Efferent projections of the periaqueductal gray in the rabbit

Abstract

The efferent projections of the periaqueductal gray in the rabbit have been described by anterograde tract-tracing techniques following deposits of tritiated leucine, or horseradish peroxidase, into circumscribed sites within dorsal, lateral or ventral periaqueductal gray. No attempts were made to place labels in the fourth, extremely narrow (medial), region immediately surrounding the aqueduct whose size and disposition did not lend itself to confined placements of label within it. These anatomically distinct regions, defined in Nissl-stained sections, corresponded to the same regions into which deposits of horseradish peroxidase were made in order for us to describe afferent projections to the periaqueductal gray. In this present study distinct ascending and descending fibre projections were found throughout the brain. Terminal labelling was detected in more than 80 sites, depending somewhat upon which of the three regions of the periaqueductal gray received the deposit. Therefore, differential projections with respect to both afferent and efferent connections of these three regions of the periaqueductal gray have now been established. Ventral deposits disclosed a more impressive system of ramifying, efferent fibres than did dorsal or lateral placements of labels. With ventral deposits, ascending fibres were found to follow two major pathways from periaqueductal gray. The periventricular bundle bifurcates at the level of the posterior commissure to form hypothalamic and thalamic components which distribute to the anterior pretectal region, lateral habenulae, and nuclei of the posterior commissure, the majority of the intralaminar and midline thalamic nuclei, and to almost all of the hypothalamus. The other major ascending pathway from the periaqueductal gray takes a ventrolateral course from the deposit site through the reticular formation or, alternatively, through the deep and middle layers of the superior colliculus, to accumulate just medial to the medial geniculate body. This contingent of fibres travels more rostrally above the cerebral peduncle, distributing terminals to the substantia nigra, ventral tegmental area and parabigeminal nucleus before fanning out and turning rostrally to contribute terminals to ventral thalamus, subthalamus and zona incerta, then continuing on to supply amygdala, substantia innominata, lateral preoptic nucleus, the diagonal band of Broca and the lateral septal nucleus. Caudally directed fibres were also observed to follow two major routes. They either leave the periaqueductal gray dorsally and pass through the gray matter in the floor of the fourth ventricle towards the abducens nucleus and ventral medulla, or are directed ventrally after passing through either the inferior colliculus or parabrachial nucleus. These ventrally directed fibres merge just dorsal to the pons on the ventral surface of the brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Projections of neurons in the periaqueductal gray to pontine and medullary catecholamine cell groups involved in the modulation of nociception

“…For example, Cameron et al (1995) reported a moderate density of labeled axons in the locus coeruleus following deposits of the anterograde tracer PHA-L in either the rostral or caudal areas of the ventrolateral PAG. These findings are also consistent with those of other reports that found projections from ventrolateral PAG to the locus coeruleus in cats (Sakai et al, 1977), rats (Cedarbaum and Aghajanian, 1978), monkeys (Mantyh, 1983), and rabbits (Meller and Dennis, 1991). However, several recent reports did not find any significant projections from ventrolateral PAG to locus coeruleus in the rat when using either anterograde tracing with PHA-L Van Bockstaele et al, 1991) or retrograde tracing with wheatgerm agglutinin conjugated to horseradish peroxidase .…”

Dendritic arborization and spines of the neurons of the cat and human periaqueductal gray: A light, confocal laser scanning, and electron microscope study

“…The analgesic effect of PAG stimulation has been demonstrated in both animals (see Behbehani, 1995) and humans (Barbaro, 1988;Meyerson, 1988). The complexity of PAG function is supported by its numerous connections (for efferent projections see Holstege G, 1991;Meller and Dennis, 1991;Mouton and Holstege G, 1994;Cameron et al, 1995a,b; for afferent projections see Meller and Dennis, 1986;Blomqvist and Craig, 1991;Shipley et al, 1991).…”

Neurogenic neuroprotection: Future perspectives

“…Some studies have revealed that FN efferents, labeled with radioactive amino acids, project into the lateral edge of the PAG in monkeys and dogs [39,44], while other authors have shown that caudal FN regions involved in oculomotor systems also have fine fibers that extend into the PAG [45]. Additionally, the PAG also has extensive ascending projections to the midline, intralaminar, and reticular thalamic nuclei, as well as the hypothalamus and basal forebrain [46][47][48][49]. The presence of these projections supports the notion that the DPAG excites the neuroprotective response of SVA, which is independent from the FN [42].…”