"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 elongate 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 III taste cells in rats. In order to determine whether these substances coexist 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 and elongate, the PGP 9.5/NSE population comprise two morphological types--one slender, the other broader and pyriform. Although gustducin-immunoreactive 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 with nerve fibers, and taper apically to a single, large microvillus. These are all characteristics of Type III taste cells described previously in rabbits (Murray [1973] Ultrastructure of Sensory Organs I. Amsterdam: North Holland. 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 single large microvillus. The other type of PGP 9.5-immunoreactive taste cell has 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, in 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, Type II taste cells come in at least two varieties: those immunoreactive for gustducin and those immunoreactive for PGP 9.5.