The ventrolateral upper cervical cell group in cat projects to all rostrocaudal levels of the periaqueductal gray matter.

The lateral ventral horn of the upper cervical (C(1-3vl)) cord in rat, cat and monkey contains many cells that project to the periaqueductal gray matter (PAG). Until now it was assumed that these cells only project to the ventrolateral part of the caudal PAG. Because the ventrolateral caudal PAG is involved in quiescence and hypotension, it was hypothesized that the C(1-3vl)-PAG projecting cells play a role in immobility behavior, possibly activated by neck muscle afferents. However, in the present anterograde and retrograde study in cat we showed that C(1-3vl) cells do not only target the caudal PAG, but terminate even more abundantly in the intermediate and rostral parts of the PAG. There, projections target the ventrolateral column, like in the caudal PAG, but also terminate in the lateral and dorsomedial columns. This finding, combined with the current, albeit limited, physiological data on C(1-3vl)-PAG and C(1-3vl)-thalamic projecting cells, sheds a new light on the possible functions of C(1-3vl) cells. It might be that the C(1-3vl) cells with complex response properties and large receptive fields have a relay function similar to cells in the dorsal column nuclei, lateral cervical or central cervical nucleus. Other C(1-3vl) cells might receive somatic or visceral input that was never tested before, or perhaps other input, like vestibular information. It might also be that these cells project to the more caudal cord to modulate visceral input, with ascending collaterals to more rostrally located structures, including mesencephalon and thalamus.

Infralimbic cortex projects to all parts of the pontine and medullary lateral tegmental field in cat.

The infralimbic cortex (ILc) in cat is the ventralmost part of the anterior cingulate gyrus. The ILc, together with the amygdala, bed nucleus of the stria terminalis and lateral hypothalamus, is involved in the regulation of fear behavior. The latter three structures are thought to take part in triggering the fear response by means of their projections to the pontine and medullary lateral tegmental field (LTF). The LTF is a large region extending from the parabrachial nuclei rostrally to the spinal cord caudally. It contains almost all the premotor interneurons for the brainstem and for some upper spinal cord motoneurons innervating the muscles of face, head and throat. The question is whether ILc also projects to the LTF. Such a pathway would allow the ILc to influence the fear response by acting directly on these premotor interneurons. Anterograde tracer injections were made in the medial surface of the cortex in four cats. Only when the injection sites involved ILc were anterogradely labeled fibers observed throughout the rostrocaudal extent of the LTF. To verify whether these projections indeed originated from ILc, in two other cases retrograde tracer injections were made in the pontomedullary LTF. The results showed many retrogradely labeled neurons in ILc, but none in adjacent cortical regions. These results show that the ILc projects to the LTF in cat and can possibly modulate the fear response not only via indirect but also via direct routes to the premotor interneurons in the brainstem.

Neurons in the guinea pig (Cavia porcellus) lateral lumbosacral spinal cord project to the central part of the lateral periaqueductal gray matter.

In order to micturate successfully, information from the bladder has to be conveyed to the brainstem. In most experimental animals, this information is relayed, via the lumbosacral spinal cord, to the periaqueductal gray matter (PAG). Although the rat is the most used experimental animal in neurourological research, urodynamic studies show that guinea pig may be a better small experimental animal because its urodynamic profile is, in contrast to that of a rat, similar to that of humans. Therefore, the present study, using anterograde and retrograde tracing, was performed to determine whether the lumbosacral spinal cord projects to the PAG in guinea pig. Results show that neurons in the lateral part of the lumbosacral spinal cord project to the central parts of the PAG. This pathway may convey information about the level of bladder filling to the PAG.

Periparabigeminal and adjoining mesencephalic tegmental field projections to the dorsolateral periaqueductal grey in cat - a possible role for oculomotor input in the defensive system.

