Indole derivatives as somatostatin agonists or antagonists

A compound of the formula:

• wherein
• ring A represents an aromatic ring optionally having substituents;
• B, Y and Ya are the same or different and each represents a bond or a spacer having a main chain of 1 to 6 atoms;
• R1 and R2 are the same or different and each represents a hydrogen atom, a hydrocarbon group optionally having substituents or a heterocyclic group optionally having substituents, or R1 and R2, together with the adjacent nitrogen atom, form a nitrogen-containing heterocyclic ring optionally having substituents, or R1 is linked with ring A together with the adjacent nitrogen atom and B to form a 5- to 7-membered nitrogen-containing heterocyclic ring;
• R3 represents a hydrogen atom, a hydrocarbon group optionally having substituents or a heterocyclic group optionally having substituents;
• R4 and R5 are the same or different and each represents a hydrogen atom or a hydrocarbon group optionally having substituents, or R4 and R5, together with the adjacent carbon atom, form a ring optionally having substituents;
• R6 represents an indolyl group optionally having substituents;
• and
• Z and Za are the same or different and each represents a hydrogen atom, a halogen atom or a cyclic group optionally having substituents;
• or a salt thereof.