The behavioural responses observed in animals after administration of the dopamine agonist, apomorphine are attributed to activation of D1
]. Mesolimbic and nigrostriatal dopaminergic pathways play key roles in the mediation of locomotor activity and stereotyped behaviour. Stereotyped behaviour is more closely associated with the caudate striatum area of the brain [31
]. Animal models used for screening antipsychotic drugs are based on the neurochemical hypothesis of schizophrenia, involving mainly the neurotransmitters dopamine and glutamate [34
]. Antagonism of dopamine D2
receptors may be a common feature of most clinically effective antipsychotic drugs, especially those active against hallucinations and delusions [35
]. The dopamine-based models usually employ apomorphine, a direct agonist, or amphetamine, a drug that increases the release of this neurotransmitter and blocks its re-uptake. In the present study, acute pre-treatment of MMC, (1-10 g/kg, p.o) in mice showed a significant dose-dependent decrease in climbing behaviour and climbing time induced by apomorphine. In addition, 7 and 21 days pre-treatment with TNJ (30, 50 and 100% v/v in daily drinking water) also significantly alleviated apomorphine-induced climbing behaviour and climbing time in mice. The reference drug, haloperidol (2 mg/kg, i.p) completely reversed the apomorphine-induced climbing behaviour and climbing time in mice. Margarita et al [24
] demonstrated neuroleptic activity of the juice of the ripe fruit of the noni in mice. Noni juice at different doses (5, 10 and 100 ml/kg, i.p) equivalent to dried juice powder doses (450, 900 and 1800 mg/kg, i.p) significantly reduced stereotypies induced by amphetamine (3 mg/kg, s.c) in a dose-dependent manner [24
]. The present study results are consistent with these findings and revealed MMC and TNJ might have dopamine D1 and/or D2 receptors antagonistic phytoconstituents.
To further confirm these findings, another set of experiment was carried out by replacing apomorphine with methamphetamine. Methamphetamine is a member of the family of phenethylamine. Methamphetamine causes the norepinephrine, dopamine, and serotonin (5HT) transporters to reverse their direction of flow. This inversion leads to a release of these transmitters from the vesicles to the cytoplasm and from the cytoplasm to the synapse, causing increased stimulation of post-synaptic receptors. Methamphetamine also indirectly prevents the reuptake of these neurotransmitters, causing them to remain in the synaptic cleft for a prolonged period [36
]. It would then produce an effect that is similar to the apomorphine-induced behaviour in mice which in this case is stereotypy. Stereotyped licking, biting, and other orofacial behaviours are known to involve nigrostriatal dopaminergic neurotransmission [37
] in distinct striatal sub regions [38
]. Importantly, it appears that control of sniffing and biting is mediated by different striatal sub regions [38
]. Overall, mesolimbic dopaminergic mechanisms have been proposed to play a critical role in the expression of stereotypy after acute psychostimulant administration [41
]. Since some of the specific behaviours produced in these models closely resemble that of humans abusing amphetamines, animals that display stereotypy have been considered as an animal model for amphetamine psychosis and are considered to be particularly relevant to schizophrenia [42
]. However, because of the compulsive and the repetitive nature of the behaviour, amphetamine-induced stereotypy have also been considered as potential animal models of obsessive–compulsive disorder [43
] and autism [44
]. Methamphetamine-induced stereotypy persists for several hours in rodents, and this abnormal behaviour can be reversed by dopamine antagonists [45
], but less so by other agents [46
]. The acute pre-treatment of MMC (1, 3 and 5 g/kg, p.o) significantly decreased the methamphetamine-induced stereotypy and cage climbing time in a dose-dependent manner. These results further confirm the antidopaminergic effect of MMC of noni unripe fruits.
Analgesic properties for commercial noni juice in rats have been reported in the literature. The results showed that rats fed with 10% and 20% noni juice had greater pain tolerance (162% and 212%, respectively) compared with the placebo group [3
]. Noni root extract (1600 mg/kg) showed significant analgesic activity in mice through the writhing and hotplate tests, similar to the effect of morphine (75% and 81% protection using noni extract and morphine, respectively), and it was also proven to be non-toxic [5
]. These studies suggested that the central pharmacological effects of noni can be observed at higher doses [24
]. In the present study, the antidopaminergic effects of MMC and TNJ were observed only at higher doses.
