Michael Schumacher

Regulation of High-Affinity GABAa Receptors in Specific Brain Regions by Ovarian Hormones

The regulation of 3H-muscimol binding to high-affinity GABAa receptors by estradiol (E) and by progesterone (P) was studied within discrete brain regions using in vitro quantitative autoradiography. Treatment of ovariectomized and adrenalectomized female rats with E resulted in a decrease of muscimol binding only in specific estrogen-sensitive brain regions like the ventromedial nuclei (VMN) of the hypothalamus, the arcuate nucleus (ARC), the medial amygdala and the midbrain central grey (MCG). When administered alone, P had no effect. However, in estrogen-primed females, P increased muscimol binding in both VMN and MCG to levels seen in control animals. Thus, E and P exert opposite effects on the GABAa receptor within these two nuclei. As both hormones facilitate female reproductive behavior as well as the release of luteinizing hormone, present results suggest that E and P affect muscimol binding by different mechanisms.

Regulation of high-affinity GABAA receptors in the dorsal hippocampus by estradiol and progesterone

The effects of estradiol benzoate (EB) and of progesterone (P) treatment on high-affinity [3H]muscimol binding in the dorsal hippocampal formation were examined in ovariectomized and adrenalectomized female rats by in vitro autoradiography. EB injected subcutaneously increased [3H]muscimol binding in specific subregions of the Ammons horn (CA1 and stratum radiatum of CA4) and of the dentate gyrus (dorsal molecular layer). In these particular regions, estrogen receptors have been shown to be present. P did not significantly affect [3H]muscimol binding in any region of the hippocampus when administered alone or in combination with EB. Results suggest that estrogens may regulate the activity of specific hippocampal neurons by modulating their sensitivity to GABA.

Regulation by dopaminergic neurotransmission of dopamine D2 mRNA and receptor levels in the striatum and nucleus accumbens of the rat.

The effect of dopamine depletion or pharmacological blockade of dopamine receptors on striatal and accumbens dopamine D2 mRNA and receptor levels was assessed by in situ hybridization histochemistry and receptor autoradiography. The time course of pharmacological blockade with haloperidol demonstrates a complex mode of regulation of dopamine D2 mRNA and receptor levels. By day 8 of haloperidol treatment, D2 mRNA and receptor levels were decreased (up to 20%) in the medial and anterior aspects of the caudate-putamen (mCPU and aCPU) and the nucleus accumbens (NAc). However, by day 21 of haloperidol treatment, D2 mRNA and receptor were increased relative to vehicle-injected controls. Likewise, unilateral dopamine depletion due to 6-hydroxydopamine (6-OHDA) lesions of mesencephalic dopaminergic neurons resulted in decreased levels of D2 receptor mRNA by day 8 post-lesion in the ipsilateral mCPU, aCPU and the NAc. However, at days 14 or 21 post-lesion, there was a reversal of the effect with increases of up to 22% in all brain regions ipsilateral to the lesion. Although no decreases in receptor level were observed at day 8, significant increases in receptor level in all three brain regions were detected at days 14 and 21 post-lesion. The results demonstrate that midbrain dopaminergic innervation exerts tonic effects on the levels of dopamine D2 receptor and mRNA in the caudate-putamen and the nucleus accumbens of the rat. Changes in receptor level are frequently accompanied by comparable changes in mRNA level, indicating a mass action relationship between receptor level and receptor biosynthesis in these forebrain regions in the rat.

Steroid and barbiturate modulation of the GABAa receptor. Possible mechanisms.

This review describes the modulation of the GABAa receptor by steroid hormones and barbiturates and proposes guidelines for further research. Having examined the complex organization of the GABAa receptor complex and the multiple allosteric interactions between its drug and transmitter/modulator binding sites, the possibility that conformational changes of the receptor molecule may explain most of its characteristics is explored. On the basis of considerable evidence, we propose that the GABAa receptor may adopt as many as five different conformations. However, the heterogeneity of central GABAa receptor binding cannot only be explained by different configurations of a single protein. It also has been shown that different GABAa receptor subtypes exist within different brain regions. These receptor subtypes may differ from each other in their subunit composition. By describing the GABAa receptor as a macromolecular complex that may adopt different conformations and whose subunit composition may vary, it becomes possible to understand the molecular mechanisms by which steroid hormones modulate the receptor. This has led to two models of hormone actions. A first model addresses the direct effects that steroids exert on the GABAa receptor and predicts that steroid hormones may cause the conformation of the receptor complex to change between active and inactive states. A second model, which addresses the observed heterogeneity of GABAa receptor binding within the brain, suggests that steroid hormones may change the expression of the different subunits of the receptor complex by acting at the genomic level. This review complements other recent reviews describing the modulation of the GABAa receptor (Olsen and Venter, 1986; Gee, 1988).

