Glutamate and kainate effects on the noradrenergic neurons innervating the rat vas deferens

Chronic levetiracetam decreases hippocampal and testicular aromatase expression in normal but not kainic acid-induced experimental model of acute seizures in rats

Reproductive disorders are more common in men with epilepsy taking anticonvulsant medications. Antiseizure/anticonvulsant drugs and seizures in medial temporal lobe structures may cause gonadal dysfunction, including infertility, decreased libido, and potency. Levels of circulating bioavailable testosterone are affected by the aromatase enzyme, which converts testosterone into estrogen and may be affected by seizure medications. However, the relationship of anticonvulsant drugs with central aromatase levels is not clear. This study investigated the possible effects of the highly efficient, new-generation antiseizure/anticonvulsant drug levetiracetam on central and gonadal aromatase expression and gonadal tissue functionality at 27 and 54 mg/kg/day doses. Epileptogenesis was generated in male Wistar rats by an intraperitoneal injection of the excitotoxic agent kainic acid. Aromatase levels were 1.5 times higher in the brain cortex of the kainic acid groups after 4 weeks and the hippocampus after 4 and 8 weeks compared with the controls. Decreased basal aromatase levels were observed after 1 week of levetiracetam treatment (27 mg/kg/day). Administration of 27 mg/kg/day levetiracetam did not alter vas deferens contractions at 1, 4, or 8 weeks compared with the controls. No histological changes were observed in the vas deferens, epididymis, or testis after 8 weeks of levetiracetam administration at both doses. Administration of 27 and 54 mg/kg/day levetiracetam downregulated testis aromatase expression at 8 weeks compared with the controls. These results suggest that levetiracetam decreases aromatase levels in the testis and increases the seizure threshold by a possible decrease in systemic estradiol levels.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Neurochemical evidence for the presence of sympathetic nerve terminals in the rat mammary gland: Changes during the lactogenic cycle

Experiments were undertaken to obtain neurochemical evidence of the presence of sympathetic nerve terminals in the rat mammary gland and the changes occurring in them during the lactogenic cycle. The norepinephrine (NE) content of the gland changed during the lactogenic cycle. Higher levels of NE were found during virginity and involution, whereas a lower level was found at 14 days of lactation. Surgical and chemical (with 6-hydroxydopamine) denervation reduced the norepinephrine content of the gland by 61 and 90%, respectively. Uptake of [(3)H]norepinephrine by the mammary gland was saturable and specifically blocked by cocaine. No changes in the maximal capacity of incorporation during the lactogenic cycle were found, but the affinity of NE for the transmembrane carrier was low during lactation, as was the NE content, suggesting a decrease in the sympathetic nerve activity during this stage of the lactogenic cycle. In support of this, we found a decrease in total NE released after stimulation with 80 mM KCI. The neurochemical evidence obtained during this research strongly suggests that rat mammary gland is innervated by sympathetic nerves and that their activity changes during the lactogenic cycle.

Some lumbar sympathetic neurons develop a glutamatergic phenotype after peripheral axotomy with a note on VGLUT₂-positive perineuronal baskets

Glutamate is the main excitatory neurotransmitter in the nervous system, including in primary afferent neurons. However, to date a glutamatergic phenotype of autonomic neurons has not been described. Therefore, we explored the expression of vesicular glutamate transporter (VGLUT) types 1, 2 and 3 in lumbar sympathetic chain (LSC) and major pelvic ganglion (MPG) of naïve BALB/C mice, as well as after pelvic nerve axotomy (PNA), using immunohistochemistry and in situ hybridization. Colocalization with activating transcription factor-3 (ATF-3), tyrosine hydroxylase (TH), vesicular acetylcholine transporter (VAChT) and calcitonin gene-related peptide was also examined. Sham-PNA, sciatic nerve axotomy (SNA) or naïve mice were included. In naïve mice, VGLUT(2)-like immunoreactivity (LI) was only detected in fibers and varicosities in LSC and MPG; no ATF-3-immunoreactive (IR) neurons were visible. In contrast, PNA induced upregulation of VGLUT(2) protein and transcript, as well as of ATF-3-LI in subpopulations of LSC neurons. Interestingly, VGLUT(2)-IR LSC neurons coexpressed ATF-3, and often lacked the noradrenergic marker TH. SNA only increased VGLUT(2) protein and transcript in scattered LSC neurons. Neither PNA nor SNA upregulated VGLUT(2) in MPG neurons. We also found perineuronal baskets immunoreactive either for VGLUT(2) or the acetylcholinergic marker VAChT in non-PNA MPGs, usually around TH-IR neurons. VGLUT(1)-LI was restricted to some varicosities in MPGs, was absent in LSCs, and remained largely unaffected by PNA or SNA. This was confirmed by the lack of expression of VGLUT(1) or VGLUT(3) mRNAs in LSCs, even after PNA or SNA. Taken together, axotomy of visceral and non-visceral nerves results in a glutamatergic phenotype of some LSC neurons. In addition, we show previously non-described MPG perineuronal glutamatergic baskets.

