Anxiolytic actions of diazepam, but not of buspirone, are influenced by gender and the endocrine stage

Abstract

The effect of diazepam (1.0 mg/kg, ip) and buspirone (5.0 mg/kg, ip) on the burying behaviour latency (denoting actions on the animals' reactivity) and on the cumulative burying behaviour (directly reflecting the experimental anxiety levels), were analyzed in male-, intact females, at proestrus and metoestrus, and in neonatally-androgenized-rats. Androgenization was performed by injecting 60 μg/rat of testosterone propionate on day 5 after delivery. Two main groups of neonatally-androgenized rats were established: A group of animals showing permanent oestrus from the vaginal opening (acyclic females) and a group presenting the delayed anovulatory syndrome. Diazepam produced a clear reduction in experimental anxiety in males and neonatally-androgenized-females. Particularly important was the anxiolytic effect of diazepam on acyclic females that was accompanied by a significant increase in burying behaviour latency. Conversely, buspirone induced a clear reduction in burying behaviour, without modifying its latency, in all groups regardless of the gender and the neonatal treatment. Data are discussed on the basis of the androgen participation on the anxiolytic drug effects. A possible age-related benzodiazepine actions in females is suggested.

Introduction

It has been demonstrated that the action of various anxiolytic drugs vary depending on the gender. Thus, we have shown that diazepam, at the same dose (1.0 mg/kg) produces anxiolytic-like effect in males but not in females. Conversely, the injection of the serotonergic anxiolytics, buspirone and 8-OH-DPAT reduced burying behaviour (a response interpreted as a reduction in anxiety) similarly regardless of the gender [5]. Additionally, Lynda Uphouse and coworkers established that some actions of these same serotonergic agonists are gender dependent and also may vary according to the female oestrous cycle phase [29]. In line with the differential drug effects along the endocrine cycle, variations in the burying behaviour response to diazepam, have been reported [5]. Moreover, we have recently observed blockade of the anxiolytic action of 8-OH-DPAT in lactating rats [8]and others have reported on the absence of the inhibition of 8-OH-DPAT on female sexual response in estrogen pretreated rats [12]. All these data, taken together, suggest that the action of benzodiazepines and serotonergic anxiolytics may differ depending both, upon the gender of the neural structures on which they act to produce their behavioural effects, and on the hormonal milieu circumscribing such structures. An additional factor that could modify the anxiolytic action of serotonergic drugs is age. Thus, we have found that in 3 week young male animals no reduction in burying behaviour is found because of presumably a floor-effect, while in relatively old animals ipsapirone and buspirone loose its tranquilizing properties whereas 8-OH-DPAT and indorenate still produce their actions [14].

In 1964 Swanson and van der Werff ten Bosch [25]reported that administration of low doses of androgens during the critical period of sexual differentiation, result in a greatly reduced period of fertility. This effect is manifested in two different manners: A large population of females showing persistent vaginal cornification from puberty onwards and a small but very consistent population of females that cease ovulating and exhibited permanent vaginal oestrus in young adulthood. This last phenomenon is referred to as the `delayed anovulatory syndrome' (DAS) [9]. One hypothesis for the permanent effects of exogenous androgen exposure on the neonatal hypothalamus is that select aspects of neural and endocrine age-related changes are accelerated [25]. Thus, androgen-sterilized female rats pass into an endocrine state at the juvenile period (persistent vaginal cornification) that would typically occur during their midlife [30]. Although controversial as an experimental animal model of reproductive senescence 4, 13, 30, such manipulation has the main advantage of including females with intact components of their physiology for the expression of various behavioural cues independently of alterations in their neuroendocrine functions. Thus, for example, it has been demonstrated that various behaviours, useful for establishing experimental anxiety in laboratory animals such as the defensive burying behaviour gradually declines after a peak present during youth 27, 28.

On these bases, the general purpose of the present work was to analyze whether injection of diazepam or buspirone, tested at single effective doses, may cause differential effects in animals in various endocrine states: Normal cycling females (at proestrus or metoestrus), intact males, acyclic females and females with one or two cycles that develop the DAS. These last two groups neonatally received low doses of androgens to produce such a state. The experimental anxiety was determined in the burying behaviour test, a paradigm which robustness has been established under various physiological and pharmacological conditions 6, 27, 28.