N-methyl-D,L-aspartate Research Articles

Visual Disfunction due to the Selective Effect of Glutamate Agonists on Retinal Cells.

One of the causes of nervous system degeneration is an excess of glutamate released upon several diseases. Glutamate analogs, like N-methyl-DL-aspartate (NMDA) and kainic acid (KA), have been shown to induce experimental retinal neurotoxicity. Previous results have shown that NMDA/KA neurotoxicity induces significant changes in the full field electroretinogram response, a thinning on the inner retinal layers, and retinal ganglion cell death. However, not all types of retinal neurons experience the same degree of injury in response to the excitotoxic stimulus. The goal of the present work is to address the effect of intraocular injection of different doses of NMDA/KA on the structure and function of several types of retinal cells and their functionality. To globally analyze the effect of glutamate receptor activation in the retina after the intraocular injection of excitotoxic agents, a combination of histological, electrophysiological, and functional tools has been employed to assess the changes in the retinal structure and function. Retinal excitotoxicity caused by the intraocular injection of a mixture of NMDA/KA causes a harmful effect characterized by a great loss of bipolar, amacrine, and retinal ganglion cells, as well as the degeneration of the inner retina. This process leads to a loss of retinal cell functionality characterized by an impairment of light sensitivity and visual acuity, with a strong effect on the retinal OFF pathway. The structural and functional injury suffered by the retina suggests the importance of the glutamate receptors expressed by different types of retinal cells. The effect of glutamate agonists on the OFF pathway represents one of the main findings of the study, as the evaluation of the retinal lesions caused by excitotoxicity could be specifically explored using tests that evaluate the OFF pathway.

Contribution of 5-HT to locomotion - the paradox of Pet-1−/− mice

Serotonin (5-HT) plays a critical role in locomotor pattern generation by modulating the rhythm and the coordinations. Pet-1, a transcription factor selectively expressed in the raphe nuclei, controls the differentiation of 5-HT neurons. Surprisingly, inactivation of Pet-1 (Pet-1(-/-) mice) that causes a 70% reduction in the number of 5-HT-positive neurons in the raphe does not impair locomotion in adult mice. The goal of the present study was to investigate the operation of the locomotor central pattern generator (CPG) in neonatal Pet-1(-/-) mice. We first confirmed, by means of immunohistochemistry, that there is a marked reduction of 5-HT innervation in the lumbar spinal cord of Pet-1(-/-) mice. Fictive locomotion was induced in the in vitro neonatal mouse spinal cord preparation by bath application of N-methyl-d,l-Aspartate (NMA) alone or together with dopamine and 5-HT. A locomotor pattern characterized by left-right and flexor-extensor alternations was observed in both conditions. Increasing the concentration of 5-HT from 0.5 to 5 μm impaired the pattern in Pet-1(-/-) mice. We tested the role of endogenous 5-HT in the NMA-induced fictive locomotion. Application of 5-HT(2) or 5-HT(7) receptor antagonists affected the NMA-induced fictive locomotion in both heterozygous and homozygous mice although the effects were weaker in the latter strain. This may be, at least partly, explained by the reduced expression of 5-HT(2A) R as observed by means of immunohistochemistry. These results suggest that compensatory mechanisms take place in Pet-1(-/-) mice that make locomotion less dependent upon 5-HT.

Failure of σ-receptor ligands to reduce the excitatory actions of N-methyl-DL-aspartate on rat spinal neurons in-vivo

Haloperidol and (+)-3-PPP, compounds with known affinity for the sigma-receptor, have been examined for their ability to reduce the excitatory actions of N-methyl-DL-aspartate (NMDLA), quisqualate and kainate on rat spinal neurons in-vivo. The actions of (-)-3-PPP were also tested. Haloperidol was injected intravenously whereas the 3-PPP enantiomers were administered by microelectrophoresis. Haloperidol had little effect on excitations evoked by NMDLA, quisqualate or kainate whereas both (+)- and (-)-3-PPP usually enhanced, non-selectively, responses to all three excitatory amino acid analogues. The results support suggestions that phencyclidine (PCP)-like compounds with affinity for both PCP and sigma-receptors reduce neuronal excitations mediated by the N-methyl-D-aspartate (NMDA) receptor via a selective effect at the PCP site.

