Release of 3H-noradrenaline by excitatory amino acids from rat mediobasal hypothalamus and the influence of aging

Prazosin blocks the glutamatergic effects of N-methyl-D-aspartic acid on lordosis behavior and luteinizing hormone secretion in the estrogen-primed female rat

We have observed that intracerebroventricular (icv) injection of selective N-methyl-D-aspartic acid (NMDA)-type glutamatergic receptor antagonists inhibits lordosis in ovariectomized (OVX), estrogen-primed rats receiving progesterone or luteinizing hormone-releasing hormone (LHRH). When NMDA was injected into OVX estrogen-primed rats, it induced a significant increase in lordosis. The interaction between LHRH and glutamate was previously explored by us and another groups. The noradrenergic systems have a functional role in the regulation of LHRH release. The purpose of the present study was to explore the interaction between glutamatergic and noradrenergic transmission. The action of prazosin, an alpha1- and alpha2b-noradrenergic antagonist, was studied here by injecting it icv (1.75 and 3.5 microg/6 microL) prior to NMDA administration (1 microg/2 microL) in OVX estrogen-primed Sprague-Dawley rats (240-270 g). Rats manually restrained were injected over a period of 2 min, and tested 1.5 h later. The enhancing effect induced by NMDA on the lordosis/mount ratio at high doses (67.06 +/- 3.28, N = 28) when compared to saline controls (6 and 2 microL, 16.59 +/- 3.20, N = 27) was abolished by prazosin administration (17.04 +/- 5.52, N = 17, and 9.33 +/- 3.21, N = 20, P < 0.001 for both doses). Plasma LH levels decreased significantly only with the higher dose of prazosin (1.99 +/- 0.24 ng/mL, N = 18, compared to saline-NMDA effect, 5.96 +/- 2.01 ng/mL, N = 13, P < 0.05). Behavioral effects seem to be more sensitive to the alpha-blockade than hormonal effects. These findings strongly suggest that the facilitatory effects of NMDA on both lordosis and LH secretion in this model are mediated by alpha-noradrenergic transmission.

Reduced cortical noradrenergic neurotransmission is associated with increased neophobia and impaired spatial memory in aged rats

In the present study, young (5-month-old (mo)) and aged (24 mo) adult male Fischer-344 (F344) rats were assigned to experimental groups based upon their performance of a reference memory task in the Morris water maze and reactivity to a novel palatable taste in a gustatory neophobia task. Levels of norepinephrine (NE) and its metabolite 3-methoxy-4-hydroxy-phenylglycol (MHPG) were assayed via high performance liquid chromatography (HPLC) in brain regions associated with the locus coeruleus (LC)-hippocampus-cortex system and A1/A2-hypothalamic system. Binding of ligands specific for alpha-1, alpha-2, beta-1, and beta-2 receptors was assessed in hippocampus and cortex with receptor autoradiography. Impaired acquisition and retention of the water maze task and gustatory neophobia in aged rats was primarily associated with decreased NE activity in cingulate cortex (CC) as indicated by a significant reduction in the MHPG/NE ratio coupled with increased NE content. No significant changes in adrenergic receptor binding were detected in any region sampled. The results suggest that an aging-related reduction in cortical NE neurotransmission is associated with the expression of increased neophobia and deficits in spatial learning and memory performance occurring with advanced age in rats.

The role of glutamate and nitric oxide in the reproductive neuroendocrine system

The preovulatory surge of gonadotropin releasing hormone (GnRH) is essential for mammalian reproduction. Recent work has implicated the neurotransmitters glutamate and nitric oxide as having a key role in this process. Large concentrations of glutamate are found in several hypothalamic nuclei known to be important for GnRH release and glutamate receptors are also located in these key hypothalamic nuclei. Administration of glutamate agonists stimulate GnRH and LH release, while glutamate receptor antagonists attenuate the steroid-induced and preovulatory LH surge. Glutamate has also been implicated in the critical processes of puberty, hormone pulsatility, and sexual behavior. Glutamate is believed to elicit many of these effects by activating the release of the gaseous neurotransmitter, nitric oxide (NO). NO potently stimulates GnRH by activating a heme containing enzyme, guanylate cyclase, which in turn leads to increased production of cGMP and GnRH release. Recent work has focused on identifying anchoring and (or) clustering proteins that target glutamate receptors to the synapse and couple the glutamate-NO neurotransmission system. The present review will discuss these new findings, as well as the role of glutamate and nitric oxide in important mammalian reproductive events, with a focus on the hypothalamic control of preovulatory GnRH release.

Key words: glutamate, nitric oxide, GnRH, postsynaptic density, hypothalamus.

