Somatovisceral projections from spinal cord and dorsal column nuclei to the thalamus in hedgehog tenrecs

Cholinergic, catecholaminergic, serotonergic, and orexinergic neuronal populations in the brain of the lesser hedgehog tenrec (Echinops telfairi)

The current study provides an analysis of the cholinergic, catecholaminergic, serotonergic, and orexinergic neuronal populations, or nuclei, in the brain of the lesser hedgehog tenrec, as revealed with immunohistochemical techniques. For all four of these neuromodulatory systems, the nuclear organization was very similar to that observed in other Afrotherian species and is broadly similar to that observed in other mammals. The cholinergic system shows the most variation, with the lesser hedgehog tenrec exhibiting palely immunopositive cholinergic neurons in the ventral portion of the lateral septal nucleus, and the possible absence of cholinergic neurons in the parabigeminal nucleus and the medullary tegmental field. The nuclear complement of the catecholaminergic, serotonergic and orexinergic systems showed no specific variances in the lesser hedgehog tenrec when compared to other Afrotherians, or broadly with other mammals. A striking feature of the lesser hedgehog tenrec brain is a significant mesencephalic flexure that is observed in most members of the Tenrecoidea, as well as the closely related Chrysochlorinae (golden moles), but is not present in the greater otter shrew, a species of the Potomogalidae lineage currently incorporated into the Tenrecoidea. In addition, the cholinergic neurons of the ventral portion of the lateral septal nucleus are observed in the golden moles, but not in the greater otter shrew. This indicates that either complex parallel evolution of these features occurred in the Tenrecoidea and Chrysochlorinae lineages, or that the placement of the Potomogalidae within the Tenrecoidea needs to be re-examined.

Amygdalar connections in the lesser hedgehog tenrec

The present study analyses the overall extrinsic connectivity of the non-olfactory amygdala (Ay) in the lesser hedgehog tenrec. The data were obtained from tracer injections into the lateral and intermediate portions of the Ay as well as several non-amygdalar brain regions. Both the solitary and the parabrachial nucleus receive descending projections from the central nucleus of the Ay, but only the parabrachial nucleus appears to project to the Ay. There is one prominent region in the ventromedial hypothalamus connected reciprocally with the medial and central Ay. Amygdalar afferents clearly arise from the dorsomedial thalamus, the subparafascicular nuclei and the medial geniculate complex (GM). Similar to other subprimate species, the latter projections originate in the dorsal and most caudal geniculate portions and terminate in the dorsolateral Ay. Unusual is the presence of amygdalo-projecting cells in the marginal geniculate zone and their virtual absence in the medial GM. As in other species, amygdalo-striatal projections mainly originate in the basolateral Ay and terminate predominantly in the ventral striatum. Given the poor differentiation of the tenrec's neocortex, there is a remarkable similarity with regard to the amygdalo-cortical connectivity between tenrec and rat, particularly as to prefrontal, limbic and somatosensorimotor areas as well as the rhinal cortex throughout its length. The tenrec's isocortex dorsomedial to the caudal rhinal cortex, on the other hand, may not be connected with the Ay. An absence of such connections is expected for primary auditory and visual fields, but it is unusual for their secondary fields.

Tracing thalamo-cortical connections in tenrecA further attempt to characterize poorly differentiated neocortical regions, particularly the motor cortex

The hedgehog tenrec (Afrosoricidae) has a very poorly differentiated neocortex. Previously its primary sensory regions have been characterized with hodological and electrophysiological techniques. Unlike the marsupial opossum the tenrec may also have a separate motor area as far as there are cortico-spinal cells located rostral to the primary somatosensory cortex. However, not knowing its thalamic input it may be premature to correlate this area with the true (mirror-image-like) primary motor cortex in higher mammals. For this reason the tenrec's thalamo-cortical connections were studied following tracer injections into various neocortical regions. The main sensory areas were confirmed by their afferents from the principal thalamic nuclei. The dorsal lateral geniculate nucleus, in addition, was connected with the retrosplenial area and a rostromedial visual region. Unlike the somatosensory cortex the presumed motor area did not receive afferents from the ventrobasal thalamus but fibers from the cerebello-thalamic target regions. These projections, however, were not restricted to the motor area, but involved the entire somatosensorimotor field as well as adjacent regions. The projections appeared similar to those arising in the rat thalamic ventromedial nucleus known to have a supporting function rather than a specific motor task. The question was raised whether the input from the basal ganglia might play a crucial role in the evolution of the mammalian motor cortex? Certainly, in the tenrec, the poor differentiation of the motor cortex coincides with the virtual absence of an entopeduncular projection to the ventrolateral thalamus.