Acute stress rapidly decreases plasma testosterone in a free-ranging male songbird: Potential site of action and mechanism

We used a free-ranging, seasonally breeding adult male songbird, the rufous-winged sparrow, Aimophila carpalis, to investigate the effects of acute stress-induced by capture followed by restraint, on the hypothalamo-pituitary–testicular axis. Intra- and interindividual comparisons revealed that males decreased their plasma testosterone (T) by 37–52% in response to acute stress. The decrease occurred within 15 min of capture and persisted for at least another 15 min. Within 15 min, the decrease in plasma T was not associated with a reduction in plasma luteinizing hormone (LH). Thirty minutes after capture and restraint, the decrease in plasma T either was likewise not associated with decreased plasma LH (intraindividual comparison) or concurred with a reduction in plasma LH (interindividual comparison). These observations indicate that effects of stress may have been mediated at the pituitary gland and also directly at the testicular levels. To address this question, we measured the hormonal response to an injection of the glutamate receptor agonist N-methyl- d, l-aspartate (NMA) to stimulate to stimulate the release of gonadotropin-releasing hormone (GnRH) or of GnRH to stimulate the release of LH. Treatment with NMA did not change plasma LH, presumably because the birds were in breeding condition and already secreting GnRH at a maximum rate. Administration of GnRH increased plasma LH equally in birds that were or were not stressed before the treatment. An injection of purified ovine LH (oLH) increased plasma T equally in birds that were or were not acutely stressed before the hormone injection. Thus, the observed acute stress-induced decrease in plasma T was apparently not mediated by decreased responsiveness of the pituitary gland to GnRH or of the testes to LH. Decreased plasma T following stress may involve a direct impairment of the testicular endocrine function.

Glutamatergic Stimulation of Luteinising Hormone Secretion in Relatively Refractory Male Songbirds

Seasonal breeding in two Sonoran desert passerines, the Cassin's (Aimophila cassinii) and Rufous-crowned (Aimophila ruficeps) Sparrows is thought to be terminated by the development of a decrease in responsiveness to photostimulation, a condition known as relative photorefractoriness. It was predicted that the development of relative refractoriness is a consequence of a decrease in gonadotrophin-releasing hormone (GnRH) synthesis and associated stores of releasable GnRH. This hypothesis was tested by determining the luteinising hormone (LH) responses to the excitatory amino acid glutamate agonist N-methyl-D,L-aspartate (NMA) in males of the two species subjected to photomanipulations aimed at generating five groups: Fully photosensitive with undeveloped testes on short days (8L : 16D); fully photosensitive with developed testes on 13L : 11D; relatively photorefractory with regressed testes on 13L : 11D, and groups with developed testes held on 15L : 9D or 16L : 8D. LH release was stimulated in the Cassin's Sparrow by NMA most in the 8L group; to a lesser, but similar extent in the two 13L groups; and not at all in the 15L and 16L groups. LH release was not stimulated by NMA in any of the photoperiodic regimes in the Rufous-crowned Sparrow. In both species, NMA induced Fos-like immunoreactivity in the anterior and basal hypothalamus, but not in GnRH cell bodies. It is concluded that the development of relative photorefractoriness in Cassin's Sparrows is a consequence of reduced GnRH synthesis, reflected in a reduction in releasable GnRH. The lack of LH response of the Rufous-crowned Sparrows to NMA administration may be a consequence of high responsiveness to handling stress.

Neuroendocrine regulation of prolactin secretion in adult female rhesus monkeys during different phases of the menstrual cycle: role of neuroexcitatory amino acid (NMA)

The present study attempts to examine the role of N-methyl-D, L-aspartate (NMDA) receptors in the central regulation of prolactin (PRL) secretion, which may be involved in ovarian function and its alteration by glutamate in various phases of the menstrual cycle of female rhesus monkeys (Macaca mulatta). The results suggest that the glutaminergic component of the control system, which governs PRL secretion by utilizing NMDA receptors, may have an important role in regulating changes in PRL secretion. The response of PRL during the luteal phase of the cycle was different from that observed in follicular and menstrual phases. Steroids may influence the NMDA-dependent drive to release PRL. N-methyl-D-aspartic acid (NMA) involvement in the regulation of PRL secretion may occur through activation of the PRL-stimulating system depending on the physiological state or steroidal milieu. It is possible, therefore, that the NMA-induced release of PRL-releasing factors (PRF) and PRL are enhanced in the presence of ovarian feedback.

