Modulation of Non-N-Methyl-D-Aspartate Receptors in Cultured Cerebellar Granule Cells

Kainic acid (KA), quisqualic acid (QUIS), and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) stimulated D-[3H]aspartate release from cultured cerebellar granule cells in a concentration-dependent way. The EC50 values were 50 microM for KA (Gallo et al., 1987) and 20 microM for both QUIS and AMPA, but the efficacy of QUIS appeared to be greater than that of AMPA. The release of D-[3H]aspartate induced by KA, QUIS, and AMPA was blocked, in a dose-dependent way, by the new glutamate receptor antagonist 6-cyano-2,3-dihydroxy-7-nitroquinoxaline (CNQX); IC50 values were 0.7 microM in the case of AMPA (50 microM) and 1 microM in the case of KA (50 microM). AMPA (50-300 microM) inhibited the effect of 50 microM KA on D-[3H]aspartate release. At 300 microM AMPA, the effect of KA plus AMPA was not antagonized by the KA receptor antagonist kynurenic acid (KYN). In contrast, when KA was used at an ineffective concentration (10 microM), the addition of AMPA at concentrations below the EC50 value (10-20 microM) resulted in a synergistic effect on D-[3H]aspartate release. In this case, the evoked release of D-[3H]aspartate was sensitive to KYN. KA stimulated the formation of cyclic GMP, whereas QUIS, AMPA, and glutamate were ineffective. The accumulation of cyclic GMP elicited by KA (100 microM) was prevented not only by the antagonists CNQX (IC50 = 1.5 microM) and KYN (IC50 = 200 microM), but also by the agonists AMPA (IC50 = 50 microM) QUIS (IC50 = 3.5 microM), and glutamate (IC50 = 100 microM). We conclude that AMPA, like QUIS, may act as a partial agonist at KA receptors. Moreover, CNQX effectively antagonizes non-N-methyl-D-aspartate receptor-mediated responses in cultured cerebellar granule cells.     

Tachykinin neurokinin 3 receptor signaling in cholecystokinin-elicited release of oxytocin and vasopressin

Neurokinin 3 receptor (NK3R) signaling has an integral role in the stimulated oxytocin (OT) and vasopressin (VP) release in response to hyperosmolarity and hypotension. Peripheral injections of cholecystokinin (CCK) receptor agonists for the CCK-A (sulfated CCK-8) and CCK-B (nonsulfated CCK-8) receptors elicit an OT release in rat. It is unknown whether NK3R contributes to this endocrine response. Freely behaving male rats were administered an intraventricular pretreatment of 250 or 500 pmol of SB-222200, a specific NK3R antagonist, or 0.15 M NaCl before an intraperitoneal or intravenous injection of CCK-8 (nonsulfated or sulfated) or 0.15 M NaCl. Blood samples were taken before intraventricular treatment and 15 min after intraperitoneal or intravenous injection, and plasma samples were assayed for OT and VP concentration. Intraperitoneal injection of both nonsulfated and sulfated CCK-8 significantly increased plasma OT levels and had no effect on plasma VP levels. Intravenous injection of sulfated CCK-8 stimulated an increase in plasma OT levels and did not alter plasma VP levels. However, intravenous injection of nonsulfated CCK-8 stimulated a significant increase in plasma levels of both OT and VP. No other studies have demonstrated CCK-8-stimulated release of VP in rat. NK3R antagonist did not alter baseline levels of either hormone. However, pretreatment of NK3R antagonist significantly blocked the CCK-stimulated release of OT in all CCK treatment groups and blocked VP release in response to intravenous injection of nonsulfated CCK-8. Therefore, central NK3R signaling is required for OT and VP release in response to CCK administration.     

