Seizures in turn selectively modulate the expression of GalR
Seizures, in turn, selectively modulate the KH CB19 of GalR in the hippocampus. Thus, pilocarpine-induced SE led to a 30% downregulation of GalR2 without affecting GalR1 (personal communication with T. Bartfai, Scripps Research Institute). Therefore, despite the fact that GalR2 could have been a more appealing target for treating seizures, GalR1 may be a more effective one. Indeed, as it is discussed below both available non-peptide GalR agonists likely mediate their anticonvulsant effects through GalR1 rather than GalR2 (see Biologically active non-peptide galanin receptor ligands below).
In addition to GalR1 and GalR2 other, yet to be identified, GalR subtypes may regulate hippocampal excitability and seizures. Thus, electrophysiological studies revealed a subpopulation of neurons in CA3 area of the hippocampus, which were hyperpolarized by galanin fragment (1–15), but responded neither to GalR1/GalR2 activation by galanin (1–29), nor to GalR2-selective agonist D-Trp-2-Galanin (1–29) (Xu et al., 1999).
Neuroprotective effects of galanin Seizures are generally accompanied by neuronal injury to various neuronal populations, due to excitotoxicity of glutamatergic transmission. Hippocampus is particularly vulnerable to seizure-induced neuronal injury. On the other hand, attenuation of seizures is commonly accompanied by mitigation of neuronal injury. Indeed, many of the antiepileptic drugs attenuate neuronal degeneration (Pitkanen and Kubova, 2004). However, since seizures and associated injury share same mechanisms (glutamate neuro- and excitotoxicity), it is quite difficult to dissociate anticonvulsant effects from neuroprotection. This is true for galanin as well. Indeed, galanin knockout mice showed more severe hippocampal neuronal damage, than wild type, while in galanin overexpressing animals the injury was attenuated (Mazarati et al., 2000). In GalR1 KO mice, not only more extensive neuronal injury was observed in the hippocampus as compared to the wild type controls, but neurodegeneration involved areas, which were spared in wild type animals (Fig. 3(b)–(e)): while wild type mice showed absolute resistance of dentate gyrus to neuronal damage, GalR1 knockouts exhibited severe injury to hilar interneurons, and mild injury to dentate granule cells after SE induced by Li-pilocarpine. Such differential susceptibility to seizure-induced neuronal injury in certain neuronal populations indirectly implicated GalR1 in neuroprotection. More definitive response to whether galanin is neuroprotective aside from being anticonvulsant, came from the studies by Haberman et al., 2003) who showed, that AAV-FIB-GAL vector-mediated unilateral overexpression of galanin in the hippocampus, while not affecting the severity of the convulsions induced by systemic kainic acid injection, selectively attenuated neuronal injury around the site of galanin overexpression and secretion in contrast to control animals, in which the severity of hippocampal injury was not different between the two hippocampi. Using an opposite approach, we found that focal downregulation of GalR2 in the hippocampus resulted in neuronal injury to hilar interneurons without exposure to seizures (Mazarati et al., 2004a, Fig. 5(a)). Thus, galanin acting through GalR2 may be required for the survival of hippocampal neurons under normal conditions. Reduction of the inhibitory drive from hilar interneurons to dentate granule cells (Fig. 1(a)) may in turn contribute to the discussed above enhanced severity of seizures induced by perforant path stimulation. Furthermore, the combination of unilateral downregulation of GalR2 in the dentate gyrus and perforant-path stimulation-induced seizures resulted in disproportionately more severe neuronal injury in the site of downregulation, than GalR2 downregulation alone, or seizures alone (Fig. 5(b)). Since perforant path stimulation-induced SE commonly leads to symmetrical activation and injury in both hippocampi (Pereira de Vasconcelos et al., 1999), the observed differences cannot be attributed to only anticonvulsant effects of galanin, but rather suggest neuroprotective effects of the peptide.