Decentralization of the superior cervical ganglion in neonates impairs the development of the innervation of the iris. A quantitative ultrastructural study

Early Exposure to Ketamine Impairs Axonal Pruning in Developing Mouse Hippocampus

Mounting evidence suggests that prolonged exposure to general anesthesia (GA) during brain synaptogenesis damages the immature neurons and results in long-term neurocognitive impairments. Importantly, synaptogenesis relies on timely axon pruning to select axons that participate in active neural circuit formation. This process is in part dependent on proper homeostasis of neurotrophic factors, in particular brain-derived neurotrophic factor (BDNF). We set out to examine how GA may modulate axon maintenance and pruning and focused on the role of BDNF. We exposed post-natal day (PND)7 mice to ketamine using a well-established dosing regimen known to induce significant developmental neurotoxicity. We performed morphometric analyses of the infrapyramidal bundle (IPB) since IPB is known to undergo intense developmental modeling and as such is commonly used as a well-established model of in vivo pruning in rodents. When IPB remodeling was followed from PND10 until PND65, we noted a delay in axonal pruning in ketamine-treated animals when compared to controls; this impairment coincided with ketamine-induced downregulation in BDNF protein expression and maturation suggesting two conclusions: a surge in BDNF protein expression "signals" intense IPB pruning in control animals and ketamine-induced downregulation of BDNF synthesis and maturation could contribute to impaired IPB pruning. We conclude that the combined effects on BDNF homeostasis and impaired axon pruning may in part explain ketamine-induced impairment of neuronal circuitry formation.

Developmental influences on vascular structure and function

Blood vessels with apparently similar structures show remarkable functional heterogeneity. Differences exist in the nature and extent of their innervation, synaptic architecture, receptor characteristics, excitation coupling systems, capacity for intrinsic tone, contractility, elasticity and calcium-pool dependence, to mention only a few variables. The underlying basis of these differences is unknown. Similarity between the distribution of particular features in the vascular bed and the early patterns of embryological development suggest that some specific functional characteristics are determined during the process of gastrulation, if not before. Examples are given of receptor characteristics and tissue sensitivity that seem to reflect the different mesenchymal origins of particular vessels. Studies on vessels from immature fetal lambs confirm that individuality of specific vessels is established early. Interruption of sympathetic nerve traffic influences the function of vascular smooth muscle cells in a complex manner which is expressed differently at different ages. If the findings of experiments on the ear artery of the rabbit reveal a general principle seen in the circulation, then the level of sympathetic nerve traffic during growth would be expected to influence both qualitatively and quantitatively the structure and reactivity of the adult circulation. By this effect the sympathetic nervous system exerts a long-term influence on blood pressure. The characteristics of a particular blood vessel in the adult depend on many factors such as the level of blood pressure and the amplitude of the pulse wave which are not discussed in this chapter. These studies show that the character of the mature vessels also reflects influences that occur during early development and growth.

Developmental axon pruning mediated by BDNF-p75NTR-dependent axon degeneration

The mechanisms that regulate the pruning of mammalian axons are just now being elucidated. Here, we describe a mechanism by which, during developmental sympathetic axon competition, winning axons secrete brain-derived neurotrophic factor (BDNF) in an activity-dependent fashion, which binds to the p75 neurotrophin receptor (p75NTR) on losing axons to cause their degeneration and, ultimately, axon pruning. Specifically, we found that pruning of rat and mouse sympathetic axons that project to the eye requires both activity-dependent BDNF and p75NTR. p75NTR and BDNF are also essential for activity-dependent axon pruning in culture, where they mediate pruning by directly causing axon degeneration. p75NTR, which is enriched in losing axons, causes axonal degeneration by suppressing TrkA-mediated signaling that is essential for axonal maintenance. These data provide a mechanism that explains how active axons can eliminate less-active, competing axons during developmental pruning by directly promoting p75NTR-mediated axonal degeneration.

Activity regulates positive and negative neurotrophin-derived signals to determine axon competition

Developmental axon competition plays a key role in sculpting neural circuitry. Here, we have asked how activity and neurotrophins could interact to select one axon over another. Using compartmented cultures of sympathetic neurons, we show that, in the presence of NGF, local depolarization confers a competitive growth advantage on the depolarized axon collaterals and at the same time disadvantages the growth of unstimulated axons from the same and competing neurons. Depolarization mediates the competitive advantage by activating a CaMKII-MEK pathway, which converges to enhance local NGF-mediated downstream growth signals. Patterned electrical stimulation also acts via this pathway to enhance NGF-promoted axonal growth. In contrast, the competitive disadvantage is due to BDNF secreted from and acting on the unstimulated, competing axons through p75NTR. Thus, activity regulates both positive and negative neurotrophin-derived signaling cascades to confer a competitive growth advantage on one axon versus another, thereby providing a cellular mechanism for developmental axon selection.