Steroid modulation of oxytocin/vasopressin receptors in the uterus

Oxytocin (OT) and V1 vasopressin (VP) receptors are present simultaneously in several tissues, including the uterus. In myometrium these receptors mediate contractility, while in endometrium they mediate the release of other uterotonic substances as endothelin (ET). In rabbit myometrium, estrogens increase, while progesterone blunts neurohypophysial hormone receptors. However, the action of sex steroids on OT and V1 VP receptors differs in terms of the ED₅₀ and maximal effect. Therefore, at parturition, only OT receptors show a dramatic rise, while V1 VP receptors do not change, suggesting a major role for OT in labor. ET is a potent stimulator of uterine activity acting through specific receptors present on myometrial cells. These receptors as well as the endometrial localization of ET are modulated by sex steroids, indicating that ET might represent a paracrine regulator of uterine activity. In humans, OT but not V1 VP receptors increase as pregnancy progresses, confirming the primary relevance of OT in timing delivery.     

Blockade of NK3R signaling in the PVN decreases vasopressin and oxytocin release and c-Fos expression in the magnocellular neurons in response to hypotension

Tachykinin neurokinin 3 receptor (NK3R) signaling has a broad role in vasopressin (VP) and oxytocin (OT) release. Hydralazine (HDZ)-induced hypotension activates NK3R expressed by magnocellular neurons, increases plasma VP and OT levels, and induces c-Fos expression in VP and OT neurons. Intraventricular pretreatment with the specific NK3R antagonist, SB-222200, eliminates the HDZ-stimulated VP and OT release. NK3R are distributed in the central pathways conveying hypotension information to the magnocellular neurons, and the NK3R antagonist could act anywhere in the pathways. Alternatively, the antagonist could act at the NK3R expressed by the magnocellular neurons. To determine whether blockade of NK3R on magnocellular neurons impairs VP and OT release to HDZ, rats were pretreated with a unilateral PVN injection of 0.15 M NaCl or SB-222200 prior to an intravenous injection of 0.15 M NaCl or HDZ. Blood samples were taken, and brains were processed for VP/c-Fos and OT/c-Fos immunohistochemistry. Intravenous injection of 0.15 M NaCl did not alter plasma hormone levels, and little c-Fos immunoreactivity was present in the PVN. Conversely, intravenous injection of HDZ increased plasma VP and OT levels and c-Fos expression in VP and OT magnocellular neurons. Intra-PVN injection of SB-222200 prior to an intravenous injection of HDZ significantly decreased c-Fos expression in both VP and OT neurons by approximately 70% and attenuated plasma VP and OT levels by 33% and 35%, respectively. Therefore, NK3R signaling in magnocellular neurons has a critical role for the release of VP and OT in response to hypotension.     

Substance P and NPY differentially potentiate ATP and adrenergic stimulated vasopressin and oxytocin release

The supraoptic nuclei are innervated by the A1 neurons of the caudal ventrolateral medulla. Substances colocalized in the A1 terminals include norepinephrine (NE), substance P (SP), ATP, and neuropeptide Y (NPY). ATP, acting at P(2x) receptors, caused rapid and unsustained stimulation of vasopressin (VP) and oxytocin (OT) release from perifused explants of the hypothalamo-neurohypophysial system. SP elicited a concentration-dependent stimulation of VP and OT release that was large and sustained compared with other stimuli. ATP, but not phenylephrine (PE, alpha(1)-adrenergic agonist), augmented the response to SP (1 microM). In contrast, NPY did not alter basal nor ATP-induced VP or OT release, but it did cause sustained potentiation of PE-induced VP and OT release. The Y(1)-agonist, [Leu(31),Pro(34)]-NPY, increased VP and OT release, suggesting that the ineffectiveness of NPY reflects opposing actions at pre- and postsynaptic receptors. However, [Leu(31),Pro(34)]-NPY did not potentiate hormone responses to ATP or PE. The differential responses to these colocalized neurotransmitters and neuropeptides illustrate the range of potential responses that stimulation of this pathway might elicit from supraoptic neurons.     

