Numbers of neurons in the developing principal sensory nucleus of the trigeminal nerve: enhanced survival of early-generated neurons over late-generated neurons

Lamina specific postnatal development of PKCγ interneurons within the rat medullary dorsal horn

Protein kinase C gamma (PKCγ) interneurons, located in the superficial spinal (SDH) and medullary dorsal horns (MDH), have been shown to play a critical role in cutaneous mechanical hypersensitivity. However, a thorough characterization of their development in the MDH is lacking. Here, it is shown that the number of PKCγ-ir interneurons changes from postnatal day 3 (P3) to P60 (adult) and such developmental changes differ according to laminae. PKCγ-ir interneurons are already present at P3-5 in laminae I, IIo, and III. In lamina III, they then decrease from P11-P15 to P60. Interestingly, PKCγ-ir interneurons appear only at P6 in lamina IIi, and they conversely increase to reach adult levels at P11-15. Analysis of neurogenesis using bromodeoxyuridine (BrdU) does not detect any PKCγ-BrdU double-labeling in lamina IIi. Quantification of the neuronal marker, NeuN, reveals a sharp neuronal decline (∼50%) within all superficial MDH laminae during early development (P3-15), suggesting that developmental changes in PKCγ-ir interneurons are independent from those of other neurons. Finally, neonatal capsaicin treatment, which produces a permanent loss of most unmyelinated afferent fibers, has no effect on the development of PKCγ-ir interneurons. Together, the results show that: (i) the expression of PKCγ-ir interneurons in MDH is developmentally regulated with a critical period at P11-P15, (ii) PKCγ-ir interneurons are developmentally heterogeneous, (iii) lamina IIi PKCγ-ir interneurons appear less vulnerable to cell death, and (iv) postnatal maturation of PKCγ-ir interneurons is due to neither neurogenesis, nor neuronal migration, and is independent of C-fiber development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 102-119, 2017.

In vivo magnetic resonance imaging at 11.7 Tesla visualized the effects of neonatal transection of infraorbital nerve upon primary and secondary trigeminal pathways in rats

Using 11.7T ultra high-field T2-weighted MRI, the present study aimed to investigate pathological changes of primary and secondary trigeminal pathways following neonatal transection of infraorbital nerve in rats. The trigeminal pathways consist of spinal trigeminal tract, trigeminal sensory nuclear complex, medial lemniscus, ventromedial portion of external medullary lamina and ventral posterior nucleus of thalamus. By selecting optimum parameters of MRI such as repetition time, echo time, and slice orientation, this study visualized the trigeminal pathways in rats without any contrast agents. Pathological changes due to the nerve transection were found at 8 weeks of age as a marked reduction of the areas of the trigeminal pathways connecting from the injured nerve. In addition, T2-weighted MR images of the trigeminal nerve trunk and the spinal trigeminal tract suggest a communication of CSF through the trigeminal nerve between the inside and outside of the brain stem. These results support the utility of ultra high-field MRI system for noninvasive assessment of effects of trigeminal nerve injury upon the trigeminal pathways.

Postnatal generation of neurons in the ventrobasal nucleus of the rat thalamus

Most CNS systems, including the trigeminal-somatosensory system, develop via a hierarchical order (from the periphery and up the neuraxis). We tested the hypothesis that development of the trigeminal system can proceed via a nonhierarchical mechanism (i.e., that neuronogenesis can occur postnatally). Preweanling rats were perfused, and brain sections were stained with cresyl violet or immunolabeled with NeuN (for neuronal counts), or processed for acetylcholinesterase (AChE) activity or p75 immunoreactivity [to identify boundaries of the ventrobasal nucleus (VB)]. Neuronal number decreased during the first postnatal week but increased 2.5-fold over the next 3 weeks. To determine whether this remarkable rise resulted from the generation of new neurons, preweanlings were given injections of bromodeoxyuridine (BrdU) on postnatal day 6 (P6) or P21. BrdU-positive VB cells were apparent on both days. Cumulative BrdU labeling showed that the cell cycle was 17.3 h on P6. Moreover, Ki-67, a protein elaborated throughout the cell cycle, was expressed by 25.8-29.3% of all VB cells on P6-P15, falling to 7.7% by P21. BrdU-positive VB cells coexpressed neuronal markers: NeuN, HuC/D, microtubule-associated protein 2, and a dextran placed in the somatosensory cortex. Note that postnatal neuronal generation was also evident in other thalamic nuclei (e.g., the lateral geniculate nucleus). Thus, the developing VB experiences two periods of neuronal generation. Prenatal neuronogenesis is part of hierarchical trigeminal-somatosensory development. Postnatal nonhierarchical neuronogenesis is intrathalamic and matches changes in neuromodulatory systems (exemplified by AChE activity and p75) and the arrival of corticothalamic afferents.

