Subchronic treatment with fluoxetine attenuates the proerectile effect of Aspidosperma ulei Markgr (Apocyanaceae).

OBJECTIVE: To evaluate the ability of acute or chronic treatment with fluoxetine to alter the proerectile effect of Aspidosperma ulei alkaloid-rich fraction (F3-5).
MATERIALS AND METHODS: In the first series of experiments, three groups of mice received either a single intraperitoneal injection of vehicle, F3-5 (25 mg/kg) or fluoxetine (10 mg/kg) + F3-5. Three behavioral responses were counted over a period of 30 min: erection, erection-like response and genital grooming. In a second series of experiments, animals treated for 13 days with fluoxetine or fluoxetine + F3-5 were assessed.
RESULTS: A. ulei has been suggested to have proerectile effect in mice. Subchronic (13-d) treatment with fluoxetine resulted in a reduction in the number erections in F3-5-treated mice.
CONCLUSION: Sexual dysfunction associated with antidepressant treatment continues to be a major compliance issue for antidepressant therapy. Acute administration of fluoxetine resulted in a near total reversal of the proerectile effect of F3-5.

Introduction

Aspidosperma ulei is a plant that largely grows in the Amazon region of Brazil and in many other parts of South America that is found to be rich in indole alkaloids. An in vitro relaxant property of the alkaloid-containing extract of A. ulei has been described on vascular and nonvascular smooth muscle in rats, guinea-pigs and rabbits.[1] We have recently demonstrated a proerectile activity of an alkaloid-rich fraction (F3–5) from A. ulei root bark that might have resulted from both central and peripheral sites of action involving α-adrenergic, dopaminergic and nitrergic receptor mechanisms.[2] Another study has demonstrated that the fraction F3–5 from A. ulei exerts a relaxant effect on rabbit corpus cavernosum smooth muscle in vitro by a mechanism probably related to the calcium antagonism.[3] These reports suggest that F3–5 may have a clinical perspective in patients with erectile dysfunction (ED).

Currently available agents for ED share the same mechanism of action, pharmacologic properties and the limitations including a principal focus on erection as an end-organ process. One of the relatively unexplored areas of research has been the potential for centrally acting agents to improve male sexual response. A variety of neurohormones and neurotransmitter systems are involved in the male sexual response, including testosterone, dopamine, serotonin and the melanocortin systems.[4] The aim of this work was to evaluate the effect of acute and chronic treatment with fluoxetine, a selective serotonin reuptake inhibitor (SSRI), on erectile responses in male mice treated with A. ulei.

Materials and Methods

Animals

Male Swiss mice (25–30 g) obtained from the central animal house of Federal University of Ceará were used. They were housed in polypropylene cages at 23±2°C before experimentation under standard environmental conditions (12-light/12-h dark cycle; 55–60% relative humidity) and had free access to pellet diet (Purina chow) and tap water. The Institutional Committee on the Care and Use of Animals for Experimentation approved the experimental protocols.

Plant Material, Extraction and Fractionation

A. ulei Markgr. was collected from the Garapa area of Acarape, Ceara, Brazil, after its identification, and a voucher specimen was deposited in Herbarium Prisco Bezerra (N°. 30823) of Federal University of Ceara, Fortaleza. The fraction, F3–5 was obtained from the ethanolic extract of A. ulei root bark according to a previously described procedure[2] 1H NMR analysis of this fraction (F3–5) revealed the presence of three major indole-type alkaloids, which have been identified as uleine, nor-uleine and tetrahydro- 3,14,4,21-elipticine based on spectral details and in comparison with literature data.[5]

Penile Erection-Related Behaviors

The method described by Rampin et al[6] was followed. Mice were habituated to the testing room for at least a 2 h period and to the test chamber (a glass aquarium, 25 ×20 ×20 cm) for 5 min, before the start of experiment. In the first series of experiments, three groups of mice (n = 8 per group) received either a single intraperitoneal injection of vehicle (3% dimethylsulfoxide, 10 mL/kg), F3-5 (25 mg/kg) or fluoxetine 10 mg/kg s.c. + F3-5. The dose selection of F3-5 was based on our previous observations.[2] Personnel blind to the treatment conducted the behavioral evaluation. Observations were commenced 30 min after the plant extract injection by placing each male in the test chamber. Three behavioral responses were counted over a period of 30 min:erection (when the male stood on its hind limbs, bent its body forward, bent its head down to reach the genital area, licked its penis and displayed hip movements); erection-like response (the male stood on its hind limbs, bent its body forward, bent its head down, licked the abdomen but not the penis and displayed hip movements; the penis was not erect); and genital grooming (the male sat on its hindquarters, bent his head down and performed grooming of the genital area). In a second series of experiments, animals treated for 13 days with fluoxetine or fluoxetine + F3-5 were assessed.