Angiotensin II-induced release of oxytocin: interaction with norepinephrine and role in lactation

These studies examined the receptors involved in angiotensin II (Ang II) stimulated secretion of systemic oxytocin (OT) and the role of this peptide in release of OT during suckling. Plasma OT concentrations were measured following intracerebroventricular (icv) injection of vehicle, Ang II, or Ang II following pretreatment with a selective AT1 (Losartan) or AT2 (PD 123319) receptor antagonist. Furthermore, we measured Ang II-induced OT release during central alpha-adrenergic receptor blockade (phentolamine). Finally, plasma OT concentrations before and during suckling were evaluated following central administration of Ang II receptor antagonists. The increase in systemic OT following central Ang II was abolished by AT1 receptor blockade and inhibited by the AT2 receptor antagonist. Furthermore, pretreatment with phentolamine significantly diminished systemic OT release in response to icv Ang II. Finally, central Ang II receptor blockade did not alter the increase in circulating OT during suckling. These data demonstrate that Ang II evoked OT release is mediated through activation of both AT1 and AT2 receptors and suggest that a component of Ang II-induced OT stimulation is due to norepinephrine release. Furthermore, central angiotensin systems do not have a direct role in stimulating OT release during suckling.     

Characterization of the Glutamate Receptors Mediating Release of Somatostatin from Cultured Hippocampal Neurons

Abstract: l -Glutamate, NMDA, dl -α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA), and kainate (KA) increased the release of somatostatin-like immunoreactivity (SRIF-LI) from primary cultures of rat hippocampal neurons. In Mg²⁺-containing medium, the maximal effects (reached at ∼100 µ M ) amounted to 737% (KA), 722% (glutamate), 488% (NMDA), and 374% (AMPA); the apparent affinities were 22 µ M (AMPA), 39 µ M (glutamate), 41 µ M (KA), and 70 µ M (NMDA). The metabotropic receptor agonist trans -1-aminocyclopentane-1,3-dicarboxylate did not affect SRIF-LI release. The release evoked by glutamate (100 µ M ) was abolished by 10 µ M dizocilpine (MK-801) plus 30 µ M 1-aminophenyl-4-methyl-7,8-methylenedioxy-5 H -2,3-benzodiazepine (GYKI 52466). Moreover, the maximal effect of glutamate was mimicked by a mixture of NMDA + AMPA. The release elicited by NMDA was sensitive to MK-801 but insensitive to GYKI 52466. The AMPA- and KA-evoked releases were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX) or by GYKI 52466 but were insensitive to MK-801. The release of SRIF-LI elicited by all four agonists was Ca²⁺ dependent, whereas only the NMDA-evoked release was prevented by tetrodotoxin. Removal of Mg²⁺ caused increase of basal SRIF-LI release, an effect abolished by MK-801. Thus, glutamate can stimulate somatostatin release through ionotropic NMDA and AMPA/KA receptors. Receptors of the KA type (AMPA insensitive) or metabotropic receptors appear not to be involved.     

Serotonin-induced enhancement of vasopressin and oxytocin secretion in rat neurohypophyseal tissue culture

The effects of serotonin (5-hydroxytryptamine; 5-HT) on vasopressin (VP) and oxytocin (OT) secretion were studied in 13-14-day cultures of isolated rat neurohypophyseal (NH) tissue. The VP and OT contents of the supernatant were determined by radioimmunoassay after a 1 or 2 h incubation. Significantly increased levels of VP and OT production were detected in the tissue culture media following 5-HT administration, depending on the 5-HT dose. The elevation of NH hormone secretion could be partially blocked by previous administration of the 5-HT antagonist ketanserin or metergoline. WAY-100635 did not influence the increased VP secretion induced by 5-HT, but the elevated OT production was prevented by WAY-100635 before 5-HT administration. The application of WAY-100635, ketanserin or metergoline, after 5-HT administration proved ineffective. The results indicate that NH hormone release is influenced directly by the serotonergic system. The serotonergic control of VP and OT secretion from the NH tissue in rats can occur at the level of the posterior pituitary.     