Glutamate mediated responses in isolated trachea preparations from control and ovalbumin sensitized guinea-pigs

We investigated whether the glutamergic system plays a role in isolated trachea from control and ovalbumin-sensitized guinea-pigs. Electrical field stimulation induced contractile responses in control group, but electrical field stimulation produced relaxation responses in ovalbumin-challenged guinea-pigs. The responses induced by electrical field stimulation in both groups were completely abolished by tetrodotoxin, but unaffected by hexamethonium. DL-2-amino-5-phosphono-valeric acid (D-AP5) caused a concentration-dependent statistically significant inhibition in the contractile responses to electrical field stimulation50 (EFS50) in control guinea-pigs. But in the ovalbumin-challenged groups, D-AP5 did not cause any significant effect on the relaxation response to frequency of field stimulation (EFS50). N(G)-monmethyl-L-argine caused a significant inhibition in the relaxation effect of EFS50. L- and D-glutamate and N-methyl-D-aspartic acid (NMDA) alone had no effect on the resting tension on the trachea in both groups. Carbachol produced concentration-dependent contractile responses in ovalbumin-challenged groups. These results suggested that responses to electrical field stimulation in control groups might be due to NMDA receptor-mediated release of any substance on prejunctional neurones and, alternatively, NMDA might exert a modulatory effect on any substance at prejunctional level. Also, responses to electrical field stimulation in ovalbumin-challenged guinea-pigs might not be mediated by NMDA but rather by increasing the production of nitric oxide by inducible nitric oxide synthase.

Release of norepinephrine from human ovary: coupling to steroidogenic response

We investigated the possibility that norepinephrine from the human ovary is released after nerve stimulation and that this neurotransmitter is coupled to a steroidogenic response. Biologically significant levels of both norepinephrine and dopamine were found in human ovarian biopsies. [3H]norepinephrine incorporated in vitro was readily released by electrical stimulation in a Ca2+-dependent process. Ovarian membrane preparations exhibited specific binding sites for the beta-adrenergic antagonist [3H]dihydroalprenolol. Displacement of [3H]dihydroalprenolol with zinterol (a specific beta2-agonist) indicated that 72% of these sites were type beta2-receptors. beta-receptors were also present on granulosa cells. Stimulation of granulosa cells with luteinizing hormone or the beta-agonist isoproterenol increased the release of progesterone after 4 d in culture. These results suggest that the sympathetic nerves present in human ovary are coupled to beta-adrenergic receptors present in endocrine cells and, as in nonprimate mammals, appear to participate in the regulation of ovarian function.

Glutamate receptors in peripheral tissues: current knowledge, future research, and implications for toxicology

We illustrate the specific cellular distribution of different subtypes of glutamate receptors (GluRs) in peripheral neural and non-neural tissues. Some of the noteworthy locations are the heart, kidney, lungs, ovary, testis and endocrine cells. In these tissues the GluRs may be important in mediating cardiorespiratory, endocrine and reproductive functions which include hormone regulation, heart rhythm, blood pressure, circulation and reproduction. Since excitotoxicity of excitatory amino acids (EAAs) in the CNS is intimately associated with the GluRs, the toxic effects may be more generalized than initially assumed. Currently there is not enough evidence to suggest the reassessment of the regulated safety levels for these products in food since little is known on how these receptors work in each of these organs. More research is required to assess the extent that these receptors participate in normal functions and/or in the development of diseases and how they mediate the toxic effects of EAAs. Non-neural GluRs may be involved in normal cellular functions such as excitability and cell to cell communication. This is supported by the wide distribution in plants and animals from invertebrates to primates. The important tasks for the future will be to clarify the multiple biological roles of the GluRs in neural and non-neural tissues and identify the conditions under in which these are up- or down-regulated. Then this could provide new therapeutic strategies to target GluRs outside the CNS.

Assessment of the adrenergic effects of orphenadrine in rat vas deferens

The peripheral adrenergic effects of orphenadrine, an antiparkinsonian drug, have been evaluated in the rat vas deferens to investigate whether these properties are the same as those of other phencyclidine ligands. In the low micromolar range, orphenadrine enhanced electrically-evoked and exogenous noradrenaline contractile responses in the epididymal portion of rat vas deferens. It also induced spontaneous activity that was inhibited by prazosin (1 microM) but not by atropine (20 nM). It inhibited accumulation of [3H]noradrenaline in rat vas deferens (IC50 = 14.2+/-2.3 microM). Orphenadrine competitively inhibited [3H]nisoxetine binding in rat vas deferens membranes (Ki = 1.05+/-0.20 microM). It can be concluded that orphenadrine, at low micromolar concentrations, interacts with the noradrenaline reuptake system inhibiting its functionality and thus potentiating the effect of noradrenaline.

Glutamate receptors on postganglionic sympathetic axons

The present study demonstrates that approximately 36% of postganglionic sympathetic axons in gray rami express receptors for the N-methyl-D-aspartate receptor 1 subunit of the N-methyl-D-aspartate receptor and 10% express the glutamate receptor 1 subunit of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor. If these receptors are active, glutamate released from primary afferent terminals could activate these receptors resulting in the release of noradrenaline and other substances from postganglionic sympathetic neurons. This interaction would constitute a non-synaptic, sensory-sympathetic, peripheral reflex that might be important in local vascular control and in pain states that have a sympathetic component.

