Neurotensin type 1 receptor (NT1_rcpt)
Type: Family

G protein-coupled receptors (GPCRs) constitute a vast protein family that encompasses a wide range of functions, including various autocrine, paracrine and endocrine processes. They show considerable diversity at the sequence level, on the basis of which they can be separated into distinct groups . The term clan can be used to describe the GPCRs, as they embrace a group of families for which there are indications of evolutionary relationship, but between which there is no statistically significant similarity in sequence . The currently known clan members include rhodopsin-like GPCRs (Class A, GPCRA), secretin-like GPCRs (Class B, GPCRB), metabotropic glutamate receptor family (Class C, GPCRC), fungal mating pheromone receptors (Class D, GPCRD), cAMP receptors (Class E, GPCRE) and frizzled/smoothened (Class F, GPCRF) . GPCRs are major drug targets, and are consequently the subject of considerable research interest. It has been reported that the repertoire of GPCRs for endogenous ligands consists of approximately 400 receptors in humans and mice . Most GPCRs are identified on the basis of their DNA sequences, rather than the ligand they bind, those that are unmatched to known natural ligands are designated by as orphan GPCRs, or unclassified GPCRs .

The rhodopsin-like GPCRs (GPCRA) represent a widespread protein family that includes hormone, neurotransmitter and light receptors, all of which transduce extracellular signals through interaction with guanine nucleotide-binding (G) proteins. Although their activating ligands vary widely in structure and character, the amino acid sequences of the receptors are very similar and are believed to adopt a common structural framework comprising 7 transmembrane (TM) helices .

Neurotensin is a 13-residue peptide transmitter, sharing significant similarity in its 6 C-terminal amino acids with several other neuropeptides, including neuromedin N. This region is responsible for the biological activity, the N-terminal portion having a modulatory role. Neurotensin is distributed throughout the central nervous system, with highest levels in the hypothalamus, amygdala and nucleus accumbens. It induces a variety of effects, including: analgesia, hypothermia and increased locomotor activity. It is also involved in regulation of dopamine pathways. In the periphery, neurotensin is found in endocrine cells of the small intestine, where it leads to secretion and smooth muscle contraction.

The existence of 2 neurotensin receptor subtypes, with differing affinities for neurotensin and differing sensitivities to the antihistamine levocabastine, was originally demonstrated by binding studies in rodent brain. Two neurotensin receptors (NT1 and NT2) with such properties have since been cloned and have been found to be G-protein-coupled receptor family members .

The NT1 receptor was cloned in 1990 from rat brain and found to act as a high affinity, levocabastine insensitive receptor for neurotensin . The affinity of neurotensin for the receptor could be decreased by both sodium ions and guanosine triphosphate (GTP) . The NT1 receptor is expressed predominantly in the brain and intestine. In the brain, expression has been found in the diagonal band of Broca, medial septal nucleus, nucleus basalis magnocellularis, suprachiasmatic nucleus, supramammillary area, substantia nigra and ventral tegmental area. The receptor is also expressed in the dorsal root ganglion neurones of the spinal cord. The predominant response upon activation of the receptor by neurotensin is activation of phospholipase C, causing an increase in intracellular calcium levels. The receptor can also stimulate cAMP formation, MAP kinase activation and the induction of growth related genes, such as krox-24 .