Neural cell adhesion molecule-associated polysialic acid potentiates alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor currents

The highly negatively charged polysialic acid (PSA) is a carbohydrate predominantly carried by the neural cell adhesion molecule (NCAM) in mammals. NCAM and, in particular, PSA play important roles in cellular and synaptic plasticity. Here we investigated whether PSA modulates the activity of the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) subtype of glutamate receptors (AMPA-Rs). Single channel recordings of affinity-purified AMPA-Rs reconstituted in lipid bilayers revealed that bacterially derived PSA, called colominic acid, prolonged the open channel time of AMPA-R-mediated currents by severalfold and altered the bursting pattern of the receptor channels but did not modify AMPA-R single channel conductance. This effect was reversible, concentration-dependent, and specific, since monomers of sialic acid and another negatively charged carbohydrate, chondroitin sulfate, did not potentiate single channel AMPA-R currents. Recombinant PSA-NCAM also potentiated currents mediated by reconstituted AMPA-Rs. In pyramidal neurons acutely isolated from the CA1 region of the early postnatal hippocampus, l-glutamate or AMPA (applied in the presence of antagonists blocking voltage-gated Na(+) and K(+) currents and N-methyl-d-aspartate and metabotropic glutamate receptors) induced inward currents, which were significantly increased by co-application of colominic acid. Chondroitin sulfate did not affect AMPA-R-mediated currents in CA1 neurons. The effect of colominic acid was age-dependent, since in pyramidal neurons from adult hippocampus, colominic acid failed to potentiate glutamate responses. Thus, our study demonstrates age-dependent potentiation of AMPA receptors by PSA via a mechanism probably involving direct PSA-AMPA-R interactions. This mechanism might amplify AMPA-R-mediated signaling in immature cells, thereby affecting their development.     

Cyclothiazide Selectively Potentiates AMPA- and Kainate-Induced [3H]Norepinephrine Release from Rat Hippocampal Slices

Abstract: Activation of the α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) subtype of ionotropic glutamate receptors has been shown to result in a rapid desensitization of the receptor in the presence of certain agonists. One effect of AMPA receptor desensitization in the hippocampus may be to decrease the efficacy of AMPA receptor agonists at stimulating the release of norepinephrine from noradrenergic terminals. Recently, cyclothiazide was reported to inhibit AMPA receptor desensitization by acting at a distinct site on AMPA receptors. We have examined the effect of cyclothiazide on AMPA- and kainate (KA)-induced norepinephrine release from rat hippocampal slices to determine whether cyclothiazide would increase the efficacy of AMPA-induced [³H]norepinephrine release by inhibiting AMPA receptor desensitization. Cyclothiazide was observed to potentiate markedly both AMPA- and KA-induced [³H]norepinephrine release. This potentiation is selective for AMPA/KA receptors as cyclothiazide did not potentiate N -methyl- d -aspartate-induced [³H]norepinephrine release or release induced by the nonspecific depolarizing agents veratridine and 4-aminopyridine. These results demonstrate that AMPA receptor-mediated modulation of [³H]norepinephrine release from rat brain slices is a useful approach to studying the cyclothiazide modulatory site on the AMPA receptor complex.     

Delayed Excitotoxic Neurodegeneration Induced by Excitatory Amino Acid Agonists in Isolated Retina

Abstract: Evidence from in vitro studies suggests that excitotoxic neuronal degeneration can occur by either an acute or delayed mechanism. Studies of the acute mechanism in isolated chick embryo retina using histological methods indicate that this process is rapidly triggered by activation of glutamate receptors of either the N-methyl-d -aspartate (NMDA) or non-NMDA subtypes. The delayed mechanism, studied primarily in cortical and hippocampal cell cultures prepared from embryonic rodent brain, requires activation of NMDA receptors. In these cell culture systems, stimulation of non-NMDA receptors does not rapidly trigger delayed neuronal degeneration, or does so only indirectly, via activation of NMDA receptors secondary to glutamate release. To provide a more valid basis for comparison of these two mechanisms, we have modified the isolated chick embryo retina model to permit studies of delayed as well as acute excitotoxic neurodegeneration. Retinas maintained for 24 h exhibited no morphological or biochemical signs of damage. Retinal damage was assessed by measuring lactate dehydrogenase (LDH) present in the medium at various times after exposure to agonists and normalized to total LDH in each retina. Glutamate exposure (1 mM, 30 min) did not result in LDH release by the end of the exposure period, but LDH was released over the following 24 h. Briefer periods also led to substantial LDH release. Incubation in the presence of NMDA, or the non-NMDA agonists kainate (KA) or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), led rapidly to delayed LDH release. NMDA and AMPA were more potent than glutamate, but high concentrations of glutamate led to more LDH release than high concentrations of these agonists. KA was a powerful excitotoxin, providing more LDH release than glutamate, NMDA, or AMPA at every concentration tested. The delayed LDH release induced by glutamate involved activation of both NMDA and non-NMDA receptors, as a combination of receptor-selective antagonists was necessary to provide complete blockade. These results indicate that glutamate, NMDA, AMPA, and KA all cause delayed as well as acute excitotoxic damage in the retina. It is interesting that brief exposure to the non-NMDA receptor agonists, in relatively low concentrations, led to delayed LDH release. This is different than in other in vitro models of delayed excitotoxic neurodegeneration.     