Emergence of intrinsic bursting in trigeminal sensory neurons parallels the acquisition of mastication in weanling rats

There is increasing evidence that a subpopulation of neurons in the dorsal principal sensory trigeminal nucleus are not simple sensory relays to the thalamus but may form the core of the central pattern generating circuits responsible for mastication. In this paper, we used whole cell patch recordings in brain stem slices of young rats to show that these neurons have intrinsic bursting abilities that persist in absence of extracellular Ca(2+). Application of different K(+) channel blockers affected duration and firing rate of bursts, but left bursting ability intact. Bursting was voltage dependent and was abolished by low concentrations of Na(+) channel blockers. The proportion of bursting neurons increased dramatically in the second postnatal week, in parallel with profound changes in several electrophysiological properties. This is the period in which masticatory movements appear and mature. Bursting was associated with the development of an afterdepolarization that depend on maturation of a persistent sodium conductance (I(NaP)). An interesting finding was that the occurrence of bursting and the magnitude of I(NaP) were both modulated by the extracellular concentration of Ca(2+). Lowering extracellular [Ca(2+)] increased both I(NaP) and probability of bursting. We suggest that these mechanisms underlie burst generation in mastication and that similar processes may be found in other motor pattern generators.

Effect of hypoxia on the expression of pro- and anti-apoptotic proteins in neuronal nuclei of the guinea pig fetus during gestation

The present study investigates the expression of apoptotic proteins Bax, Bad, Bcl-2, and Bcl-xl following hypoxia in the cerebral cortex of the guinea pig fetus as a function of gestational age. Normoxic (Nx, n = 6) and hypoxic (Hx, n = 6) guinea pig fetuses at 35 and 60 days gestation were studied. Bax expression (OD X mm(2)) was 96.9 +/- 9.5 (Nx 35 days), 116.5 +/- 8.3 (Hx 35 days), P < 0.05 and 116.2 +/- 3.4 (Nx 60 days, 144.6 +/- 11.7 (Hx 60 days), P < 0.05. Bad expression (OD X mm(2)) was 78.6 +/- 2.6 (Nx 35 days), 102.9 +/- 5.8 (Hx 35 days), P < 0.05 and 101.5 +/- 4.3 (Nx 60 days), 139.8 +/- 7.9 (Hx 60 days), P < 0.05 vs. Nx 60 days, also significantly higher from preterm hypoxia P < 0.007. Expression of Bcl-2 (OD X mm(2)) was 27.4 +/- 2.0 (Nx 35 days), 28.0 +/- 2.4 (Hx 35 days), and 27.4 +/- 2.7 (Nx 60 days), 29.7 +/- 2.3 (Hx 60 days). Expression of Bcl-xl (OD X mm(2)) was 51.0 +/- 4.4 (Nx 35 days), 46.1 +/- 8.0 (Hx 35 days) and 50.0 +/- 1.4 (Nx 60 days), 54.9 +/- 7.4 (Hx 60 days). Hypoxia resulted in increased expression of the proapoptotic proteins Bax and Bad by 20% and 30% in the preterm as compared to 24% and 38% at term, without altering the expression of anti-apoptotic proteins Bcl-2 and Bcl-xl. We conclude that the hypoxia-induced increased expression of Bax and Bad is greater at term compared to preterm. Furthermore, the hypoxia-induced increase in proapoptotic as compared to antiapoptotic proteins at term will accelerate the ongoing active process of programmed cell death at term compared to preterm gestation.

Optical mapping of the functional organization of the rat trigeminal nucleus: initial expression and spatiotemporal dynamics of sensory information transfer during embryogenesis

We examined the functional organization of the rat trigeminal nuclear complex and its developmental dynamics using a multiple-site optical recording technique. Brainstem preparations were dissected from embryonic day 12 (E12)-E16 rat embryos, and stimulation was applied individually to the three branches of the trigeminal nerve (V1-V3). The action potential activity of presynaptic fibers was detected from E13, and the glutamate-mediated postsynaptic response was significantly observed from E15 on. At E14, the evoked signals usually consisted of only the action potential-related fast component. However, when extracellular Mg2+ was removed, a significant dl-2-amino-5-phosphonovaleric acid-sensitive slow component appeared. These results suggest that postsynaptic function mediated by NMDA receptors is latently generated as early as E14. The response area of the three branches of the trigeminal nerve showed some functional somatotopic organization, with the ophthalmic (V1) nerve area medially located and the mandibular (V3) nerve area laterally located. The center of the trigeminal nuclear complex in which the activity of neurons and synaptic function was greatest shifted caudally with development, suggesting that the functional architecture of the trigeminal nuclear complex is not fixed but changes dynamically during embryogenesis. By electron microscopy, we could not observe clear correlations between functional data and morphological information; when we surveyed E16 preparations, we could not identify typical synaptic structures between the 1,1'-dioctyldecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate-labeled trigeminal nerve terminals and the neurons in the trigeminal nuclear complex. This implies that postsynaptic function in the trigeminal nuclear complex is generated before the appearance of the morphological structure of conventional synapses.

