dopamine (DA) D2-like receptors that was initially developed as a potential antipsychotic agent. A great deal of research has utilized this drug to better understand central DA receptor function, the role of D2-like receptors in behavior, and the influence of blockade of these receptors on several preclinical animal models. This review highlights research utilizing this drug and compares it to typical and atypical antipsychotics used clinically. First, we describe structure-activity relationships as it relates to binding at DA receptors and the consequences on behavior. This is followed by a discussion of several imaging strategies including the use of eticlopride for in vivo, in vitro, and ex vivo examination of DA D2-like receptor densities and function. Finally, we discuss the use of eticlopride in several behavioral models predictive of antipsychotic activity, extrapyramidal side effects (EPS), and learning and memory. While eticlopride is not used clinically, it remains a viable research tool for understanding DA receptor function and behavior.

Effects of age and sex on ketamine-induced hyperactivity in rats

Investigators tested the hypothesis that administration of the NMDA antagonist ketamine would result in differential effects on activity levels in rats depending upon the age and the sex of the animal. Twenty-two-, 35-, and 50-day-old rats were given doses of ketamine (0.0 or 10.0 mg/kg) and tested for open-field activity and frequencies of reverse locomotion, rearing, turning, and head weaving. Results indicated that ketamine produced hyperactivity in both males and females at 22 days of age but only in females at 35 days of age. There was no effect of ketamine on locomotor activity in 50-day-old rats, regardless of sex. Effects of ketamine on turning, reverse locomotion, and head weaving were similar with administration, in general, causing increments in these behaviors in both sexes at 22 days and in females at all ages tested. Ketamine resulted in reductions in rearing in both sexes regardless of age.

QF2004B, a potential antipsychotic butyrophenone derivative with similar pharmacological properties to clozapine

The aim of the present work was to characterize a lead compound displaying relevant multi-target interactions, and with an in vivo behavioral profile predictive of atypical antipsychotic activity. Synthesis, molecular modeling and in vitro and in vivo pharmacological studies were carried out for 2-[4-(6-fluorobenzisoxazol-3-yl)piperidinyl]methyl-1,2,3,4-tetrahydro-carbazol-4-one (QF2004B), a conformationally constrained butyrophenone analogue. This compound showed a multi-receptor profile with affinities similar to those of clozapine for serotonin (5-HT2A, 5-HT1A, and 5-HT2C), dopamine (D1, D2, D3 and D4), alpha-adrenergic (alpha1, alpha2), muscarinic (M1, M2) and histamine H1 receptors. In addition, QF2004B mirrored the antipsychotic activity and atypical profile of clozapine in a broad battery of in vivo tests including locomotor activity (ED50 = 1.19 mg/kg), apomorphine-induced stereotypies (ED50 = 0.75 mg/kg), catalepsy (ED50 = 2.13 mg/kg), apomorphine- and DOI (2,5-dimethoxy-4-iodoamphetamine)-induced prepulse inhibition (PPI) tests. These results point to QF2004B as a new lead compound with a relevant multi-receptor interaction profile for the discovery and development of new antipsychotics.

Generalization of clozapine as compared to other antipsychotic agents to a discriminative stimulus elicited by the serotonin (5-HT)2A antagonist, MDL100,907

Employing a two-lever, food-reinforced FR10 procedure, rats were trained to recognize a discriminative stimulus (DS) elicited by the 5-HT(2A) receptor antagonist and potential antipsychotic agent, MDL100,907 (0.16 mg/kg, i.p.). In generalization tests, by analogy to MDL100,907 itself (Effective Dose(50) (ED(50)), 0.002 mg/kg, s.c.), the 'atypical' antipsychotic, clozapine, which displays high affinity for 5-HT(2A) as compared to D(2) receptors, dose-dependently and fully generalized to MDL100,907 (ED(50), 0.2 mg/kg, s.c.). S16924 (0.05 mg/kg, s.c.), S18327 (0.09 mg/kg, s.c.), quetiapine (1.8 mg/kg, s.c.), risperidone (0.02 mg/kg, s.c.) and ziprasidone (0.01 mg/kg, s.c.), antipsychotics which possess-like clozapine-marked affinity for 5-HT(2A) versus D(2) receptors, also generalized to MDL100,907. In distinction, raclopride, an antipsychotic which selectively interacts with D(2) versus 5-HT(2A) receptors, did not display significant generalization. Interestingly, haloperidol, which shows only modest affinity for 5-HT(2A) versus D(2) sites, generalized to MDL100,907 (ED(50), 0.02 mg/kg, s.c.). In light of the antagonist properties of haloperidol, clozapine and all other antipsychotics tested (except raclopride) at alpha(1)-adrenoceptors (ARs), the selective alpha(1)-AR antagonists, prazosin and WB4101, were examined. Both dose-dependently and fully generalized to MDL100,907 (ED(50)s, 0.07 and 0.11 mg/kg, s.c., respectively). At doses showing pronounced generalization to MDL100,907, the only drugs which significantly suppressed response rates were haloperidol and, weakly, quetiapine. Raclopride also markedly decreased response rates. In conclusion, the antipsychotic agents, clozapine, ziprasidone, risperidone, S16924, S18327, quetiapine and haloperidol, all generalized to a DS elicited by MDL100,907. While D(2) receptors are not implicated in their actions, in addition to antagonist properties at 5-HT(2A) receptors, blockade of alpha(1)-ARs and other, as yet unidentified, mechanisms may be involved. These data underpin interest in MDL100,907 as a potential antipsychotic agent.

Influence of the cannabinoid agonist HU 210 on cocaine- and CQP 201-403-induced behavioural effects in rat

Acute injection of the cannabinoid agonist HU 210 (6.25-100 microg/kg, i.p.) dose-dependently inhibited rat locomotor activity and rearing, while subchronic treatment with the drug (once daily for 7 days) at the same doses only diminished locomotion. Acute but not subchronic administration of HU 210 (12.5-50 microg/kg, i.p.) potently counteracted acute and subchronic cocaine (15 mg/kg, i.p.)-induced hyperlocomotion and enhanced rearing. The acute cannabinoid (6.25-100 microg/kg, i.p.) also inhibited locomotor activity, stereotyped behaviour and shaking elicited by the D1/D2 agonist CQP 201-403 (500 microg/kg, i.p.). On the contrary, subchronic treatments with HU 210 enhanced CQP 201-403-induced locomotor activity and potently stimulated escape attempts. Discussion centers on the influence of cannabinoids on experimental models of psychosis.