Ricardo Cabrera

Hormonal Regulation of Female Reproduction

Reproduction is an event that requires the coordination of peripheral organs with the nervous system to ensure that the internal and external environments are optimal for successful procreation of the species. This is accomplished by the hypothalamic-pituitary-gonadal axis that coordinates reproductive behavior with ovulation. The primary signal from the central nervous system is gonadotropin-releasing hormone (GnRH), which modulates the activity of anterior pituitary gonadotropes regulating follicle stimulating hormone (FSH) and luteinizing hormone (LH) release. As ovarian follicles develop they release estradiol, which negatively regulates further release of GnRH and FSH. As estradiol concentrations peak they trigger the surge release of GnRH, which leads to LH release inducing ovulation. Release of GnRH within the central nervous system helps modulate reproductive behaviors providing a node at which control of reproduction is regulated. To address these issues, this review focuses on several critical questions. How is the HPG axis regulated in species with different reproductive strategies? What internal and external conditions modulate the synthesis and release of GnRH? How does GnRH modulate reproductive behavior within the hypothalamus? How does disease shift the activity of the HPG axis?

Allopregnanolone modulates striatal dopamingergic activity of rats under different gonadal hormones conditions.

Abstract Objectives: Progesterone modulates dopamine (DA) release in corpus striatum. Our objective was to evaluate the effect of the i.c.v injection of the neurosteroid allopregnanolone (ALL), a progesterone metabolite on dopaminergic activity in the corpus striatum of rats under different gonadal hormonal conditions. Methods: We have measured the concentrations of DOPA, DA and DOPAC (main metabolite of DA) in the corpus striatum in estrus and diestrus rats and in ovariectomized rats without hormonal replacement (OVX group) and primed with estrogen and progesterone (OVXi group). Additionally, we have used the aromatic acid decarboxylase inhibitor NSD in order to evaluate the function of tyrosine hydroxylase (TH), the rate-limiting enzyme of dopamine synthesis. Results: ALL significantly decreased the striatal concentrations of both DA and DOPAC in the estrus. On the other hand, ALL increased significantly the levels of DA in the OVXi group. The DOPA accumulation in OVXi after NSD treatment in the ALL-treated groups was greater than in the vehicle group. However, the estrus group did not modify the DOPA accumulation after NSD injection. Discussion: Our results suggest that ALL could modulate the dopaminergic transmission in the corpus striatum by causing changes in the activity of TH and/or in the pre- and post-synaptic dopaminergic terminals in the corpus striatum. This neurosteroidal mechanism could be a new kind of neurotransmitter systems modulation accomplished on TH activity itself and/or on the second messengers not related to ionic channels. Additionally, our results reinforce the idea of a close relationship between the fast non-genomic mechanism of ALL and the genomic actions of estrogen and progesterone.

Allopregnanolone prevents memory impairment: Effect on mRNA expression and enzymatic activity of hippocampal 3-α hydroxysteroid oxide-reductase

In this work we investigated how the neurosteroid allopregnanolone can modulate learning and memory processes. For this purpose, we used ovariectomized (OVX) rats subcutaneously injected with oestradiol benzoate (E) alone or E and progesterone (P). Then, rats were injected in dorsal hippocampus with allopregnanolone or vehicle. Animals were tested in inhibitory avoidance task (IA task). After behavioural test hippocampal mRNA expression and enzymatic activity of 3α-HOR, the enzyme responsible of allopregnanolone synthesis, were analysed. In IA task OVX-EP rats spent less time on platform, compared to those OVX or OVX-E. Regression analyses revealed that there was a significant negative relationship between E-P infusion and performance in this task. Pre-training allopregnanolone administration to OVX-EP rats increased the time spent on the platform. Interestingly, when enzymatic activity of 3α-HOR was tested, OVX-EP rats showed a significant decrease in the enzymatic activity, compared with OVX and OVX-E rats. In addition, OVX-EP group showed a significant increase in the enzymatic activity after intrahippocampal infusion of allopregnanolone. On the other hand, when mRNA expression of 3α-HOR was analysed no differences were observed when the hippocampal allopregnanolone injection was done. These results suggest that E and P have amnesic effects on female rats, being reversed by allopregnanolone through its modulation on hippocampal 3α-HOR activity.

