Postnatal development of the corticospinal tract in the rat

Summary Horseradish-peroxidase was used to anterogradely label and thus to trace the growth of corticospinal axons in rats ranging in age from one day to six months. Three to eight HRP-gels were implanted in the left cerebral hemisphere of the cortex. In each spinal cord three levels were studied, the cervical intumescence (C5), the mid-thoracic region (T5) and the lumbar enlargement (L3). The methodology employed for the electron microscopic visualization of HRP has been described previously (Joosten et al. 1987a).The outgrowth of labelled unmyelinated corticospinal tract axons in the rat spinal cord primarily occurs during the first ten postnatal days. The outgrowth of the main weve of these fibres is preceded by a number of pathfinding axons, characterized by dilatations at their distal ends, the growth cones. By contrast, later appearing unmyelinated axons, which presumably grow along the pathfinding axons, do not exhibit such growth cones. The first labelled pioneer axons can be observed in the cervical intumescence at postnatal day one (P1), in the mid-thoracic region at day three (P3) and in the lumbar enlargement at day five (P5).Prior to the entrance of the axons, the prospective corticospinal area or the pre-arrival zone is composed of fascicles consisting of unlabelled, unmyelinated fibres surrounded by lucent amorphous structures. During the outgrowth phase of the corticospinal fibres some myelinated axons could be observed within the outgrowth area even before day 14. These axons, however, were never labelled. These findings strongly suggest that the outgrowth area, which is generally denoted as the pyramidal tract, contains other axons besides the corticospinal fibres (and glial cells). The process of myelination of the labelled corticospinal tract axons in the rat spinal cord starts rostrally (C5) at about day 14 and progresses caudally during the third and fourth postnatal weeks. Although myelination seems to be largely complete at day 28 at all three spinal cord levels, some labelled unmyelinated axons are still present in the adult stage.     

Postnatal development of excitatory postsynaptic currents in nucleus accumbens medium spiny neurons

We have recorded excitatory postsynaptic currents (EPSCs) evoked by local electrical stimulation in 243 nucleus accumbens (nAcb) neurons in vitro during postnatal development from the day of birth (postnatal day 0; P0) to P27 and in young adults rats (P59-P71). An EPSC sensitive to glutamatergic antagonists was found in all neurons. In the majority of cases (189/243), the EPSC had two distinct components: an early one sensitive to 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and a late one that was sensitive to d-2-amino-5-phosphonovaleric acid (APV) showing that early and late components of the EPSC were mediated by AMPA/kainate (KA) and N-methyl-d-aspartate (NMDA) receptors respectively. During the first four postnatal days, the amplitudes of both the AMPA/KA and NMDA components of the EPSC were relatively small and then began to increase until the end of the second postnatal week. Whereas the amplitude of the early component appeared to stabilize from that point on, the late component began to decrease and became virtually undetectable in preparations from animals older than 3 weeks unless the AMPA/KA response was blocked with CNQX. In addition, the ratio between the amplitude of the NMDA and AMPA/KA receptor-mediated components of the EPSC followed a developmental pattern parallel to that of the NMDA receptor component showing an increase during the first two postnatal weeks followed by a decrease. Together, these results show that, during postnatal development, there is a period when NMDA receptor-mediated EPSC are preeminent and that time frame might represent a period during which the development of the nAcb might be sensitive to environmental manipulation.     

Execution-dependent modulation of corticospinal excitability during action observation

Excitability of the corticospinal pathway increased during observation of an action. The present study investigated whether corticospinal excitability during an action observation is modulated by physical movement executed by the observers themselves. Participants alternately repeated the observation of a pinching action with the thumb and index finger and execution (30 times) of the same action. Motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation were recorded from the first dorsal interosseous muscle during observation of the action. MEP amplitude gradually increased with the number of executions of the same action. However, MEP amplitude did not change when the participants only repeated observation of the pinching action or when the participants alternately repeated observation of the pinching action and execution of the isometric abduction of the index finger. These findings suggest that enhancement of cotricospinal excitability is dependent on the execution of a particular action that is very similar to the observed action.     

