Molecular characterization of the ligand binding site of the human galanin receptor type 2, identifying subtype selective interactions.

To define the specific role of the galanin receptors when mediating the effect of galanin, effective tools for distinct activation and inhibition of the different receptor subtypes are required. Several of the physiological effects modulated by galanin are implicated to be mediated via the GalR2 subtype and have been distinguished from GalR1 effects by utilizing the Gal(2-11) peptide, recognizing only GalR2 and GalR3. In this study, we have performed a mutagenesis approach on the GalR2 subtype and present, for the first time, a molecular characterization of the interactions responsible for ligand binding and receptor activation at this receptor subtype. Our results identify four residues, His(252) and His(253) located in transmembrane domain 6 and Phe(264) and Tyr(271) in the extracellular loop 3, to be of great significance. We show evidence for the N-terminal tail of GalR2 to participate in ligand binding and that selective binding of Gal(2-11) includes interaction with the Ile(256) residue, located at the very top of TM 6. In conclusion, we present a mutagenesis study on GalR2 and confer interactions responsible for ligand binding and receptor activation as well as selective recognition of the Gal(2-11) peptide at this receptor subtype. The presented observations could be of major importance for the design and development of new and improved peptide and non-peptide ligands, selectively activating the GalR2 subtype.

Activation of peripheral galanin receptors: differential effects on nociception.

Numerous reports suggest a significant role of peripheral galanin (GAL) in pain transmission; however, due to the lack of selective galanin receptor agonists and antagonists, the role of GAL receptors (GalR1-3) in pain transmission remains unclear. In this study, a new agonist, M617, that preferentially binds to GalR1, a GalR2 agonist (AR-M1896), and a GalR2 antagonist (M871) were tested in the periphery to elucidate the role of peripheral GalR1 and GalR2 in nociception. Ipsilateral, but not contralateral, hindpaw injection of M617 reduced capsaicin (CAP)-induced flinching by approximately 50%, suggesting that GalR1 activation produces anti-nociception. This anti-nociceptive effect was blocked by intraplantar injection of the non-selective GalR antagonist M35. In contrast ipsilateral, but not contralateral, intraplantar injection of GalR2 agonist AR-M1896 enhanced the CAP-induced nociception (1.7-fold). The GalR2 antagonist M871 blocked the pro-nociceptive effect of AR-M1896 in a dose-dependent manner. This antagonist had no effect on nociceptive behaviors induced by CAP alone. The data demonstrate that activation of peripheral GalR1 results in anti-nociception but activation of peripheral GalR2 produces pro-nociception. Thus, the use of these pharmacological tools may help to elucidate the contribution of GalR subtypes in nociceptive processing, identifying potential drug targets for the treatment of peripheral pain.

Determining receptor-ligand interaction of human galanin receptor type 3.

Galanin is a neuropeptide found throughout the central and peripheral nervous systems of a wide range of species, ranging from human and mouse to frog and tuna. Galanin mediates its physiological roles through three receptors (GalR1-3), all members of the G-protein coupled receptor family. In mapping these roles, receptor subtype selective ligands are crucial tools. To facilitate the ligand design, data on receptor structure and interaction points are of great importance. The current study investigates the mechanism by which galanin interacts with GalR3. Mutated receptors were tested with competitive binding analysis in vitro. Our studies identify six mutagenic constructs that lost receptor affinity completely, despite being expressed at the cell surface. Mutations of the Tyr103(3.33) in transmembrane helix (TM) III, His251(6.51) in TM VI, Arg273(7.35) or His277(7.39) in TM VII, Phe263(6.63) or Tyr270(7.32) in the extracellular loop III all result in complete reduction of ligand binding. In addition, docking studies of an in silico model of GalR3 propose that four of the identified residues interact with pharmacophores situated within the galanin(2-6) sequence. This study provides novel insights into the interaction between ligands and GalR3 and highlights the requirement for correct design of targeting ligands.

Selective stimulation of GalR1 and GalR2 in rat substantia gelatinosa reveals a cellular basis for the anti- and pro-nociceptive actions of galanin.

Galanin modulates spinal nociceptive processing by interacting with two receptors, GalR1 and GalR2. The underlying neurophysiological mechanisms were examined by whole-cell recording from identified neurons in the substantia gelatinosa of young adult rats. GalR1 was activated with a 'cocktail' containing the GalR1/2 agonist, AR-M 961 (0.5 microM), in the presence of the GalR2 antagonist, M871 (1.0-2.5 microM). GalR2 was activated with the selective agonist, AR-M 1896 (0.5-1.0 microM). Application of the 'GalR1 agonist cocktail' often activated an inwardly-rectifying conductance in delay firing (excitatory) and tonically firing (inhibitory) neurons. This conductance was not activated by AR-M 1896 which instead decreased or increased an outwardly-rectifying conductance at voltages positive to -70 mV. Despite this variability in its actions on current-voltage relationships, AR-M 1896 very consistently decreased membrane excitability, as measured by cumulative action potential latency in response to a depolarizing current ramp. This strong GalR2-mediated effect was seen in neurons where membrane conductance was decreased, and where membrane excitability might be predicted to increase. GalR2 was also located presynaptically, as AR-M 1896 increased the interevent interval of spontaneous EPSCs in both delay and tonic cells. By contrast, the 'GalR1 agonist cocktail' had little effect on spontaneous EPSCs, suggesting that presynaptic terminals do not express GalR1. These diverse actions of GalR1 and GalR2 activation on both inhibitory and excitatory neurons are discussed in relation to the known spinal antinociceptive and pro-nociceptive actions of galanin, to the possible association of GalR1 with the inhibitory G-protein, G(i/o) and to report that GalR2 activation suppresses Ca2+ channel currents.