Several oral toxicity studies in Sprague-Dawley rats, using the widely consumed commercial noni fruit juice, TNJ have been assessed [3
]. Acute and sub chronic (13 wks.) oral toxicity studies revealed no diverse effects from consuming doses equivalent to 80 ml/kg body weight/d [48
]. Pureed noni fruit from Tahiti was administered by oral gavage at a dose of 15 g/kg to Sprague-Dawley rats. All animals survived and showed no signs of toxicity or behavioural changes when observed for two weeks. Conversely, all animals appeared healthy and gained weight. Gross necropsies of all animals at the end of 2 weeks revealed no pathological effects. Consequently, the LD50 of noni fruit was found to be greater than 15 g/kg [49
]. Compounds are considered nontoxic if the acute oral LD50 is greater than 5 g/kg, or if the acute intraperitoneal LD50 is greater than 2 g/kg. The LD50 of noni fruit juice and its crude extract are greater than the minimum criteria for nontoxic status [48
]. The present acute toxicity results are consistent with these earlier reports. In the present study, acute oral treatment of TNJ at 200 ml/kg and MMC at 20 g/kg respectively did not show any toxic effects and behavioural changes when observed for 14 days.
It has been demonstrated that M. citrifolia
possess antiemetic property in patients who are considered high risk for postoperative nausea and vomiting (PONV) after various types of surgery. The 600 mg dose of noni extract (equivalent to 20 g of dried noni fruit/ 8.712 mcg of scopoletin) was the minimum dose that effectively reduced the incidence of postoperative nausea in the early postoperative period [50
]. However, this study could not reveal the possible mechanism of action of M. citrifolia
for antiemetic action. Our study result suggests that the antiemetic activity of M. citrifolia
might be mediated through dopaminergic pathways. It is well known that dopamine D2
receptors in the area postrema play an important role in the regulation of emetic responses in ferrets, dogs and humans [51
]. From a clinical point of view, dopamine receptor antagonists such as phenothiazines, butyrophenones and benzamides, which have affinity for dopamine D2
receptors, are used as antiemetic agents [53
]. The contributions of dopaminergic abnormalities to the pathophysiology of schizophrenia have been studied most intensively. The focus on dopaminergic abnormalities in schizophrenia was prompted by the complementary observations in humans that psychosis can be elicited by psychostimulant medications such as d-amphetamine, especially when they are abused, whereas the ability to inhibit competitively the binding of dopamine to the D2
type of dopamine receptor is a pharmacological property shared by all of the conventional antipsychotic medications [54
]. The obtained antidopaminergic activity of M. citrifolia
in the present studies could be utilized in the treatment of schizophrenia.
Conversely, it has been demonstrated that ethyl acetate fraction of crude methanol extract of Morinda citrifolia
at a daily dose of 200 and 400 mg/kg when administered to rats for 15 days significantly increased the brain levels of serotonin, dopamine and noradrenaline, which could be attributed to the significant protection offered against MES induced seizures in rats [22
]. These opposing effects could be due to dose differences. This kind of biphasic response was extensively studied and reported in the literature [55
]. However, further neurochemical studies in the brain are necessary to confirm this hypothesis and such studies are indeed underway in our laboratory.
Phytochemical studies using high performance liquid chromatographic (HPLC) fingerprint profile of the MeOH extracts of noni fruit revealed three major peaks representing scopoletin, rutin and quercetin (retention times: 25.72, 28.57 and 32.70 min, respectively), together with several minor peaks. These are major bioactive constituents of noni responsible for various pharmacological activities [20
]. The antipsychotic like-effect of MMC and TNJ observed in the present study might be attributed to the presence of these phytoconstituents. Investigation of antidopaminergic active phytoconstituents responsible for noni's antipsychotic effect by using bioassay-guided chromatographic fractionation is currently underway in our laboratory.