The presence and absence of prosencephalic cell groups relaying striatal information to the medial and lateral thalamus in tenrec

Although there are remarkable differences regarding the output organization of basal ganglia between mammals and non-mammals, mammalian species with poorly differentiated brain have scarcely been investigated in this respect. The aim of the present study was to identify the pallidal neurons giving rise to thalamic projections in the Madagascar lesser hedgehog tenrec (Afrotheria). Following tracer injections into the thalamus, retrogradely labelled neurons were found in the depth of the olfactory tubercle (particularly the hilus of the Callejal islands and the insula magna), in subdivisions of the diagonal band complex, the peripeduncular region and the thalamic reticular nucleus. No labelled cells were seen in the globus pallidus. Pallidal neurons were tentatively identified on the basis of their striatal afferents revealed hodologically using anterograde axonal tracer substances and immunohistochemically with antibodies against enkephalin and substance P. The data showed that the tenrec's medial thalamus received prominent projections from ventral pallidal cells as well as from a few neurons within and ventral to the cerebral peduncle. The only regions projecting to the lateral thalamus appeared to be the thalamic reticular nucleus (RTh) and the dorsal peripeduncular nucleus (PpD). On the basis of immunohistochemical data and the topography of its thalamic projections, the PpD was considered to be an equivalent to the pregeniculate nucleus in other mammals. There was no evidence of entopeduncular (internal pallidal) neurons being present within the RTh/PpD complex, neuropils of which did not stain for enkephalin and substance P. The ventrolateral portion of RTh, the only region eventually receiving a striatal input, projected to the caudolateral rather than the rostrolateral thalamus. Thus, the striatopallidal output organization in the tenrec appeared similar, in many respects, to the output organization in non-mammals. This paper considers the failure to identify entopeduncular neurons projecting to the rostrolateral thalamus in a mammal with a little differentiated cerebral cortex, and also stresses the discrepancy between this absence and the presence of a distinct external pallidal segment (globus pallidus).

Thalamo-striatal projections in the hedgehog tenrec

Unlike the basal ganglia input from the midline and intralaminar nuclei, the origin and prominence of striatal projections arising in the lateral thalamus varies considerably among mammals being most restricted in the opossum and monkey, most extensive in the rat. To get further insight into the evolution of thalamo-striatal pathways the Madagascar lesser hedgehog tenrec (Afrotheria) was investigated using anterograde and retrograde flow techniques. An extensive medial thalamic region (including presumed equivalents to the paraventricular, parataenial and dorsomedial nuclei as well as the reuniens complex), the rostral (central) and caudal (parafascicular) intralaminar nuclei were shown to give rise to striatal projections. Additional projections originated in the ventral anterolateral nuclear group and regions within and around the medial geniculate complex. Similar to the rat there was also substantial projections from the lateral posterior-pulvinar complex and the ventral posterior nucleus. The fibers terminated extensively across the striatum in a mainly homogeneous fashion. Isolated patches of low-density terminations were found in the caudoputamen. This inhomogeneous labeling pattern appeared similar to one described in the cat with the unlabeled islands showing features of striosomes. The medial and intralaminar nuclei also projected heavily upon the olfactory tubercle. Differential innervation patterns were noted in the polymorphous layer, the deep and the superficial molecular layer.

The striatum in the hedgehog tenrec: histochemical organization and cortical afferents

