Dopaminergic Profile of New Heterocyclic N-phenylpiperazine Derivatives

Dopamine constitutes about 80% of the content of central catechol-amines and has a crucial role in the etiology of several neuropsychiat-ric disorders, including Parkinsons disease, depression and schizo-phrenia. Several dopaminergic drugs are used to treat these patholo-gies, but many problems are attributed to these therapies. Within this context, the search for new more efficient dopaminergic agents with less adverse effects represents a vast research field. The aim of the present study was to report the structural design of two N-phenyl-piperazine derivatives, compound 4:

Dopamine has a crucial role in the etiology of several neuropsychiatric disorders, including Parkinsons disease, depression and, mainly, schizophrenia.Schizophrenia is a chronic psychiatric illness that affects approximately 1% of the world population.Usually, a schizophrenic patient presents two main kinds of symptoms: positive (delusions, hallucinations) and negative (avolition, anhedonia, attentional impairment) symptoms.Most of the drugs used for treating this disorder are D 2 -like receptor antagonists and are called typical antipsychotics.These drugs are effective only for treating the positive symptoms and present an important profile of adverse effects, mainly the extrapyramidal disturbances such as tardive dyskinesia (2,3).
Clozapine ( 1) is an atypical antipsychotic drug with high affinity for D 4 and 5-HT 2 receptors that has a pharmacological profile different from that of classic antipsychotics, being effective in the treatment of positive and negative symptoms and lacking the extrapyramidal effects.On the other hand, clozapine (1) tends to induce agranulocytosis in 1 to 2% of the patients, which restricts its clinical use in patients that do not respond to classical therapy (2,4).For this reason, the search for new more efficient dopaminergic agents with lower adverse effects is still an extremely active research field.
Male Wistar rats (200 to 250 g) and male Swiss CF-1 mice (20 to 30 g), both from the breeding colony of Fundação Estadual de Produção e Pesquisa em Saúde (Porto Alegre, RS, Brazil), were used.The animals had free access to food and water and were kept at constant room temperature (22 ± 1ºC) under a 12-h light cycle (lights off at 7:00 pm).All drugs were administered by the intraperitoneal (ip) route (injection volume of 1 ml/kg rat body weight and 1 ml/100 g mouse body weight).Compounds 4 and 5 were suspended in saline with the aid of no more than 5% (v/v) polysorbate 80.The experimental groups consisted of at least 10 animals each.
Amphetamine-induced stereotypy was evaluated according to the protocol and doses described by Carlini (7).Thirty minutes after amphetamine (30 mg/kg, ip) administration, the results were scored from 0 to 3 according to increasing intensity of stereotypy (7).This behavior is classically blocked by drugs that inhibit the dopaminergic functions such as neuroleptics (D 2 antagonists) (8).Conversely, atypical antipsychotics like clozapine (1) have little or no effect on stereotypy (9).The results are shown in Table 1.Only compound 4 (30 mg/kg, ip) significantly blocked the stereotyped behavior, although compound 5 (30 mg/ kg, ip) also showed a tendency to block it.
The induction of catalepsy test was adapted from those described by Carlini (7).The mice were placed gently on a wood bar elevated 6.5 cm from the floor.The time spent by the animals in this position was measured 30 and 60 min after the treatments.The results are shown in Table 1.Both derivatives induced a time-and dose-dependent cataleptic behavior in the animals.Compound 5 presented a longer-lasting effect, indicating a possible pharmacokinetic difference between drugs.Despite having virtually no strength as an animal behavioral model for antipsychotics, catalepsy tests do have value in the study of neuropharmacology of extrapyramidal function and as a rapid behavioral screening for predicting the motor side effects of potentially antipsychotic drugs (10,11).The extrapyramidal side effects of antipsychotic drugs are related to a blockade of D 2 striatal dopamine receptors (12).However, selective D 1 blockers such as SCH23390 also induce marked catalepsy behavior in animals similar to those of opioids and cholinergic agonists (10).
