Prominent expression of Narp in central vestibular pathways: selective effect of labyrinth ablation

Narp Mediates Antidepressant-Like Effects of Electroconvulsive Seizures

Growing recognition of persistent cognitive defects associated with electroconvulsive therapy (ECT), a highly effective and commonly used antidepressant treatment, has spurred interest in identifying its mechanism of action to guide development of safer treatment options. However, as repeated seizure activity elicits a bewildering array of electrophysiological and biochemical effects, this goal has remained elusive. We have examined whether deletion of Narp, an immediate early gene induced by electroconvulsive seizures (ECS), blocks its antidepressant efficacy. Based on multiple measures, we infer that Narp knockout mice undergo normal seizure activity in this paradigm, yet fail to display antidepressant-like behavioral effects of ECS. Although Narp deletion does not suppress ECS-induced proliferation in the dentate gyrus, it blocks dendritic outgrowth of immature granule cell neurons in the dentate molecular layer induced by ECS. Taken together, these findings indicate that Narp contributes to the antidepressant action of ECT and implicate the ability of ECS to induce dendritic arborization of differentiating granule cells as a relevant step in eliciting this response.

Selective expression of Narp in primary nociceptive neurons: role in microglia/macrophage activation following nerve injury

Neuronal activity regulated pentraxin (Narp) is a secreted protein implicated in regulating synaptic plasticity via its association with the extracellular surface of AMPA receptors. We found robust Narp immunostaining in dorsal root ganglia (DRG) that is largely restricted to small diameter neurons, and in the superficial layers of the dorsal horn of the spinal cord. In double staining studies of DRG, we found that Narp is expressed in both IB4- and CGRP-positive neurons, markers of distinct populations of nociceptive neurons. Although a panel of standard pain behavioral assays were unaffected by Narp deletion, we found that Narp knockout mice displayed an exaggerated microglia/macrophage response in the dorsal horn of the spinal cord to sciatic nerve transection 3days after surgery compared with wild type mice. As other members of the pentraxin family have been implicated in regulating innate immunity, these findings suggest that Narp, and perhaps other neuronal pentraxins, also regulate inflammation in the nervous system.

Disruption of the head direction cell signal after occlusion of the semicircular canals in the freely moving chinchilla

Head direction (HD) cells in the rat anterodorsal thalamic nucleus (ADN) fire relative to the animal's directional heading. Lesions of the entire vestibular labyrinth have been shown to severely alter VIIIth nerve input and disrupt these HD signals. To assess the specific contributions of the semicircular canals without altering tonic VIIIth nerve input, ADN cells were recorded from chinchillas after bilateral semicircular canal occlusion. Although ADN HD cells (and also hippocampal place cells and theta cells) were identified in intact chinchillas, no direction-specific activity was seen after canal occlusions. Instead, "bursty" cells were observed that exhibited burst-firing patterns similar to normal HD cells but with firing unrelated to the animal's actual head direction. Importantly, when pairs of bursty cells were recorded, the temporal order of their firing was dependent on the animal's turning direction, as is the case for pairs of normal HD cells. These results suggest that bursty cells are actually disrupted HD cells. The present findings further suggest that the HD cell network is still able to generate spiking activity after canal occlusions, but the semicircular canal input is critical for updating the network activity in register with changes in the animal's HD.

Narp regulates long-term aversive effects of morphine withdrawal

Although long-lasting effects of drug withdrawal are thought to play a key role in motivating continued drug use, the mechanisms mediating this type of drug-induced plasticity are unclear. Because Narp is an immediate early gene product that is secreted at synaptic sites and binds to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, it has been implicated in mediating enduring forms of synaptic plasticity. In previous studies, the authors found that Narp is selectively induced by morphine withdrawal in the extended amygdala, a group of limbic nuclei that mediate aversive behavioral responses. Accordingly, in this study, the authors evaluate whether long-term aversive effects of morphine withdrawal are altered in Narp knockout (KO) mice. The authors found that acute physical signs of morphine withdrawal are unaffected by Narp deletion. However, Narp KO mice acquire and sustain more aversive responses to the environment conditioned with morphine withdrawal than do wild type (WT) controls. Paradoxically, Narp KO mice undergo accelerated extinction of this heightened aversive response. Taken together, these studies suggest that Narp modulates both acquisition and extinction of aversive responses to morphine withdrawal and, therefore, may regulate plasticity processes underlying drug addiction.