In order to get insight into the striopallidal organization in mammals with little differentiated brain the striatum of the lesser hedgehog tenrec (Afrotheria) was characterized histochemically and analysed with regard to its cortical afferents using axonal tracer substances. The majority of neocortical cells projecting to the striatum were found bilaterally in the layers 2 and 3 of the frontal hemisphere; caudalwards the relative number of cells increased somewhat in the upper layer 5. There was a topographical organization as far as the allocortical projections appeared confined to the ventral striatum, and the efferents from hippocampal, posterior paleocortical, somatosensory and audiovisual areas were distributed in largely different striatal territories. Projections from the anterior frontal cortex, on the other hand, terminated extensively upon the caudate-putamen and also involved the nucleus accumbens and the olfactory tubercle. In the latter region the molecular layer was especially involved. The entorhinal cortex also projected heavily to the olfactory tubercle but unlike other species it scarcely involved the nucleus accumbens. The cortical fibers were distributed in a relatively homogenous fashion within their striatal territory and there was little evidence for patches of high density terminations. Islands of low density labeling, however, were noted occasionally in the caudate-putamen. These islands were partly similar in size as the patches of neuropil staining obtained with anti-calretinin and anti-substance P. There were also hints for the presence of a shell-like region in the nucleus accumbens stained with anti-dopamine transporter and NADPh-diaphorase. The classical striosome-matrix markers such as calbindin, acetylcholinesterase and enkephalin, however, failed to reveal any compartmental organization.

Parabrachio-cortical connections with the lateral hemisphere in the madagascan hedgehog tenrec: prominent projections to layer 1, weak projections from layer 6

The present study was undertaken to further characterize and subdivide the rhinal cortex (insular and perirhinal areas) in the hedgehog tenrec (Echinops telfairi), a placental mammal with a rather low encephalisation index. Injections of wheat germ agglutinin-horseradish peroxidase into the dorsolateral pontine tegmentum revealed a prominent layer 1 projection to several rhinal target areas, while the rhinal cortex only stained weakly for the calcitonin gene-related peptide. Among the regions retrogradely labeled following tracer injections into the rhinal cortex, the parabrachial nucleus was considered the main origin of the tegmento-cortical projection. This conclusion was based on the circumscribed pattern of termination, as well as the differences noted between the pattern of anterograde labeling and the pattern obtained by thyrosine hydroxylase immunohistochemistry. The tracer injections into the dorsolateral tegmentum also revealed numerous retrogradely labeled cells in the layer 5 of the dorsomedial frontal cortex. In contrast, the rhinal cortex only showed few labeled cells and most of these cells were located in the layer 6/7. A comparison with other species indicates that the tenrec's parabrachial nucleus gives rise to the most extensive cortical projections but receives the least prominent input from the lateral cerebral hemisphere. The layer 6/7 projection may be a common mammalian feature but it is overshadowed by the layer 5 projection in higher mammals.

Trigeminal projections to thalamus and subthalamus in the hedgehog tenrec

The objective of the present study was the identification and characterization of the trigemino-diencephalic target areas in the Madagascan lesser hedgehog tenrec in order to get a more comprehensive view on the mammalian somatosensory thalamus, its evolution and representation in different species. Such an analysis has been considered important because in lower mammals the head and face are relatively well represented, but their ascending trigeminal projections have scarcely been analysed. Following injections of different tracer substances into the rostral and caudal portions of the trigeminal nuclear complex the most prominent area of termination was found in the medial ventroposterior nucleus. These projections were patchy and scarcely overlapped the region previously shown to receive spinal and dorsal column nuclear afferents. On the basis of the laterality and the intensity of the projections, two subdivisions were distinguished, the principal portion and the accessory portion receiving a dense contralateral and a weak bilateral input, respectively. They were considered equivalents to the magnocellular and parvocellular subdivisions of the medial ventroposterior nucleus in more differentiated mammals. In the latter species, however, the overlap between trigeminal and parabrachial fibres appears less extensive than in the tenrec. In addition, a weak bilateral projection was shown from the caudal trigeminal nucleus to the caudal and dorsal subdivision of the nucleus submedius. There was little, if any evidence for a trigeminal projection to the intralaminar nuclei and we failed to identify a correlate to the posterior nuclear complex of higher mammals. On the other hand, there was a distinct contralateral projection to the ventral portion of the zona incerta. This projection was of similar strength as the projection to the medial ventroposterior nucleus; it supports the notion that the zona incerta may play a crucial role in relaying trigeminal information.

