In the squirrel monkey (Saimiri sciureus), electrodes were implanted into the PAG at sites yielding vocalization when electrically stimulated. Then, injections of kynurenic acid (5 µg in 0.25 µl), a non-specific glutamate antagonist, were made stereotactically at numerous positions within the pons and rostral medulla, looking for sites capable of blocking PAG-induced vocalization.

Out of the 188 injection sites tested, 26 were able to block periaqueductally elicited vocalization. These sites were found in the caudal pons around the ventral nucleus of lateral lemniscus and superior olive as well as in the dorsolateral pontine reticular formation. Bilateral injections were not more effective than ipsilateral injections. Injections made contralateral to the stimulation sites were without effect. The blockade was limited to one particular class of call types, the so-called clucking/cackling/twitter calls - which are all characterized by extensive frequency modulations. PAG-induced purring, groaning, shrieking and peeping calls could not be abolished by pontine injections. Injections into the area of the rostral nucl. ambiguus, even if bilateral did not abolish vocalization, but changed its acoustic structure.

The results suggest that the nucl. retroambiguus is not the only relay station of the vocalization-controlling pathway descending from the PAG to the phonatory motoneurones. Critical processing steps, at least with respect to frequency-modulated calls, also seem to take place in the caudolateral pons.

Figure 1.

Figure 2.

Figure 3.

Figure 4.