Reciprocal connections between the claustrum and the auditory cortical fields in the cat. An experimental study using light- and electron microscopic anterograde degeneration methods, and the horseradish peroxidase retrograde axonal transport.

The reciprocal connections between the claustrum and the auditory cortical fields AI, AII and Ep were investigated by means of Nauta and Fink-Heimer selective silver impregnation procedures, electron microscopic identification of degenerated axons and synaptic boutons, and with the Mesulam horseradish peroxidase retrograde tracing technique. The course and termination of degenerating corticoclaustral axons were investigated following circumscript lesions of the AI, AII and Ep areas in 19 cats. The greatest amount of degeneration debris was observed following destruction of the AII area. The central third of the claustrum (stereotaxic level A13-A15) is filled with degenerating terminals (d. t.), with greatest concentration in the lateral wedge of the nucleus, and along its inferolateral border. Rostrally and caudally the density of degeneration diminishes but scattered d. t. were observed up to the rostral pole, and a moderate number - up to the caudal pole of the claustrum. Slightly lesser amount of d. t. was observed following Ep destruction. The caudal portion of the claustrum is filled with d. t. In the central third the degeneration field occupies mainly the ventrolateral zone of the nucleus. The rostral pole of the claustrum is free of degeneration. The projection from the AI field is considerably more moderate, and is diffusely organized. A substantial number of d. t. is encountered only in the lateral parts of the central claustral third. The crossed corticoclaustral connections mirror the ipsilateral ones but are far more modest. The AII area projects mainly to the central claustral third, the Ep area--to the caudal third. The projection of the AI area to the contralateral claustrum is very weak. The electron microscopic examination of the claustrum following auditory cortex destruction in 9 cats revealed an appreciable number of degenerating synaptic boutons. They undergo dark and more rarely light degenerative changes. The cortical terminals are classified in two types: "small round" (SR), comprising approximately 70 to 75% of the corticoclaustral boutons, and "large round" (LR)-25-30%, resp. The SR boutons measure 0.6-1.2 micron, contain tightly packed round synaptic vesicles (380-420 A), and form asymmetrical axodendritic contacts. The LR boutons measure 1-2.5 microns, contain round vesicles (400-500 A) and form asymmetrical axodendritic and (far more rarely) axosomatic contacts. The claustrocortical connection was investigated in 13 cats with selective injections of 30% HRP in the three subdivisions of the auditory cortex.(ABSTRACT TRUNCATED AT 400 WORDS)