Abstract
Lectin-conjugated horseradish peroxidase was injected or implanted in crystalline form into various parts of the periaqueductal gray matter (PAG) in the cat. After varying survival periods, the animals were fixed and the mesencephalon was sectioned and incubated for HRP histochemistry. Outside PAG, labelled cells and terminal labelling were observed in the cuneiform, parabrachial and intercollicular nuclei, in the deep and intermediate gray layers of the superior colliculus, in the anterior and posterior pretectal nuclei and in the nucleus of Darkschewitsch. This study has shown that the region of PAG that is known to receive heavy ascending somatosensory input from the spinal cord and to be part of descending pain-inhibiting systems, also has reciprocal connections with other somatosensory areas of the midbrain. The results are discussed in relation to nociception and nociceptive inhibiting mechanisms.
Highlights
It is well known that the periaqueductal gray matter (PAG) receives a heavy projection from the spinal cord in several species including man,and recent studies have revealed somatotopy with the lumbar spinal cord projecting to the caudal region of the PAG and with the cervical spinal cord projecting to the middle region of the PAG [8, 36, 39]
In the present study such transport seems unlikely, since [1] the injected amounts were small, [2] the survival periods were kept short and [3] labelled fibres were traced from the injection site to the terminal areas, Anatomical projections; In this study PAG was found to have reciprocal connections with CNF, PBN, intercollicular nucleus (Inc), SGI, SGP, PTA, PTP and D
In two earlier studies in the cat [20, 21] with use of a degeneration technique, efferent projections were observed from PAG to CNF, SGI, SGP and the pretectum, projections that have since been shown to exist in the rat and monkey ( 6, 19, 26 )
Summary
It is well known that the periaqueductal gray matter (PAG) receives a heavy projection from the spinal cord in several species including man (see e.g. 39),and recent studies have revealed somatotopy with the lumbar spinal cord projecting to the caudal region of the PAG and with the cervical spinal cord projecting to the middle region of the PAG [8, 36, 39]. As it has been demonstrated that other areas of the midbrain, especially the intercollicular region, receive pronounced input from the dorsal column and lateral cervical nuclei [7, 9, 18, 37], the present study was undertaken to determine whether there are any connections between these latter sites of projection and the site of the spinal projection to PAG, providing a possible means of activating descending pain-inhibiting systems
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