Changes in cortical and cerebellar bcl-2 mRNA levels in the developing hydrocephalic rat (LEW-HYR) as measured by a real time quantified RT-PCR

In order to evaluate the appearance of brain bcl-2 during development of the hydrocephalus, we measured levels of bcl-2 mRNA in the cortex and cerebellum of congenital hydrocephalic rats (LEW-HYR) at 1 and 2 weeks after birth using the quantified reverse transcriptase-polymerase chain reaction (RT-PCR) with TaqMan fluorogenic detection system. Normal and hydrocephalic siblings were killed 7 and 14 days after birth, and their cortices and cerebella were homogenized with the Isogen-chloroform mixtured solution. By means of the RT-PCR with genetic analyzer, the sequence of bcl-2 mRNA detected in the LEW-HYR was identified to be the same as that of the registered rat brain (L14680). During the development of normal siblings of LEW-HYR, the levels of bcl-2 mRNA detected in the cortex and cerebellum 7 days after birth were significantly higher than those seen on day 14 after birth. In the hydrocephalic rats, however, these levels were not significantly different during development. On days 7 and 14 after birth, the cortical levels of bcl-2 mRNA detected in the hydrocephalic rats were significantly higher than those in normal rats. In the cerebellum, these levels in the hydrocephalic rats were higher, but not significantly, than those of normal rats. These results indicate that the significant appearance of bcl-2 mRNA in the developing normal rat brain is related to sprouting and to the diminished number of neurons, whereas the significant increase of bcl-2 levels seen in the developing hydrocephalic rats is indicative of an excess activity of glutamate neurons in cerebral cortex and the protection of neurons from cell death induced by cerebral ventricular dilatation in the cortex after bcl-2 levels.

Vulnerability of the developing brain. Neuronal mechanisms

Despite the improved survival of tiny preterm neonates, their neurodevelopmental outcomes remain a cause for grave concern. The authors propose two primary mechanisms leading to enhanced neuronal cell death in the immature brain: (1) NMDA-mediated excitotoxicity resulting from repetitive or prolonged pain, and (2) enhanced naturally occurring neuronal apoptosis during early development due to multiple metabolic stresses or lack of social stimulation. The pattern and magnitude of abnormalities will depend on genetic variability as well as the timing, intensity, and duration of adverse environmental experiences. Thus, cumulative brain damage during infancy will finally lead to reductions in brain volume, abnormal behavioral and neuroendocrine regulation, and poor cognitive outcomes during childhood and adolescence. The public health and economic importance of preventing or ameliorating the subtle brain damage caused by these mechanisms cannot be overestimated. This certainly justifies concerted efforts by neuroscientists and clinicians to investigate the mechanisms underlying early neuronal injury, to minimize the impact of adverse experiences and environmental factors in neonates, and to develop novel therapeutic strategies for improving the cognitive and behavioral outcomes of ex-preterm neonates.

L-type calcium channel-mediated plateau potentials in barrelette cells during structural plasticity

Development and maintenance of whisker-specific patterns along the rodent trigeminal pathway depends on an intact sensory periphery during the sensitive/critical period in development. Barrelette cells of the brain stem trigeminal nuclei are the first set of neurons to develop whisker-specific patterns. Those in the principal sensory nucleus (PrV) relay these patterns to the ventrobasal thalamus, and consequently, to the somatosensory cortex. Thus PrV barrelette cells are among the first group of central neurons susceptible to the effects of peripheral damage. Previously we showed that membrane properties of barrelette cells are distinct as early as postnatal day 1 (PND 1) and remain unchanged following peripheral denervation in newborn rat pups (Lo and Erzurumlu 2001). In the present study, we investigated the changes in synaptic transmission. In barrelette cells of normal PND 1 rats, weak stimulation of the trigeminal tract (TrV) that was subthreshold for inducing Na(+) spikes evoked an excitatory postsynaptic potential-inhibitory postsynaptic potential (EPSP-IPSP) sequence that was similar to the responses seen in older rats (Lo et al. 1999). Infraorbital nerve transection at birth did not alter excitatory and inhibitory synaptic connections of the barrelette cells. These observations suggested that local neuronal circuits are already established in PrV at birth and remain intact after deafferentation. Strong stimulation of the TrV induced a sustained depolarization (plateau potential) in denervated but not in normal barrelette neurons. The plateau potential was distinct from the EPSP-IPSP sequence by 1) a sustained (>80 ms) depolarization above -40 mV; 2) a slow decline slope (<0.1 mV/ms); 3) partially or totally inactivated Na(+) spikes on the plateau; and 4) a termination by a steep decay (>1 mV/ms) to a hyperpolarizing membrane level. The plateau potential was mediated by L-type Ca(2+) channels and triggered by a N-methyl-D-aspartate (NMDA) receptor-mediated EPSP. gamma-aminobutyric acid-A (GABA(A)) receptor-mediated IPSP dynamically regulated the latency and duration of the plateau potential. These results indicate that after neonatal peripheral damage, central trigeminal inputs cause a large and long-lasting Ca(2+) influx through L-type Ca(2+) channels in barrelette neurons. Increased Ca(2+) entry may play a key role in injury-induced structural remodeling, and/or transsynaptic cell death.