Activity-dependent secretion of neuronal activity regulated pentraxin from vasopressin neurons into the systemic circulation

Neuronal activity regulated pentraxin (Narp) is a secreted, synaptic protein that has been implicated in modulating synaptic transmission. However, it is unclear how Narp secretion is regulated. Since we noted prominent Narp immunostaining in vasopressin neurons of the hypothalamus and in the posterior pituitary, we assessed whether it, like vasopressin, is released into the systemic circulation in an activity-dependent fashion. Consistent with this hypothesis, electron microscopic studies of the posterior pituitary demonstrated that Narp is located in secretory vesicles containing vasopressin. Using affinity chromatography, we detected Narp in plasma and found that these levels are markedly decreased by hypophysectomy. In addition, we confirmed that injection of a viral Narp construct into the hypothalamus restores plasma Narp levels in Narp knockout mice. In checking for activity-dependent secretion of Narp from the posterior pituitary, we found that several stimuli known to trigger vasopressin release, i.e. hypovolemia, dehydration and endotoxin, elevate plasma Narp levels. Taken together, these findings provide compelling evidence that Narp is secreted from vasopressin neurons in an activity-dependent fashion.

Developmental expression of the SRF co-activator MAL in brain: role in regulating dendritic morphology

The dynamic changes in dendritic morphology displayed by developing and mature neurons have stimulated interest in deciphering the signaling pathways involved. Recent studies have identified megakaryocytic acute leukemia (MAL), a serum response factor (SRF) co-activator, as a key component of a signaling pathway linking changes in the actin cytoskeleton to SRF-mediated transcription. To help define the role of this pathway in regulating dendritic morphology, we have characterized the pattern of MAL expression in the developing and adult brain, and have examined its role in regulating dendritic morphology in cultured cortical neurons. In histological studies of mouse brain, we found prominent expression of MAL in neurons in adult hippocampus and cerebral cortex. MAL immunostaining revealed localization of this protein in neuronal cell bodies and apical dendrites. During development, an increase in MAL expression occurs during the second post-natal week. Expression of dominant negative MAL constructs or MAL siRNA in cortical neurons grown in primary culture reduces the number of dendritic processes and decreases the basal level of SRF-mediated transcription. Taken together, these findings indicate that the MAL-SRF signaling pathway plays a key role in regulating dendritic morphology.

Hypocretin (orexin): role in normal behavior and neuropathology

The hypocretins (Hcrts, also known as orexins) are two peptides, both synthesized by a small group of neurons, most of which are in the lateral hypothalamic and perifornical regions of the hypothalamus. The hypothalamic Hcrt system directly and strongly innervates and potently excites noradrenergic, dopaminergic, serotonergic, histaminergic, and cholinergic neurons. Hcrt also has a major role in modulating the release of glutamate and other amino acid transmitters. Behavioral investigations have revealed that Hcrt is released at high levels in active waking and rapid eye movement (REM) sleep and at minimal levels in non-REM sleep. Hcrt release in waking is increased markedly during periods of increased motor activity relative to levels in quiet, alert waking. Evidence for a role for Hcrt in food intake regulation is inconsistent. I hypothesize that Hcrt's major role is to facilitate motor activity tonically and phasically in association with motivated behaviors and to coordinate this facilitation with the activation of attentional and sensory systems. Degeneration of Hcrt neurons or genetic mutations that prevent the normal synthesis of Hcrt or of its receptors causes human and animal narcolepsy. Narcolepsy is characterized by an impaired ability to maintain alertness for long periods and by sudden losses of muscle tone (cataplexy). Administration of Hcrt can reverse symptoms of narcolepsy in animals, may be effective in treating human narcolepsy, and may affect a broad range of motivated behaviors.

Opiate withdrawal induces Narp in the extended amygdala

The negative affective states associated with drug withdrawal produce long-lasting behavioral effects thought to play a central role in the development and maintenance of dependence. However, little is known about the molecular mechanisms mediating the long-term effects of drug withdrawal. Neuronal activity-regulated pentraxin (Narp) is a secreted neuronal immediate early gene (IEG) product that regulates AMPA receptor clustering at synapses. As both IEGs and changes in AMPA receptor trafficking mediate enduring forms of neuronal plasticity, we have assessed whether Narp could be involved in the molecular adaptations accompanying drug withdrawal. To this end, we checked the effect of opiate withdrawal on Narp expression in the extended amygdala, a brain region closely linked to the aversive effects of drug withdrawal. We found a marked increase in the number of Narp-positive cells in this region following opiate withdrawal triggered by either low doses of opiate antagonists or by 'natural withdrawal', removal of the morphine pellets used to induce dependence. In contrast, Arc, another 'effector' IEG, was not induced by opiate withdrawal. As expected, pretreatment of animals with clonidine, which blocks opiate withdrawal, suppresses Narp induction in this paradigm. These results implicate Narp in mediating the long-term, aversive behavioral effects induced by opiate withdrawal.