Tyrosine hydroxylase mRNA is increased in old age and norepinephrine uptake transporter mRNA is decreased in middle age in locus coeruleus of Brown-Norway rats

In normal aging, cell loss occurs in the locus coeruleus (LC), the major noradrenergic nucleus in the brain. This study examined changes in the LC of aged rats by measuring mRNA expression for tyrosine hydroxylase (TH) and the norepinephrine uptake transporter (NET). TH and NET mRNA expression were measured by in situ hybridization in young, middle-aged and aged rats. It appears that in middle age, the transporter system responds initially to LC cell loss by decreasing NET mRNA expression. Then, with further aging and cell loss, TH mRNA expression increases which may potentially increase NE synthesis in the remaining neurons. These findings suggest that multiple regulatory components are used to maintain stable noradrenergic synaptic levels despite neuronal loss. Published by Elsevier Science B.V.

Opioid-glutamate-nitric oxide connection in the regulation of luteinizing hormone secretion in the rat

Opioid neurons are recognized to be an important component of the inhibitory "brake" in the CNS that restrains LHRH secretion. Opioid inhibition could be exerted directly on LHRH neurons, or it could be achieved via indirect mechanisms involving restrainment of excitatory "accelerator" neurons that facilitate LHRH release. The purpose of the present study was to explore the second hypothesis by investigating whether removal of opioid inhibition by administering the opioid antagonist, naloxone leads to enhanced activation of glutamate and nitric oxide (NO) neurons, which are known to be important excitatory "accelerator" components for the control of LHRH secretion. Naloxone administration (2.5 mg/kg) to adult male rats induced a significant elevation of serum LH levels at 20 min post injection. NOS activity in preoptic area (POA) and medial basal hypothalamic (MBH) fragments was demonstrated to be significantly elevated 20 min post naloxone injection. Administration of a glutamate (NMDA) receptor antagonist (MK-801, 0.2 mg/kg) abolished the naloxone-induced increase in NOS activity in the POA and MBH, with a corresponding block of the naloxone-induced LH release. Glutamate appears to only be involved in LH surge generation and not to regulate basal LH levels, as MK-801 had no effect on basal LH release. Because previous work by our laboratory and others have provided evidence that NO is a mediator of glutamate effects in the hypothalamus, these findings are interpreted to mean that opioid inhibition is mediated on glutamate neurons that are upstream of NO neurons. In support of this contention, we found that NMDA treatment enhanced NOS activity in the male rat POA and MBH fragments in vitro, an effect that was specific as it was completely blocked by the NMDA receptor antagonist, MK-801. Additionally, in vivo microdialysis studies revealed that naloxone treatment significantly enhances glutamate release in the preoptic area (POA) at 15 min post injection in conscious, unanesthetized, freely moving male rats. Release rates of the control amino acid, serine did not change significantly following naloxone injection. Taken as a whole, these findings provide evidence for an opioid-glutamate-NO pathway in the control of LHRH secretion, and they demonstrate the importance of "brake-accelerator" interactions in the control of LHRH and LH secretion.

Effects of nitric oxide on neuroendocrine function and behavior

Nitric oxide (NO) is an unusual chemical messenger. NO mediates blood vessel relaxation when produced by endothelial cells. When produced by macrophages, NO contributes to the cytotoxic function of these immune cells. NO also functions as a neurotransmitter and neuromodulator in the central and peripheral nervous systems. The effects on blood vessel tone and neuronal function form the basis for an important role of NO on neuroendocrine function and behavior. NO mediates hypothalamic portal blood flow and, thus, affects oxytocin and vasopression secretion; furthermore, NO mediates neuroendocrine function in the hypothalamic-pituitary-gonadal and hypothalamic-pituitary-adrenal axes. NO influences several motivated behaviors including sexual, aggressive, and ingestive behaviors. Learning and memory are also influenced by NO. Taken together, NO is emerging as an important chemical mediator of neuroendocrine function and behavior.

The enhancement and the inhibition of noradrenaline-induced cyclic AMP accumulation in rat brain by stimulation of metabotropic glutamate receptors

