Involvement of amino acids, opioids, nitric oxide, and NMDA receptors in learning and memory consolidation in crickets

Is TNF a link between aging-related reproductive endocrine dyscrasia and Alzheimer's disease?

This commentary addresses a novel mechanism by which aging-related changes in reproductive hormones could mediate their action in the brain. It presents the evidence that dyotic endocrine signals modulate the expression of tumor necrosis factor (TNF) and related cytokines, and that these cytokines are a functionally important downstream link mediating neurodegeneration and dysfunction. This convergence of dyotic signaling on TNF-mediated degeneration and dysfunction has important implications for understanding the pathophysiology of AD, stroke, and traumatic brain disease, and also for the treatment of these diseases.

Life span extension in Drosophila melanogaster induced by morphine

The influence of morphine on the life span of Drosophila melanogaster fruit flies has been investigated. Morphine hydrochloride (MH) at concentrations of 0.01, 0.05 and 0.25 mg/ml was added to a medium starting from day 5 or 54 of imaginal life. Supplementation with MH starting from day 5 of imaginal life has resulted in significant increases in the mean life span of males at all concentrations studied. In females, a significant increase in life span compared with control was obtained only for those treated with 0.25 mg/ml MH. In flies with MH feeding from day 54, residual life span was significantly increased in both males and females after treatment with 0.05 mg/ml MH. The present data, together with those of our earlier study in mice (Dubiley et al. Probl Aging Longvity 9:331–332, 2000) suggest that morphine supplementation can result in life extension in both vertebrate and invertebrate animal species.

Retrograde amnesia for extinction: similarities with amnesia for original acquisition memories

Two experiments were conducted using rats to determine whether extinction is susceptible to a traditional amnestic agent (i.e., hypothermia) and to examine whether amnesia for extinction follows the same characteristics as those that occur with original memories. In Experiment 1, rats received hypothermia immediately, 60 min, or 120 min after extinction. When tested, the subjects cooled shortly after extinction showed little memory of the extinction training. This amnesia for extinction disappeared with longer postextinction delays, demonstrating a temporal gradient. Experiment 2 replicated the basic finding and demonstrated that an amnestic-extinguished memory could be recovered by reexposing the subjects to the amnestic agent and that the recovered extinction memory did not persist. These findings provide more evidence that extinction is a form of new learning and are consistent with retrograde amnesia research for original memories showing a temporal gradient and alleviation of retrograde amnesia by reexposure to the amnestic agent.

Focal and temporal release of glutamate in the mushroom bodies improves olfactory memory in Apis mellifera

In contrast to vertebrates, the role of the neurotransmitter glutamate in learning and memory in insects has hardly been investigated. The reason is that a pharmacological characterization of insect glutamate receptors is still missing; furthermore, it is difficult to locally restrict pharmacological interventions. In this study, we overcome these problems by using locally and temporally defined photo-uncaging of glutamate to study its role in olfactory learning and memory formation in the honeybee, Apis mellifera. Uncaging glutamate in the mushroom bodies immediately after a weak training protocol induced a higher memory rate 2 d after training, mimicking the effect of a strong training protocol. Glutamate release before training does not facilitate memory formation, suggesting that glutamate mediates processes triggered by training and required for memory formation. Uncaging glutamate in the antennal lobes shows no effect on memory formation. These results provide the first direct evidence for a temporally and locally restricted function of glutamate in memory formation in honeybees and insects.

The effects of sodium nitrite on neocortical neuron activity during performance of defensive and inhibitory conditioned reflexes

Administration of the NO-generating substance sodium nitrite to conscious non-immobilized rabbits at a dose of 11 mg/kg (s.c.) decreased the intensity and duration of the short-latency "modality-specific" components of the responses of neurons in the visual cortex (to flashes of light) and sensorimotor cortex (to pain reinforcement). Decreases in neuron activation in the visual cortex in response to flashes of light occurred independently of their biological significance. i.e., as the signal for a defensive reflex and on the background of conditioned inhibition. The long-latency activatory components of the response of sensorimotor and visual cortex neurons to pain reinforcement, the inhibitory pause in the responses of visual cortex neurons to flashes of light, and the disinhibitory effect of pain reinforcement showed smaller changes after sodium nitrite. The results obtained here support the suggestion that different neuromediators are involved in transmitting "modality-specific" and "modality-non-specific" influences to neocortical neurons during learning, and provide the basis for suggesting that sodium nitrite has a neurotropic action when given systemically.