The dorsolateral column of the mesencephalic periaqueductal grey (PAGdl) differs from its adjacent columns in terms of afferent and efferent connections and the distribution pattern of different histochemical substances. Functionally, PAGdl is associated with aversive and defensive behaviours, but in an earlier study of this laboratory [E.M. Klop et al. (2005) J. Comp. Neurol., 492, 303-322], it was found that PAGdl specifically receives input from the nucleus prepositus hypoglossi, which plays a role in oculomotor control. In search for other oculomotor-related brainstem structures projecting to PAGdl we studied the projections from the parabigeminal nucleus (PBGN) and its medially adjoining periparabigeminal area (PPBGA). In three cats, injections of wheatgerm agglutinin-horseradish peroxidase involving PAGdl did not, or to only a very limited extent, result in retrogradely labelled neurons in PBGN. When the peripheral parts of PAGdl were involved in the injection site, labelled neurons were located in PPBGA, while after an injection involving only the more central parts of PAGdl they were located in the tegmentum medial to the PPBGA. An anterograde tracing study using [3H]-leucine and biotinylated dextran amine affirmed that neurons in PPBGA project to more peripheral parts of PAGdl, while neurons located in the tegmentum medial to PPBGA project mainly to its central parts. These results provide further evidence for the existence of two different subdivisions of PAGdl. We hypothesize that PAGdl is alerted by sudden changes in the visual field, and that the PAGdl defensive system is inhibited when these changes are caused by eye movements.

Segmental and laminar organization of the spinothalamic neurons in cat: evidence for at least five separate clusters.

The spinothalamic tract (STT), well known for its role in the relay of information about noxe, temperature, and crude touch, is usually associated with projections from lamina I, but spinothalamic neurons in other laminae have also been reported. In cat, no complete overview exists of the precise location and number of spinal cells that project to the thalamus. In the present study the laminar distribution of retrogradely labeled cells in all spinal segments (C1-Coc2) was investigated after large WGA-HRP injections in the thalamus. The results show that this distribution of STT cells differed greatly between the different spinal segments. Quantitative analysis showed that there exist at least five separate clusters of spinothalamic neurons. Lamina I neurons in cluster A and lamina V neurons in cluster B are mainly found contralaterally throughout the length of the spinal cord. Cluster C neurons are located bilaterally in the ventrolateral part of laminae VI-VII and lamina VIII of the C1-C3 spinal cord. Cluster D neurons were found contralaterally in lamina VI in the C1-C2 segments, and cluster E neurons were located mainly contralaterally in the medial part of laminae VI-VII and lamina VIII of the lumbosacral cord. Most spinothalamic neurons are not located in the enlargements and most spinothalamic neurons are not located in lamina I, as suggested by several other authors. The location of the spinothalamic neurons shows remarkable similarities, but also differences, with the location of spino-periaqueductal gray neurons.

Two parts of the nucleus prepositus hypoglossi project to two different subdivisions of the dorsolateral periaqueductal gray in cat.

The dorsolateral column of the mesencephalic periaqueductal gray (PAG) is a separate part of the PAG. Its afferent sources, efferent targets, and neurochemical properties differ from the adjacent PAG columns. The dorsolateral PAG is thought to be associated with aversive behaviors, but it is not yet understood how these behaviors are brought about. To elucidate the function of the PAG further, in the present study we investigated which brainstem regions project to the dorsolateral PAG. Wheat germ agglutinin-horseradish peroxidase (WGA-HRP) injections involving the dorsolateral PAG, but extending into the lateral part, resulted in many retrogradely labeled cells in the pontine and medullary tegmentum bilaterally. However, it was concluded that these neurons were labeled from the lateral PAG, because no anterograde labeling was found in the dorsolateral PAG after a large injection into the tegmentum. Retrogradely labeled cells were also found in the nucleus prepositus hypoglossi (PPH), mainly contralaterally. Injections of [3H]leucine or WGA-HRP in the PPH resulted in anterogradely labeled fibers in the dorsolateral PAG. Two separate distribution patterns were found. The caudal and intermediate PPH projected to a small region on the dorsolateral edge of the dorsolateral column, whereas the supragenual PPH distributed labeled fibers to all other parts of the dorsolateral PAG, except the area on the dorsolateral edge. These separate PPH projections suggest that two subdivisions exist within the dorsolateral PAG. The present findings suggest a role for the dorsolateral PAG in the oculomotor system.

Neurons in the lateral sacral cord of the cat project to periaqueductal grey, but not to thalamus.

Previous work of our laboratory has shown that neurons in the lateral sacral cord in cat project heavily to the periaqueductal grey (PAG), in all likelihood conveying information from bladder and genital organs. In humans this information usually does not reach consciousness, which raises the question of whether the lateral sacral cell group projects to the thalamus. After wheatgerm agglutinin-horseradish peroxidase (WGA-HRP) injections into the sacral cord, anterogradely labelled fibers were found in the thalamus, specifically in the ventral anterior and ventral lateral nuclei, the medial and intralaminar nuclei, the lateral ventrobasal complex/ventroposterior lateral nucleus, and the nucleus centre median, lateral to the fasciculus retroflexus. Much denser projections were found to the central parts of the PAG, mainly to its dorsolateral and ventrolateral parts at caudal levels and lateral parts at intermediate levels. In a subsequent retrograde tracing study, injections were made in those parts of the thalamus that received sacral fibers, as found in the anterograde study. Labelled neurons were observed in the sacral cord, but not in the lateral sacral cell group. In contrast, a small control injection in the caudal PAG resulted in many labelled neurons in the lateral sacral cord. These results suggest that afferent information regarding micturition and sexual behaviour is relayed to the PAG, rather than to the thalamus.

