PTK7 and panic disorder: Second, although CCK-8 in its sulfated form is the predominant cerebral form of CCK, and may by exogenous administration (of CCK-8S itself or its analogue, caerulein) cause anxiety and panic attacks [24,30,31], the short C-terminal tetrapeptide fragment, CCK-4, appears on a molar basis to be a more potent panicogenic peptide [24,30].