Origin and terminal distribution of the trigeminal projections to the inferior and superior colliculi in the lesser hedgehog tenrec

The trigemino-tectal projections were investigated with anterograde and retrograde tracing techniques in the Madagascan lesser hedgehog tenrec, Echinops telfairi. There were prominent contralateral projections to the inferior colliculus (CoI) and the superior colliculus (CoS), each showing its own characteristic pattern of terminations. While the projections to the CoI were confined consistently to a circumscribed region in its ventrolateral, external portion, the projections from particularly the rostral trigeminal subdivision to the CoS were distributed inhomogenously across almost the entire rostro-caudal and mediolateral extents. Comparing these data with the spino-tectal projections published previously, it demonstrates that the somatotopic organization of ascending tectal afferents is more distinct in the CoI than in the CoS. There were roughly twice as many trigeminal neurones projecting to the CoS than to the CoI. This difference might be due to the fact that the cells projecting to CoS were distributed extensively across the trigeminal nuclear complex (peak densities in the principal and interpolar subdivisions), while the neurones projecting to the CoI were largely confined to the interpolar and caudal trigeminal subdivisions. The latter cells were located adjacent to the spinal trigeminal tract; the neurones projecting to the CoS occupied preferentially the ventral trigeminal regions at rostral levels, while from the interpolar subdivision caudalward the labelled cells shifted dorsolaterally. In comparison to other mammals the trigeminal projection to the tenrec's CoI is unique. There is evidence for such a projection in other species too, but it is poorly documented, presumably due to technical reasons.

Cortical and medullary somatosensory projections to the cochlear nuclear complex in the hedgehog tenrec

Various tracer substances were injected into the spinal cord, the dorsal column nuclei, the trigeminal nuclear complex and the somatosensory cortex in Madagascan hedgehog tenrecs. With the exception of the cases injected exclusively into the spinal cord all injections gave rise to sparse, but distinct anterograde projections to the cochlear nuclear complex, particularly the granular cell domain within and outside of the dorsal cochlear nucleus. Among these cochlear afferents the projection from the primary somatosensory cortex is the most remarkable because the hedgehog tenrec has one of the lowest encephalisation indices among mammals and a similar cortico-cochlear connection has not been demonstrated so far in other species.

Diencephalic connections of the superior colliculus in the hedgehog tenrec

Using different tracer substances the pathways connecting the superior colliculus with the diencephalon were studied in the Madagascan hedgehog tenrec (Echinops telfairi), a nocturnal insectivore with tiny eyes, a small and little differentiated superior colliculus and a visual cortex with no obvious fourth granular layer. The most prominent tecto-thalamic projection terminated in the ipsilateral dorsal lateral geniculate nucleus. The entire region receiving contralateral retinal afferents was labeled with variable density. In addition, there was a widespread, homogeneously distributed collicular input to the lateralis posterior-pulvinar complex and a distinct tectal projection to the suprageniculate nucleus. The latter projections were bilateral with a clear ipsilateral predominance. Among the intra- and paralaminar nuclei the centralis lateralis complex was most heavily labeled on both sides, followed by the nucleus centralis medialis. The paralamellar portion of the nucleus medialis dorsalis and the nucleus parafascicularis received sparse projections. A clear projection to the nucleus ventralis medialis could not be demonstrated but its presence was not entirely excluded either. There were also projections to medial thalamic nuclei, particularly the reuniens complex and the nucleus paraventricularis thalami. The main tecto-subthalamic target regions were the zona incerta, the dorsal hypothalamus and distinct subdivisons of the ventral lateral geniculate nucleus. These regions also gave rise to projections to the superior colliculus, as did the intergeniculate leaflet. The pathways oriented toward the visual or frontal cortex and the projections possibly involved in limbic and circadian mechanisms were compared with the connectivity patterns reported in mammals with more differentiated brains. Particular attention was given to the tenrec's prominent tecto-geniculate projection, the presumed W- or K-pathway directed toward the supragranular layers.

Crossed thalamocortical connections in the Madagascan hedgehog tenrec: dissimilarities to erinaceous hedgehog, similarities to mammals with more differentiated brains

The adult erinaceous hedgehog, unlike other mammals, has recently been shown to have prominent crossed projections from the thalamus to the motor cortex. There are suggestions relating this unique pattern of connectivity to the overall degree of brain differentiation and/or the poorly developed corpus callosum. The present tracing study demonstrates that the Madagascan lesser hedgehog tenrec, with its tiny corpus callosum and one of the lowest neocorticalization indices among insectivores, has extensive crossed cortico-thalamic projections, but essentially the same sparse thalamic projections to the contralateral cortex as have placental mammals with more differentiated brains. The implications of the findings and the relevance of extracallosal pathways are discussed.