Statistical Analysis

Comparisons between groups were performed using one-way analysis of variance (ANOVA) followed by Student-Newman-Keul's test. Differences were considered statistically significant when P < 0.05.

Results

Our results show that acute administration of fluoxetine reduced the proerectile effect (no. of erections) of F3-5 by 50%. A similar effect was also seen on erection-like and genital grooming reponses (data not shown). The proerectile effect of F3-5 remained equally altered in mice receiving fluoxetine for 13 days [Table 1].

Table 1

Effect of acute and subchronic treatment with fluoxetine on proerectile action of F3-5 in mice

Discussion

Impairment of sexual activity is one of the most frequent side effects of antidepressant drugs and the increase in the synaptic concentrations of serotonin seems to be mainly responsible.[7] Furthermore, the activation of some serotonin (5-HT) receptor subtypes, is known to inhibit (5-HT1A), but others stimulate (5-HT2C) and penile erection.[8]

In previous work, we have demonstrated that the fraction F3–5 from A. ulei root bark elicits penile erection-related behaviors in male mice. Presence of indole alkaloids closely related to yohimbine in this fraction and previous studies that show the clinical efficacy of an yohimbine rich extract from the bark of Aspidosperma quebracho blanco[9] prompted us to undertake this study. Indole alkaloids can exert potent central and peripheral pharmacological effects by influencing various neurotransmitter systems. F3–5-induced erections in mice were abolished by clonidine, an α2-adrenoceptor agonist, suggesting that it may function as an α2-adrenoceptor blocker.

Systemic administration of α2-antagonists has been shown to enhance serotonergic neurotransmission via direct inhibition of the α2-heteroceptors located on the serotonergic nerve terminals and indirect stimulation of α1-receptors via inhibition of α2-autoreceptors.[10] The coadministration of an SSRI and mirtazapine (an agent with α2-antagonist activity) shortens the delay in enhancing the tonic activation of the postsynaptic 5-HT1A receptor.[11]

In spite of its selective effect on 5-HT transport system, it has been reported that fluoxetine could inhibit noradrenaline uptake in brain.[12] It was observed by Busch et al[12] that long-term treatment with fluoxetine-modified isolated rat vas deferens contractility, and suggested an alteration of vas deferens contractile response to adrenergic neurotransmission. It seems then possible that some of the untoward effects associated with fluoxetine therapy could be due to an alteration of the adrenergic neurotransmission at a peripheral level.

Inhibitory effects of an acute treatment with fluoxetine on sexual behavior are consistent with prior reports.[13] Fluoxetine rapidly increases extracellular 5-HT in a variety of brain areas including the hypothalamus, thus, acute effects of fluoxetine are consistent with 5-HT's ability to inhibit the sexual behavior.[14] Since 5-HT's inhibiting effect resides, in part, from the neurotransmitter's activation of 5-HT1A receptors,[15] it is tempting to speculate that fluoxetine acutely reduces the proeretile effect of F3-5 via enhanced activation of 5-HT1A receptors.

The reduced effectiveness of the acute treatment with F3-5 in mice subchronically treated with fluoxetine is consistent with prior findings that 7-10 days of fluoxetine treatment reduced the effect of a 5-HT1A receptor agonist on activation of the hypothalamic–pituitary–adrenal axis.[16] The current findings lead to the suggestion that subchronic treatment with fluoxetine reduces negative effects of fluoxetine on mouse sexual behavior. Serum concentrations of fluoxetine were not measured in the subjects. Therefore, we cannot rule out the possibility that F3-5 is having a pharmacokinetic effect that results in altered metabolism of fluoxetine.