The effect of estrogen on dopamine and serotonin receptor and transporter levels in the brain: An autoradiography study

Abstract

The aim of the present study was to elucidate the effect of estrogen on dopaminergic and serotonergic regulation of prepulse inhibition (PPI) by measuring its effects on the density of dopamine transporters (DAT), dopamine D₁ and D₂ receptors, serotonin transporters (SERT), serotonin-1A (5-HT_1A) and 5-HT_2A receptors using radioligand binding autoradiography. Three groups of female Sprague–Dawley rats were compared: sham-operated controls, untreated ovariectomized (OVX) rats and OVX rats with a 17β-estradiol implant (OVX + E). These groups were identical to our previous prepulse inhibition (PPI) studies, allowing comparison of the results. Results showed that in the nucleus accumbens, DAT levels were 44% lower in OVX rats than in intact controls. Estrogen treatment completely reversed the effect of OVX in this brain region to levels similar to those in intact controls. Dopamine D₂ receptor density was increased in OVX rats by 28% in the nucleus accumbens and 25% in the caudate nucleus compared to intact controls. Estrogen treatment reversed this increase and, in addition, reduced dopamine D₂ receptor levels by a further 25% and 20%, respectively, compared to intact control rats. There were no differences between the groups with respect to the densities of dopamine D₁ receptors, SERT, 5-HT_1A receptors or 5-HT_2A receptors. These results show effects of estrogen treatment on central indices of dopaminergic, but not serotonergic function. The observed changes do not provide a direct overlap with the effects of these estrogen treatment protocols on drug-induced disruptions of PPI, but it is possible that a combination of effects, i.e. on both DAT and dopamine D₂ receptor density, is involved. These data could also be relevant for our understanding of the potential protective effect of estrogen treatment in schizophrenia.

Introduction

Recently, a clinical trial revealed that adjuvant transdermal estradiol treatment attenuated positive and general psychopathological symptoms, but not negative symptoms, in women with schizophrenia (Kulkarni et al., 2001, Kulkarni et al., 2008). This confirmed and extended earlier epidemiological, neurochemical and pre-clinical findings, that estrogen may play a role in schizophrenia (Seeman, 1997, Stevens, 2002). For example, it has been recognized for several years that there are gender differences in the age-of-onset, symptom severity and treatment response in schizophrenia (for references, see Castle et al., 1998, Häfner, 2003, Seeman, 1997). However, despite many reports on the effects of estrogen on brain neurotransmitter activity (for reviews, see Bethea et al., 1998, Di Paolo, 1994), the mechanism by which estrogen might be involved in schizophrenia, remains unclear.

We previously focused on prepulse inhibition (PPI) of acoustic startle to study the potential role of estrogen in schizophrenia. PPI is the reduction of a startle response to a loud, sudden noise pulse, if it is preceded by a low-intensity prepulse. PPI has been widely used as a model for sensory gating, i.e. the ability to filter sensory information and allow focused attention (Braff et al., 2001). PPI is reduced in patients with schizophrenia and other neurological/psychiatric illnesses (Braff and Geyer, 1990, Braff, 1993). One advantage of PPI is, that it can be studied across species with similar methodology, stimulus characteristics and behavioral responses (Geyer and Swerdlow, 1998). In rats and mice, PPI is disrupted by several dopaminergic and serotonergic drugs (Geyer et al., 2001). We initially studied the effect of the serotonin-1A (5-HT_1A) receptor agonist, 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) and observed that in both species, estrogen treatment modulates its effects on PPI (Gogos and Van den Buuse, 2004, Gogos et al., 2006a). For example, in ovariectomized rats chronically treated with a high dose, but not a low dose of estradiol, the disruption of PPI induced by treatment with 8-OH-DPAT was prevented (Gogos and Van den Buuse, 2004). A subsequent proof-of-concept study in human volunteers showed the same effect and confirmed the translational relevance of these results (Gogos et al., 2006b). Thus, in healthy women treated with estradiol, the disruption of PPI caused by treatment with the partial 5-HT_1A receptor agonist, buspirone, was prevented (Gogos et al., 2006b), similar to the effect of 8-OH-DPAT in rats. Subsequent studies in rats extended these findings to suggest that the action of 8-OH-DPAT (and presumably buspirone) was mediated by modulating dopaminergic activity (Gogos et al., in press). In these experiments, chronic estrogen treatment similarly inhibited the effect of 8-OH-DPAT and the dopamine receptor agonist, apomorphine, on PPI (Gogos et al., in press). In other studies we had already observed that dopamine D₂ receptor antagonists, such as haloperidol, could block the action of 8-OH-DPAT on PPI (Van den Buuse and Gogos, 2007), suggesting that serotonergic pathways and 5-HT_1A receptors are functionally ‘in series’ with dopaminergic pathways and dopamine D₂ receptors to modulate PPI.

In the present study we use the chronic estradiol treatment rat model to further investigate mechanisms involved in the effect of estrogen on the brain, particularly transporter and receptor densities. Because of our previous results (see above), we focused on serotonergic and dopaminergic parameters. With respect to serotonin, we included 5-HT_1A receptor levels, as well as the other major serotonergic receptor in the brain, the 5-HT_2A receptor, and the serotonin transporter (SERT). With respect to dopamine, we included dopamine D₂ receptors, as well as the other major dopaminergic receptor, the dopamine D₁ receptor, and dopamine transporter (DAT). Brain regions chosen were those with the highest level of the respective transporters or receptors. The results show significant changes in dopamine D₂ receptor and DAT levels caused by chronic estrogen treatment. These results may help to explain our in vivo results using PPI and, potentially, elucidate the role of estrogen in schizophrenia.