"Type III" cells of rat taste buds: Immunohistochemical and ultrastructural studies of neuron-specific enolase, protein gene product 9.5, and serotonin

Taste buds contain a variety of morphological and histochemical types of el ongate cells. Serotonin, neuron-specific enolase (NSE), ubiquitin carboxyl terminal hydrolase (PGP 9.5), and neural cell adhesion molecule (N-CAM) all have been described as being present in the morphologically defined Type I II taste cells in rats. In order to determine whether these substances coex ist in a single cell, we undertook immunohistochemical and ultrastructural analysis of taste buds in rats. Double-label studies show that PGP 9.5 and NSE always colocalize. In contrast, PGP 9.5 and serotonin seldom colocalize . Further, whereas the serotonin-immunoreactive cells are always slender an d elongate, the PGP 9.5/NSE population comprise two morphological types - o ne slender, the other broader and pyriform. Although gustducin-immunoreacti ve taste cells appear similar in overall shape to the pyriform PGP 9.5/NSE population, gustducin never colocalizes with PGP 9.5 or NSE. The serotonin- immunoreactive taste cells have an invaginated nucleus, synaptic contacts w ith nerve fibers, and taper apically to a single, large microvillus. These are all characteristics of Type III taste cells described previously in rab bits (Murray [1973] Ultrastructure of Sensory Organs L Amsterdam: North Hol land. p 1-81). PGP 9.5-immunoreactive taste cells exhibit two morphological varieties. One type is similar to the serotonin-immunoreactive population, containing an invaginated nucleus, synapses with nerve fibers, and a singl e large microvillus. The other type of PGP 9.5-immunoreactive taste cell ha s a large round nucleus and the apical end of the cell tapers to a tuft of short microvilli, which are characteristics of Type II taste cells. Thus, i n rats, some Type III cells accumulate serotonin but do not express PGP 9.5 , whereas others express PGP 9.5 but do not accumulate amines. Similarly, T ype Il taste cells come in at least two varieties: those immunoreactive for gustducin and those immunoreactive for PGP 9.5. (C) 2001 Wiley-Liss, Inc.