Propriospinal Circuitry Underlying Interlimb Coordination in Mammalian Quadrupedal Locomotion

Soon after birth, freely moving quadrupeds can express locomotor activity with coordinated forelimb and hindlimb movements. To investigate the neural mechanisms underlying this coordination, we used an isolated spinal cord preparation from neonatal rats. Under bath-applied 5-HT, N-methyl-d,l-aspartate (NMA), and dopamine (DA), the isolated cord generates fictive locomotion in which homolateral cervicolumbar extensor motor bursts occur in phase opposition, as does bursting in homologous (left-right) extensor motoneurons. This coordination corresponded to a walking gait monitored with EMG recordings in the freely behaving animal. Functional decoupling of the cervical and lumbar generators in vitro by sucrose blockade at the thoracic cord level revealed independent rhythmogenic capabilities with similar cycle frequencies in the two locomotor regions. When the cord was partitioned at different thoracic levels and 5-HT/NMA/DA was applied to the more caudal compartment, the ability of the lumbar generators to drive their cervical counterparts increased with the proportion of chemically exposed thoracic segments. Blockade of synaptic inhibition at the lumbar level caused synchronous bilateral lumbar rhythmicity that, surprisingly, also was able to impose bilaterally synchronous bursting at the unblocked cervical level. Furthermore, after a midsagittal section from spinal segments C1 to T7, and during additional blockade of cervical synaptic inhibition, the cord exposed to 5-HT/NMA/DA continued to produce a coordinated fictive walking pattern similar to that observed in control. Thus, in the newborn rat, a caudorostral propriospinal excitability gradient appears to mediate interlimb coordination, which relies more on asymmetric axial connectivity (both excitatory and inhibitory) between the lumbar and cervical generators than on differences in their inherent rhythmogenic capacities.

Serotonin refines the locomotor‐related alternations in the in vitro neonatal rat spinal cord

Serotonergic projections from raphe nuclei arrive in the lumbar enlargement of the spinal cord during the late fetal period in the rat, a time window during which the locomotor-related left/right and flexor/extensor coordinations switch from synchrony to alternation. The goal of the present study was to investigate the role played by serotonin (5-HT) in modulating the left/right and flexor/extensor alternations. Fictive locomotion was induced by bath application of N-methyl-D,L-aspartate (NMA) in the in vitro neonatal rat spinal cord preparation. By means of cross-correlation analysis we demonstrate that 5-HT, when added to NMA, improves left/right and flexor/extensor (recorded from the 3rd and 5th lumbar ventral roots, respectively) alternations. This effect was partly reproduced by activation of 5-HT(2A/2C) receptors. We then tested the contribution of endogenous 5-HT to NMA-induced fictive locomotion. Reducing the functional importance of endogenous 5-HT, either by inhibiting its synthesis with daily injections of p-chloro-phenylalanine (PCPA), starting on the day of birth, or by application of ketanserin (a 5-HT(2) receptor antagonist) or SB269970 (a 5-HT(7) receptor antagonist), disorganized the NMA-induced locomotor pattern. This pattern was restored in PCPA-treated animals by adding 5-HT to the bath. Blocking 5-HT(7) receptors disorganized the locomotor-like rhythm even in the absence of electrical activity in the brain stem, suggesting that NMA applied to the spinal cord does not cause 5-HT release by activating a spino-raphe-spinal loop. These results demonstrate that 5-HT is critical in improving the locomotor-related alternations in the neonatal rat.