Histamine-induced enhancement of vasopressin and oxytocin secretion in rat neurohypophyseal tissue cultures

The effects of histamine (HA) on vasopressin (VP) and oxytocin (OT) secretion were studied in 13-14-day cultures of isolated rat neurohypophyseal (NH) tissue. The VP and OT contents of the supernatant were determined by radioimmunoassay (RIA) after a 1 or 2-h incubation. Significantly increased levels of VP and OT production were detected in the tissue culture media following HA administration, depending on the HA dose. The elevation of NH hormone secretion could be partially blocked by previous administration of the HA antagonist mepyramine (MEP, an H1 receptor antagonist) or cimetidine (CIM, an H2 receptor antagonist). Thioperamide (TPE, an H3-H4 receptor antagonist) did not influence the VP or OT secretion increase induced by HA. The application of MEP, CIM or TPE after HA administration proved ineffective. The H1 and H2 receptors are mainly involved in the HA-induced increase of both VP and OT secretion in isolated NH tissue cultures. The results indicate that NH hormone release is influenced directly by the histaminergic system, and the histaminergic control of VP and OT secretion from the NH tissue in rats can occur at the level of the posterior pituitary.     

Aluminum, Iron, and Zinc Ions Promote Aggregation of Physiological Concentrations of β-Amyloid Peptide

Abstract: Mechanisms of non-NMDA receptor-mediated excitotoxicity were studied in embryonic rat hippocampal cultures using kainic acid (KA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) as agonists. Under basal culture conditions, overnight treatment with AMPA resulted in negligible excitotoxicity as assessed by phase-contrast microscopy and measurement of lactate dehydrogenase (LDH) release. In contrast, similar treatment with KA resulted in marked excitotoxic morphologic changes and release of LDH. Cotreatment of cultures with AMPA but not NMDA effectively blocked KA toxicity, suggesting that AMPA-induced rapid desensitization of the AMPA/KA receptor could account for the lack of prominent direct toxicity as well as AMPA's ability to block KA toxicity. To test this hypothesis, cultures were briefly pretreated with 10 μ M cyclothiazide, a drug reported to block desensitization of the AMPA/KA receptor, and then exposed overnight to cyclothiazide plus AMPA and/or KA. Cyclothiazide-treated cultures were now vulnerable to AMPA as well as KA; moreover, AMPA was unable to block KA toxicity completely, suggesting that cyclothiazide impaired AMPA/KA receptor desensitization. These and related studies suggest that a regulatory site may exist on the AMPA/KA receptor that modulates non-NMDA receptor-mediated excitotoxicity.     

Kainic acid-induced neurotrophic activities in developing cortical neurons

Using primary cultured cortical neurons from embryonic rat brains, we elucidated an alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainic acid (KA) receptor-mediated neuroprotective mechanism through actions of nerve growth factor (NGF) in developing neurons. Neurotoxicity of KA in early days in vitro neurons was quite low compared with the mature neurons. However, pretreatment with anti-NGF antibody or TrkA inhibitor AG-879 profoundly raised KA toxicity. Furthermore, KA stimulation resulted in an increase of TrkA expression and phosphorylation, which was blocked not only by the AMPA/KA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and AG-879, but also by the phospholipase C inhibitor U73122 and the intracellular calcium chelator BAPTA. A study of polyphosphoinositide turnover showed that KA-stimulated phospholipase C (PLC) activity was directly triggered by the AMPA/KA receptor activity, but not by the activity of TrkA or other excitatory amino acid receptor subtypes. Sources of KA-increased intracellular calcium levels were contributed by both extracellular calcium influx and intracellular calcium release and were partially sensitive to guanosine 5'-O-(2-thiodiphosphate). These results indicate that in developing cortical neurons, activation of AMPA/KA receptors by KA may induce expression, followed by activation of TrkA via PLC signaling and intracellular calcium elevation and hence increase reception of NGF on KA-challenged neurons. A G protein-coupled AMPA/KA receptor may be involved in these metabotropic events for neuronal protection.     