Expression of bcl-2, bax, and caspase-3 in the brain of the developing rat

Naturally occurring neuronal death (NOND) is generally considered to be apoptotic. Apoptosis is an active form of cell death in which the regulation of specific proteins produces anti- or pro-apoptotic signals. Two of the protein families involved in this regulation are the bcl proteins and caspases. A quantitative immunoblotting technique was used to examine the temporal expression of bcl-2, bax, and two isoforms of caspase 3 (an active 20 kDa isoform and the inactive 32 kDa precursor) throughout the developing neuraxis. Long-Evans rat fetuses were collected on gestational day (G) 16 and G19, and pups were harvested on postnatal day (P) 0, P3, P6, P12, P21, and P30. Brains were divided into five segments: cortex, thalamus, midbrain, medulla/pons, and cerebellum. In general, the expression of bax increased and the ratio of bcl-2 expression to bax expression decreased concurrent with published data on the onset of NOND in a given area. The timing of these events was paralleled by an increase in the expression of active caspase 3. Unlike the bcl proteins, caspase 3 expression returned toward fetal levels as the brain matured. The timing of the changes in bcl protein and caspase expression show that both protein families are involved in promoting neuronal death. Reductions in caspase expression (and not bcl-2 and bax expression) are key to ending the period of NOND.

Neurotrophin receptor (p75) in the trigeminal thalamus of the rat: development, response to injury, transient vibrissa-related patterning, and retrograde transport

We report on the transient, patterned expression of p75 in the ventrobasal (VB) thalamus, the major thalamic relay for somatosensation. We immunostained the brains of developing rats ranging in age from embryonic day (E) 14.5 to postnatal day (PD) 15 with an antibody against p75. To compare p75 expression with the developing synaptic organization within VB, we also immunolocalized the synaptic-vesicle-associated protein, synaptophysin (SYN), on alternate sections. p75-immunoreactivity (IR) was dense and uniform in the ventroposterior medial nucleus (VPM) in the late embryonic and early postnatal periods (E 16.5 to PD 3). In contrast, from PD 4-10, p75-IR in the VPM was patterned, reminiscent of cytochrome-oxidase-stained barreloids, a characteristic feature of the VB in rodents. By PD 14, p75-IR in the VPM was no longer detectable. The ventroposterior lateral nucleus (VPL), in contrast, exhibited no p75-IR. No p75-IR was detected in the ventroposterior lateral nucleus (VPL) at any developmental stage in which VPM could be distinguished from VPL. Light, but clearly patterned SYN-IR, first detectable on PD 2-3, increased in intensity in both VPL and VPM through PD 15. Sectioning the infraorbital nerve on PD 0 resulted in blurred patterns of p75- and SYN-IR within VPM in PD 7-9 rat pups. Removing large portions of the somatosensory cortex on PD 0 resulted in subsequent greatly reduced p75- and SYN-IR within VB. To specify the source of the p75-IR terminals, we stereotaxically injected into the VPM of PD 4-5 rats a monoclonal antibody to p75. One to 2 days later, IR of retrogradely transported p75 antibodies could be traced within axons and cell bodies of neurons associated with the trigeminothalamic pathway through the caudal diencephalon and mesencephalon; labelling was confined to the contralateral trigeminal principal sensory nucleus. The observed, transiently patterned p75-IR in VPM the early postpartum period suggests a role for p75 in synaptogenesis and pattern formation.

