Galanin-Like Peptide

Overview

Galanin-like peptide (GALP) is a 60-amino acid neuropeptide isolated in 1999 from porcine hypothalamus by Ohtaki and colleagues during a screen for novel endogenous ligands at the galanin receptor GalR2. Its name derives from the striking sequence identity between GALP residues 9-21 and the N-terminal 13 residues of galanin, the classical neuropeptide whose receptors GALP activates.

Unlike galanin, which is broadly expressed throughout the central and peripheral nervous system, GALP expression is remarkably restricted. In the brain, GALP-expressing neurons are confined almost exclusively to the arcuate nucleus of the hypothalamus, with a small population also in the median eminence and posterior pituitary. This focal expression pattern places GALP at a strategic node for integrating peripheral metabolic signals — particularly leptin — with central control of feeding, reproduction, and energy homeostasis.

GALP neurons are regulated by leptin: leptin administration upregulates GALP expression, and GALP expression falls in leptin-deficient ob/ob mice and fasted animals. This leptin-responsiveness, combined with GALP's reciprocal projections to GnRH neurons, NPY/AgRP neurons, and POMC neurons, has led to its conceptualization as a "second-order" leptin target integrating metabolic status with reproductive and feeding outputs.

Structure/Sequence

Human GALP: APAHRGRGGWTLNSAGYLLGPVLHLPQMGDQDGKRETALEILDLWKAIDGLPYSHPPQPS

• Length: 60 amino acids
• Molecular weight: ~6.5 kDa
• Gene: GALP (chromosome 19q13.43)
• Shared galanin motif: Residues 9-21 (WTLNSAGYLLGPV) are identical to galanin residues 1-13 — the canonical pharmacophore for GalR binding
• Unique N- and C-terminal extensions: Distinguish GALP from galanin and may contribute to differential receptor interactions

Shared Sequence with Galanin

The core GWTLNSAGYLLGP motif is the minimal region required for GalR1 and GalR2 activation. GALP and galanin converge on this pharmacophore but have distinct flanking sequences that alter pharmacokinetics, secondary receptor interactions, and cellular trafficking.

Species Conservation

The galanin-like core is highly conserved across mammals. The species-variable flanking regions may explain some inter-species differences in GALP effects reported in the literature.

Mechanism of Action

Receptor Binding

GALP binds and activates:

• GalR1 — Gi/o-coupled; inhibits adenylyl cyclase; primary central galanin receptor
• GalR2 — Gq/11 and Gi/o-coupled; broader tissue distribution
• Weak/no activity at GalR3

Binding affinities are similar to or slightly lower than galanin. GALP may also engage other, as-yet-unidentified receptors to account for effects not reproduced by galanin.

Arcuate Nucleus Circuitry

GALP neurons in the arcuate nucleus:

• Express the leptin receptor (ObRb); are directly leptin-responsive
• Project to GnRH neurons, paraventricular nucleus, and other hypothalamic nuclei
• Integrate peripheral metabolic signals with central neuroendocrine output

Feeding Behavior

GALP has biphasic effects on food intake:

• Acute central administration: Often stimulates feeding (GalR1-mediated)
• Chronic or peripheral administration: Reduces food intake and body weight
• The pattern has been interpreted as reflecting differential receptor subtype engagement and circuit-level integration with NPY and POMC systems

Reproduction

• GALP stimulates LH and FSH release, likely via effects on GnRH neurons
• Rescues LH secretion in leptin-deficient and fasted animals
• Acts upstream of or parallel to kisspeptin signaling
• Implicated in coupling metabolic status to reproductive function

Energy Expenditure and Thermogenesis

GALP administration increases locomotor activity, core body temperature, and energy expenditure in rodent models, effects that may contribute to its weight-reducing effects with chronic administration.

Sexual Behavior

GALP has been reported to modulate sexual behavior in male rodent models, consistent with its integration into reproductive hypothalamic circuits.

Research Summary

Common Discussion Topics

1. Arcuate focal expression — GALP's nearly exclusive expression in arcuate nucleus neurons is unusual among neuropeptides and positions it as a specialized relay rather than a diffuse modulator. This focal expression has facilitated study of GALP-specific circuits.

2. Metabolic-reproductive integration — GALP sits at the interface between metabolic sensing (leptin responsiveness) and reproductive output (GnRH/LH regulation). This makes it conceptually related to kisspeptin but through a different receptor system.

3. Biphasic feeding effects — The acute-stimulatory/chronic-inhibitory pattern of GALP on food intake has been a puzzle. Proposed explanations include differential receptor engagement (GalR1 acute feeding stimulation, GalR2 chronic metabolic effects) and circuit-level adaptive responses.

4. Relationship to galanin and spexin — Three peptides — galanin, GALP, and spexin — converge on galanin receptors with different subtype selectivities (galanin: 1/2/3, GALP: 1/2, spexin: 2/3). This constitutes an elaborated ligand family with combinatorial receptor selectivity.

5. Leptin effector — In leptin-deficient states, GALP signaling is suppressed. GALP administration partially rescues reproductive and metabolic phenotypes, suggesting GALP is a key downstream effector of leptin action in the hypothalamus.

Related Compounds

• Galanin — parent neuropeptide sharing receptors and core pharmacophore
• Spexin — related galanin-receptor-selective peptide (GalR2/3)
• Leptin — upstream regulator of GALP expression
• Kisspeptin — parallel reproductive integrator
• Neuropeptide Y — arcuate neuron circuit partner