Effect of glutamate and somatostatin-14 on basal and cAMP-stimulated steroidogenesis by rainbow trout ( Oncorhynchus mykiss) ovarian follicles, in vitro

The effects of glutamate and somatostatin-14 (SRIF) on the in vitro basal and cAMP-stimulated steroid production of mid-vitellogenic rainbow trout ( Oncorhynchus mykiss) ovarian follicles were investigated. cAMP-stimulation was achieved by the addition of the adenylyl cyclase activator, forskolin (FS), or a membrane permeate cAMP agonist, 8-bromo-cAMP (BA), to the incubation medium. Testosterone (T) and 17β-estradiol (E 2) secretion was measured using radioimmunoassay. Solid phase extraction (SPE) was used to measure the relative formation of unconjugated and conjugated steroids, and high performance liquid chromatography (HPLC) was used to examine the steroid metabolites formed from the metabolism of a tritium labelled precursor, pregnenolone (P 5). The accumulations of T and E 2 in the medium were suppressed in the presence of the glutamate agonists, N-methyl- d, l-aspartate (NMA) or l-glutamic acid (GA), and by the presence of SRIF. The suppression was evident for both basal and cAMP-stimulated steroidogenesis except for T concentrations of GA treatments following basal steroidogenesis, when there were no treatment effects. No significant effects of treatment on conjugated:unconjugated steroid ratios were found. For all treatments E 2 was the major end product steroid synthesized from P 5, and the steroid profiles were similar except for trace amounts of radiolabelled androgens in the medium following cAMP-stimulated steroidogenesis that were not present following basal steroidogenesis. The findings suggest that glutamate and SRIF reduce end point steroid production, possibly by reducing P 5 production. However, since the inhibitory affect was found for basal and cAMP-stimulated steroidogenesis, the response does not appear to be due to the inhibition of cAMP synthesis.

Seasonal Differences in the Secretion of Luteinising Hormone and Prolactin in Response to N‐Methyl‐dl‐Aspartate in Starlings (Sturnus vulgaris)

In birds, unlike mammals, seasonal changes in reproductive function are associated with marked changes in the amount of gonadotrophin-releasing hormone (GnRH) stored in the hypothalamus. Prolonged exposure to long photoperiods leads to photorefractoriness after the breeding season. Photorefractory birds have low hypothalamic concentrations of chicken GnRH-I (cGnRH-I). Exposure to short photoperiods results in renewed cGnRH-I synthesis and increased hypothalamic stores. Birds are then photosensitive and subsequent exposure to an increase in photoperiod results in increased cGnRH-I secretion and gonadal maturation. However, it is unclear whether the reverse is true at the time of gonadal regression during long photoperiods (i.e. that a decrease in GnRH-I synthesis precedes regression). Hypothalamic stores of cGnRH-I, and possibly therefore of releasable GnRH-I, decrease after regression. Single injections of the glutamate agonist N-methyl-DL-aspartate (NMA) were used as a probe to assess releasable stores of cGnRH-I in male starlings at four physiologically different reproductive stages. Treatment induced the greatest increase in luteinising hormone (LH) in photosensitive birds in January, and a slight increase in sexually mature birds in April. There was a slight but significant increase in June, immediately after testicular regression, but no increase in fully photorefractory birds in September. These data confirm that photorefractoriness is associated with a lack of releasable cGnRH-I, but that decreased synthesis of cGnRH-I is not the proximate cause of regression. There was an increase in prolactin in response to NMA at all times. The magnitude of the response was proportional to pre-treatment concentrations, with the greatest response in June. It is suggested that high circulating prolactin may fine-tune the timing of gonadal regression in advance of the inhibition of cGnRH-I synthesis.

Peptidergic neuromodulation of the lumbar locomotor network in the neonatal rat spinal cord

It is now well established that a dynamic balance of neurotransmitters and neuromodulators finely influence the output of neuronal networks and subsequent behaviors. In the present study, to further understand the modulatory processes that control locomotor behavior, we investigated the action of 11 neuropeptides, chosen among the various peptide subfamilies, on the lumbar neuronal network in the in vitro neonatal rat spinal cord preparation. Peptides were bath-applied alone, in combination with N-methyl- d, l-aspartate (NMA) or with the classical ‘locomotor cocktail’ of NMA and serotonin. Using these different experimental paradigms, we show that each peptide can neuromodulate the lumbar locomotor network and that peptides exhibit different neuromodulatory profiles and potencies even within the same family. Only vasopressin, oxytocin, bombesin and thyrotropin releasing hormone triggered tonic or non-organized rhythmic activities when bath-applied alone. All the neuropeptides modulated NMA induced activity and/ or ongoing sequences of fictive locomotion to varying degrees. These results suggest that neuropeptides play an important role in the control of the neural network for locomotion in the neonatal rat. Their various profiles of action may account in part for the great flexibility of motor behaviors.