Apoptotic cascade of neurons in the subcortical sensory relay nuclei following the neonatal infraorbital nerve transection

A terminal transferase-mediated dUTP nick end labeling (TUNEL) method was utilized for detection of neuronal death in the subcortical relay nuclei of the trigeminosensory system following the infraorbital nerve transection in newborn rats. At 18-24 h after injury, numerous TUNEL-positive profiles were found within the ventroposteromedial thalamic nucleus (VPM) contralateral to the injury, whereas the VPM on the ipsilateral side and of the age-matched normal control contained only a few profiles per section. Electron microscopy revealed that the TUNEL-positive profiles were apoptotic neurons. The ventral part of the ipsilateral brainstem sensory trigeminal nuclear complex (the nucleus principalis, and the subnuclei oralis and interpolaris) exhibited statistically significant 65-70% increase in number of apoptotic neurons compared to the contralateral side. Taken together with our previous study [T. Sugimoto, C. Xiao, H. Ichikawa, Neonatal primary neuronal death induced by capsaicin and axotomy involves an apoptotic mechanism, Brain Res. 807 (1998) 147-154], the present results demonstrated a cascade of apoptosis in the primary, secondary and tertiary order sensory neurons along the neuroaxis.

Expression of p53 and ALZ-50 immunoreactivity in rat cortex: effect of prenatal exposure to ethanol

Neuronal death is an active process that results in the upregulation of antigens recognized by ALZ-50 and p53. Since prenatal exposure to ethanol can induce the postnatal death of cortical neurons, we examined the effects of ethanol on the in vivo expression of both the ALZ-50-positive antigen and p53. Pregnant rats were fed one of three diets, a liquid diet containing ethanol (Et), an isocaloric and isonutritive diet (Ct), or chow and water (Ch). Segments of frontoparietal cortex from fetuses and pups were examined for ethanol-induced changes (a) in the expression of ALZ-50 and p53 immunoreactivity using a quantitative immunoblotting assay and (b) in the distribution of ALZ-50- and p53-positive cells using immunohistochemistry. In control rats, ALZ-50 identified a 56-kDa peptide that was transiently expressed postnatally and peak expression occurred on postnatal day (P) 6 to P12. In Et-treated rats, peak expression was attained earlier (on P3) and was about three times of that achieved in the controls. The anti-p53 antibody identified three proteins (28, 56, and 58 kDa). Peak expression in control rats occurred during the second postnatal week and only the 58-kDa protein was expressed in appreciable amounts in adult cortex. Each p53-positive protein was affected by ethanol exposure. The 28- and 56-kDa p53-positive proteins were affected by ethanol much in the same way as was the ALZ-50-positive antigen. That is, the timing and amount of peak expression were earlier and lower, respectively, in the Et-treated rats. The postnatal expression of the 58-kDa protein was halved following prenatal exposure to ethanol. Both ALZ-50 and anti-p53 immunoprecipitated proteins are p53- and ALZ-50-positive, respectively. Thus, ethanol alters the expression of the ALZ-50- and p53-positive proteins and presumably the timing of neuronal death in the developing cortex. The parallel effects of prenatal ethanol exposure on the 56-kDa ALZ-50-positive antigen and the 28- and 56-kDa p53-positive proteins and the coprecipitation of the proteins are consistent with the notion that ALZ-50 recognizes a form of p53.

Neuronal changes in the basolateral complex during development of the amygdala of the rat

Neuronal changes in the amygdala basolateral complex were studied during development and maturation in fetal and postnatal rat brains using morphometrical methods. Forty brains of animals of various ages were fixed in formalin, frozen and cut into 25 microm thick sections and stained with cresyl violet or haematoxylin and eosin (H&E). In cresyl violet preparations, the complex appeared for the first time on embryonic day (E)17 and was composed of two homogeneous nuclei lateral and basolateral. On about the seventh postnatal day, each of these nuclei was divided into two parts the first one into the dorsolateral and ventromedial and the second one into the anterior and posterior. Morphometric investigations showed a different increase of the neuronal and nuclear size in various parts of the basolateral complex up to postnatal day (P)14; after that time these parameters did not change significantly. The neuronal density and the total number of neurons stabilized at P7 in all parts of this complex, except for the dorsolateral part of the lateral nucleus in which a 30% decrease of the total number of cells was observed. From P14, in all nuclei under study, the total number of neurons did not change significantly.

Neonatal transection of the infraorbital nerve increases the expression of proteins related to neuronal death in the principal sensory nucleus of the trigeminal nerve

