The monoaminergic innervation of the telencephalon of the frog, Rana pipiens.

A large number of serotonin (5-hydroxytryptamine, 5-HT)-type axon terminals has been visualized in the so-called 'striatum' (pars ventralis and dorsalis) of Rana pipiens by means of the Falck-Hillarp histochemical method. The frog striatum contains, however, a much smaller number of catecholamine (CA) type axon terminals and differs strikingly in that aspect from the neostriatum of 'higher' vertebrates which is known to receive a massive CA innervation. In the telencephalon of Rana pipiens the highest density of CA axon terminals occurs at the level of the medial wall, which appears highly hypertrophied in comparison to the thin lateral wall. The CA terminals are mainly concentrated within nucleus accumbens septi and in the ventrolateral portion of nucleus lateralis septi where they surround intimately the non-fluorescent neuronal somata of both septal nuclei. The three pallial fields (medial, dorsal and lateral) of Rana pipiens are nearly devoid of CA axon terminals. A moderate amount of 5-HT terminals is present, however, within the caudal half of both the medial and dorsal pallial fields. Finally, a few CA-type neuronal somata are found lying amongst the mitral cells of the olfactory bulb. As a whole, the telecenphalon of Rana pipiens appears more profusely innervated by axons of the 5-HT-type than by those of the CA-type. This finding has been related to the fact that in the frog brain stem the CA neuronal somata are scarce in comparison to the highly developed 5-HT neuronal systems.