Somatostatin (SRIF = somatotropin release inhibiting factor) is a hormone/ neuropeptide with multiple endocrine and exocrine effects, effects in inhibition of hormone release, cognitive functions, behaviour, sleep activity and inhibition of tumour growth. The SRIF analogue octreotide is used to treat acromegaly, hormone-secreting tumours, and AIDS-related diarrhoe. In mammals, the SRIF family includes SRIF₁₄, SRIF₂₈, and the recently identified and putative neuropeptide cortistatin (CST); non-mammalian vertebrates posses SRIF₁₄ and a second SRIF variant.
A class of G protein-coupled receptors, the somatostatin receptors, mediate the actions of SRIF, and five mammalian subtypes (sst_1-5) have been cloned; SRIF receptors are specifically expressed in brain, periphery, and many tumours. The third cytoplasmatic loop if G protein-coupled receptors is suggested to link the receptors to G proteins, which couple the receptors to specific intracellular signalling cascades. Many cellular effector proteins such as phospholipase C (PLC), phospholipase A₂, calcium channels, potassium channels, Na⁺/H⁺ exchanger, adenylate cyclase (AC), protein tyrosine phosphatases, mitogen-activated protein kinase (MAPK) or p53 are reported to be specifically modulated by SRIF receptor subtypes. In this study, the five human SRIF receptor subtypes, and the first cloned non-mammalian SRIF receptor, fish sst₃ receptor (of Apteronotus albifrons), were characterised by analysing their binding and transductional features under the same environment, i.e. by stable receptor expression in CCL39 hamster lung fibroblast cells.

CST_14/17 and the iodinated analogue [¹²⁵I][Tyr¹⁰]CST₁₄ bound with similar high affinity to all five human SRIF receptors, and thus the pharmacological profiles of the iodinated peptide was established with a number of SRIF/ CST analogues; the affinity profiles were comparable to those established using [¹²⁵I]LTT-SRIF₂₈. This underlines the close relation of CST and SRIF peptides, although specific CST receptors may also exist.
Very marked differences in peptide affinities for SRIF receptors have been described in the literature; therefore additional synthetic radioligands([¹²⁵I]CGP 23996, [¹²⁵I][Tyr³]octreotide) were used to establish affinity profiles. Surprisingly, [¹²⁵I][Tyr³]octreotide labelled beside human sst₂ also sst₅ receptor sites with high affinity, and some other classically sst₂-selective compounds (octreotide, seglitide etc.) showed high affinity to sst₅ receptors; hence, sst₅ receptors may mediate physiological effects of octreotide, which previously were attributed to the sst₂ subtype solely.
Ligand affinities and receptor densities were radioligand-dependent at human sst₅, but not at sst_1-4 receptors: e.g. [¹²⁵I]LTT-SRIF₂₈ labelled seven times more sst₅ receptor sites than [¹²⁵I][Tyr³]octreotide, and the affinity of e.g. octreotide defined with [¹²⁵I]LTT-SRIF₂₈ was 100-fold lower compared to that defined with [¹²⁵I][Tyr³]octreotide.
Although the non-iodinated analogues of the four radioligands are full agonists in functional studies, their binding to sst₅ receptors was differently modulated by the GTP-analogue, guanylylimidodiphosphate (GppNHp): e. g. [¹²⁵I][Tyr³]octreotide binding was highly affected suggesting selective labelling of G protein-coupled sst₅ receptors, whereas [¹²⁵I]LTT-SRIF₂₈ and [¹²⁵I][Tyr¹⁰]CST₁₄ seem to label rather uncoupled receptors and hence a higher receptor density; radioligand binding at sst₂/ sst₃ receptors was markedly inhibited, and rather unaffected at sst₁/ sst₄ receptors.
The data do not fit the ternary complex model, instead the existence of multiple G protein-coupled/ -uncoupled agonist-specific receptor states may be proposed.

In functional studies, all five SRIF receptors inhibited forskolin-stimulated adenylate cyclase (FSAC) activity, hsst_2-5 receptors stimulated [³⁵S]GTPgS binding (G protein activation), and sst₃/ sst₅ activated PLC activity (measured by IP_x accumulation, intracellular Ca²⁺ increase), the latter effect being only partially pertussis toxin (PTX) sensitive (i.e. partly mediated by G_i/o).
Pharmacological profiles of human SRIF receprors established in these various functional assays correlated significantly, but to various extents even at the same receptor with the different radioligand binding profiles; functional data of sst₁/ sst₂ receptors correlated only modestly with the affinity profiles suggesting other effector pathways to be more important. In addition, the potency rank orders of SRIF/ CST ligands examined at each human SRIF receptor subtype was distinct from one functional assay to the other, and compared to the affinity profiles. These findings support the hypothesis of receptor-induced effector trafficking by presumably different agonist-specific receptor states.

In the brain, fish sst₃ receptor trancripts, as deteced by RT-PCR, and fish sst₃ protein seem to be present, since the profile of fsst₃ receptors expressed in CCL39 cells correlates highly with that of native brain receptors [¹²⁵I]LTT-SRIF₂₈ binding; however, biphasic curves in brain and low correlation with liver sites suggest additional fish SRIF receptors. At recombinant fsst₃ receptors radioligand-dependency of receptor densities, affinities, and GppNHp-sensitivity was documented using [¹²⁵I]LTT-SRIF₂₈, [¹²⁵I][Tyr¹⁰]CST₁₄, [¹²⁵I]CGP 23996, [¹²⁵I][Tyr³]octreotide, similar to the human sst₅ receptor. Coupling to signalling pathways seems to be highly conserved between fish and mammalian SRIF receprors: fsst₃ receptors mediate stimulation of [³⁵S]GTPgS binding, inhibition of FSAC via G_i/o, and PLC activation partly via G_i/o. Pharmacological profiles of radioligand binding and functional tests correlated with each other with variations, supporting again a model of specific agonist-induced receptor conformations; species differences on pharmacology: fsst₃ profiles fitted best with the hsst₅ receptor profiles, in spite of the highest sequence homology with the hsst₃ subtype.

Taken together, the results suggests: (1)that multiple agonist-specific receptor conformations can be achieved at recombinat SRIF rceptor subtypes, which are G protein-coupled and/ or G protein-uncoupled, (2) that the nature of the agonist-modulated receptor/ G protein/ effector interactions might be more complex than initially suggested by the ternary complex model, and (3) different rank orders of apparent potency can be observed at SRIF receptors depending on the nature of the ligand/ receptor interaction studied (radioligand binding, [³⁵S]GTPgS binding, AC activity, PLC activity); this suggests that receptor-effector trafficking can take place, as proposed for the human 5-HT_2C receptor (see literature), i.e. that each agonist-specific receptor conformation triggers specifically the modulated signalling pathways.