Effects of fentanyl and low doses of alcohol on neuropsychological performance in healthy subjects

The effects of the opioid fentanyl and low doses of alcohol on neuropsychological functions in healthy volunteers were measured. Twenty-four healthy male volunteers participated in this study. Two randomised placebo-controlled cross-over trials were conducted. In group 1, 6 subjects received fentanyl (0.2 microgram/kg body weight) in the order of fentanyl/placebo and 6 subjects in the order of placebo/fentanyl. Group 2 received alcohol in a similar procedure by continuous intravenous infusion, leading to a blood alcohol concentration (BAC) of 0.03%. Impairment was measured via different neuropsychological tests. The results indicate that fentanyl in concentrations commonly used in out-patient surgical procedures produces pronounced cognitive impairment (auditory reaction time, signal detection, sustained attention, recognition) in comparison to placebo. After application of low doses of alcohol (BAC 0.03%) only visual reaction time was impaired in comparison to placebo.

Spinal plasticity of acute opioid tolerance

Spinal acute opioid tolerance remains mechanistically undercharacterized. Expanded clinical use of direct spinal administration of opioids and other analgesics indicates that studies to further understand spinal mechanisms of analgesic tolerance are warranted. Rodent models of spinal administration facilitate this objective. Specifically, acute spinal opioid tolerance in mice presents a plasticity-dependent, rapid, and efficient opportunity for evaluation of novel clinical agents. Similarities between the pharmacology of acute and chronic spinal opioid tolerance, neuropathic pain, and learning and memory suggest that this model may serve as a high through-put predictor of bioactivity of novel plasticity-modifying compounds.

Effect of sodium nitrite on the execution of defensive and inhibitory conditioned reflexes

Administration of sodium nitrite (an NO-generating preparation) doses of 11 mg/kg to rabbits was followed by motor disinhibition, with increases in myogram amplitudes in responses to nonreinforced flashes presented on a background of continuous illumination which served as a conditioned inhibitory stimulus; these changes appeared 1-1.5 h after administration and lasted throughout the 4-h observation period. After sodium nitrite doses of 5.5 mg/kg, responses to conditioned inhibition showed only a tendency to motor disinhibition. These results can be explained in terms of the ability of sodium nitrite to inhibit the function of GABAergic receptors, as development of internal inhibition is accompanied by increased inhibitory hyperpolarization processes in which the GABAergic mediator system is involved. The lower and higher sodium nitrite doses produced no increases in myogram amplitudes in responses to combinations of flashes of light and pain reinforcement, as occurred when these stimuli were presented to controls.

Nitric oxide in invertebrates

Nitric oxide (NO) is considered an important signaling molecule implied in different physiological processes, including nervous transmission, vascular regulation, immune defense, and in the pathogenesis of several diseases. The presence of NO is well demonstrated in all vertebrates. The recent data on the presence and roles of NO in the main invertebrate groups are reviewed here, showing the widespread diffusion of this signaling molecule throughout the animal kingdom, from higher invertebrates down to coelenterates and even to prokaryotic cells. In invertebrates, the main functional roles described for mammals have been demonstrated, whereas experimental evidence suggests the presence of new NOS isoforms different from those known for higher organisms. Noteworthy is the early appearance of NO throughout evolution and striking is the role played by the nitrergic pathway in the sensorial functions, from coelenterates up to mammals, mainly in olfactory-like systems. All literature data here reported suggest that future research on the biological roles of early signaling molecules in lower living forms could be important for the understanding of the nervous-system evolution.

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.