Vasopressin and oxytocin release evoked by NaCl loads are selectively blunted by area postrema lesions

The present study investigated the effect of area postrema lesions (APX) on stimulated neurohypophysial secretion of vasopressin (VP) and oxytocin (OT) in conscious rats. Blunted increases in plasma levels of both pituitary hormones were observed when rats with APX were infused intravenously with 1 M NaCl solution (2 ml/h for 6 h). In contrast, plasma VP and OT increased normally in rats with APX when equivalent increases in plasma osmolality (but not plasma Na(+)) resulted from intravenous infusion of an equiosmotic solution of 1 M mannitol and 0.5 M NaCl. Furthermore, APX did not affect increases in plasma VP and OT stimulated by plasma volume deficits, nor did APX disrupt OT secretion stimulated by intravenous injection of cholecystokinin. These findings suggest that the area postrema plays an important role in mediating secretion of VP and OT in response to an NaCl load, but not in response to an equiosmotic load that does not cause substantial hypernatremia, and not in response to other stimuli of neurohypophysial hormone secretion. Together with previous reports, these results suggest that APX impairs Na(+) regulation in rats.     

Neuronal nitric oxide synthase proteolysis limits the involvement of nitric oxide in kainate-induced neurotoxicity in hippocampal neurons

In this work, we investigated the role of nitric oxide (NO) in neurotoxicity triggered by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor activation in cultured hippocampal neurons. In the presence of cyclothiazide (CTZ), short-term exposures to kainate (KA; 5 and 15 min, followed by 24-h recovery) decreased cell viability. Both NBQX and d-AP-5 decreased the neurotoxicity caused by KA plus CTZ. Long-term exposures to KA plus CTZ (24 h) resulted in increased toxicity. In short-, but not in long-term exposures, the presence of NO synthase (NOS) inhibitors (l-NAME and 7-NI) decreased the toxicity induced by KA plus CTZ. We also found that KA plus CTZ (15-min exposure) significantly increased cGMP levels. Furthermore, short-term exposures lead to decreased intracellular ATP levels, which was prevented by NBQX, d-AP-5 and NOS inhibitors. Immunoblot analysis revealed that KA induced neuronal NOS (nNOS) proteolysis, gradually lowering the levels of nNOS according to the time of exposure. Calpain, but not caspase-3 inhibitors, prevented this effect. Overall, these results show that NO is involved in the neurotoxicity caused by activation of non-desensitizing AMPA receptors, although to a limited extent, since AMPA receptor activation triggers mechanisms that lead to nNOS proteolysis by calpains, preventing a further contribution of NO to the neurotoxic process.     

Glutamate receptors of the AMPA type modulate neurotransmitter release and peristalsis in the guinea-pig isolated colon

To assess the role of AMPA and kainate receptors in modulating neurotransmitter release from the myenteric plexus, the effect of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) and kainic acid on endogenous acetylcholine (ACh) and noradrenaline (NA) overflow from the guinea-pig isolated colon was studied. AMPA inhibited spontaneous ACh overflow and increased electrically-evoked NA overflow. Kainic acid did not influence both ACh and NA overflow. AMPA-mediated effects on ACh and NA overflow were significantly reduced by the AMPA/kainate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, CNQX. The inhibitory effect of AMPA on ACh overflow could be due, at least in part, to the AMPA-induced NA overflow as it was greatly reduced after adrenoceptor blockade and virtually abolished in sympathetically-denervated animals. The possible functional significance of these findings was studied by measuring the efficiency of the peristaltic reflex in the presence of the different agonists. The efficiency of peristalsis was enhanced by AMPA, whereas it was not modified by kainic acid. In conclusion, AMPA receptors, but not kainate receptors, may play a role in the modulation of ACh and NA release and of peristalsis in the guinea-pig colon.     