Experience-dependent modulation of motor corticospinal excitability during action observation

The primate premotor cortex is endowed with an "action observation/execution matching system", that is, the same premotor neurons discharge when actions are performed and when actions are observed. Hence, this system predicts a strong visual input to the motor system. Whether this input is dependent on visual experience or not has not been previously investigated. To address this issue we compared corticospinal excitability while subjects viewed frequently observed and less frequently observed hand actions of others and of themselves. Motor corticospinal excitability was larger when the action orientations were as they are frequently observed (Self-away, subject's own hand facing out from the subject, or Other-toward, an unknown hand facing toward the subject) compared with less frequently observed actions (Self-toward, subject's own hand facing "toward" the subject, or Other-away, an unknown hand facing out from the subject). This finding suggests that the modulation of motor corticospinal excitability during action observation and hence the "action observation/execution matching system" is largely dependent upon visual experience. Electronic Publication     

The early development of corticobulbar and corticospinal systems

Summary The North American opossum is born 12 days after conception and is therefore available for experimental manipulation in an immature state. We have used the opossum to study the growth of cortical axons into the brainstem and spinal cord and have obtained evidence that such growth occurs in an orderly fashion. Cortical axons reach the ventral mesencephalon 12 days after birth and some of them have grown into the caudal medulla where they decussate by 23 days. At the latter stage immature cortical axons also distribute to the midbrain tegmentum, the basilar pons, the inferior olive and the hilum of the nucleus cuneatus. Cortical axons first enter the spinal cord about 30 days after birth where they are present in the white matter before growing into the dorsal horn. The forelimb placing reaction does not develop until well after cortical axons have reached cervical levels. Axons from the cerebral cortex grow into the spinal cord before there is evidence for cortical innervation of either the red nucleus or the bulbar reticular formation and well before pyramidal cells of the neocortex are mature. The relatively late development of corticospinal and corticobulbar systems contrasts markedly with the early growth of bulbospinal axons.This investigation was supported by NS-07410 and BNS-8008675 as well as the WVa Medical Corporation     

Postnatal development of the pterygoid process

The postnatal increase of the paramedian distances of the medial and lateral plate of the pterygoid process, its elongation and also the position of the plates to the median sagittal plane were investigated. Moreover the development of height and width of the pyramidal process and the scaphoid fossa were surveyed.     

Postnatal development of bovine Sertoli cells

Summary The fine structure of bovine Sertoli cells was studied from the 4th to the 40th week post natum in order to correlate the progressive acquisition of normal adult morphology with functional development. The considerable increase in tubular size during the first 20 weeks is due to the proliferation of both presumptive Sertoli and germ cells. Aside from this, the presumptive Sertoli cells are seen to expand radially and lengthen considerably. From then on however, the observed increase in tubular diameter during the later period of postnatal development is solely due to the great increase in the number of germ cells. Presumptive Sertoli cells undergo morphological differentiation to mature Sertoli cells during the first 28 weeks of proliferative development. The maturation process includes distinct changes in cell shape, nucleus and cellular organelles, as well as an increase in and differentiation of Sertoli cell surface specializations. At 24 weeks the development of inter-Sertoli cell junctions has reached a point of differentiation where, in our opinion, a functional blood-testis barrier can be expected. During the first 8 weeks an extensive development of rough endoplasmic reticulum and a well-developed Golgi apparatus can be observed, which suggests a high secretory activity in the presumptive Sertoli cells at this time. We speculate that these secretory activities may play a role in the formation of the basal lamina which is extremely well developed during early postnatal life. The subsequent reduction of the basal lamina correlates well with diminished secretory activity in the Sertoli cells.     

Neuroprotective action of bacterial melanin in rats after corticospinal tract lesions

Experiments were performed on 48 albino rats. Part of the experimental animals were initially trained to a balancing instrumental conditioned reflex (ICR). Unilateral bulbar pyramidotomy performed in all rats caused contralateral hemiparesis. On the next day following the operation 24 rats were injected intramuscularly with bacterial melanin solution. 12 of these rats were initially trained to ICR. Recovery periods of ICR and paralyzed hindlimb movements were registered for melanin injected rats (n=24) and for operated rats, not treated with melanin (n=24). In rats injected with bacterial melanin the posttraumatic recovery is shorter than in animals not treated with melanin. The fastest and complete recovery was registered in rats initially trained to ICR and injected after the operation with bacterial melanin. Electrophysiological experiments were performed in transected animals treated with melanin, transected animals without melanin treatment and intact animals. Spiking activity of motoneurons was registered in lumbar motoneurons of rats in response to high frequency stimulation above the corticospinal tract transection. Spiking activity was very similar in motoneurons of melanin injected and intact or non operated animals. In animals, not dosed with bacterial melanin after the operation, areactivity or no change in firing rate was registered in response to stimulus. Stimulation of the corticospinal tract of melanin injected rats produced potentiation of the motoneuronal firing rate and is an evidence of regeneration in corticospinal tract. Similarity in spiking activity of intact and melanin injected rats shows the recovery of conductance in pyramidal tract. Morphohistochemical examination was carried out to confirm the results of behavioral and electrophysiological experiments. Medulla slices were prepared to trace the regeneration of nerve fibers. Examination of transection area revealed that bacterial melanin increases vascularization, dilates the capillaries in nervous tissue and stimulates the process of sprouting.     