Effect of glutamate on basal steroidogenesis by ovarian follicles of rainbow trout ( Oncorhynchus mykiss)

The effects of glutamate on the in vitro basal steroid production of three maturational stages of rainbow trout ( Oncorhynchus mykiss) ovarian follicles were investigated. Radioimmunoassays were used to measure the rates of synthesis of testosterone (T) and 17-estradiol (E 2). High performance liquid chromatography (HPLC) was used to examine the steroid metabolites produced from a tritium labeled precursor, pregnenolone (P 5). The glutamate agonist, N-methyl- d, l-aspartate (NMA) had a dose-dependent suppressive effect on T and E 2 synthesis in mid-vitellogenic (MV) follicles, but had no significant effect on early- (EV) and late-vitellogenic (LV) follicles. l-glutamic acid (GA) had a dose-related suppressive effect on T synthesis by MV follicles, suppressing both T and E 2 synthesis by LV follicles, but having no effect on EV follicles. HPLC separation of the steroid metabolites synthesized from P 5 showed clear evidence of differences in rates of overall steroidogenesis of the three follicular stages, but no effect of either NMA or GA on the nature or the amount of the metabolites produced by the three developmental stages examined. The findings suggest that glutamate may act via a reduction in the production of P 5, which is the principal rate-limiting step in the steroidogenic cascade, and not via modulation of steroidogenic enzyme activities.

Neuroprotective effect of developmental docosahexaenoic acid supplement against excitotoxic brain damage in infant rats

Long-chain polyunsaturated fatty acid (LC-PUFA) composition of neural membranes is a key factor for brain development, in chemical communication of neurons and probably also their survival in response to injury. Viability of cholinergic neurons was tested during brain development following dietary supplementation of fish oil LC-PUFAs (docosahexaenoic acid [DHA], eicosapentaenoic acid, arachidonic acid) in the food of mother rats. Excitotoxic injury was introduced by N-methyl- d,l-aspartate (NMDA) injection into the cholinergic nucleus basalis magnocellularis of 14-day-old rats. The degree of loss of cholinergic cell bodies, and the extend of axonal and dendritic disintegration were measured following immunocytochemical staining of cell bodies and dendrites for choline acetyltransferase and p75 low-affinity neurotrophin receptor and by histochemical staining of acetylcholinesterase-positive fibres in the parietal neocortex. The impact of different feeding regimens on fatty acid composition of neural membrane phospholipids was also assayed at 12 days of age. Supplementation of LC-PUFAs resulted in a resistance against NMDA-induced excitotoxic degeneration of cholinergic neurones in the infant rats. More cholinergic cells survived, the dendritic involution of surviving neurons in the penumbra region decreased, and the degeneration of axons at the superficial layers of parietal neocortex also attenuated after supplementing LC-PUFAs. A marked increment in DHA content in all types of phospholipids was obtained in the forebrain neuronal membrane fraction of supplemented rats. It is concluded that fish oil LC-PUFAs, first of all DHA, is responsible for the neuroprotective action on developing cholinergic neurons against glutamate cytotoxicity.

The respective contribution of lumbar segments to the generation of locomotion in the isolated spinal cord of newborn rat.

