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

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.

Developmental gene expression in the mouse clarifies the organization of the claustrum and related endopiriform nuclei

Studies on gene expression in the developing claustrum of the mouse have clarified the relationships and identity of the claustrum proper and related endopiriform nuclei. The cells of the claustrum primordium express Nr4a2; they are formed in combination with the Nr4a2-labeled subplate cells in the lateral pallium at the site of the future insular cortex. The insular cortex cells, which are born later and which are Nr4a2-negative, migrate through the claustrum toward the pial surface to form layers (2-6a) of the insular cortex. The claustrum is made up of distinct deep (subplate-like) and superficial (principal) parts. The cells of the dorsal endopiriform nucleus (which are also Nr4a2-positive) are formed in the deep part of the claustrum primordium in the lateral pallium, but they migrate ventrally to reach the ventral pallium deep to the piriform cortex at E14.5 in the mouse. On the other hand, the ventral endopiriform nucleus is formed by radially migrating Nr4a2-negative cells in the ventral pallium; it is therefore developmentally distinct from the Nr4a2-postive dorsal endopiriform nucleus, which is a lateral pallial derivative. J. Comp. Neurol. 525:1499-1508, 2017. © 2016 Wiley Periodicals, Inc.

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 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.