To test the apomorphine (1 mg/kg ip)induced hypothermia, two treatments were administered to each animal: the first one immediately after measuring the basal rectal temperature (T0) and the second one 30 min later.The rectal temperature was recorded 15 and 30 min after the second treatment (T45 and T60, respectively) (9,13).The results are shown in Table 2.A single administration of compounds 4 (30 mg/kg, ip) and 5 (30 mg/kg, ip) induced a hypothermic response which was not abolished by haloperidol (0.5 mg/kg, ip) administration.Furthermore, a synergistic hypothermic effect between apomorphine (1 mg/kg, ip) and compound 5 was demonstrated.
Production of hypothermia is a major pharmacological effect of apomorphine in animals.It has been demonstrated that, at low doses (1 mg/kg), this effect results from the direct agonistic action of apomorphine on both D 1 and D 2 dopamine receptors located in the temperature-regulating centers of the hypothalamus (13)(14)(15).Functional interaction between D 1 and D 2 receptors has been well documented and these receptors are generally considered to interact synergistically.Nevertheless, there is also evidence for independent roles of dopamine D 1 and D 2 receptors in thermoregulation (14,16).
On the other hand, the role of serotonin receptors in thermoregulation should also be considered (17).
At the tested dose, haloperidol efficiently blocked the apomorphine effect on the core temperature of the animals.In contrast, it has been reported that higher doses of haloperidol induce a hypothermic response which is antagonized by the administration of 5-HT 2 agonists (18).Thus, since haloperidol did not affect the N-phenylpiperazine derivative-induced hypothermia, it is plausible to suggest that the mechanism of the hypothermic effect of compounds 4 and 5 is other than dopamine D 2 /D 3 receptor activation.A hypothermic effect that was not abolished by a selective D 2 antagonist was also reported The treatments were SAL (saline + polysorbate 80 up to 5%, 1.0 ml/100 g mouse body weight), APO (apomorphine, 1.0 mg/kg), HAL (haloperidol, 0.5 mg/kg), C4 (compound 4, 30.0 mg/kg), and C5 (compound 5, 30.0 mg/kg).T0 is the basal temperature of the animals before any treatment, T45 the rectal temperature 15 min after the second treatment, and T60 the rectal temperature 30 min after the second treatment.for the selective D 1 agonist A68930 (14).In addition, it was reported that clozapine (1)induced hypothermia in rats is blockaded by selective D 1 antagonists, suggesting that this drug may be a partial D 1 agonist (19,20).On the other hand, Menon et al. (15) demonstrated that the selective D 1 antagonist SCH23390 was not effective in blocking the apomorphine-induced hypothermia but caused a potentiation of the hypothermic effect of quinpirole, a selective D 2 agonist.Since compounds 4 and 5 were planned based on the chemical structure of clozapine, it is possible that their hypothermic effects are also mediated by D 1 receptor occupation or, perhaps, by interaction with the serotonergic system.The hypothermic effects of compounds 4 and 5 have a magnitude comparable to that of subcutaneous 10 mg/kg clozapine in rats (20).Other experiments are in progress to elucidate the putative participation of D 1 and 5-HT 2 receptors in the hypothermic effect of N-phenylpiperazine derivatives.
In conclusion, the N-phenylpiperazine derivatives 4 and 5 seem to have a peculiar profile of action on dopaminergic functions.On the basis of the results of catalepsy and amphetamine-induced stereotypy, the compounds demonstrated an inhibitory effect on dopaminergic behaviors.However, their hypothermic effect is compatible with the stimulation of dopaminergic function which seems not to be mediated by D 2 /D 3 receptors.

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N-phenylpiperazine derivatives • Dopamine • Apomorphine hypothermia • Amphetamine stereotypy • Catalepsy Brazilian Journal of Medical and Biological Research (2003) 36: 625-629 ISSN 0100-879X Short Communication Dopamine belongs to the catecholamine neurotransmitter group and constitutes about 80% of the content of these substances in the brain.At least five different forms of dopamine receptors have been cloned from the brain.These receptors are divided into two classes: the D 1 -like class (including D 1 and D 5 receptors) and the D 2 -like class (including D 2 , D 3 and D 4 receptors), differentiated by anatomical, pharmacological and biochemical parameters (1).

Table 1 .
Effect of compounds 4 and 5 on the induction of catalepsy and blockade of amphetamine (30 mg/kg)-induced stereotypy in rodents.

Table 2 .
Effect of compounds 4 and 5 on the rectal temperature of mice.