Various studies on isolated neonatal rat spinal cord have pointed to the predominant role played by the rostral lumbar area in the generation of locomotor activity. In the present study, the role of the various regions of the lumbar spinal cord in locomotor genesis was further examined using compartmentalization and transections of the cord. We report that the synaptic drive received by caudal motoneurons following N-methyl-d-l-aspartate (NMA)/5-HT superfusion on the entire lumbar cord is different from that triggered by the same compounds specifically applied on the rostral segments. These differences appear to be due to the direct action of NMA/5-HT on motoneuron membrane potential, rather than on premotoneuronal input activation. In order to assess the possible participation of the caudal lumbar segments in locomotor rhythm generation, the segments were over-stimulated with high concentrations of NMA or K+. We find that significant variations in motor cycle period occurred during the over-activation of the rostral segments. Over-activation of caudal segments only si+gnificantly increased the caudal ventral roots burst amplitude. We find that low 5-HT concentrations were unable to induce fictive locomotion under our experimental conditions. When a hemi-transection of the cord was performed between the L2-L3 segments, rhythmic bursting in the ipsilateral L5 disappeared while rhythmicity persisted on the contralateral side. Sectioning of the remaining L2-L3 side totally suppressed rhythmic activity in both L5 ventral roots. These results show that the thoracolumbar part of the cord constitutes the key area for locomotor pattern generation.

Variability analyses suggest that supraspino–spinal interactions provide dynamic stability in motor control

Effects of supraspino–spinal feedforward–feedback (FF-FB) interactions on variability in locomotor rhythm and coordination were examined in in vitro brain–spinal cord lamprey preparations. Spinal locomotor networks were activated by applying 0.2 mM N-methyl- dl-aspartate (NMA) to three spinal pools: gill, rostral and caudal. Bathing the brain with zero Ca 2+ saline altered supraspinal–spinal drive and FF-FB interaction while spino–supraspinal feedback was changed by applying NMA to the caudal pool only. Wavelet analyses indicated a non-uniform energy distribution in ventral root (VR) activity that shifted between frequency bands on FF-FB interruption. Wavelet analysis was used to extract 300-s long epochs of low frequency burst rhythm. These were analyzed using a sliding-window time-varying covariance method. From the autocovariance in each window, the cycle period and height of the first side lobe peak were determined. Rostral VR variability (determined from standard deviation and coefficient of variation of all cycle periods and the mean peak height) was significantly higher than caudal VR variability. FF-FB interruption significantly decreased the rostral VR cycle period and variability but the rostro-caudal gradient remained. The intersegmental delay was also affected. The caudal VR rhythm with NMA in the caudal pool only was slower but more variable than with NMA over the entire cord. These results indicate that the locomotor rhythm in the presence of supraspino–spinal interactions is slower but has a higher variability. The higher variability may reflect a dynamic stability of the system. Additionally, differences in local neural organization likely contribute to rostro-caudal differences in variability of the motor output.

Pituitary hormone and insulin responses to infusion of amino acids and N-methyl-D,L-aspartate in horses.

Thirty-nine adult light horse mares, geldings, and stallions were used in two experiments to assess the pituitary hormone and insulin responses to infusions of arginine, aspartic acid, lysine, glutamic acid, and N-methyl-D,L-aspartate (NMA). In Exp. 1, 27 horses were assigned to one of three infusion treatments: 1) physiological saline (1 L); 2) 2.855 mmol of arginine/kg BW in 1 L of water; or 3) 2.855 mmol of aspartic acid/kg BW in 1 L of water. In Exp. 2, 12 horses were assigned, in a multiple-square 4 x 4 Latin square design, to one of four infusion treatments: 1) 2 mL of saline/kg BW; 2) 2.855 mmol of lysine/kg BW in water; 3) 2.855 mmol of glutamic acid/kg BW in water; or 4) 1 mg of NMA/kg BW in water. In Exp. 1, an acute (within 20 min) release of growth hormone (GH) was induced (P = 0.002) by aspartic acid. In contrast, acute release of prolactin (P = 0.001) and insulin (P = 0.002) was induced only by arginine; moreover, the arginine effect on insulin was present only in mares (P = 0.011). In Exp. 2, an acute release of GH was induced (P = 0.001) by glutamic acid and NMA. In males, the glutamic acid-induced GH release was greater than that of NMA; in mares, the NMA-induced GH release was greater than that of glutamic acid (P = 0.069). Both lysine and glutamic acid induced (P = 0.001) acute release of prolactin, whereas an acute release of insulin was elicited (P = 0.002) only by lysine. The NMA-induced LH response was due almost entirely to the response in mares and stallions (P = 0.016), and the NMA-induced FSH release was due almost entirely to the response in mares (reproductive status effect; P = 0.004). In the horse, aspartic acid, glutamic acid, and NMA seem to stimulate GH release; arginine and lysine seem to stimulate prolactin and insulin release; and NMA seems to stimulate LH and FSH release. It seems that N-methyl-D-aspartate glutamate receptors are involved in controlling GH, LH, and FSH secretion, whereas other mechanisms are involved with prolactin secretion. These results also indicate that gonadal steroids interact with amino acid-induced pituitary hormone release in adult horses.