Neonatal lesion of the primary afferents in the infraorbital nerve causes the death of one-third of the neurons in the second-order target, the principal sensory nucleus of the trigeminal nerve (PSN). We examined the expression of two candidate 'death' proteins, p53 and the antigen recognized by the antibody ALZ-50, in the normal and deafferented PSN. In addition, the effect of neonatal transection of the infraorbital nerve (a major component of the trigeminal nerve) on protein expression was examined. The expression of c-fos in the developing PSN was also studied as an index of metabolic activity. Protein expression was measured using quantitative analyses of immunoblots and immunohistochemical preparations. The expression of p53- and ALZ-50-immunoreactivity in the normal PSN peaked during the first postnatal week. Transection of the infraorbital nerve directly affected the expression of p53 and the ALZ-50-positive antigen. The immunoblots showed that whereas p53 amounts were unaffected by the lesion, ALZ-50 expression was significantly upregulated in the ipsilateral PSN 2 h and 2 days postlesion. The density of p53- and ALZ-50-immunoreactive neurons was significantly higher in the ventral ipsilateral PSN (i.e., the target of the transected infraorbital nerve) than in the contralateral PSN. c-fos expression selectively and transiently rose in the ventral ipsilateral PSN within 2 h of the lesion. Thus, both p53 and the ALZ-50-positive antigen are involved in neuronal death. In light of data suggesting that ALZ-50 recognizes a phosphorylated form of p53, we conclude that neuronal death in the developing nervous system involves the post-translational modification of an existing protein, p53. The increase in ALZ-50 expression apparently occurs during a catabolic phase of neuronal death, as indicated by the increase in c-fos expression.

Development of trigeminal nucleus principalis in the rat: effects of target removal at birth

Little is known about how neurons develop in the trigeminal nucleus principalis (PrV) despite their acknowledged role in establishing whisker-related patterns in the thalamus and cortex. Golgi-impregnated PrV cells were studied in newborn, 4-day-old and adult rats. Adult neurons typically had short dendrites that were confined to a hemisphere around the soma. In contrast, at birth PrV neurons had radial trees and more primary dendrites than did adults, but adult-like numbers of dendritic spines. By day 4, most neurons had eccentric dendritic trees and the numbers of primary dendrites per neuron were adult-like, yet spines were more prevalent than in adults and newborns. Thus, it appears that there is a pruning of the dendritic tree during the first postnatal week. To assess the role of retrograde signals from the thalamus on PrV development, the right thalamus was destroyed at birth. By postnatal day 6, the number of neurons in the left PrV was 59% of that in the right PrV, PrV transverse area was reduced by 21%, cell density was reduced by 48%, and somatic diameter was increased by 36%, relative to the intact right PrV. By contrast, in the left V subnucleus interpolaris, which has only a weak thalamic projection, these measures were unaffected. Thus, neonatal thalamic lesions selectively depopulated the PrV. The morphology of PrV neurons was affected by the thalamic lesions: e.g. the total dendritic length, the number of dendritic branch points and the total number of spines were increased. The number of primary dendrites and the tree's eccentricity, area, and volume of influence were unaffected by the lesion. The structure of neurons in subnucleus interpolaris was unaffected by the lesion. Thus, normal afferent patterning is insufficient for normal development of PrV cells. Interactions among dendrites and retrograde signals from a target are also important.

Genesis and fate of the perireticular thalamic nucleus during early development

A striking feature of the internal capsule during early development is that it is full of small neurones. Later, this group of neurones, called the perireticular thalamic nucleus, appears to have reduced in size, and only a few scattered cells are seen. In an effort to understand better the developmental history of the perireticular nucleus this study examines: i) the period of cell generation in the nucleus, ii) the magnitude of cell loss in the nucleus, and iii) the subsequent fate of cells in the nucleus during development. The perireticular cells are generated very early in development, being among the first generated in the thalamus (rats: E13-14; cats: E21-30). In rats, the first perireticular cells are generated at about the same developmental stage as the first subplate cells, which are among the first generated cells of the cortex: in cats, the first perireticular cells are generated well before those in the subplate (E24-30). In rats, the number of perireticular cells during developmental peaks at P5 (approximately 30,000) and then declines sharply (approximately 98%) by P15 (approximately 750), when adult-like patterns are seen. This dramatic loss of perireticular cells is due to both cell death and a migration of cells into the adjacent globus pallidus. The majority of the perireticular cells which migrate into the globus pallidus, however, are likely to die also. The presence of pyknotic profiles (indicators of dying cells) in the rat perireticular nucleus points to cell death as a contributor to the reduction in cell number during development. In this study, a period of relatively high pyknotic profile incidence (number of pyknotic cells per 1,000 "living" cells) is recorded in the perireticular nucleus over a 5 day period, from P2 to P7 (13.5-15.5). Similar values and patterns are recorded in the reticular nucleus and globus pallidus, except that in these structures, a period of relatively high pyknotic profile incidence (15-20) occurs over a shorter period (3 days; P2-5). Previous studies have suggested that some perireticular cells migrate into and settle within the adjacent globus pallidus. This study, with the use of long-term survivals after tracer injections in rats, shows that none (or very few) of these perireticular cells which migrate into the globus pallidus survive into more mature postnatal stages. Tracer (biotinylated dextran) was injected into the sensory nuclei of the dorsal thalamus at early stages (P7) and the rats were allowed to survive for either a day thereafter (to P8) or until well after the period of cell death was complete (to P16 or P21). In the short-term survivals (to P8), there are many dextran-labelled cells seen in the globus pallidus and in the perireticular nucleus. In the long-term survivals (to P16 or P21), by contrast, there are no dextran-labelled cells apparent in the globus pallidus or in the perireticular nucleus. It is likely that these cells in the globus pallidus, as with those in the perireticular nucleus, undergo cell death during development.

