Abstract

The binding of the histamine autoreceptor (H3) agonist [3H]-N alpha-methyl-histamine ([3H]-N-MeHA) was examined in 25 micron thick rat forebrain sections. The specific binding was saturable and of high affinity: Scatchard analysis indicated a Kd of 2 nM and a Bmax of 25 +/- 3 fmol/section. Under similar conditions, [3H]-histamine [( 3H]-HA) bound with a Kd of 8 nM and a Bmax of 20 +/- 2 fmol/section. Competition studies indicated that both ligands bound an identical site which had the pharmacological characteristics of the H3 binding site. The high affinity binding of [3H-N-MeHA was sensitive to the presence of 5'-guanylyl-imidodiphosphate, indicating that the binding site is likely coupled to a G-protein. Autoradiographic studies indicated the [3H]-N-MeHA binding to be greatest in the nucleus accumbens, striatum, substantia nigra pars reticulata, and certain cortical areas. Striatal quinolinic acid lesions greatly reduced binding in both the striatum and ipsilateral substantia nigra, while 6-hydroxydopamine lesions of the nigrostriatal dopamine system were without effect on binding. Therefore, most of the H3 binding sites in the basal ganglia are on striatonigral projection neurons. Cortical quinolinic acid lesions greatly reduced H3 binding in cortex, indicating that the binding in cortex, as in striatum, is largely on intrinsic neurons, rather than on afferents such as histamine nerve terminals.