Marina Bentivoglio

The history of radial glia

Radial glial cells are now recognized as a transient population that serves as scaffolding for neuronal migration. The recognition of the existence and role of radial glia has not been smooth, and here we provide a brief historical overview on the pioneering studies on this subject. The histologists and embryologists Albert Kölliker and Wilhelm His performed seminal investigations on cortical morphogenesis in the last decades of the 19th century. However, the introduction of the silver impregnation Golgi technique, and its diffusion in the late 1880s, played a crucial role in the detection of radial glial processes. The radial arrangement of fibers emerging from the neuroepithelium lining the central canal was initially detected in the embryonic spinal cord by Camillo Golgi himself. The first Golgi impregnation of the cerebral cortex of mammalian fetuses was performed by Giuseppe Magini, who detected radial fibers extending from the ventricular neuroepithelium, and observed cells intercalated along these processes. Radial fibers, regarded as epithelial or ependymal processes, were then observed in the developing spinal cord and cerebral cortex by several investigators. Santiago Ramón y Cajal was the first to suggest that radial fibers were modified astrocytic processes functioning as a support during cortical histogenesis. Cajal acknowledged Maginis findings, but he criticized Maginis observations on the existence of neurons along radial fibers. With the advent of electron microscopy, the existence of radially arranged glial processes along which young neurons migrate was finally ascertained in the early 1970s by Pasko Rakic, thus opening a new era in the cellular and molecular biology of radial glia.

The thalamo-caudate versus thalamo-cortical projections as studied in the cat with fluorescent retrograde double labeling

SummaryThe distribution of thalamic cells projecting to the head of the caudate and their interrelations with thalamo-cortical cells were studied in the cat with different combinations of fluorescent tracers. Injections in the head of the caudate were combined with the injections in the pericruciate, proreal, suprasylvian, anterior cingulate, occipital and ectosylvian cortices. The following results were obtained: (i) Injections in the head of the caudate resulted in retrograde labeling of thalamic cells medially and laterally to the anteromedial (AM) nucleus, and in the medioventral part of the ventral anterior (VA) nucleus. Further, labeled cells were distributed throughout the anterior intralaminar central medial (CeM), paracentral (Pc) and central lateral (CL) nuclei, and the posterior intralaminar center median-parafascicular complex (CM-Pf). Labeled cells were mainly grouped in the mediodorsal parts of the anterior intralaminar nuclei; they were also found in the more dorsal part of the mediodorsal (MD) nucleus, ventral to the thalamic paraventricular (Pv) nucleus and to the habenular complex, (ii) Thalamo-cortical and thalamo-caudate cells overlapped in the medial part of the VA; in the anterior intralaminar nuclei they were either intermingled or were distributed in separate clusters or longitudinal bands. The two cell populations also overlapped in the posterior intralaminar complex. The greatest overlap occurred with the thalamic cell population projecting to the pericruciate cortex. (iii) Thalamic cells bifurcating to the head of the caudate and to the pericruciate cortex were found lateral to the AM, within the VA, and throughout the anterior intralaminar nuclei, especially in the CeM and in the posterior part of the CL; a few branched cells were also found in the CM. Thalamic cells bifurcating to caudate and anterior suprasylvian cortex were also found in the VA. Very few cells (scattered in the anterior thalamus lateral to the AM, as well as in the CeM, Pc and CL) were found to bifurcate to the head of the caudate and the other cortical fields here examined.

UPREGULATION OF SPINAL GLUTAMATE RECEPTORS IN CHRONIC PAIN

Recent studies indicate that glutamate binding to N-methyl-D-aspartate receptors in the spinal cord is involved in triggering the development of chronic pain However, the processes which directly underlie the increased pain remain unclear. Here we report that, following peripheral nerve injury (ligation of the sciatic nerve) in the rat, there is an increase in immunoreactive labelling of non-N-methyl-D-asparatate, AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate), glutamate receptors in the superficial laminae of the lumbar spinal cord ipsilateral to the ligation. The increase in AMPA receptor expression peaks 14 days after nerve ligation and decreases 35 days post-ligation, corresponding to the time-course of heightened sensitivity to mechanical and thermal noxious stimuli (hyperalgesia) induced by the ligation. Given evidence that AMPA receptors in the superficial laminae mediate fast nociceptive transmission in the spinal cord, our findings suggest that an upregulation of spinal AMPA receptors contributes to hyperalgesia following peripheral nerve injury.

