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The existence of a neuronal-glial signalling through the activation of neurotransmitter receptors expressed in glia is well-documented. In excitatory synapses, glutamate released from presynaptic terminals activates not only postsynaptic receptors, but also ionotropic and metabotropic glutamate receptors localized in the glia ensheathing the synapses. The medial nucleus of the trapezoid body of the auditory system is involved in the localization of sounds in the space. In this nucleus, the large excitatory synaptic terminals formed by the calyces of Held on the principal globular cell bodies are wrapped by astrocytic processes. Since these synapses are functional from early postnatal days, glia receiving excitatory synaptic signals from the calyces may participate in modulating the maturation and development of the system. Groups I and II of metabotropic glutamate receptors (mGluRs) have been localized in glial cells in different brain regions. To investigate whether group II mGluRs are present in the medial nucleus of the trapezoid body, we have studied the pattern of expression of mGluR2/3 in the developing and mature nucleus by means of immunocytochemichal methods. The most remarkable finding was the switch in the occurrence of mGluR2/3 from glial to neuronal compartments. Thus, a preferential localization of mGluR2/3 immunoreactivity was observed in astrocytic processes surrounding the calyces of Held during the early postnatal development. In contrast, the main feature in adult rats was the presence of the group II mGluRs in presynaptic calyces of Held and postsynaptic principal globular cells. From these observations we suggest a role for group II mGluRs in neuronal-glial signalling in the calyx of Held-principal globular neuron synapses. Activation of these receptors might be relevant to the maturation and modulation of synaptic transmission in the medial nucleus of the trapezoid body.

A pre-embedding immunocytochemical method was used to study the subcellular distribution of the voltage-dependent potassium channel Kv3.1b in the medial nucleus of the trapezoid body (MNTB) in developing and adult rat. The main finding was the localization of the channel in specific membrane compartments of the calyces of Held and principal globular neurons. Thus, at postnatal day (P) 9 immunoparticles were densely localized in plasma membranes of globular cell bodies and their main dendrites. At P16, a strong Kv3.1b labeling was still observed in these globular cell compartments, but the most remarkable feature was the presence of immunoparticles in synaptic terminal membranes of the calyces of Held. However, the presynaptic and postsynaptic specializations of the calyx of Held–globular cell synapses were virtually devoid of immunoparticles. This same subcellular distribution of Kv3.1b was seen in adult, with membranes of calycine terminals more uniformly labeled. The developmental profile of Kv3.1b expression in MNTB coincides with the functional maturation of the calyx of Held–principal globular neuron synapse. The presence of the channel in this system is crucial for the high-frequency synaptic transmission of auditory signals.

The feasibility of excluded and everted small intestine segments as reservoirs for adrenal tissue transplantation was compared between biadrenalectomized adult rats with isogenic transplants of adrenal quarters or allogenic transplants of neonatal adrenal glands. Small clusters of adrenocortical cells, located between the adipocytes and the mesenteric vessels of the segments’ lumen, were observed following isogenic transplantation. Several large regenerated adrenocortical nodules, surrounded by a connective capsule, were present in each of the rats transplanted with neonatal adrenal glands. Medullary tissue did not regenerate. It was concluded that the excluded and everted segments of small intestine were optimal sites for adrenal tissue transplantation and that the regeneration process of the adrenocortical tissue takes place more quickly after the transplantation of neonatal adrenal glands than after the transplantation of adrenal quarters.

After WGA-HRP injections in the pontine grey involving the dorsolateral pontine nucleus, a great number of labeled cells were found in the superficial layers of the ipsilateral superior colliculus. The majority of these cells were located in the stratum griseum superficiale (SGS). Few labeled cells were found in the stratum opticum, and the stratum zonale (SZ) showed no labeled cells. Labeled cells in the SGS formed a rather homogeneous population as most of them had fusiform somata with an upper dendritic process which runs vertically to reach the SZ. These cells were mainly located in the middle third of the SGS, forming a sublamina in this layer. These results demonstrate the participation of the superficial tectal layers in the ipsilateral descending pathway of the superior colliculus, allowing visual information to reach precerebellar stations at the dorsolateral pontine nucleus.

This study examines the thalamo-cortical projections to the most superficial neocortical layers in the hedgehog (Erinaceus europaeus) after small injections of horseradish peroxidase and horseradish peroxidase conjugated to wheat germ agglutinin in the somato-sensory cortex. The injections were limited to layers I, II and upper parts of layer III/IV. Retrogradely labeled cells were plotted in serial sections through the thalamus. Injections in the somato-sensory cortex gave a pattern of elongated columns of labeled cells, extending rostro-caudally in the nucleus ventralis thalami. In the neocortex, labeled fibers extended for considerable distances running horizontally in layer I. Complementary observations demonstrate the thalamic origin of certain, coarse ascending bundles observed previously in Golgi preparations of the hedgehog. It is concluded that a major cortical input to layer I originates in the hedgehog in the principal thalamic (relay) nuclei. After injections in the somato-sensory cortex, retrogradely labeled cells were also found in the nucleus ventro-medialis thalami and very few in a zone medial to the nucleus ventralis thalami corresponding to the intralaminar thalamic nuclei. The contributions of this latter system seem to be limited in comparison with other mammals.