Reinnervation of denervated iris by transplanted sympathetic ganglia: effect of neuronal age

Neuronal outgrowth in vivo is aggressive postnatally, but is diminished with increasing age. This may be attributable to intrinsic features of the neuron or its interaction with other components of the developing organism. The purpose of this study was to determine if there is an age-dependent reduction in the intrinsic ability of sympathetic neurons to initiate fiber outgrowth. Superior cervical ganglia from donor rats aged 3-4, 11-12, 27-28 and 45-46 days were removed and transplanted to the anterior chamber of the sympathectomized eye of host rats 85-89 days of age. Ganglia with host irides were removed at 3, 6 and 10 days post-transplant and whole mounts were analysed using catecholamine histofluorescence for maximum sympathetic fiber density, length and initial rate of outgrowth. Fluorescent fibers were present in host irides of donors of all ages and at all post-transplant times. However, maximum fiber density was less for the 3-4-day-old donor ganglia (e.g. 43-71% of 11-46-day-old donor ganglia at 600 microns, 10 days post-transplant). Maximum fiber length was also less in the youngest group (e.g. 35-49% of 11-46-day-old donor ganglia, 10 days post-transplant). Further, the initial rate of outgrowth was decreased for the 3-4-day-old donor ganglia (128 +/- 46 microns/day for the 3-4-day-old ganglia vs 253 +/- 48 microns/day for the 11-12-day-old ganglia, 307 +/- 35 microns/day for the 27-28-day-old ganglia and 260 +/- 22 microns/day for the 45-46-day-old ganglia).(ABSTRACT TRUNCATED AT 250 WORDS)

Plasticity of the sympathetic nervous system innervating the cerebral arteries in rats

In order to investigate the neuroplasticity of the peripheral sympathetic nervous system innervating the cerebral blood vessels, we observed and traced the sprouting nerve fibers originating in the contralateral superior cervical ganglion (SCG) into the previously denervated cerebral arteries following unilateral excision of the SCG and/or decentralization of the contralateral SCG in young rats (4 weeks old). These nerve fibers were labeled anterogradely with wheat germ agglutinin-horseradish peroxidase or stained immunohistochemically with anti-tyrosine hydroxylase antibody. Eight weeks after the right SCG excision, reinnervating nerve fibers originating in the contralateral ganglion formed a circular pattern of nerve plexus only on the wall of the main cerebral arteries of the circle of Willis in the ganglionectomized side. However, the decentralization of the contralateral SCG, which was performed simultaneously with a unilateral SCG excision, prevented the nerve sprouting into the denervated regions. Unilateral decentralization of SCG itself failed to affect their distribution pattern or their density of nerve fibers originating in the ganglion. It is concluded that in the young rat the outgrowth of the sympathetic nerve fibers into the denervated cerebral arteries was strongly impeded by the disconnection of ganglion cells from the central nervous system, while the decentralization alone could not affect the innervation pattern of the postganglionic fibers which have been already built-up in the cerebral arterial system.

Dendritic development in the rat superior cervical ganglion

Previous studies from our laboratory have shown that synaptogenesis in the superior cervical sympathetic ganglion (SCG) of the rat occurs predominantly during the first weeks after birth. The purpose of the present study was to examine the normal development of dendrites of the ganglion neurons, and to assess the importance of the afferent input in shaping this development. Two independent methods for examining dendritic morphology were used. One was to label neurons in the SCG by injecting a conjugate of horseradish peroxidase and wheat germ agglutinin (HRP-WGA) into a target of the SCG neurons (the submandibular gland). This procedure results in a 'Golgi-like' filling of the retrogradely labelled cell bodies and their dendrites. The second method was stereologic analysis (point-counting) of electron micrographs of sections of the SCG. At birth, the ganglion neurons give rise to an average of 2.4 primary dendrites and 1.2 secondary branches. No tertiary branches are observed at this age. The total dendritic length is 15.3 microns. Electron microscopic stereology reveals that the mean volume occupied by dendrites in the newborn SCG is 0.0093 mm3. In the adult, there is an average of 5.2 primary, 6.3 secondary, 4.8 tertiary and 1.9 quaternary dendrites. The total dendritic length is increased 23-fold to 347 microns. The mean volume occupied by dendrites is 0.0771 mm3, representing an 8-fold increase. In ganglia from adult rats which were deafferented at birth, an essentially normal dendritic form is attained. There are 4.5 primary, 6.2 secondary, 2.8 tertiary and 2.2 quaternary dendrites, and the total dendritic length is 297 microns. The mean volume of dendrites is 0.0571 micron3.(ABSTRACT TRUNCATED AT 250 WORDS)