The effect of ketamine hydrochloride anesthesia on basal and N-methyl- D,L-aspartate induced plasma prolactin secretion in the adult male rhesus monkey

The excitatory amino acids (EAAs), glutamate and aspartate, acting predominantly on N-methyl-D-aspartate (NMDA) receptor, have been shown to be involved in the central regulation of the secretion of several anterior pituitary hormones including prolactin (PRL), whereas ketamine hydrochloride (KH), a widely used anesthetic, has been reported to antagonize a variety of NMDA receptor mediated actions of these EAAs. In the present study, the effect of KH on basal PRL levels as well as on N-methyl-D,L-aspartate (NMA), an agonist of NMDA receptor, induced plasma PRL secretion was investigated in the adult male rhesus monkey. The values were compared to those obtained from the same animals restrained in primate chairs. The plasma PRL concentrations were higher in animals receiving KH administered either intramuscularly (2.5 mg/kg BW at 30 min intervals) or intravenously (10 mg/kg BW) as compared to those observed in the unanesthetized chair-restrained monkeys. NMA induced an unequivocal increase in plasma PRL concentrations in both conscious chair-restrained and KH anesthetized monkeys, but the response was greater in anesthetized animals than the conscious monkeys. The present findings suggest that KH has stimulatory effects on both basal and NMA induced plasma PRL secretion.

Neuroendocrine aging in the female rat: the changing relationship of hypothalamic gonadotropin-releasing hormone neurons and N-methyl-D-aspartate receptors.

The reproductive axis undergoes alterations during aging, resulting in acyclicity and the loss of reproductive function. In the hypothalamus, changes intrinsic to GnRH neurons may play a critical role in this process, as may changes in inputs to GnRH neurons from neurotransmitters such as glutamate. We investigated the effects of age and reproductive status on neuroendocrine glutamatergic NMDA receptors (NRs), their regulation of GnRH neurons, and their expression on GnRH neurons, in female rats. First, we quantified NR subunit messenger RNAs (mRNAs) in preoptic area-anterior hypothalamus (POA-AH) and medial basal hypothalamus (MBH), the sites of GnRH perikarya and neuroterminals, respectively. In POA-AH, NR1 mRNA levels varied little with age or reproductive status. NR2a and NR2b mRNA levels decreased significantly between cycling and acyclic rats. In MBH, NR mRNAs all increased with aging, particularly in acyclic animals. Second, we tested the effects of N-methyl-D,L-aspartate (NMA) on GnRH mRNA levels in POA-AH of aging rats. NMA elevated GnRH mRNA levels in young rats, but decreased them in middle-aged rats. Third, we quantified expression of the NR1 subunit on GnRH perikarya in aging rats using double label immunocytochemistry. NR1 expression on GnRH cell bodies varied with age and reproductive status, with 30%, 19%, and 46% of GnRH somata double labeled with NR1 in young proestrous, middle-aged proestrous, and middle-aged persistent estrous rats, respectively. Thus, 1) the expression of hypothalamic NR subunit mRNAs correlates with reproductive status; 2) changes in NR subunit mRNA levels, if reflected by changes in protein levels, may result in alterations in the stoichiometry of the NR during aging, with possible physiological consequences; 3) the effects of NR activation on GnRH mRNA switches from stimulatory to inhibitory during reproductive aging; and 4) expression of the NR1 subunit on GnRH perikarya changes with reproductive status. These molecular, physiological, and cellular neuroendocrine changes are proposed to be involved in the transition to acyclicity in aging female rats.

