Effects of a benzodioxan derivative MKC-242 on the firing activity of the dorsal hippocampus CA1 pyramidal neurons in awake and urethane- anaesthetized rats: in vivo electrophysiological evidence for a 5-HT 1A agonistic property.

The aim of the present in vivo study was to determine whether a benzodioxan derivative MKC-242, (S)-5-[3-[(1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-benzodioxole hydrochloride, possesses an agonistic activity at postsynaptic serotonin (5-HT)1A receptors in the rat hippocampus when administered systemically. We examined the effects of acute administrations of MKC-242 on the firing activities of dorsal hippocampus CA1 pyramidal neurons. In quiet awake rats, s.c. administrations of MKC-242 significantly decreased the spontaneous firing activity in a dose-dependent manner at doses of 0.3-6 mg/kg. In urethane-anaesthetized rats, i.v. injections of MKC-242, at cumulative doses of 0.3-3 mg/kg, also significantly and dose-dependently inhibited the firing activity induced by microiontophoretically applied quisqualate. These decreasing effects were antagonized by the selective 5-HT1A antagonists WAY-100135 (5 mg/kg, s.c.) and WAY-100635 (0.2 mg/kg, s.c. to the awake rats and 0.4 mg/kg, i.v. to the anaesthetized rats), thereby confirming that MKC- 242 decreased the firing activities by stimulating 5-HT1A receptors. The selective depletion of 5-HT produced by the 3-d administration of the 5-HT synthesis inhibitor, parachlorophenylalanine (500 mg/kg.d, i.p.), did not affect the decreasing effect of MKC-242 in the awake animals, indicating that postsynaptic 5-HT1A receptors mediated the decreasing effect. The present results provided the first in vivo electrophysiological evidence that MKC-242, when systemically administered, exerts a 5-HT1A agonistic action at the postsynaptic level.