Region- and stage-specific patterns of melanocortin receptor ontogeny in rat central nervous system, cranial nerve ganglia and sympathetic ganglia

Observations on developmental actions of melanotropic peptides in nervous system have been difficult to interpret in the absence of data on receptor ontogeny. We investigated binding of [125I]Nle4,D-Phe7-alpha-MSH ([125I]NDP) in developing Long Evans rats from gestational day (E) 13 by quantitative autoradiography. Regional [125I]NDP binding characteristics were assessed by competition experiments in early postnatal brain. The study revealed region- and stage-specific, often transient ontogenetic patterns. Sympathetic ganglia exhibit high [125I]NDP binding from E13, with a peak in superior cervical ganglion at E16-E18. The first central [125I]NDP binding sites transiently appear in parts of thalamus between E13 and E15. The early fetal period is characterized by prominent peaks of receptor density in somatosensory and viscerosensory nuclei (trigeminal sensory nuclei, solitary tract nucleus), paralleled by receptor expression in 5th, 7th, 9th and 10th cranial nerve ganglia. During late fetal life, receptor density peaks in dorsal motor nucleus of vagus and inferior olive; binding sites transiently appear in cerebellum. Caudate-putamen, nucleus accumbens, olfactory tubercle and septohippocampal nucleus show a high perinatal maximum. Starting with late fetal piriform cortex, [125I]NDP binding peaks sequentially in cerebral cortical areas, with highest levels in entorhinal cortex. Preoptic, septal, hypothalamic and amygdaloid areas known for elevated receptor densities in adulthood, exhibit a slow, peri- and postnatal receptor ontogeny. Temporal relations to regional developmental processes support the idea of a role of melanocortins during ontogeny.

Effect of pre- or postnatal exposure to ethanol on the total number of neurons in the principal sensory nucleus of the trigeminal nerve: cell proliferation and neuronal death

Early exposure to ethanol reduces the number of neurons in many CNS structures in vivo. The present study determined whether such reductions are caused by the death of neurons. Three groups of ethanol-treated rats were prepared: those exposed to ethanol from gestational day (G) 11 to G19 (during the period of neuronal generation and migration), from postnatal day (P) 4 to P12 (during the period of synaptogenesis), or from P31 to P39 [after the mature structure and function of neurons in the principal sensory nucleus (PSN) of the trigeminal nerve was established]. During these times, pregnant dams or pups were fed a liquid ethanol-containing diet that produced peak blood ethanol concentrations of 137-157 mg/dl. The number of PSN neurons in mature rats exposed to ethanol pre- or postnatally was determined using stereological procedures. The number of PSN neurons was also calculated for rats pair-fed an isocaloric liquid control diet or fed chow and water and libitum. The volume of the PSN was not affected by pre- or postnatal ethanol exposure. The number of PSN neurons, however, was significantly affected by ethanol exposure in a time-dependent manner. Prenatal exposure lead to a 27.1% decrease in neuronal number. Early postnatal exposure led to a smaller decrease (-15.1%), and late postnatal exposure had no affect on the number of PSN neurons. These data show not only that ethanol directly depresses the proliferation of neuronal precursors, but also that ethanol causes the death of neurons during the period of synaptogenesis.

The concept of trophic units in the central nervous system

The present paper proposes that trophic interplay among cells may represent the final common pathway for both genetic and environmental influences, and hence new criteria for the understanding of central nervous system (CNS) connectivity can be suggested. In particular, trophic signals may make up the common "language" through which genetic and epigenetic influences mold the CNS during development and the adult life. Furthermore, it will put forward the hypothesis that the developmental trophic interplay among cells leads to the formation of trophic units in the adult brain. A trophic unit is defined as the smallest set of cells, within the CNS, which act in a complementary way to support each other's trophism. The trophic units consist of neurons, glial cells, blood vessels, extracellular matrix (ECM). In particular, ECM gives support to the thin elongated cell processes and gives rise to selective chemical bridges between cell surfaces or between cell surfaces and the extracellular milieu. The trophic unit is a plastic device that not only assures neuronal survival, but also operates to adapt neuronal networks to new tasks by controlling extension of neuronal processes, synapse turnover and ECM characteristics. These plastic responses depend on the interplay of all the elements that constitute the trophic units. The concept of trophic unit may help to understand some features of neurodegenerative diseases, for example, the clustering of tangles in the neocortex and in the entorhinal cortex of Alzheimer's patients [corrected].