Differential effects of emotional and physical stress on the central and peripheral secretion of neurohypophysial hormones in male rats

The effects of various stressful conditions on the levels of oxytocin (OT) and vasopressin (VP) in plasma and cisternal cerebrospinal fluid (CSF) of male rats were investigated. Three experimental models were used: exposure to a novel environment for 5 min, immobilization for 15 min, and ether inhalation for 10 min resulting in anaesthesia. Novelty and immobilization induced a slight but significant increase in OT levels in the CSF immediately after the stress. The effect of ether was considerably more pronounced. The concentration of VP in the CSF was elevated only by ether stress. In plasma, the level of OT was increased immediately following immobilization and ether stress but not after novelty stress, whereas VP only showed a delayed response 20 min after immobilization. These results indicate a rapid preferential release of OT in the periphery in response to physical and pharmacological stress. In addition, they provide evidence that release of OT into the CSF is triggered by physical, pharmacological as well as emotional stress, while the central release of VP is rather resistant to emotional stress. The data suggest that OT is a stress hormone in the central nervous system.     

Glutamatergic Regulation of Basal and Stimulus-Activated Dopamine Release in the Prefrontal Cortex

Abstract: The present study was undertaken to determine whether basal and stimulus-activated dopamine release in the prefrontal cortex (PFC) is regulated by glutamatergic afferents to the PFC or the ventral tegmental area (VTA), the primary source of dopamine neurons that innervate the rodent PFC. In awake rats, blockade of NMDA or α-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors in the VTA, or blockade of AMPA receptors in the PFC, profoundly reduced dopamine release in the PFC, suggesting that the basal output of dopamine neurons projecting to the PFC is under a tonic excitatory control of NMDA and AMPA receptors in the VTA, and AMPA receptors in the PFC. Consistent with previous reports, blockade of cortical NMDA receptors increased dopamine release, suggesting that NMDA receptors in the PFC exert a tonic inhibitory control on dopamine release. Blockade of NMDA or AMPA receptors in the VTA as well as blockade of AMPA receptors in the PFC reduced the dopaminergic response to mild handling, suggesting that activation of glutamate neurotransmission also regulates stimulus-induced increase of dopamine release in the PFC. In the context of brain disorders that may involve cortical dopamine dysfunction, the present findings suggest that abnormal basal or stimulus-activated dopamine neurotransmission in the PFC may be secondary to glutamatergic dysregulation.     

Tachykinin NK3 receptor contribution to systemic release of vasopressin and oxytocin in response to osmotic and hypotensive challenge

Activation of the neurokinin 3 receptor (NK3R) by a receptor agonist, hypotension, and hyperosmolarity results in the internalization of NK3R expressed by magnocellular neurons and the release of vasopressin (VP) and oxytocin (OT) into the circulation. The contribution of NK3R activation to the release of VP and OT in response to hyperosmolarity and hypotension was evaluated by measuring the release of both hormones following pretreatment with a selective NK3R antagonist, SB-222200. Freely behaving male rats were given an intraventricular injection of either 0.15 M NaCl or 250, 500, or 1,000 pmol SB-222200, and then were administered an intravenous infusion of 2 M NaCl or 0.15 M NaCl (experiment 1), or a bolus intra injection of 0.15 M NaCl or hydralazine (HDZ), a hypotension-inducing drug (experiment 2). Blood samples were taken from indwelling arterial catheters at various time points for 1-2 h, both before and after treatments. Plasma VP and OT levels were determined by ELISA. Blockade of NK3R did not affect the baseline levels of either hormone. In contrast, pretreatment with SB-222200 significantly reduced ( approximately 60%) or abolished the release of VP and OT, respectively, to 2 M NaCl infusion. HDZ-induced VP and OT release was eliminated by pretreatment with 500 pmol SB-222200. Therefore, NK3R activation contributes significantly to the systemic release of both VP and OT in response to osmotic and hypotensive challenges.