ABSTRACT
Immunocytochemistry with an antiserum raised against serotonin was used to provide detailed description of the serotonin (5-HT)-containing cell bodies and fibers in the CNS of the hedgehog (Erinaceus europeaus). The distribution of the 5-HT-positive cells and the pattern of 5-HT innervation resembled those described in other mammalian species but several differences were also noticed. The nine B groups of Dahlstrom and Fuxe (1964) were observed, except for the B 4 group which was not detected in the hedgehog. Neuron-like immunoreactive cells were also observed in brain regions located outside the brainstem reticular formation. Among the latter the olfactory tubercle and the thalamus have not been described to contain 5-HT cell bodies in other mammals. The number and the "lateralization" of 5-HT-positive neurons in areas outside the midline raphe were more limited than those described in the rodents and much more limited than in the carnivores and the primates. 5-HT-positive axons were unevenly distributed throughout the hedgehog's CNS. From all neocortical areas, which displayed the highest innervation density in the layer I, somatosensory and visual areas were the most heavily and lightly innervated ones respectively. Other regions of the forebrain exhibiting dense serotoninergic innervation were: olfactory cortex; olfactory tubercle; subiculum; lateral septal nucleus; stratum; medial and basal amygdaloid nuclei. Dense serotoninergic innervation were also found to receive the following brainstem structures: lateral hypothalamic area; dorsal part of the suprachiasmatic nucleus; arcuate nucleus; median eminence; habenular nuclei; centromedian thalamic nucleus; ventral tegmental area; medial part of the substantia nigra; interpeduncular nucleus; locus coeruleus; cuneiform nucleus; motor nucleus of the trigeminal nerve; facial nucleus; parabrachial nucleus; prepositus hypoglossi nucleus; medial subnucleus of the nucleus of the solitary tract; hypoglossal nucleus; dorsal motor nucleus of the vagus. In the spinal cord, laminae IX and X received the heaviest innervation.