Increase in striatal dopamine D2 receptor mRNA after lesions of haloperidol treatment

In the present study, we show that both destruction of nigral dopamine neurons by 6-hydroxydopamine (6-OHDA) and chronic treatment with haloperidol increase the content of D 2 mRNA and D 2 receptors within distinct parts of the striatum

Steroid Regulation and Sex Differences in [3H]Muscimol Binding in Hippocampus, Hypothalamus and Midbrain in Rats

The gonadal steroids estradiol and progesterone have previously been shown to modulate the specific binding of the GABAA agonist, [3H]muscimol, in the CA1 region of the hippocampus, the ventromedial nucleus of the hypothalamus and the midbrain central gray of ovariectomized female rats. In this report we show a sex difference in the level of binding in the very caudal ventromedial nucleus of the hypothalamus. In contrast to females, there is no steroid modulation of [3H]muscimol binding in the ventromedial nucleus of the hypothalamus and midbrain central gray of males. These effects may be functionally related to GABAergic control of female sexual behavior. In contrast, steroid modulation of [3H]muscimol binding in the CA1 region of the hippocampus occurred to the same degree in males and females, and there was no difference in the level of binding in any region of the hippocampus between gonadectomized males and females.

Ovarian steroid modulation of [3H]muscimol binding in the spinal cord of the rat

[3H]Muscimol binding was measured in the lumbar spinal cord of female rats by in vitro quantitative autoradiography. Ovariectomized rats were treated subcutaneously with either oil, estradiol benzoate (EB) or EB plus progesterone (P) in a regime known to reliably induce sexual receptivity. The level of [3H]muscimol binding was highest in laminae I-III and in the region around the central canal. Binding was lower in laminae IV-VI and was frequently undetectable in the ventral horn. There was a significant increase in the level of binding in laminae I-III after EB treatment. There was also a significant increase after treatment with EB+P in comparison to both the ovariectomized and EB-treated groups in this same region of the spinal cord.

Sex differences in the regulation of oxytocin receptors by ovarian steroids in the ventromedial hypothalamus of the rat.

The facilitation of sexual receptivity by oxytocin (OT) in female rats is related to the regulation of oxytocin receptors (OTR) by ovarian steroids in the ventromedial nuclei (VMN) of the hypothalamus. In a previous study, we have shown that estradiol benzoate (EB) causes a twofold increase in OTR binding in the VMN. Progesterone (P) then modulates levels of the estrogen-induced OTR and increases the area occupied by the receptors by acting on the neuronal membrane. In the present study, we compared the effects of EB and P on OTR binding between males and females. In both sexes, EB increased the density of OTR and the area covered by the receptors at the level of the medial and caudal VMN. In estrogen-primed females, P further increased OTR levels in the medial VMN and the area covered by OTR at the level of the caudal VMN. By contrast, P did not modulate OTR binding in estrogen-primed males. Thus, the behavioral insensitivity of male rats to ovarian hormones, in particular to P, may be related to sex differences affecting the modulation of OTR binding.

Light-dark differences in behavioral sensitivity to oxytocin

Ovariectomized female rats treated with estradiol benzoate (EB) and progesterone (P) were infused intracerebroventricularly with a low (200 ng) or high (1 microgram) dose of oxytocin (OT). The low dose of OT facilitated lordosis behavior only during the dark phase of the light-dark cycle in females that were pretreated with low doses of EB (2 micrograms) and P (250 micrograms). In contrast, the high dose of OT facilitated lordosis behavior during both the light and the dark phases but only in long-term ovariectomized females that were primed with large amounts of EB (2 x 10 micrograms) and P (500 micrograms). In females that were primed with lower amounts of ovarian steroids, the high dose of OT failed to increase levels of lordosis responding in either the dark or light phase. Thus, when female rats are treated with physiological amounts of ovarian hormones and OT, they are more sensitive to the facilitative effects of the OT on lordosis behavior during the dark phase.

Binding of [3H]cholecystokinin in the ventromedial hypothalamus modulated by an afferent brainstem projection but not by ovarian steroids

The ventromedial nuclei (VMN) of the hypothalamus are innervated by cholecystokinin-immunoreactive (CCK-IR) fibers originating in the dorsal parabrachial nuclei (PBS). They also contain high levels of receptors for CCK and binding of [125I]CCK to these receptors is modulated by estrogen. In the present study, we show that unilateral lesion of the PBS increases the binding of the sulphated octapeptide of [3H]cholecystokinin ([3H]CCK) within the ipsilateral VMN of the hypothalamus, but not within other brain nuclei that contain receptors for CCK. Thus, CCK fibers originating in the PBS selectively innervate the VMN and CCK receptors within the VMN are postsynaptic to parabrachial afferents. However, treatment of ovariectomized rats with estradiol benzoate and progesterone did not affect the binding of [3H]CCK in the VMN, even after lesion of the parabrachial afferents.