The expression of the neuropeptide galanin is markedly up-regulated in many areas of the central and peripheral nervous system after injury. We have recently demonstrated that peripheral sensory neurons depend on galanin for neurite extension after injury, mediated by activation of the second galanin receptor subtype (GALR2). We therefore hypothesized that galanin might also act in a similar manner in the CNS, reducing cell death in hippocampal models of excitotoxicity. Here we report that galanin acts an endogenous neuroprotective factor to the hippocampus in a number of in vivo and in vitro models of injury. Kainate-induced hippocampal cell death was greater in both the CA1 and CA3 regions of galanin knockout animals than in WT controls. Similarly, exposure to glutamate or staurosporine induced significantly more neuronal cell death in galanin knockout organotypic and dispersed primary hippocampal cultures than in WT controls. Conversely, less cell death was observed in the hippocampus of galanin overexpressing transgenic animals after kainate injection and in organotypic cultures after exposure to staurosporine. Further, exogenous galanin or the previously described high-affinity GALR2 agonist, both reduced cell death when coadministered with glutamate or staurosporine in WT cultures. These results demonstrate that galanin acts an endogenous neuroprotective factor to the hippocampus and imply that a galanin agonist might have therapeutic uses in some forms of brain injury.T he 29-aa neuropeptide galanin (1) is widely expressed in both the central and peripheral nervous system and has strong inhibitory actions on synaptic transmission by reducing the release of a number of classical neurotransmitters (2-7). These inhibitory actions result in a diverse range of physiological effects including, an impairment of working memory (8) and long-term potentiation (9); a reduction in hippocampal excitability with a decreased predisposition to seizure activity (10); and a marked inhibition of nociceptive responses in the intact animal and after nerve injury (11). These neuromodulatory actions of galanin have long been regarded as the principal role played by the peptide in the nervous system. However, there is now a large body of evidence to indicate that injury to many of these neuronal systems markedly induces the expression of galanin at both the mRNA and peptide levels. Examples of such lesion studies include the up-regulation of galanin in (i) the dorsal root ganglion after peripheral nerve axotomy (12), (ii) magnocellular secretory neurons of the hypothalamus after hypophysectomy (13), (iii) the dorsal raphe and thalamus after removal of the frontoparietal cortex (decortication) (14), (iv) the molecular layer of the hippocampus after an entorhinal cortex lesion (15), and (v) the medial septum and vertical limb diagonal-band after a fimbria fornix bundle transection (16). These studies have led a number of investigators to speculate that galanin might play a trophic role in addition to its classical neuromodulatory ...

Expression of the neuropeptide galanin is markedly up-regulated within the adult dorsal root ganglia (DRG) following peripheral nerve injury. We have previously demonstrated that galanin knockout (Gal-KO) mice have a developmental loss of a subset of DRG neurons. Galanin also plays a trophic role in the adult animal, and the rate of peripheral nerve regeneration and neurite outgrowth is reduced in adult Gal-KO mice. Here we describe the characterization of mice with an absence of GalR2 gene transcription (GalR2-MUT) and demonstrate that they have a 15% decrease in the number of calcitonin gene-related peptide (CGRP) expressing neuronal profiles in the adult DRG, associated with marked deficits in neuropathic and inflammatory pain behaviours. Adult GalR2-MUT animals also have a one third reduction in neurite outgrowth from cultured DRG neurons that cannot be rescued by either galanin or a high-affinity GalR2/3 agonist. Galanin activates extracellular signal-regulated kinase (ERK) and Akt in adult wild-type (WT) mouse DRG. Intact adult DRG from GalR2-MUT animals have lower levels of pERK and higher levels of pAkt than are found in WT controls. These data suggest that a lack of GalR2 activation in Gal-KO and GalR2-MUT animals is responsible for the observed developmental deficits in the DRG, and the decrease in neurite outgrowth in the adult.