C-fos mRNA is Spontaneously Induced in the Rat Brain During the Activity Period of the Circadian Cycle

The basal expression of the proto‐oncogene c‐fos was studied by Northern blot analysis in different regions of the rat brain during 24 h. A striking spontaneous oscillation of c‐fos mRNA expression was detected in animals kept in basal conditions with a 12 h light/12 h dark cycle. In these animals c‐fos mRNA was just detectable during the rest hours (morning through afternoon), and was high during the activity hours (night). The periodicity of this oscillation persisted and became free‐running when the animals were exposed for 6 consecutive days to constant light or darkness. It was thus demonstrated that the fluctuation of c‐fos expression is circadian and is not created by the light‐dark cycle, but the latter exerts a synchronizing effect. The oscillation of c‐fos mRNA was modified by manipulations of the rest‐activity cycle. In particular, the fluctuation observed in basal conditions was inverted, keeping the animals awake during the rest hours (diurnal) and allowing them to sleep in the activity period (nocturnal). These data indicated a close relationship between the oscillation of c‐fos expression and the rest‐activity cycle. Finally, electroencephalographic (EEG) monitoring was performed under behavioural control for 3 h before the animals were killed. These experiments confirmed that, irrespective of the time of day, the EEG pattern typical of a state of sleep (including both slow waves and paradoxical sleep) was associated with low or undetectable c‐fos levels, whereas the protracted EEG desynchronization corresponding to wakefulness was associated with high c‐fos expression. Altogether these results indicate that c‐fos mRNA undergoes during the circadian cycle spontaneous oscillations related to the activation of genomic expression subserving the behavioural and electrical correlates of wakefulness. On the other hand, the behavioural and electrical correlates of sleep are associated with low c‐fos expression.

The cortical projections of the thalamic intralaminar nuclei restudied by means of the HRP retrograde axonal transport.

Following multiple injections of HRP in different cortical areas in the cat, labeled cells were, in some cases, found in the thalamic intralaminar nuclei. The following cortical zones were found to constitute the preferential target for the projections from the respective intralaminar nuclei: motor and anterior suprasylvian areas for the nucleus centralis lateralis, cingulate cortex for the nuclei paracentralis and centralis medialis, sensory and motor areas for the nucleus centrum medianum. These data are compared with the results previously obtained by means of the retrograde degeneration technique.

The Thalamic Intralaminar Nuclei and the Cerebral Cortex

The use of the term “intralaminar nuclei” or “nuclei of the internal medullary lamina” dates back to Vogt (1909) and Friedemann (1911). According to its literal definition, the term “intralaminar nuclei” refers to the nuclear structures which lie within the internal medullary lamina of the thalamus. However, in its most common use, this term indicates the nuclear structures which are topographically related to three distinct regions of the thalamus of mammals. The first, the central medial nucleus (CeM), is located at the midline between the internal medullary laminae of the two sides. The second is located laterally in the anterior part of each internal medullary lamina, and includes the paracentral (Pc) and central lateral (CL) nuclei. The third expands posteriorly in a splitting of the internal medullary lamina and includes the posterior intralaminar, centre median (CM), and parafascicular (Pf) nuclei, which are commonly designated as the CM-Pf complex in the species in which they can both be identified (see Section 2).

The cortical projections of the thalamic intralaminar nuclei, as studied in cat and rat with the multiple fluorescent retrograde tracing technique.

Two retrograde fluorescent tracers were injected in two different areas of the cerebral cortex in rats and in cats. In all the experiments many single labeled cells and only some double labeled ones were seen in the thalamic intralaminar nuclei. The present results suggest that the diffusely distributed intralaminar-cortical projections mainly consist of axons of separate cells, and only to a minor extent of axon collaterals of the same cells.

