Effects of p-chlorophenylalanine on hypothalamic indoleamine levels and the associated changes which occur in catecholamine dynamics and LH surges in estrogen-treated ovariectomized rats

Serotonin stimulates female preoptic area kisspeptin neurons via activation of type 2 serotonin receptors in mice

Background

The neuroendocrine control of ovulation is orchestrated by neuronal circuits that ultimately drive the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus to trigger the preovulatory surge in luteinizing hormone (LH) secretion. While estrogen feedback signals are determinant in triggering activation of GnRH neurons, through stimulation of afferent kisspeptin neurons in the rostral periventricular area of the third ventricle (RP3VKISS1 neurons), many neuropeptidergic and classical neurotransmitter systems have been shown to regulate the LH surge. Among these, several lines of evidence indicate that the monoamine neurotransmitter serotonin (5-HT) has an excitatory, permissive, influence over the generation of the surge, via activation of type 2 5-HT (5-HT2) receptors. The mechanisms through which this occurs, however, are not well understood. We hypothesized that 5-HT exerts its influence on the surge by stimulating RP3VKISS1 neurons in a 5-HT2 receptor-dependent manner.

Methods

We tested this using kisspeptin neuron-specific calcium imaging and electrophysiology in brain slices obtained from male and female mice.

Results

We show that exogenous 5-HT reversibly increases the activity of the majority of RP3VKISS1 neurons. This effect is more prominent in females than in males, is likely mediated directly at RP3VKISS1 neurons and requires activation of 5-HT2 receptors. The functional impact of 5-HT on RP3VKISS1 neurons, however, does not significantly vary during the estrous cycle.

Conclusion

Taken together, these data suggest that 5-HT2 receptor-mediated stimulation of RP3VKISS1 neuron activity might be involved in mediating the influence of 5-HT on the preovulatory LH surge.

Prevention of inhibitory effect of dorsal raphe nucleus lesions on ovulation and LH surge by 5-HT 2A/2C receptor agonists in female rats

In order to investigate the role of the dorsal raphe nucleus and the serotonergic system in the regulation of ovulation, the number of ova and plasma luteinizing hormone (LH) concentrations were measured in female rats after making lesions in this nucleus (DRL) and/or treatment with 5-hydroxytryptamine (5-HT) receptor agonists or antagonists. DRL or sham lesion was made on the afternoon of proestrous (12:00-14:00 h) under ether anesthesia and the number of ova in the oviduct was counted on the next estrous and diestrous morning. In some animals, blood samples were taken via the atrial cannula during the proestrous evening for the radioimmunoassay of LH. All intact control and sham-operated females ovulated and plasma LH increased between 19:00 and 21:00 h. In contrast, ovulation was seen in only 36% of DRL rats. LH surge did not occur in this group. However, 80% of DRL rats ovulated after treatment with (+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride [(+/-)-DOI; a 5-HT 2A/2C receptor agonist] at 15:00 h on proestrous day. LH surge was also observed in the DRL rats with (+/-)-DOI. On the other hand, only 8% of DRL rats ovulated after treatment with buspirone (5-HT 1A receptor agonist). Furthermore, when mianserin (5-HT 2A/2C receptor antagonist) was administered at 16:00 h on proestrous day, ovulation was not seen in all rats without DRL. These results suggest that the dorsal raphe nucleus plays an important role in induction of LH surge and ovulation and the 5-HT 2A/2C receptor system is involved in this mechanism.

The effect of alpha-adrenergic receptor blockers prazosin and yohimbine on cerebral metabolism and biogenic amine content of traumatized brain

Widespread decrease in local cerebral glucose utilization (LCGU) previously shown to occur 3 days after a local freezing lesion was interpreted as reflecting a depression of functional activity in the affected areas. In parallel experiments, cortical norepinephrine (NE) content of traumatized brain was found to be decreased. The effects of prazosin (PZ), an alpha 1-adrenergic receptor blocker, and yohimbine (YOH), an alpha 2-blocker, on glucose use and biogenic amine content of lesioned rat brain were studied to determine if the changes in the noradrenergic system associated with injury are of functional importance, to identify the receptors that may be involved in mediating the action of NE in injured brain, and to look for evidence of interaction between the noradrenergic and the serotonergic systems in traumatized brain. PZ (1 mg/kg) given 30 min before the lesion ameliorated the subsequent metabolic cortical depression seen in untreated animals. PZ given for 3 days starting before the lesion (3 mg/kg/day) was also effective in normalizing LCGU in areas where it was depressed by lesioning, despite the fact that this regimen induced significant global decrease in LCGU in normal animals. Once cortical metabolic depression had developed 3 days after the lesion, it could not be modified by PZ. YOH was less effective than PZ and was so only when given for 3 days (22.5 mg/kg/day in three divided doses). PZ (3 mg/kg/day in three divided doses) slightly but significantly decreased the accumulation of the serotonin (5-HT) metabolite 5-hydroxyindoleacetic acid in the traumatized hemisphere. These results provide evidence that blockage of alpha 1-adrenergic receptors prevents the development of cortical dysfunction associated with brain trauma. This implies that the noradrenergic system plays a role in the functional consequences of injury and that this effect is, at least in part, mediated by alpha 1-adrenergic receptors. Furthermore, alpha 1-adrenergic receptor blockage appears to modulate cortical turnover of 5-HT, previously also implicated in functional consequences of brain injury. The data are compatible with inhibitory effects of NE in the cortex and suggest a potential of alpha 1-adrenergic blockage in development of novel therapeutic approaches to brain injury.