On the two subdivisions and intrinsic synaptic connexions in the submandibular ganglion of the rat

Parasympathetic neurones in the submandibular ganglion of the rat innervate the submandibular and sublingual salivary glands. Neurones which innervate the submandibular gland (s.m. neurones) are usually located along the salivary ducts which drain both glands. Neurones which innervate the sublingual gland (s.l. neurones) are located in the thin sheet of tissue which lies between the salivary ducts and the lingual nerve. The existence and characteristics of intrinsic synaptic connexions were studied electrophysiologically in these two divisions of the submandibular ganglion. Three days or more after denervating the ganglion two types of excitatory intrinsic synaptic potentials--chemical and electrical--were recorded in ganglion cells. Chemical synaptic responses were reversibly blocked by nicotinic antagonists such as hexamethonium (10 microM) and D-tubocurarine (100 microM). Intrinsic chemical synapses were common among s.m. neurones (present in 72% of neurones) but only 12% of s.l. neurones were coupled with chemical synapses. Electrical coupling was found among 31% of s.m. neurones but was not observed between s.l. neurones. Electrotonic coupling in s.m. neurones in denervated and intact ganglia was directly demonstrated by impaling adjacent neurones with separate micro-electrodes. The average coupling ratio for current pulses injected into one cell and recorded in the adjacent cell was 0.06. During the first 30 days after birth, the number of synaptic inputs from preganglionic (chorda tympani) axons was markedly reduced in both s.m. and s.l. neurones, whereas the incidence of electrical synaptic connexions remained unchanged. The effect of long-term denervation (up to 4 months) on intrinsic synapses was examined. The membrane properties of the parasympathetic neurones and the intrinsic synaptic connexions were maintained without marked changes. It is concluded that the submandibular ganglion in the rat consists of two distinct populations of parasympathetic neurones. The two classes of neurones differ in (1) their location within the ganglion, (2) their target organs and (3) the incidence of intrinsic synapses. Possible mechanisms for the development, maintenance and function of these intrinsic synapses are discussed.

Differentiation of alpha-adrenergic responsiveness in the neonatal rat iris after decentralization or extirpation of the superior cervical ganglion

The effects of stimulating alpha-adrenergic receptors in the iris of neonatal rats (up to 30 days of age) whose superior cervical ganglion (SCG) has been decentralized or extirpated 3 days after birth were investigated by measuring changes in pupil diameter. The responses of these experimental animals were compared with those of normal animals of similar age. Alpha-adrenergic stimulation was effected by topical application of noradrenaline (NA) after blockade of beta-adrenoceptors with dichlorisoproterenol and cholinoceptors with atropine. The results reveal that although decentralization of the SCG depletes the NA content of the ipsilateral iris as demonstrated by radio-chemical assay and formaldehyde-induced fluorescence, it does not affect the development of alpha-adrenoceptor responsiveness. Even the more severe disruption of the innervation caused by extirpation of the SCG did not affect the responsiveness of the ipsilateral iris.

Synapse formation in the rat superior cervical ganglion during normal development and after neonatal deafferentation

Ten per cent of the adult number of synapses are present in the neonatal rat superior cervical sympathetic ganglion on day 1 (day of birth taken as day 0). Synapses are formed rapidly over the first postnatal week, and then more slowly, reaching 80% of adult numbers by the end of the third week. Adult numbers are present at the end of the third month. Prominent axosomatic synapses are present for the first two weeks of life, but disappear later. Of the adult numbers of synapses, 20-40% are already present on days 2 and 4, and transection of the preganglionic chain on these days causes disappearance of all synapses by 2 days after operation. However, by 2 months after operation the numbers of synapses are the same as in unoperated ganglia from rats of the same age. Unoccupied postsynaptic densities were not seen either in normal development or after lesions.