Expression of the neuropeptide galanin is up-regulated in many brain regions following nerve injury and in the basal forebrain of patients with Alzheimer's disease. We have previously demonstrated that galanin modulates hippocampal neuronal survival, although it was unclear which receptor subtype(s) mediates this effect. Here we report that the protective role played by galanin in hippocampal cultures is abolished in animals carrying a loss-of-function mutation in the second galanin receptor subtype (GalR2-MUT). Exogenous galanin stimulates the phosphorylation of the serine/ threonine kinase Akt and extracellular signal-regulated kinase (ERK) in wild-type (WT) cultures by 435 ± 5% and 278 ± 2%, respectively. The glutamate-induced activation of Akt was abolished in cultures from galanin knockout animals, and was markedly attenuated in GalR2-MUT animals, compared with WT controls. In contrast, similar levels of glutamate-induced ERK activation were observed in both loss-of-function mutants, but were further increased in galanin over-expressing animals. Using specific inhibitors of either ERK or Akt confirms that a GalR2-dependent modulation in the activation of the Akt and ERK signalling pathways contributes to the protective effects of galanin. These findings imply that the rise in endogenous galanin observed either after brain injury or in various disease states is an adaptive response that reduces apoptosis by the activation of GalR2, and hence Akt and ERK.

The neuropeptide galanin is expressed in the dorsal root ganglia (DRG) and spinal cord and is thought to be involved in the modulation of pain processing. However, its mechanisms of action are complex and poorly understood, as both facilitatory and inhibitory effects have been described. To understand further the role played by galanin in nociception, we have generated two transgenic lines that overexpress galanin in specific populations of primary afferent DRG neurons in either an inducible or constitutive manner. In the first line, a previously defined enhancer region from the galanin locus was used to target galanin to the DRG (Gal-OE). Transgene expression recapitulates the spatial endogenous galanin distribution pattern in DRG neurons and markedly overexpresses the peptide in the DRG after nerve injury but not in the uninjured state. In the second line, an enhancer region of the c-Ret gene was used to constitutively and ectopically target galanin overexpression to the DRG (Ret-OE). The expression of this second transgene does not alter significantly after nerve injury. Here, we report that intact Ret-OE, but not Gal-OE, animals have significantly elevated mechanical and thermal thresholds. After nerve damage, using a spared nerve-injury model, mechanical allodynia is attenuated markedly in both the Gal-OE and Ret-OE mice compared with WT controls. These results support an inhibitory role for galanin in the modulation of nociception both in intact animals and in neuropathic pain states.T he neuropeptide galanin is widely distributed within the somatosensory system, where it has been implicated in the modulation of nociception (reviewed in refs. 1-3). In the adult dorsal root ganglia (DRG), galanin is expressed at relatively low levels in Ͻ5% of neurons that are predominantly small-diameter C fiber type (4). Expression of the peptide also is detected in the primary afferent terminals of the dorsal spinal cord and in subsets of dorsal horn interneurons (5). Galanin mRNA and peptide levels markedly increase in the DRG after axotomy and in several neuropathic pain models of sciatic nerve injury (4, 6-9). After axotomy, galanin is expressed in Ϸ40-50% of all DRG neurons (4, 10). In the dorsal horn, axotomy-induced galanin immunoreactivity increases in the afferent terminals, with a concurrent enhancement of synaptic release of the peptide (11).The role of galanin in pain signaling is complex (reviewed in refs. 2, 12, and 13). Electrophysiological and behavioral studies in the intact, uninjured peripheral nervous system have demonstrated both facilitatory (14-21) and inhibitory (22, 23) effects of exogenously applied galanin on nociception, sometimes in a modality-specific manner (14,16,20,21), tending toward inhibition at higher doses.A number of groups have used rodent models of neuropathic pain to study the role of galanin in the modulation of chronic pain behavior. The oldest and least understood pain model involves the scoring of autotomy behavior (self-mutilation of the denervated limb) after sciatic nerve a...