Involvement of dopamine in the differences in sexual behaviour between roman high and low avoidance rats: behavioral, pharmacological and neurochemical findings

INVOLVEMENT OF DOPAMINE IN THE DIFFERENCES IN SEXUAL BEHAVIOUR BETWEEN ROMAN HIGH AND LOW AVOIDANCE RATS: BEHAVIORAL, PHARMACOLOGICAL AND NEUROCHEMICAL FINDINGS. The Roman lines of rats (High – RHA, and Low Avoidance - RLA) display opposite behavioral traits: RHA rats are active copers, impulsive and prone to abuse drugs while RLA rats are reactive copers, hyperemotional and prone to develop depressive-like symptoms. These differences are linked to differences in brain monoamine function and neuroendocrine responses to stress. RHA and RLA rats differ also in sexual behavior, with RHA rats displaying higher motivation and better copulatory performances than RLA rats [1]. Recently, we found that the differences observed in sexual behavior between the two Roman lines occur concomitantly to differences in the activation of the mesolimbic dopaminergic system, which plays a key role in motivated behaviors (i.e. goal-directed behaviors), as assessed by the differences in dopamine concentrations in the dialysates obtained from the nucleus accumbens shell by means of intracerebral microdialysis before and during sexual activity [2]. Together with the well known role of dopamine in sexual behavior [3], these findings suggest that the sexual differences between RHA and RLA rats may be due to differences in dopamine neurotransmission. Another brain area involved in motivated behaviors is the medial prefrontal cortex (mPFC), which receives, as the nucleus accumbens, dopaminergic projections from the ventral tegmental area (VTA). The mPFC is involved in several functions, from decision making to coping to stress, from behavioral inhibition to incentive evaluation. This raises the possibility that the mPFC might be another brain area in which the Roman lines display differences in dopaminergic (monoaminergic) activity related to the observed differences in sexual behavior. In order to test this hypothesis, naïve (never exposed to a receptive female) and sexually experienced (which underwent five copulation tests) RHA and RLA rats implanted with a microdialysis probe aimed at the mPFC, were used in a classical appetitive/consummatory test of sexual behavior, during which copulatory parameters were recorded and dialysate aliquots collected from the mPFC for the determination of dopamine and noradrenaline by HPLC-ECD. The results show that the higher sexual motivation and better performance of RHA vs. RLA rats occurred concomitantly with a higher dopamine and noradrenaline release in the mPFC, as shown by the higher concentrations found in the mPFC dialysates of RHA vs. RLA rats. These differences between the two lines were greater in naïve animals and persisted, although attenuated, in experienced animals. Furthermore, sexually experienced rats of both lines displayed higher dopamine, and to a lesser extent noradrenaline, release in the mPFC compared to their sexually naïve counterparts. These findings confirm that a different mesocorticolimbic dopaminergic tone exists in RHA and RLA rats, which may be responsible, together with differences in noradrenaline release, at least in part, for their different copulatory patterns, and could provide further insights into the differences among individuals in the neural basis of motivated behaviors and their relationship with vulnerability to abuse natural and/or drug rewards or to develop depressive disorders.