The cytoarchitecture of the claustrum in the hedgehog (Erinaceus europaeus) brain, the morphology of its neurons, and the efferent connections with cortical and subcortical structures were studied with the Nissl and Klüver-Barrera, the Golgi, and the horseradish peroxidase methods. It was found that the claustrum is a well developed nucleus in the hedgehog telencephalon and, as in other mammals, is divided into dorsal and ventral parts. In Golgi-stained sections, spiny multipolar cells are the predominant neurons of both the dorsal and the ventral claustrum and are projection neurons. Aspiny multipolar neurons with fewer, often beaded, dendrites constitute a minority in both divisions and are interneurons. Injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) in the prefrontal, motor, somatosensory, auditory and visual areas, and HRP or WGA-HRP injections in the thalamus showed that: (1) the claustroneocortical projections originate in the dorsal claustrum and are distributed to the entire neocortex; these projections are mainly ipsilateral but some also originate contralaterally; (2) the claustroneocortical projections show a rough topographic organization; there exists a substantial degree of overlap; and (3) the claustrothalamic projection, arising throughout the dorsal claustrum, is strictly ipsilateral. No evidence of a thalamoclaustral projection was found. The present results suggest that, although the hedgehog has been referred to as a "paleocortical mammal" owing to the great development of its rhinencephalic structures in comparison with its small neocortex, the dorsal claustrum is well developed and is connected with all neocortical areas as well as with the thalamus, establishing it as a key structure in the hedgehog forebrain.

The distribution pattern and the morphology of vasoactive intestinal polypeptide (VIP)- and cholecystokinin (CCK)-like-immunoreactive neurons were studied in the brain of the hedgehog and the sheep by means of the peroxidase-antiperoxidase immunocytochemical method. A total of 34 hedgehogs and 26 sheep of both sexes were used. Fourteen hedgehogs and 13 sheep received an intracerebroventricular injection of colchicine that enhanced the immunostaining and revealed "new" immunoreactive cell bodies. VIP-immunoreactive bipolar and multipolar neurons were observed in both species in the cerebral cortex, hippocampal formation, amygdaloid complex, hypothalamus, and central gray substance of the midbrain. CCK-immunoreactive bipolar, bitufted, and multipolar neurons displayed a broader distribution in both mammals than VIP neurons and were found in the cerebral cortex, the hippocampal formation, the amygdaloid complex, the hypothalamus, the mesencephalon, and the pons. In the cortex, in both the hedgehog and the sheep, VIP neurons were located in all layers but were concentrated in layers II and III, with the majority being typical bipolar. CCK neurons were more numerous in the superficial layers (I-III) but were found in the deep layers as well. They were bipolar, bitufted, or multipolar in morphology. From these neurons a small percentage, which were located almost exclusively in layers II and III of the visual cortex, exhibited also VIP immunoreactivity. Perikarya of such double-labeled cells were ovoid or round in shape with one or two main processes emanating from each pole of the cell body and oriented perpendicularly to the pia. The coexistence of the two peptides within individual neurons of the cortex has not been reported in other species and its physiological significance is discussed in relation to the GABAergic neurons of the cortex.

Using immunocytochemistry, we have examined the morphology and distribution of vasoactive intestinal polypeptide-like immunoreactive neurons in the visual cortex of albino rats whose ages were closely spaced in time between the first postnatal day and adulthood. In the adult, immunoreactive neurons were located in layers II to VI but were concentrated in layers II and III. All labelled neurons had the morphological characteristics of cortical non-pyramidal cells with the majority being of the bipolar variety as described in Golgi preparations. Some multipolar forms were also present. Vasoactive intestinal polypeptide-immunoreactivity appeared to develop in postnatal life. Labelled cells were first seen in layers V and VI at day 4. During the subsequent few days, some labelled cells were observed in the more superficial layers and by day 8 they were predominantly present in layers II and III. Although the distribution of immunoreactive cells at this time resembled that of adult animals, their morphology displayed immature features. The size and extent of their dendritic branching appeared to increase considerably during the second and third weeks and their morphological maturation was attained by the middle of the fourth postnatal week.

When cat visual cortex (area 17) is reacted with an antibody to vasoactive intestinal polypeptide (VIP) a variety of neuronal types is labelled. Many of the labelled neurons are bipolar in form and are most common in layers II and III, although significant numbers of bipolar neurons are also encountered in layer V. Multipolar cells are also labelled. These are most frequent in layer IV and have a variety of shapes. In layer I, the labelled cells are of three varieties, i.e. horizontal bipolar cells, horizontal bitufted cells and multipolar neurons, while in layer VI the few VIP-positive neurons are horizontal bipolar cells. This suggests that all of the VIP-labelled neurons in cat area 17 are non-pyramidal in form, and this has been confirmed by electron microscopy. In these preparations, axon terminals are also labelled and under the light microscope it can be seen that these terminals occur both within the neuropil and around the cell bodies of some neurons, particularly neurons in layers II and III. Electron microscopy has shown that all of the labelled axon terminals form symmetric synapses and that those in the neuropil synapse with the shafts of smooth dendrites. These axodendritic synapses account for about 90% of the synapses formed by the labelled axon terminals. The remainder of the labelled axon terminals synapse with the cell bodies of pyramidal neurons. Parallels are drawn between these results and those previously obtained by examining those neuronal elements labelled with VIP antibodies in rat visual cortex.

Using conventional immunocytochemical techniques, we have examined the morphology and distribution of somatostatin-like immunoreactive neurons in the visual cortex of albino rats between the first postnatal day and maturity. In the adult, somatostatin-immunoreactive neurons were observed in layers II to VI but were concentrated in layers II and III. These cells displayed morphological features characteristic of the multipolar and bitufted varieties of cortical non-pyramidal neurons as described in Golgi preparations of rat visual cortex. On the first postnatal day and in the subsequent few days, immunoreactivity was confined to immature bipolar and multipolar neurons concentrated in layers V and VI. Labelled cells first appeared in the more superficial layers at the beginning of the second postnatal week and attained a distribution similar to that observed in adult animals at the end of this week. At this time they closely resembled their adult counterparts from which they appeared indistinguishable by the end of the third postnatal week. The late appearance of labelled cells in the superficial layers, where they are predominantly located in adult animals, suggests that the somatostatin immunoreactivity exhibited by most of these neurons develops several days after they have completed their migration and assumed their positions in the visual cortex.