Inhibition of the biochemical and morphological maturation of adrenergic neurons by nicotinic receptor blockade

Re-establishment of neurochemical coding of preganglionic neurons innervating transplanted targets

We investigated the effect on neurochemical phenotype of changing the targets innervated by sympathetic preganglionic neurons. In neonatal rats, the adrenal gland was transplanted into the neck, to replace the postganglionic neurons of the superior cervical ganglion. Transplanted adrenal glands survived, and contained noradrenergic and adrenergic chromaffin cells, and adrenal ganglion cells. Retrograde tracing from the transplants showed that they were innervated by preganglionic neurons that would normally have supplied postganglionic neurons of the superior cervical ganglion. The neurochemical phenotypes of preganglionic axons innervating transplanted chromaffin cells were compared with those innervating the normal adrenal medulla or superior cervical ganglion neurons. As in the normal adrenal gland, preganglionic nerve fibres apposing transplanted chromaffin cells were cholinergic. The peptide and calcium-binding protein content of preganglionic fibres was similar in normal and transplanted adrenal glands. In both cases, cholinergic fibres immunoreactive for enkephalin targeted adrenergic chromaffin cells, whilst cholinergic fibres with co-localised calretinin-immunoreactivity innervated noradrenergic chromaffin cells and adrenal ganglion cells. In contrast to the innervation of normal adrenal glands, these axons lacked immunoreactivity to nitric oxide synthase. In a set of control experiments, the superior cervical ganglion was subjected to preganglionic denervation in rat pups the same age as those that received adrenal transplants, and the ganglion was allowed to be re-innervated over the same time course as the adrenal transplants were studied. When the superior cervical ganglion was re-innervated by preganglionic nerve fibres, we observed that all aspects of chemical coding were restored, including cholinergic markers, nitric oxide synthase, enkephalin, calcitonin gene-related peptide and calcium binding proteins in predicted combinations, although the density of nerve fibres was always lower in re-innervated ganglia. These data show that the neurochemical phenotypes expressed by preganglionic neurons re-innervating adrenal chromaffin cells are selective and similar to those seen in the normal adrenal gland. Two explanations are advanced: either that contact of preganglionic axons with novel target cells has induced a switch in their neurochemical phenotypes, or that there has been target-selective reinnervation by pre-existing fibres of appropriate phenotype. Regardless of which of these alternatives is correct, the restoration of normal preganglionic codes to the superior cervical ganglion following denervation supports the idea that the target tissue influences the neurochemistry of innervating preganglionic neurons.

Transneuronal regulation of neuronal specific gene expression in the mouse olfactory bulb

Peripheral afferent denervation (deafferentation) of the rodent main olfactory bulb produces a marked decrease in tyrosine hydroxylase (TH) activity and immunoreactivity in a population of juxtaglomerular dopaminergic neurons. Preservation of activity and immunostaining for aromatic L-amino acid decarboxylase implies that these cells do not die, but change phenotype. We now report that the steady-state level of TH mRNA markedly decreases in the adult mouse olfactory bulb in response to deafferentation. This reduction is permanent following intranasal irrigation with 0.17 M zinc sulphate (ZnSO4) but reversible following deafferentation produced by intranasal irrigation with 0.7% Triton X-100. The initial declines in TH activity, protein and mRNA of dopaminergic juxtaglomerular neurons observed after Triton X-100 treatment are all reversible as the steady-state level of TH mRNA gradually returns to control levels. Steady-state levels of mRNA for olfactory marker protein (OMP), a protein found in high concentrations in olfactory receptor neurons and their processes which innervate the olfactory bulb, were also monitored following deafferentation. Following treatment with either ZnSO4 or Triton X-100, the pattern of changes in steady-state levels of OMP mRNA was similar to that observed for TH. The steady-state level of PEP19 mRNA, a peptide previously localized to granule cells in the olfactory bulb, was not altered by deafferentation. These data indicate selective and parallel regulation of TH and OMP message and protein levels following deafferentation.