Postnatal development of rat motor nerve terminals

Summary The nerve terminals of neuromuscular junctions in the rat diaphragm, extensor digitorum longus muscle and soleus muscle have been studied in animals between 3 weeks and 2.5 years of age using methylene blue stain and light microscopy. Dimensions, structure and organization of the nerve terminals were shown to change during life at various rates in different muscles and postnatal periods. The area and length of the terminals increase in all three muscles until young adult age. Later these dimensions continue to increase in the extensor digitorum longus and soleus muscles. In the diaphragm only the length increases, and this occurs late in adult life. The area also increases in relation to the diameter of the corresponding muscle fiber. Adult soleus terminals are more elongated than terminals in the diaphragm and extensor digitorum longus muscle. During adult life the extension of nerve terminals in relation to muscle fiber length increases in the extensor digitorum longus and soleus muscles, but is almost unchanged in the diaphragm. The nerve terminal branches are mainly coarse and irregular in young animals, but possess varying numbers of varicosities in adult animals. The number of varicosities is high in the extensor digitorum longus muscle and low in the diaphragm. In old animals the number of varicosities tends to be reduced. With increasing age the nerve terminal branches become organized in distinct groups with increasing distance between the groups. This is prominent in the soleus.     

Possible postsynaptic action of aminoglycosides in the frog rectus abdominis

The present study was undertaken to investigate the postsynaptic effects of aminoglycosides on contractions evoked by acetylcholine (ACh), KCl, electrical field stimulation (EFS) and Na(+)- and Ca(2+)-free Ringer solution with 0.2 mM Na2 EDTA (NaFCaFR) in the isolated frog rectus abdominis. Neomycin inhibited contraction elicited by ACh, NaFCaFR, and EFS at the higher frequencies (8 and 10 Hz) but not those elicited by KCl and EFS at the lower frequencies (2, 3 and 5 Hz). D-tubocurarine inhibited ACh-induced contractions in a concentration-dependent manner. In addition, drug reduced EFS-evoked contractions to a limited extent. Lower concentrations (10(-5), 5 x 10(-5), 10(-4), 2 x 10(-4) and 3 x 10(-4) M) but not higher concentrations (4 x 10(-4) and 5 x 10(-4) M) of methoxyverapamil exhibited a concentration-dependent inhibitory action on NaFCaFR-induced contractions. Similar inhibitions of the same type of contraction were displayed by aminoglycosides (neomycin, streptomycin, netilmycin, gentamycin and amikacin). These results suggest that in addition to their antagonistic action on nicotinic receptors in the frog rectus abdominis, aminoglycosides may exert stabilizing effects on some functional components contributing to contractions at the membrane.     

The action of ammonium on postsynaptic inhibition of cat spinal motoneurons

Summary Potential changes were investigated which follow the intra- and extracellular electrophoretic injection of NH₄ ⁺ and also the intravenous application of neutral ammonium salts. The common effect found in all kinds of application is a depolarizing shift of the IPSP-equilibrium potential (E_IPSP) towards the resting potential. Inhibitory transmission is preserved but produces considerably reduced hyperpolarizing potential changes at normal resting potentials. In addition, ammonium ions produce short term shifts of resting and overshoot potentials with intracellular application. The latter changes can be explained under the assumption that NH₄ ⁺ partly substitutes for Na⁺ as for the action potential generation.The observed reversible reduction of E_IPSP has high sensitivity towards extracellular and systemic NH₄ ⁺ application as compared to intracellular injection. The time course of the depolarizing shift of E_IPSP, especially its restitution, was shortest in extracellular application. It is suggested that ammonium release from the internal site produces the long term effects observed on the IPSP after intracellular injection. The externally effective dosages correspond to intracerebral NH₄ ⁺-concentrations which are reported for preconvulsive states of various kinds of metabolically induced epilepsies.We thank Miss C. Lueg for careful technical assistance.     

Postnatal development of phenylethanolamine-N-methyltransferase activity of rat retina

The postnatal development of rat retinal phenylethanolamine-N-methyltransferase (PNMT) activity was measured by radiometric assay. Activity was detected on day 1 of life. Retinal PNMT activity of day 1 neonates approximated 10% that of the adult. There is an increase in enzyme activity before eye opening. By day 30, enzyme activity has peaked. The enzyme during this early period possesses the same substrate specificity and inhibitor sensitivity as that of the adult enzyme. PNMT activity is detected before tyrosine hydroxylase activity.     