Alternative splicing of the NMDAR1 subunit affects modulation by calcium

Four splice variants of the NR1 receptor subunit, characterized by the presence or absence of cassettes encoding inserts of 21 (Insert 1) and 37 (Insert 2) amino acids were expressed in Xenopus oocytes and studied using voltage-clamp techniques. In 1.8 mM Ca2+, a slow inward current (Islow), which peaked 20 s after exposure to NMDA was evident when Insert I was present, but not when absent. However, in elevated external Ca2+ medium a similar Islow was observed in variants missing Insert I. The Ca2+ dependency of Islow reflected a requirement for intracellular accumulation of Ca2+. The divalent ion permeability of Insert I containing and Insert 1 lacking receptor channels expressed alone, as well as in heteromeric assemblies with NR2A and NR2B, was similar for all combinations tested. Thus, the lower Ca2+ dependency for Islow in oocytes expressing Insert I was not due to higher calcium entry. Islow was less sensitive to blockers of ICl(Ca) than were endogenous calcium-activated chloride currents (ICl(Ca)). Also, Islow was not abolished in Cl(-)-free external medium, when voltage was manipulated such that Islow was outward-going. Thus, Islow, while containing a component due to activation of endogenous ICl(Ca), is primarily due to current flowing through the receptor ion channel. Development of Islow was unaffected by PKC or PKA inhibitors. The modulation of the Ca2+ dependency of Islow by Insert I occurs in a range of Ca2+ concentrations which are physiologically relevant, and may provide an important means of modulation of glutamate transmission under normal and pathological conditions.

Inhibitory learning and memory in newborn rats influenced by nitric oxide

The impact of nitric oxide on learning, memory processing and retrieval was studied in the neonatal rats. For comparison, spontaneous motor activity and changes of brain temperature were also studied after nitric oxide manipulations in identical age groups. The nitric oxide availability was either increased by a systemic or intracerebroventricular application of L-arginine, a substrate of nitric-oxide synthase, or decreased by nitro-L-arginine, its inhibitor. L-arginine, 20 mM or nitro-L-arginine, 10 or 5 mM were given intraperitoneally, 1 ml/100 g weight, or in the amounts of 11 into both lateral cerebral ventricles. Intact and saline injected pups were used as controls. Spontaneous motor behavior of newborn pups were not unambiguously affected by nitric oxide, and the same applies to changes of brain and body temperature or heart rate. In no case any correlation with changes of learning and/or memory could be established. Learning was dose dependently impaired relative to controls by intraperitoneal application of nitroarginine. L-arginine only slightly decreased numbers of trials to both criteria and partially abolished the blocking effect of nitroarginine on nitric oxide synthase. With the use of intracerebroventricular injections the positive impact of L-arginine on learning became highly significant. In 24-h memory, intraperitoneal injections of L-arginine enhanced the retention indexes. The impairing effect of nitro-L-arginine significantly increased with delaying after-learning application intervals, being more pronounced at the 3-h than at 0-h interval. Here also, its effect was partially abolished by L-arginine. Effects of nitric-oxide availability in brain after intracerebroventricular application of these substances at 16 various post-learning intervals were assessed on memory processing and retrieval. A general enhancing effect of increased nitric-oxide supply on 24-h retention indexes was found through all studied intervals, which was not, however, monotonous, but several peaks appeared with application at 3, 6, 18 and 23.5 h after learning. On the other hand, the suppressive effect of NArg was not evident relative to saline before the 6-h post-learning injection delay. These results show that nitric oxide exerts a considerable central modulatory effect on learning, memory processing and retrieval at the very early postnatal period of the rat. The efficiency of nitric-oxide manipulations depends on its actual bioavailability in the brain and the stage of memory processing.

G-protein-coupled receptors in insect cells

The main classes of transmembrane signaling receptor proteins are well conserved during evolution and are encountered in vertebrates as well as in invertebrates. All members of the G-protein-coupled receptor superfamily share a number of basic structural and functional characteristics. In both insects and mammals, this receptor class is involved in the perception and transduction of many important extracellular signals, including a great deal of paracrine, endocrine, and neuronal messengers and visual, olfactory and gustatory stimuli. Therefore, most of the receptor subclasses appear to have originated several hundred million years ago, before the divergence of the major animal Phyla took place. Nevertheless, many insect-specific molecular interactions are encountered and these could become interesting tools for future applications, e.g., in insect pest control. Insect cell lines are well suited for large-scale expression and characterization of cloned receptor genes. Furthermore, novel methods for the production of stably transformed insect cells may form a major breakthrough for insect signal transduction research.

Endogenous opiates: 1994

This article is the 17th installment of our annual review of research concerning the opiate system. It includes papers published during 1994 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics covered this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.