Presence and connections of auditory neurons in the rostrodorsal and rostrolateral parts of the thalamic reticular nucleus.

OBJECTIVE: Auditory neurons have been identified in the caudoventral part of the thalamic reticular nucleus (TRN). We examined the acoustic input to single cells in the rostrodorsal part of the TRN.

MATERIAL AND METHODS: In alpha-chloralose-anesthetized cats, we extracellularly recorded the responses of single neurons in the rostral TRN to acoustic and light stimuli. Next, to examine efferent projections of auditory neurons in the rostral TRN, we injected wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into other thalamic nuclei where auditory neurons were detected, including the lateral posterior nucleus (LP), the lateral medial and suprageniculate nuclei and the centromedian nucleus. Finally, intracortical microstimulation of the LP was performed to demonstrate antidromic activation of the auditory neurons in the rostral TRN.

RESULTS: In the rostral TRN, 2 types of response to auditory stimuli were observed: brief, short-latency bursts (13-20 ms; mean 16.5 ms) and longer bursts with a long latency (38.8-50 ms; mean 44.8 ms). Injection of WGA-HRP into the medial LP labeled cells only in the rostrodorsal TRN, while extending the injection to the other nuclei labeled cells in the rostrodorsal and rostrolateral parts of the nucleus. Auditory neurons in the rostral TRN were activated antidromically by microstimulation of auditory neurons in the LP, with a latency of 1.2 ms.

CONCLUSIONS: These results strongly suggest that auditory neurons in the rostrodorsal TRN project to auditory neurons in the LP. The rostral auditory TRN may be involved in transmission of auditory information via the non-specific association system of the thalamus.