Allopregnanolone induces LHRH and glutamate release through NMDA receptor modulation

LHRH release from hypothalamus is influenced by the neurotransmitter glutamate that acts, among others, on NMDA receptors present in LHRH neurons. On the other hand, the neurosteroid allopregnanolone can modulate the activity of specific neurotransmitter receptors and affect neurotransmitter release. We examined the role of allopregnanolone on in vitro LHRH and glutamate release from mediobasal hypothalamus and anterior preoptic area of ovariectomized rats with estrogen and progesterone replacement. Moreover, we evaluated whether the neurosteroid might act through modulation of NMDA receptors. Allopregnanolone induced an increase in LHRH release. This effect was reversed when the NMDA receptors were blocked by the NMDA antagonist 2-amino-7-phosphonoheptanoic acid (AP-7) indicating that this neurosteroid would interact with NMDA receptors. Moreover allopregnanolone induced an augment in K+ evoked [3H]-glutamate release from mediobasal hypothalamus-anterior preoptic area explants and this effect was also reversed when NMDA receptors were blocked with AP-7. These results suggest an important physiologic function of allopregnanolone on the regulation of neuroendocrine function in female adult rats. Not only appears to be involved in enhancing LHRH release through modulation of NMDA receptors but also in the release of glutamate which is critical in the control of LHRH release.

Glutamate neurotransmission is affected in prenatally stressed offspring.

Previous studies from our laboratory have shown that male adult offspring of stressed mothers exhibited higher levels of ionotropic and metabotropic glutamate receptors than control rats. These offspring also showed long-lasting astroglial hypertrophy and a reduced dendritic arborization with synaptic loss. Since metabolism of glutamate is dependent on interactions between neurons and surrounding astroglia, our results suggest that glutamate neurotransmitter pathways might be impaired in the brain of prenatally stressed rats. To study the effect of prenatal stress on the metabolism and neurotransmitter function of glutamate, pregnant rats were subjected to restrain stress during the last week of gestation. Brains of the adult offspring were used to assess glutamate metabolism, uptake and release as well as expression of glutamate receptors and transporters. While glutamate metabolism was not affected it was found that prenatal stress (PS) changed the expression of the transporters, thus, producing a higher level of vesicular vGluT-1 in the frontal cortex (FCx) and elevated levels of GLT1 protein and messenger RNA in the hippocampus (HPC) of adult male PS offspring. We also observed increased uptake capacity for glutamate in the FCx of PS male offspring while no such changes were observed in the HPC. The results show that changes mediated by PS on the adult glutamatergic system are brain region specific. Overall, PS produces long-term changes in the glutamatergic system modulating the expression of glutamate transporters and altering synaptic transmission of the adult brain.

Progesterone prevents depression-like behavior in a model of Parkinson's disease induced by 6-hydroxydopamine in male rats

Hemiparkinsonism induced by 6-hydroxydopamine (6-OHDA) injected in left corpus striatum is a recognized model of motor deficits in rats. Some reports concerning motor deficits indicate a favorable response to steroid administration in hemiparkinsonian animals. However, there is no much information regarding progesterone administration in relation to cognitive and affective dysfunctions. Here we could confirm earlier reports regarding a mild deficit of memory and a noticeable depressive-like behavior 4 weeks after injecting 6-OHDA. We also present some evidence that progesterone could be - when administered 7 days after the injection of 6-OHDA - a possible neuroprotector concerning both motor deficits as well as cognitive - memory- and depression-like behaviors. The affective deficit was reverted by administering the tricyclic antidepressant imipramine. Since Parkinsons disease is a conspicuous cause of psycho-organic decline in human beings, it would be important to be able of dealing early with non-motor indicators in order to use prospective neuroprotectors to prevent the progression of the disease.

Ghrelin inhibited serotonin release from hippocampal slices.

Ghrelin (Ghr) is a peptide produced peripherally and centrally. It participates in the modulation of different biological processes. In our laboratory we have shown that (a) Ghr administration, either intracerebroventricular or directly into the hippocampus enhanced memory consolidation in a step down test in rats (b) the effect of Ghr upon memory decreases in animals pretreated with a serotonin (5-HT) reuptake inhibitor, Fluoxetine, suggesting that Ghr effects in the hippocampus could be related to the availability of 5-HT. It has been demonstrated that Ghr inhibits 5-HT release from rat hypothalamic synaptosomes. Taking in mint these evidences, we studied the release of radioactive 5-HT to the superfusion medium from hippocampal slices treated with two doses of Ghr (0.3 and 3 nm/μl). Ghr inhibited significantly the 5-HT release in relation to those superfused with artificial cerebrospinal fluid (ACSF) (H = 9.48, df = 2, p ≤ 0.05). In another set of experiments, Ghr was infused into the CA1 area of hippocampus of the rats immediately after training in the step down test and the 5-HT release from slices was studied 24h after Ghr injection showing that in this condition also the 5-HT release was inhibited (H = 11.72, df = 1, p ≤ 0.05). In conclusion, results provide additional evidence about the neurobiological bases of Ghr action in hippocampus.

