LOCAL CONNECTIONS BETWEEN THE COLUMNS OF THE PERIAQUEDUCTAL GRAY-MATTER - A CASE FOR INTRINSIC NEUROMODULATION

Chemical stimulation of the midbrain periaqueductal gray matter (PAG) in conscious animals produces opposite responses (viz., defensive beha vior and presser responses from the lateral PAG column vs. quiescence and depressor responses from the ventrolateral PAG column), raising th e possibility that PAG columns are interconnected. To test this hypoth esis, two types of anatomical experiments were performed in rats. Firs t, the anterograde axonal marker Phaseolus vulgaris is leuco-agglutini n (PHA-L) was injected into individual PAG columns or adjoining region s which included the Edinger-Westphal, dorsal raphe, and precommissura l nuclei. Ten days later, the intrinsic PAG projections were studied. Our results show that each PAG column projects bilaterally to all of t he other PAG columns, and also provides local connections within its o wn column. Furthermore, the Edinger-Westphal and precommissural nuclei project to all four PAG columns, while the dorsal raphe nucleus proje cts only to the ventrolateral and lateral columns. In a second experim ent, we found that cardiovascular-related PAG projection neurons of bo th the lateral and ventrolateral columns receive inputs from the recip rocal PAG column. This was demonstrated by a double tracer neuroanatom ical study in which PHA-L was first iontophoretically ejected into eit her the lateral or ventrolateral PAG columns and then, several days la ter the retrograde transneuronal viral tracer, pseudorabies virus, was injected into the stellate sympathetic ganglion. Intra-PAG circuits w ere visualized by a dual immunohistochemical procedure. These results suggest that during the fight-or-flight response when the 'fight' prog ram is activated, the 'flight' PAG network may be inhibited and the co nverse situation may occur during the flight response. (C) 1998 Elsevi er Science B.V.