The variety of synaptic vesicles in the interpeduncular nucleus (ITP) of the frog

The electron microscope has revealed a large variety of synaptic vesicles in the interpeduncular nucleus (ITP) of the frog "Rana esculenta". They vary in shape, size and electron density. There are two types of synapses which show only translucent spherical vesicles: in one type the vesicles are 40 nm, in the other type they are 70 nm in diameter. In other types of synapses the translucent vesicles may be mixed with those with dense core. Large granules, 160 nm in diameter, already reported in the ITP (KEMALI 1977a), are also shown as well as tiny flat mixed with large flat dense core vesicles of dumb-bell shape. Two types of axo-axonic synapses are illustrated while no crest synapses have been demonstrated. The results suggest that the afferents to the ITP might be more numerous than those reported in the literature or that--as in the case of the habenular afferents which consist of cholinergic and peptergic fibres--each projecting nucleus to the ITP has different types of fibres with more than one type of transmitter. Furthermore, due to the vesicles sizes, we may consider the ITP as a site in the vertebrate central nervous system where conventional neurotransmitter structures coexist with probable neurohumoral elements.