Postnatal Administration of Allopregnanolone Modifies Glutamate Release but Not BDNF Content in Striatum Samples of Rats Prenatally Exposed to Ethanol

Ethanol consumption during pregnancy may induce profound changes in fetal CNS development. We postulate that some of the effects of ethanol on striatal glutamatergic transmission and neurotrophin expression could be modulated by allopregnanolone, a neurosteroid modulator of GABAA receptor activity. We describe the acute pharmacological effect of allopregnanolone (65 μg/kg, s.c.) administered to juvenile male rats (day 21 of age) on the corticostriatal glutamatergic pathway, in both control and prenatally ethanol-exposed rats (two ip injections of 2.9 g/kg in 24% v/v saline solution on gestational day 8). Prenatal ethanol administration decreased the K+-induced release of glutamate regarding the control group. Interestingly, this effect was reverted by allopregnanolone. Regarding BDNF, allopregnanolone decreases the content of this neurotrophic factor in the striatum of control groups. However, both ethanol alone and ethanol plus allopregnanolone treated animals did not show any change regarding control values. We suggest that prenatal ethanol exposure may produce an alteration of GABAA receptors which blocks the GABA agonist-like effect of allopregnanolone on rapid glutamate release, thus disturbing normal neural transmission. Furthermore, the reciprocal interactions found between GABAergic neurosteroids and BDNF could underlie mechanisms operating during the neuronal plasticity of fetal development.

Neuromodulatory effect of progesterone on the dopaminergic, glutamatergic, and GABAergic activities in a male rat model of Parkinson's disease.

Abstract Objectives: Progesterone has been reported to have a neuroprotective role in depression-like rats in a hemiparkinsonian model of the disease. In this work, we investigate if this hormone affects the three principal neurochemicals striatal systems (dopaminergic, glutamatergic, and GABAergic) that are involved in the physiopathology of the disease in a hemiparkinsonim male rat model at 8 weeks post-chemical injury. Methods: For this purpose, we design three experimental groups: (1) sham group; (2) hemiparkinsonian group; and (3) hemiparkinsonian group subcutaneously injected with progesterone at 7 days post-chemical injury. Animals were tested in an automated rotational device at 8 weeks post-chemical injury. After behavioral test, K+-evoked [3H]-dopamine, [3H]-glutamate, and [3H]-gamma aminobutyric acid release from striatum slices were analyzed by superfusion experiments. Results: The hemiparkinsonian group showed distinctive alterations that are produced by neurodegeneration of left nigrostriatal dopaminergic pathway by 6-hydroxydopamine hydrobromide (6-OHDA). On the other hand, the administration of progesterone 7 days after the injection of the neurotoxin was able to (1) improve the K+-evoked [3H]-dopamine release from the damaged striata (left); (2) avoid significant increase in the K+-evoked [3H]-glutamate release from the left striata; and (3) progesterone does not modify the K+-evoked [3H]-gamma aminobutyric acid release from the left striata. Discussion: These results suggest that progesterone does have neuroprotective and neuromodulatory effects on striatal neurotransmission systems in the hemiparkinsonian male rats. The possible mechanisms would involve genomic and non-genomic actions of this neuroactive steroid which would modulate the activity of dopaminergic, glutamatergic, and GABAergic pathways.

Progesterone Exerts a Neuromodulatory Effect on Turning Behavior of Hemiparkinsonian Male Rats: Expression of 3 α -Hydroxysteroid Oxidoreductase and Allopregnanolone as Suggestive of GABAA Receptors Involvement.

There is a growing amount of evidence for a neuroprotective role of progesterone and its neuroactive metabolite, allopregnanolone, in animal models of neurodegenerative diseases. By using a model of hemiparkinsonism in male rats, injection of the neurotoxic 6-OHDA in left striatum, we studied progesterones effects on rotational behavior induced by amphetamine or apomorphine. Also, in order to find potential explanatory mechanisms, we studied expression and activity of nigrostriatal 3α-hydroxysteroid oxidoreductase, the enzyme that catalyzes progesterone to its active metabolite allopregnanolone. Coherently, we tested allopregnanolone for a possible neuromodulatory effect on rotational behavior. Also, since allopregnanolone is known as a GABAA modulator, we finally examined the action of GABAA antagonist bicuculline. We found that progesterone, in addition to an apparent neuroprotective effect, also increased ipsilateral expression and activity of 3α-hydroxysteroid oxidoreductase. It was interesting to note that ipsilateral administration of allopregnanolone reversed a clear sign of motor neurodegeneration, that is, contralateral rotational behavior. A possible GABAA involvement modulated by allopregnanolone was shown by the blocking effect of bicuculline. Our results suggest that early administration of progesterone possibly activates genomic mechanisms that promote neuroprotection subchronically. This, in turn, could be partially mediated by fast, nongenomic, actions of allopregnanolone acting as an acute modulator of GABAergic transmission.