European Journal of Pharmacology, 238 (1993) 173-180 173 © 1993 Elsevier Science Publishers B.V. All rights reserved 0014-2999/93/$06.00 F_JP 53170 Multiple mechanisms of serotonin 5-HT 2 receptor desensitization Shafiqur Rahman and Richard S. Neuman Faculty of Medicine, Memorial University, St. John's, Newfoundland, Canada Received 21 December 1992, revised MS received 5 April 1993,accepted 20 April 1993 Desensitization of serotonin 5-HT2 receptor-mediated enhancement of the N-methyl-D-aspartate (NMDA) depolarization was studied in rat cortical neurons. Serotonin and (+)-l-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) induced long term desensitization. Staurosporine, a nonspecific protein kinase C inhibitor, potentiated the serotonin and DOI facilitation, suggesting acute desensitization was operative. In the case of DOI, long term desensitization was prevented by staurosporine. Activators of protein kinase C abolished the serotonin facilitation, an action prevented by staurosporine. Concanavalin A potentiated the facilitation at 100 /zM, but not 30 /zM serotonin, suggesting these receptors undergo dose dependent internalization. Calmodulin antagonists prevent long term desensitization induced by serotonin. The depolarization induced by NMDA alone was not altered by staurosporine, protein kinase C activators, concanavalin A or calmodulin antagonists. Serotonin at 100/zM, but not 30/zM, induced heterologous desensitization of phenylephrine and carbachol induced facilitation of the NMDA depolarization. We conclude that serotonin 5-HT2 receptors both induce and undergo several forms of desensitization. 5-HT (5-hydroxytryptamine, serotonin); Transmitter interaction; Desensitization; Receptor internalization; Protein kinase C 1. Introduction Homologous desensitization is a widespread phe- nomenon in which initial exposure to an agonist results in decreased cellular responsiveness on second expo- sure. Serotonin 5-HT receptors, like many other recep- tors, are susceptible to agonist induced desensitization. Thus, serotonin 5-HT 2 receptors located on cortical neurons (Leysen et al., 1989; Araneda and Andrade, 1991), cerebellar granule cells (Dillon-Carter and Chuang, 1989), facial motoneurons (Aghajanian, 1990) and platelets (Kagaya et al., 1990) undergo desensitiza- tion. As with other receptors leading to activation of protein kinase C, desensitization of serotonin 5-HT z receptors results from feedback inhibition mediated by protein kinase C (Roth et al., 1986; Aghajanian, 1990). Such inhibition may serve (i) to protect cells from over stimulation, (ii) as a modulatory substrate by other regulators of cellular activity. A second mechanism leading to loss of receptor mediated responsiveness is internalization or seques- tration of receptors by which hydrophilic agonists are prevented access to receptors. Sequestration of/3-ad- Correspondence to: R.S. Neuman, Faculty of Medicine, Memorial University, St. John's, NewfoundlandA1B 3V6, Canada. renoceptors has been extensively studied (Waldo et al., 1983; Toews et al., 1984; Kassis et al., 1986). Internal- ization of /3-adrenoeeptors occurs in the presence of high concentrations of agonist (Waldo. et al., 1983). Sequestration of serotonin 5-HT4 receptors has been recently demonstrated (Ansanay et al., 1992), but to our knowledge, sequestration of serotonin 5-HT 2 re- ceptors has not been reported (Roth et al., 1990). Recently, while characterizing enhancement of N- methyl-D-aspartate (NMDA) depolarization of cortical neurons by 5-HT (Neuman and Rahman, 1992; Rah- man and Neuman, 1993), we observed that the 5-HT response exhibited tachyphylaxis and acute desensitiza- tion. In the present report, we investigated mechanisms underlying desensitization of serotonin 5-HT 2 recep- tors. Part of this material has been presented in ab- stract form (Rahman and Neuman, 1992). 2. Materials and methods 2.1. Wedge preparation and recording Wedges from rat sensory-motor cortex were pre- pared for recording as described (Rahman and Neu- man, 1993). Briefly, male Sprague-Dawley rats (Charles Rivers, Montreal, Quebec) weighing 100-300 g were