Presynaptic modulation by L-glutamate and GABA of sympathetic co-transmission in rat isolated vas deferens

1. The modulatory effects of L-glutamate and its structural analogues, and of gamma-aminobutyric acid (GABA), on sympathetic co-transmission were studied in the rat isolated vas deferens exposed to electrical field stimulation (EFS). 2. Application of exogenous L-glutamate caused a concentration-dependent (1 microM-3 mM) inhibition of the rapid twitch component of the biphasic EFS contraction. However, L-glutamate (1 microM-3 mM) had a minimal effect on the phasic contraction induced by exogenous adenosine 5'-triphosphate (ATP, 150 microM) and noradrenaline (50 microM). Unlike L-glutamate, D-glutamate had no effect on the EFS contraction. 3. The L-glutamate-induced inhibition of the EFS contractions was significantly attenuated by the glutamate decarboxylase (GAD) inhibitor 3-mercapto-propionic acid (150 microM) and was abolished in the presence of the GABA transaminase (GABA-T) inhibitor, 2-aminoethyl hydrogen sulphate (500 microM). 4. The L-glutamate-induced inhibition of the electrically evoked contraction was not affected by the adenosine A1-receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine (DPCPX)(30 nM), reactive blue 2 (30 microM) or the GABAA receptor antagonist bicuculline (50 microM). However, the GABAB receptor antagonist 2-hydroxysaclofen (50 microM) significantly inhibited the L-glutamate effect. 5. Similar to L-glutamate, GABA also caused a concentration-dependent (0.1-100 microM) inhibition of the EFS contractions. This GABA-induced inhibition was not affected by either the GABAA receptor antagonist bicuculline (50 microM) or reactive blue 2 (30 microM). However, a significant attenuation of the GABA-mediated effect was recorded with the GABAB receptor antagonist 2-hydroxysaclofen (50 microM). Contractions of the vas deferens induced by exogenous ATP and noradrenaline were not affected by GABA (0.1-100 microM). 6. The L-glutamate analogues, N-methyl-D-aspartate (NMDA) (1 microM-1 mM) and quisqualate (Quis 0.1 microM-0.3 mM) had no effect, whilst kainate (Kain, 1 microM-1 mM) caused an inhibition of the EFS-induced contractions. Effects of Kain could be abolished by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dioxine (CNQX, 10 microM). NMDA, Quis and Kain had no effect on the exogenous ATP- or noradrenaline-induced contractions. 7. It is concluded that the excitatory amino acid L-glutamate modulates the electrically evoked vas deferens contraction through conversion to the inhibitory amino acid GABA by a specific GABA transaminase. The GABA formed may then act on GABAB receptors and cause inhibition of the contraction through a presynaptic mechanism.

Biochemical, autoradiographic and functional studies on a unique glutamate binding site in adrenal gland

L-Glutamate is known to function as a major excitatory neurotransmitter in the mammalian central nervous system, and recent reports suggest the existence of receptors for glutamate in several peripheral tissues. In the present study, the characteristics of the binding of [3H]L-glutamate to sections of bovine adrenal gland were studied, and the localisation of this binding was investigated in adrenal glands from cow, dog, rat and guinea pig. In addition, the effects of glutamate on catecholamine release from the perfused isolated bovine adrenal gland were investigated. Binding of [3H]L-glutamate to slide-mounted sections of bovine adrenal gland was of high affinity (Kd 0.4 microM), rapid, saturable, reversible, stereospecific and to a single population of sites. The pharmacological profile of this binding site appeared to be unique, and did not correspond to any of the central receptor subtypes for glutamate so far identified. In the adrenal gland of the cow, rat and guinea pig, the binding density of [3H]L-glutamate was higher in cortex than medulla, while this pattern was reversed in the canine adrenal gland. Glutamate had no effect on the basal secretion of noradrenaline or adrenaline from the perfused isolated bovine adrenal gland, and neither glutamate nor the glutamate receptor antagonist kynurenate altered the nicotine-stimulated release of these catecholamines. These results suggest the existence of a novel peripheral binding site for glutamate in the adrenal gland. The differential autoradiographic localisation of this binding site in the adrenal glands of the various species studied may reflect different functional properties of glutamate in these species, and suggests possible roles for glutamate in the modulation of adrenal function.

Excitatory amino acid receptors in the mammalian periphery

Aspartate and glutamate occur ubiquitously in free and chemically bound forms and have been considered primarily as substances of metabolic relevance. This focus has changed with the more recent discovery of their specific role as excitatory synaptic transmitters in the mammalian CNS. Enthusiasm for this concept has overshadowed the possibility that glutamate and aspartate may also have specific, receptor-mediated functions in the periphery. In this review, Sándor Erdö summarizes the current knowledge of excitatory amino acid (EAA) receptors outside the CNS, through which EAAs may modulate various functions in peripheral organs and tissues.