1. The actions of several metabotropic glutamate receptor and antagonists on noradrenaline (NA)-stimulated [3H]-cyclic AMP accumulation were investigated in rat cerebral cortical slices. 2. Quisqualate (QUIS), L-2-amino-3-phosphonopropionic acid (L-AP3) and glutamate (GLU) elicited concentration-dependent inhibition of (NA)-stimulated [3H]-cyclic AMP accumulation, with IC50 values of 105 +/- 29, 275 +/- 36 and 944 +/- 150 microM respectively. In contrast (Rs)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) (0.5 mM) and N-methyl-D-aspartic acid (NMDA) (0.5 mM) had no effect. 3. (2S,3S,4S)-alpha-(Carboxycyclopropyl)glycine (L-CCGI), 1-Aminocyclo-pentane-1S,3R-dicarbo-xylate (1S,3R-ACPD), ibotenate (IBO) and (RS)-4-carboxy-3-hydroxy-phenylglycine (CHPG)elicited a concentration-dependent enhancement of NA-stimulated [3H]-cyclic AMP accumulation, with EC50 values of 2.5 +/- 0.11, 42 +/- 1.3, 97.8 +/- 2.1 and 157 +/- 13.4 microM, respectively. 4. (S)-3-carboxy-4-hydroxyphenylglycine (3C4HPG) and (S)-4-carboxy-3-hydroxyphenyl-glycine (4C3HPG) produced a biphasic effect, at concentrations up to 100 and 500 microM, respectively, they significantly enhanced the action of NA (100 microM), at 1mM concentration both compounds as well as alpha-methyl-4-carboxyphenylglycine (MCPG) produced a significant inhibition of NA-stimulated cyclic AMP accumulation. 5. A putative mGluR antagonist-L-AP3, inhibited the 1S,3R-ACPD (100 microM) induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation in a biphasic manner with an IC50 of 4.5 microM for the high affinity site, which represented 65% of the total and an IC50 of 283 microM for the low affinity site. 6. beta-adrenoceptor antagonist propranolol inhibited the interaction between 1S,3R-ACPD (100 microM) and NA (100 microM) on [3H]-cyclic AMP accumulation by about 80%, with an IC50 of 0.52 +/- 0.011 microM, to the level observed after 1S,3R-ACPD alone. Prazosin, an alpha 1-adrenoceptor antagonist was more potent (IC50 of 0.091 +/- 0.012 microM) but less efficacious (60% inhibition) as an inhibitor of the interaction either between NA and 1S,3R-ACPD while yohimbine, na alpha 2-adrenoceptor antagonist (up to 1 microM) had no effect. 7. Neither the protein kinase C inhibitor - staurosporine (10 microM) nor thapsigargin (1 microM), which depletes IP3 sensitive calcium stores, inhibited significantly the 1S,3R-ACPD (100 microM)-induced enhancement of the action of NA (100 microM) on [3H]-cyclic AMP accumulation. 8. Adenosine deaminase (0.5 U/ml) abolished both the 1S,3R-ACPD (100 microM)-induced [3H]-cyclic AMP accumulation and the synergistic interaction of this compound with NA (100 microM). 9. These results indicate the existence of different subtypes of metabotropic glutamate receptors in rat brain which either inhibit or enhance the NA-stimulated [3H]-cyclic AMP accumulation. The enhancement in cerebral cortical slices is mediated via receptors which are blocked with high affinity by L-AP3 and occurs via interactions with endogenous adenosine; the inhibition is mediated by receptors sensitive to quisqualate, L-AP3 and glutamate and may represent a predominant interaction between NA and excitatory amino acids (EAA), which in cerebral cortical slices is masked by excitatory effects.

Glutamic acid induces luteinizing hormone releasing hormone release via alpha receptors

Glutamic acid (GA) and norepinephrine (NE) stimulate luteinizing hormone-releasing hormone (LHRH) release via release of nitric oxide (NO) from NOergic neurons in the arcuate-median eminence region. To determine if GA releases LHRH via direct stimulation of NOergic neurons, or via stimulation of noradrenergic terminals, arcuate median eminence explants from male rats were incubated with various compounds, and the LHRH release into the medium was measured. GA-induced release of LHRH was completely blocked by phentolamine (1 microM), an alpha receptor blocker, which, by itself, had no effect on the release. Nitroprusside (NP), which spontaneously releases NO, more than doubled LHRH release. To determine if alpha receptors on the LHRH neuron are required for the action of NP, the tissue was incubated with phentolamine, plus NP. Phentolamine had no effect on the LHRH-releasing action of NP. The results are interpreted to mean that GA activates the release of NE from the noradrenergic terminals. This acts on alpha receptors on the NOergic neuron to produce the release of NO. This NO diffuses to the LHRH terminals and induces release of LHRH.

Interaction between glutamate and GABA on 3H-noradrenaline release from rat hypothalamus

Glutamate has been shown to stimulate noradrenaline (NA) release from hypothalamic nerve terminals. In the present study, we evaluated the possible interaction between the excitatory amino acid glutamate and gamma-aminobutyric acid (GABA), an inhibitory transmitter, on noradrenaline (NA) release from mediobasal hypothalamus (MBH) of adult male rats. Hypothalamic slices loaded in vitro with 3H-NA were superfused and exposed to glutamate, N-methyl-D-aspartic acid (NMDA), or kainate (KA). We found that 3H-NA release evoked by the excitatory amino acids glutamate and NMDA was dramatically decreased by GABA. The facilitatory effects of NMDA and KA were prevented concentration-dependently by the GABAB receptor antagonist 2-hydroxy saclofen which restored the NMDA effect. In addition, baclofen blocked K(+)-induced 3H-NA release. Activation of GABAA receptors by muscimol and THIP was ineffective. In conclusion, glutamate and GABA, through GABAB receptors, may interact to modulate NA release from the rat mediobasal hypothalamus.