388 Brain Research. 418 (1987) 388- 39t Elsevier BRE 22444 The histaminergic of meeencephaiic n teus of e trigeminal nerve in rat ain: alight microscopical stu Naoyuki Inagaki I , Atsushi Yamatodani 1, Koh Shinoda 3, Yahe Shiotani 3, Masaya Tohyama 2, Takehiko Watanabe 4 and Hiroshi Wada ~ 1Department of Pharmacology ll and 2Department of Anatomy 11, Osaka University Medical School, and ~Department of Neuroanatomy, Institute of Higher Nervous Activity, Osaka University Medical School, Osaka (Japan) and 4Department of Pharmacology I, Tohoku University School of Medicine, Sendai (Japan) (Accepted 19 May 1987) Key words." Histaminergic innervation; Histidine decarboxylase-like immunoreactivity; Mesencephalic nuclexts of the trigeminal nerve; Light microscopy; Electron microscopy; Immunocytochemistry; Rat Histaminergic fibers in the mesencephalic nucleus of the trigeminal nerve of Long-Evans rats were examined by light and electron microscopy after peroxidase-antiperoxidase immunocytochemical staining for histidine decarboxy!ase (HDC) asia marker. By light microscopy, neurons in the mesencephatic nucleus of the trigeminal nerve were seen to be surrounded by a number of HDC-like im- munoreactive (HDCI) fibers, suggesting the presence of axo-somatic contact. This finding was supported by immunoelectron micro. scopic demonstration of synaptic contact of some HDCI fibers with the soma of the neurons in this nucleus. These findings indicate that histamine is involved in the sensory regulation of movement of the masticatory muscles at the level of the trigeminal mesencephal. ic nucleus. Histamine is considered to function as a neuro- transmitter or a neuromodulator in the mammalian brain 1°-13. We have reported a central histaminergic neuron system demonstrated with histidine decar- boxylase (HDC, l-histidine carboxylase. EC 4.1.1.22), a histamine-synthesizing enzyme, as a marker 19 and demonstrated the wide. but uneven dis- tribution of this system in rat brain 22'23. We found that histamine neurons were concentrated in magno- cellular nuclei of the posterior hypothalamic area. such as the tuberal magnocellular nucleus, the caudal magnocellular nucleus and the postmammillary cau- dal magnocellular nucleus, and that extensive net- works of histaminergic fibers of various densities were present in many other areas of the brain 23. Simi- lar results were obtained by using antibodies against histamine itself as a marker sA6. For determination of the functions of the histaminergic system in the cen- tral nervous system, it is indispensable to clarify the targets of histaminergic innervation in the brain. Re- cently, we reinvestigated the distribution of the hista- minergic neuron system in the brain of Long-Evans rats and found particularly rich innervation of the mesencephalic nucleus of the trigeminal nerve (Mes V). The Mes V consists of large cells which are pro- prioceptive primary afferents of the trigeminal nerve with peripheral branches to the masticatory muscle spindles 1,2,4,18,24. In the present study, to establish the relationship between the histaminergic fibers and the sensory system, we examined the synaptic contacts of the fibers with the sensory neurons of the Mes V im- munocytochemicalty with HDC as a marker. Eight male Long-Evans rats (150-200 g) were used. Preparation of tissue sections and immunocyto- chemical procedures were as reported in detail pre- viously 5,2°. Briefly, animals were anesthetized with pentobarbital sodium (30 mg/kg, i.p.) and perfused through the ascending aorta first with saline, and Correspondence: N. Inagaki. Department of Pharmacology II. Osaka University Medical School. Nakanoshima 4-3-57. Kita-ku. Osaka 530, Japan. 0006-8993/87/$03.50 © 1987 Elsevier Science Publishers B.V. (Biomedical Division)