The specificity of the nonspecific thalamus: The midline nuclei

Publisher Summary The concept of a nonspecific thalamus has been substantially revised in the past several years, so that it is composed of a heterogeneous collection of individually signatured cell groups. The term “nonspecific” has been dismissed, but it has left behind a great deal of confusion and the embarrassing impression that the nonspecific thalamus has not found a satisfactory anatomical identity. This chapter reviews the midline thalamic region, which is part of this complex territory. It aims to define the midline thalamus as a collection of cell groups whose structural features and circuits differ greatly from those of adjacent structures and from all other portions of the “nonspecific” thalamus. The chapter discusses midline in relation to the intralaminar thalamus because these two regions have been unified historically in the same entity, and they are still often considered as a single group. This is due to the common belief that midline and intralaminar nuclei share functional features, based on their preferential innervation from the brain stem core and their robust connections with the basal ganglia.

African trypanosome infections of the nervous system: Parasite entry and effects on sleep and synaptic functions.

The extracellular parasite Trypanosoma brucei causes human African trypanosomiasis (HAT), also known as sleeping sickness. Trypanosomes are transmitted by tsetse flies and HAT occurs in foci in sub-Saharan Africa. The disease, which is invariably lethal if untreated, evolves in a first hemo-lymphatic stage, progressing to a second meningo-encephalitic stage when the parasites cross the blood-brain barrier. At first, trypanosomes are restricted to circumventricular organs and choroid plexus in the brain outside the blood-brain barrier, and to dorsal root ganglia. Later, parasites cross the blood-brain barrier at post-capillary venules, through a multi-step process similar to that of lymphocytes. Accumulation of parasites in the brain is regulated by cytokines and chemokines. Trypanosomes can alter neuronal function and the most prominent manifestation is represented by sleep alterations. These are characterized, in HAT and experimental rodent infections, by disruption of the sleep-wake 24h cycle and internal sleep structure. Trypanosome infections alter also some, but not all, other endogenous biological rhythms. A number of neural pathways and molecules may be involved in such effects. Trypanosomes secrete prostaglandins including the somnogenic PGD2, and they interact with the hosts immune system to cause release of pro-inflammatory cytokines. From the sites of early localization of parasites in the brain and meninges, such molecules could affect adjacent brain areas implicated in sleep-wakefulness regulation, including the suprachiasmatic nucleus and its downstream targets, to cause the changes characteristic of the disease. This raises challenging issues on the effects of cytokines on synaptic functions potentially involved in sleep-wakefulness alterations.

Postnatal development of calbindin and parvalbumin immunoreactivity in the thalamus of the rat.

The maturation of the calcium binding proteins calbindin-D28k (CB) and parvalbumin (PV) during the first 3 postnatal weeks was studied in the rat thalamus using immunohistochemistry. These two proteins display a non-homogeneous distribution in the adult thalamus. In the rat, CB is mainly localized in the neurons and neuropil of the thalamic midline, intralaminar, and ventromedial nuclei, as well as in the posterior complex. At birth, CB-immunoreactive cell bodies were evident in thalamic midline structures, and especially in the nucleus reuniens. The number of thalamic CB-positive cell bodies, as well as the intensity of the neuropil immunostaining, increased progressively in the first postnatal weeks. This quantitative increase was first apparent in the midline structures and then in the other thalamic territories which are CB-positive in adulthood, and followed a mediolateral gradient. The mature pattern was achieved by the end of the third postnatal week. In the adult rat thalamus the neurons of the reticular nucleus display PV-immunostaining and PV-positive fibers densely innervate most of the dorsal thalamic domains. PV-immunoreactivity was clearly evident at birth in the cell bodies of the reticular nucleus. The density of PV-containing fibers increased progressively after birth in the dorsal thalamus, with a lateromedial gradient. At the end of the third postnatal week the ventroposterior (VP) complex appeared heavily innervated by PV-positive fibers, whose density in more medial structures was still lower than in the adult thalamus. A transient hyperinnervation of PV-immunoreactive fibers, displaying a dishomogenous organization in distinct segments, was observed in VP, and especially in the ventroposteromedial nucleus, during the second postnatal week. Altogether these findings indicate that the maturation of CB and PV requires postnatally a relatively prolonged period of time. The possible involvement of these proteins in different functional aspects of thalamic neuronal maturation is discussed.