The hypofunction of NMDA receptors (NMDARs) in schizophrenia could arise from the occlusion of GluN2B-containing NMDARs by the CABT complex,
which consists of a CRMP2 monomer, an alpha and a beta tubulin. Tubulin is the canonical binding partner of CRMP2. Binding between GluN2B and CRMP2 has been demonstrated experimentally
(See Paper 25 for details).
A class of hallucinogenic drugs are agonists of 5-HT2A receptors (5-HT2ARs), yet several 5-HT2AR agonists are not hallucinogens.
It has been shown that their major difference could lie in the capacity of hallucinogens to induce the Gi/o pathway. Since the Gi/o pathway inhibits protein kinase A (PKA),
this finding supports the CABT Hypothesis where PKA plays a key role in preventing NMDAR hypofunction (i.e., CABT occlusion) by (1) phosphorylating GluN2B at Ser-1166,
and (2) inhibiting glycogen synthase kinase 3 (GSK-3). The importance of PKA in NMDAR hypofunction is further underscored by the emergence of phosphodiesterase (PDE)
as a therapeutic target for schizophrenia. PDE catalyzes the breakdown of cAMP and/or cGMP. Its inhibitors may elevate cAMP level, thereby enhancing PKA activity.
(See Paper 26 for details).