Serotonin synthesis inhibition or receptor antagonism reduces pregnancy-induced nocturnal prolactin secretion

During early pregnancy, two surges of prolactin (PRL) designated as nocturnal (N) and diurnal (D) are displayed by the rat. We previously reported the positive influence of serotonin (5-HT) in regulating the D surge. Its role in the N surge remained inconclusive due to the contradictory results obtained with the 5-HT synthesis inhibitor parachlorophenylalanine (PCPA) and 5-HT2 receptor antagonists. This study further characterizes the involvement of 5-HT in regulating the N surge. The effectiveness of different doses of ketanserin (KET), a 5-HT2 receptor antagonist, to reduce plasma PRL levels during the surge was established. Sub-threshold (1 mg/kg BW) or just maximally effective (10 mg/kg BW) doses of KET were administered to rats that had been pre-treated with PCPA (250 mg/kg BW) for 24h. The lower dose of KET was ineffective in reducing the N surge even though less 5-HT was available due to PCPA treatment 24h earlier. The higher dose was effective in blocking the surge. Subsequently, the effect of one compared to two injections of PCPA 24 hours apart on plasma PRL levels and concentrations of 5-HT, dopamine (DA) and their respective metabolites 5-hydroxy-indoleacetic acid (5-HIAA) and dihydroxyphenylacetic acid (DOPAC) in the medial basal hypothalamus (MBH) and the medial dorsal hypothalamus (MDH) was studied. Two injections of PCPA but not one abolished the N PRL surge. Levels of 5-HT and 5-HIAA were significantly (p less than .005) reduced following either one or two injections of PCPA. Nevertheless, there was a greater (50 fold) decrease in 5-HIAA following 2 injections compared to one injection (10 fold), resulting in lower 5-HT turnover as indicated by lower 5-HIAA/5-HT ratios. Levels of DA in the MBH were reduced significantly only following two injections of PCPA, suggesting that the lack of effect of PCPA after one injection on the N surge was not due to a decrease in DA.

An analysis of serotonin secretion in hypothalamic regions based on 5-hydroxytryptophan accumulation or push-pull perfusion. Effects of mesencephalic raphe or locus coeruleus stimulation and correlated changes in plasma luteinizing hormone

This study evaluates the effect of electrical stimulation (ES) of the dorsal (DRN) and median raphe (MRN) nuclei serotoninergic systems on luteinizing hormone (LH) release in estrogen-treated, ovariectomized rats. To show that ES increased serotonin (5-hydroxytryptamine: 5-HT) secretion into hypothalamic regions known to contain luteinizing hormone-releasing hormone (LH-RH) cell bodies and terminals. 5-hydroxytryptophan (5-HTP) accumulation was measured in microdissected hypothalamic areas after blockade of aromatic-L-amino-acid decarboxylase with NSD-1015. DRN-ES produced a significant increase in 5-HTP accumulation in the medial preoptic (MPN) and paraventricular nuclei (PVN), but not in the suprachiasmatic nucleus (SCN), arcuate nucleus (ARC) or median eminence (ME). In contrast, MRN-ES produced a significant rise in 5-HTP accumulation only in the PVN and ARC, not in the SCN, MPN or ME. Because the DRN receives noradrenergic innervation from the locus coeruleus (LC) we also evaluated the effect of LC stimulation on 5-HT secretion into these hypothalamic regions. LC stimulation, like DRN-ES, resulted in increases in 5-HTP accumulation in MPN and PVN, but not in SCN or ME. In addition, using push-pull perfusion methodology, we observed that LC-ES results in a 240% increase in 5-HT and a decrease of approximately 40% in 5-hydroxyindoleacetic acid (5-HIAA) in MPN perfusates collected 10-20 min after LC-ES began. With evidence that DRN- and MRN-ES evoke 5-HT release, we next examined whether such stimulation affects basal LH release. Because we were unable to find any effect, we determined whether 5-HT release would augment or suppress electrochemically evoked LH release. MPN electrochemical stimulation (ECS) induced a significant increase in plasma LH with a peak seen at 45 min. When the MPN was ECS and 30 min later the DRN was ES for 30 min no augmenting or suppressive effect was seen during the first 60 min. However, while plasma LH declined towards baseline in the MPN-ECS group, it remained significantly elevated in MPN-ECS + DRN-ES rats. MPN-ECS + MRN-ES had no such effect: instead. LH levels were transiently decreased 45 min after beginning MPN-ECS. This study provides additional information on hypothalamic sites which receive axonal projections from the DRN and MRN and clearly indicate that an increase in 5-HT secretion occurs in the MPN following DRN- or LC-ES.(ABSTRACT TRUNCATED AT 400 WORDS)