Relationship of the time of origin and death of neurons in rat somatosensory cortex: barrel versus septal cortex and projection versus local circuit neurons

The birth of a neuron initiates a series of ontogenetic events, e.g., neuronal migration and differentiation. The outcomes of these events are neurons that successfully integrate into the cortical circuitry and neurons that are unsuccessful and ultimately die. The present study determined whether there is a relationship between the generation and death of cortical neurons. The decrease in the density of postmigratory neurons (heavily labeled by a single injection of [3H]thymidine) during normal development was used as an index of neuronal death. The survival indices of neurons varied with their times of origin. Neurons born from gestational day (G) 15 to G18 had the highest rates of survival. In contrast, the earliest and latest generated neurons (i.e., those born on G12-G13 and those born on G19-G21, respectively) had the lowest survival rates. The role of neuronal death in the formation of cortical patterns was determined by assessing the survival of neurons in the barrels and septa of somatosensory cortex. No differences in the survival index were determined for neurons in the C-row barrels and adjacent septa with a particular time of origin. The survival rate of projection and local circuit neurons was determined with a double-labeling technique. One label, [3H]thymidine, was used to determine the time of origin of the neurons. The second label was used to identify the chemical or hodological characteristics of a neuron; projection neurons were labeled either by retrograde transport of horseradish peroxidase or by glutamate immunohistochemistry, and local circuit neurons were immunohistochemically identified with an antibody directed against gamma-aminobutyric acid (GABA) antibody.(ABSTRACT TRUNCATED AT 250 WORDS)

Orderly migration of neurons to the principal sensory nucleus of the trigeminal nerve of the rat

As nuclei in the central nervous system develop, neurons actively migrate from their site of generation to their permanent residence. This study examines the spatiotemporal sequence of the migration of neurons to the principal sensory nucleus of the trigeminal nerve (PSN) of the rat. Tritiated thymidine autoradiography and bromodeoxyuridine immunohistochemistry were used to examine the spatiotemporal patterns of migration of PSN neurons born on gestational day (G) 12 (early-generated neurons) and of those born on G14 (late-generated neurons). The final residence of early- and late-generated neurons was determined by injecting a thymidine analog into a pregnant rat on G12 or G14 and sacrificing the pups on postnatal day (P) 30. Early- and late-generated neurons were distributed medially and laterally, respectively. The schedule of the migration of PSN neurons was also determined. A few pioneer neurons born on G12 reached the PSN by G14; however, the last of the neurons born on G12 arrived in the PSN by G18. The migration of neurons born on G14 was completed 2-6 days later than that of the early-generated neurons. The path followed by migrating neurons was delineated by radial glial fibers. These processes were identified in the developing metencephalon by RAT-401 immunohistochemistry. Radial glial fibers extended from the lateral part of the ventricular zone through the tegmentum and the PSN to the surface of the metencephalon external to the sensory tract of the trigeminal nerve. RAT-401-immunoreactive processes were detected during the period of neuronal migration, but disappeared by P5. Thus, the migration of PSN neurons follows an inside-to-outside sequence, which apparently is organized by radial glial fibers. The inside-to-outside sequence of neuronal migration directly opposes the outside-to-inside gradient of synaptogenesis.

Development of the principal sensory nucleus of the trigeminal nerve of the rat and evidence for a transient synaptic field in the trigeminal sensory tract

The early development of the principal sensory nucleus of the trigeminal nerve (PSN) was examined to determine whether spatiotemporal patterns of synaptogenesis coincide with patterns in neuronal generation, migration, and death. The morphogenesis of PSN neurons during the period from G16 to P14 was studied with a Golgi method. Prenatally, PSN neurons had dendrites that extended into the sensory tract of the trigeminal nerve (s5), and from as early as G18, these dendrites were studded with spines. The dendrites in the s5 degenerated or regressed in the early postnatal period so that the s5 was free of dendrites by P14. The development of anti-synapsin I immunoreactivity was traced from G14 to P10. Immunoreactive puncta (synaptic boutons) appeared in the medial third of the s5 transiently between G18 and P5. On the other hand, puncta in the PSN did not appear until G20, at which time they were confined to the lateral margin of the PSN. By P0, puncta were distributed throughout the PSN. Cytochrome oxidase activity in the PSN was low and unpatterned prenatally. Postnatally, cytochrome oxidase activity intensified and a segmented pattern of barreloids appeared in the ventral PSN on the day of birth. By P5, the complete pattern of barreloids, spanning the full width of the ventral PSN, was evident. The development of cytochrome oxidase activity in the PSN followed the lateral-to-medial gradient of synaptogenesis revealed by the development of synapsin 1 immunoreactivity. This gradient is opposite of that for neuronal generation, migration, and death. Moreover, the s5 serves as a transient synaptic field.