Properties of rhythmic activity generated by the isolated spinal cord of the neonatal mouse.

We examined the ability of the isolated lumbosacral spinal cord of the neonatal mouse (P0-7) to generate rhythmic motor activity under several different conditions. In the absence of electrical or pharmacological stimulation, we recorded several patterns of spontaneous ventral root depolarization and discharge. Spontaneous, alternating discharge between contralateral ventral roots could occur two to three times over a 10-min interval. We also observed other patterns, including left-right synchrony and rhythmic activity restricted to one side of the cord. Trains of stimuli delivered to the lumbar/coccygeal dorsal roots or the sural nerve reliably evoked episodes of rhythmic activity. During these evoked episodes, rhythmic ventral root discharges could occur on one side of the cord or could alternate from side to side. Bath application of a combination of N-methyl-D,L-aspartate (NMA), serotonin, and dopamine produced rhythmic activity that could last for several hours. Under these conditions, the discharge recorded from the left and right L(1)-L(3) ventral roots alternated. In the L(4)-L(5) segments, the discharge had two peaks in each cycle, coincident with discharge of the ipsilateral and contralateral L(1)-L(3) roots. The L(6) ventral root discharge alternated with that recorded from the ipsilateral L(1)-L(3) roots. We established that the drug-induced rhythm was locomotor-like by recording an alternating pattern of discharge between ipsilateral flexor and extensor hindlimb muscle nerves. In addition, by recording simultaneously from ventral roots and muscle nerves, we established that ankle flexor discharge was in phase with ipsilateral L(1)/L(2) ventral root discharge, while extensor discharge was in phase with ipsilateral L(6) ventral root discharge. Rhythmic patterns of ventral root discharge were preserved following mid-sagittal section of the spinal cord, demonstrating that reciprocal inhibitory connections between the left and right sides of the cord are not essential for rhythmogenesis in the neonatal mouse cord. Blocking N-methyl-D-aspartate receptors, in both the intact and the hemisected preparation, revealed that these receptors contribute to but are not essential for rhythmogenesis. In contrast, the rhythm was abolished following blockade of kainate/AMPA receptors with 6-cyano-7-nitroquinoxalene-2,3-dione. These findings demonstrate that the isolated mouse spinal cord can produce a variety of coordinated activities, including locomotor-like activity. The ability to study these behaviors under a variety of different conditions offers promise for future studies of rhythmogenic mechanisms in this preparation.

Effect of testosterone on n-methyl-d, l-aspartate-induced growth hormone secretion in male swine

Previous research from our laboratory demonstrated that n-methyl-d, l-aspartate (NMA), a potent agonist of glutamate, increased growth hormone (GH) secretion in barrows and boars. To determine if testosterone modulates NMA-induced GH secretion, Poland China × Yorkshire swine were challenged with NMA in a model that compared GH responses in boars with those of barrows or barrows treated with testosterone propionate (TP). Boars and barrows weighing 112.6 ± 1.4 kg (mean ± SE) were fitted with indwelling jugular vein catheters. Barrows (n = 16) were given i.m. injections of TP (25 mg in corn oil) twice daily from d 0 to d 6. Boars (n = 16) and control barrows (n = 15) received twice daily injections of corn oil. On d 6, blood was sampled every 15 min for 4 h. Two h after sampling began, all animals received an i.v. injection of NMA at a dose of 2.5 mg/kg body weight. Mean testosterone concentrations (ng/ml serum) were similar ( P > .1) for boars (8.1 ± 0.8) and barrows receiving TP (7.3 ± 0.3), but were greater in both cases ( P < .05) than for barrows receiving corn oil (.2 ± .01). Prior to NMA injections, mean GH concentrations were similar ( P > .1) among groups and averaged 2.7 ± .2 ng/ml serum across treatments. Serum concentrations of GH after NMA increased ( P < .05) similarly among groups and averaged 6.3 ± 0.3 ng/ml across treatments during the 2-h period after injection. These results were not supportive of a role for testosterone as a modulator of NMA-induced GH secretion in male swine.