MODULATION OF RETINOTECTAL TRANSMISSION BY PRESYNAPTIC 5-HT1B RECEPTORS IN THE SUPERIOR COLLICULUS OF THE ADULT HAMSTER

1. Radioligand binding with [I-125]-cyanopindolol in the presence of i soproterenol was used to define the distribution of 5-HT1B receptors i n the superior colliculus (SC) of adult hamsters. There was a high den sity of these receptors in the stratum griseum superficiale ( SGS), an d they were much less dense in other SC laminae. Enucleation of one ey e produced a marked reduction in the density of these receptors in the contralateral SGS, suggesting that they are located primarily on reti notectal axon terminals. 2. Intracellular recording techniques were us ed to evaluate the effects of serotonin (5-HT) on the excitatory posts ynaptic potentials (EPSPs) evoked in SC cells of adult hamsters by sti mulation of the optic tract (OT) in vitro. Application of 5-HT produce d a reduction of greater than or equal to 50% in OT-evoked EPSPs in 79 % of the 67 cells tested. The average EPSP amplitude was 7.8 +/- 2.1 ( SD) mV under control conditions and 2.7 +/- 1.9 mV in the presence of 5-HT (P < 0.01). For most of these neurons, application of 5-HT had li ttle effect on either membrane potential or input resistance. The aver age percent change in membrane potential for cells tested with 5-HT wa s 0.5 +/- 6.0% and the average percent change in input resistance was 0.6 +/- 22.9%. 3. For four of six cells tested, application of 5-HT ha d no significant effects on the responses evoked by application of glu tamate, either under normal bathing conditions or when the medium incl uded low Ca2+ and high Mg2+. 4. Pharmacologic experiments indicated th at the effects of 5-HT on retinotectal transmission were mimicked by t he 5-HT1B agonists 1-[3-(trifluoromethyl) phenyl]-piperazine and 7-tri fluoromethyl-4 (4-methyl-1-piperazinyl) - pyrrolo[1,2-a]-quinoxaline m aleate and antagonized by the 5-HT1A/1B antagonists (-)-pindolol and m ethiothepin. The effects of 5-HT on the OT-evoked EPSP were not antago nized by either spiperone, ketanserine, hoxyphenyl)-4-[4-(2-phthalimmi do)butyl]-piperazine HBr, or [1-H-3 alpha-5 alpha-tropan-3-yl]-3,5-dic hlorobenzoate. 5. Both the anatomic and physiological results are cons istent with the conclusion that 5-HT presynaptically inhibits retinote ctal transmission and that this effect is mediated by the 5-HT1B recep tor.