C1-C3 spinal cord projections to periaqueductal gray and thalamus: a quantitative retrograde tracing study in cat.

By far, the strongest spinal cord projections to periaqueductal gray (PAG) and thalamus originate from the upper three cervical segments, but their precise organization and function are not known. In the present study in cat, tracer injections in PAG or in thalamus resulted in more than 2400 labeled cells, mainly contralaterally, in the first three cervical segments (C1-C3), in a 1:4 series of sections, excluding cells in the dorsal column and lateral cervical nuclei. These cells represent about 30% of all neurons in the entire spinal cord projecting to PAG and about 45% of all spinothalamic neurons. About half of the C1-C3 PAG and C1-C3 thalamic neurons were clustered laterally in the ventral horn (C(1-3vl)), bilaterally, with a slight ipsilateral preponderance. The highest numbers of C(1-3vl)-PAG and C(1-3vl)-thalamic cells were found in C1, with the greatest density rostrocaudally in the middle part of C1. A concept is put forward that C(1-3vl) cells relay information from all levels of the cord to PAG and/or thalamus, although the processing of specific information from upper neck muscles and tendons or facet joints might also play a role.

Lateral cervical nucleus projections to periaqueductal gray matter in cat.

The midbrain periaqueductal gray matter (PAG) integrates the basic responses necessary for survival of individuals and species. Examples are defense behaviors such as fight, flight, and freezing, but also sexual behavior, vocalization, and micturition. To control these behaviors the PAG depends on strong input from more rostrally located limbic structures, as well as from afferent input from the lower brainstem and spinal cord. Mouton and Holstege (2000, J Comp Neurol 428:389-410) showed that there exist at least five different groups of spino-PAG neurons, each of which is thought to subserve a specific function. The lateral cervical nucleus (LCN) in the upper cervical cord is not among these five groups. The LCN relays information from hair receptors and noxious information and projects strongly to the contralateral ventroposterior and posterior regions of thalamus and to intermediate and deep tectal layers. The question is whether the LCN also projects to the PAG. The present study in cat, using retrograde and anterograde tracing techniques, showed that neurons located in the lateral two-thirds of the LCN send fibers to the lateral part of the PAG, predominantly at rostrocaudal levels A0.6-P0.2. This part of the PAG is known to be involved in flight behavior. A concept is put forward according to which the LCN-PAG pathway alerts the animal about the presence of cutaneous stimuli that might represent danger, necessitating flight. J. Comp. Neurol. 471:434-445, 2004.

Nucleus retroambiguus projections to the periaqueductal gray in the cat.

The nucleus retroambiguus (NRA) of the caudal medulla is a relay nucleus by which neurons of the mesencephalic periaqueductal gray (PAG) reach motoneurons of pharynx, larynx, soft palate, intercostal and abdominal muscles, and several muscles of the hindlimbs. These PAG-NRA-motoneuronal projections are thought to play a role in survival behaviors, such as vocalization and mating behavior. In the present combined antero- and retrograde tracing study in the cat, we sought to determine whether the NRA, apart from the neurons projecting to motoneurons, also contains cells projecting back to the PAG. After injections of WGA-HRP in the caudal and intermediate PAG, labeled neurons were observed in the NRA, with a slight contralateral preponderance. In contrast, after injections in the rostral PAG or adjacent deep tectal layers, no or very few labeled neurons were present in the NRA. After injection of [(3)H]leucine in the NRA, anterograde labeling was present in the most caudal ventrolateral and dorsolateral PAG, and slightly more rostrally in the lateral PAG, mainly contralaterally. When the [(3)H]leucine injection site extended medially into the medullary lateral tegmental field, labeling was found in most parts of the PAG as well as in the adjoining deep tectal layers. No labeled fibers were found in the dorsolateral PAG, and only a few were found in the rostral PAG. Because the termination pattern of the NRA fibers in the PAG overlaps with that of the sacral cord projections to the PAG, it is suggested that the NRA-PAG projections play a role in the control of motor functions related to mating behavior.