ORIENTING-LIKE REACTION AFTER IBOTENIC ACID INJECTIONS INTO THE THALAMIC CENTER MEDIAN NUCLEUS IN THE CAT

1. The excitotoxin ibotenic acid was injected bilaterally into the int ralaminar centre median nucleus of chronically implanted cats in order to study the effects of early excitation of centre median population on electrographic correlates of behavioral states and to compare them to those induced by injection into intralaminar centralis lateralis nu cleus, previously shown. 2. Immediately and during the first 24h after injection, highly aroused behavior with electrocortical activation, m yoclonic jerks, enhancement of ocular movements and ponto-geniculo-occ ipital waves were observed. 3. Surprisingly, in contrast to the increa se of REM sleep episodes in centralis lateralis cases, REM sleep was o bliterated. 4. The injection sites were histologically confirmed. The different connectional properties of the two intralaminar population m ay explain the differential results. 5. Tn conclusion, the present beh avioral observations and electrographic findings taken together with t he known afferent and efferent projections suggest that the caudal gro up of intralaminar nuclei is involved in orienting-like behavior.