Postnatal development of the rat organ of Corti

Summary The development of the rat organ of Corti was studied during the first postnatal weeks. The temporal and the spatial patterns of cochlear development were investigated between 4 and 24 days after birth by means of semi-thin sections at approx. ten equidistant positions along the entire cochlear duct. At all examined positions width, thickness and cross sectional area of basilar membrane, cross-sectional area of tectorial membrane, of cells of Hensen, Claudius and Boettcher and of the organ of Corti were quantitatively analyzed. The most conspicuous maturational changes occur between 8 and 12 days after birth. These are the detachment of the tectorial membrane, the first appearance of filaments within the basilar membrane, the formation of the tunnel of Corti and the opening of the inner spiral sulcus. Quantitative analysis revealed that structures of a given position along the cochlear duct do not develop synchronously. Width of the basilar membrane and cross-sectional area of the tectorial membrane are already mature at the onset of hearing (10–12 days after birth). Length, thickness and cross-sectional area of the basilar membrane as well as cross-sectional area of the organ of Corti and of the cells of Hensen, Claudius and Boettcher still develop after the onset of hearing (up to 20–24 days after birth). We suggest that basic cochlear function is established by structures which are mature before the onset of hearing. Cochlear structures which develop after the onset of hearing might be involved in this improvement during this period.     

Postnatal testicular development in the Chinchilla rabbit

The Chinchilla rabbit is a breed with high commercial value and nowadays is increasingly used in various fields of biomedical research, however, its postnatal reproductive biology has been little studied. The aim of the present study was to investigate the postnatal development of the testis in this rabbit breed to determine both the proliferative periods and apoptosis. 30 rabbits aged 3–100 days old were used in the study. Determination of the period of differentiation of gonocytes to spermatogonia (50 dpp), the periods of proliferation and apoptosis of their cells, as well as the beginning of spermatogenesis (60 dpp) and the different stages of the seminiferous epithelium cycle were made. We found that these testicular developments were closer to that of humans when compared with rats, a species commonly employed in reproductive research. On comparing these results with those obtained from other breeds, there are clear differences favoring the use of this species as a research model in the field of male reproductive biology.     

Postnatal development of the renin-angiotensin system in rats

The purpose of this investigation was to correlate the development of the various enzyme activities associated with the renin-angiotensin system with age-related differences in the steady-state concentrations of angiotensin I (AI) and II (AII). Angiotensin was quantified by radioimmunoassay. Plasma renin activity and concentration increased between birth and 3 wk of age, and declined thereafter to adult values. Renal renin content, on the other hand, increased throughout the first 6 wk of postnatal life. The concentration of AII in plasma also increased following birth; however, maximum concentrations were not attained until 5 wk of age. In contrast, plasma AI did not increase between 3 and 6 wk of age. These data suggest that the steady-state concentration of AII in neonatal rat plasma may be partially limited by the low plasma renin substrate concentration. The increase in AII between 3 and 6 wk of age may reflect the increasing converting enzyme activity.     

Postnatal development of the hypothalamic neuropeptide Y system

In the adult rat, arcuate-neuropeptide Y/agouti-related protein neurons have efferent projections throughout the hypothalamus and provide a potent orexigenic stimulus. At birth neuropeptide Y fibers are also present throughout the hypothalamus; however, the source of these fibers has been unknown. The present studies determined the postnatal ontogeny of arcuate-neuropeptide Y fibers into the paraventricular nucleus and dorsomedial hypothalamic nucleus, as well as the ontogeny of neuropeptide Y1 receptor expression within these areas. Agouti-related protein messenger RNA and protein expression was present exclusively in cell bodies in the arcuate throughout postnatal development, starting at P2, and was colocalized in the vast majority of arcuate-neuropeptide Y neurons. This exclusive colocalization of agouti-related protein with arcuate-neuropeptide Y neurons makes it an excellent marker for these neurons and their projections. Even though single-label neuropeptide Y fibers were abundant in the dorsomedial hypothalamic nucleus and paraventricular nucleus as early as P2, arcuate-neuropeptide Y/agouti-related protein fibers did not significantly innervate these areas until P5-6 and P10-11, respectively. In contrast, a portion of the neuropeptide Y fibers within the paraventricular nucleus as early as P2 originated from the brainstem, as indicated by their colocalization with dopamine beta hydroxylase. It remains to be determined if local sources of neuropeptide Y-expressing cells within the dorsomedial hypothalamic nucleus and paraventricular nucleus also contribute to the neuropeptide Y-immunoreactive fibers within these regions prior to the development of arcuate-neuropeptide Y/agouti-related protein projections. In addition to the dramatic change in arcuate-neuropeptide Y/agouti-related protein projections, there is also a striking change in Y1 protein expression in the hypothalamus during the first two postnatal weeks. Taken together these data suggest that the early postnatal period, during which there is a dynamic change in the hypothalamic neuropeptide Y system, may constitute a critical period in the development of this important feeding circuit.