Developmental interactions between sweat glands and the sympathetic neurons which innervate them: effects of delayed innervation on neurotransmitter plasticity and gland maturation

The neurotransmitter properties of the sympathetic innervation of sweat glands in rat footpads have previously been shown to undergo a striking change during development. When axons first reach the developing glands, they contain catecholamine histofluorescence and immunoreactivity for catecholamine synthetic enzymes. As the glands and their innervation mature, catecholamines disappear and cholinergic and peptidergic properties appear. Final maturation of the sweat glands, assayed by secretory competence, is correlated temporally with the development of cholinergic function in the innervation. To determine if the neurotransmitter phenotype of sympathetic neurons developing in vivo is plastic, if sympathetic targets can play a role in determining neurotransmitter properties of the neurons which innervate them, and if gland maturation is dependent upon its innervation, the normal developmental interaction between sweat glands and their innervation was disrupted. This was accomplished by a single injection of 6-hydroxy-dopamine (6-OHDA) on Postnatal Day 2. Following this treatment, the arrival of noradrenergic sympathetic axons at the developing glands was delayed 7 to 10 days. Like the gland innervation of normal rats, the axons which innervated the sweat glands of 6-OHDA-treated animals acquired cholinergic function and their expression of endogenous catecholamines declined. The change in neurotransmitter properties, however, occurred later in development than in untreated animals and was not always complete. Even in adult animals, some fibers continued to express endogenous catecholamines and many nerve terminals contained a small proportion of small granular vesicles after permanganate fixation. The gland innervation in the 6-OHDA-treated animals also differed from that of normal rats in that immunoreactivity for VIP was not expressed in the majority of glands. It seems likely that following treatment with 6-OHDA sweat glands were innervated both by neurons that would normally have done so and by neurons that would normally have innervated other, noradrenergic targets in the footpads, such as blood vessels. Contact with sweat glands, therefore, appears to suppress noradrenergic function and induce cholinergic function not only in the neurons which normally innervate the glands but also in neurons which ordinarily innervate other targets. Effects of delayed innervation were also observed on target development. The appearance of sensitivity to cholinergic agonists by the sweat glands was coupled with the onset of cholinergic transmission.(ABSTRACT TRUNCATED AT 400 WORDS)

On the role of normal acetylcholine metabolism in the formation and maintenance of the Drosophila nervous system

We have examined the requirement for normal acetylcholine metabolism in the formation and maintenance of the larval and adult central nervous system in Drosophila melanogaster. By using mutations at the Ace and Cha loci, which respectively encode the degradative and synthetic enzymes for acetylcholine (ACh), acetylcholinesterase (AChE), and choline acetyltransferase (ChAT), we have been able to disrupt acetylcholine metabolism in situ. An ultrastructural analysis of embryonic nervous tissue lacking either enzymatic function has indicated that while neither function is required for the formation of the larval central nervous system, each is required for the subsequent maintenance of its structural integrity and function. Using temperature sensitive mutations at the Cha locus, the normal developmental profile of ChAT activity during the late larval and pupal stages was disrupted. Subsequent examination of the morphology and behavior of the treated animals has indicated that normal acetylcholine metabolism is not required for the initial formation of the adult nervous system, but is required for the subsequent maintenance of its structural integrity and function. The results obtained in these studies are discussed with respect to data presented on the adult distribution of the cholinergic markers' AChE activity and ChAT immunoreactivity. The projections of adult peripheral neurons innervating Ace+ tissue from Ace cuticular clones has been examined to address the nature of the structure of Ace neuropil. Normal projections are apparently achieved and maintained, suggesting that the defects seen in adult Ace mosaics arise as an aberrant intracellular organization of morphologically normal cells.

Development of the sex difference in neuron numbers of the superior cervical ganglion: effects of transection of the cervical sympathetic trunk

The number of superior cervical ganglion (SCG) neurons does not differ for males and females on the day of birth, but by 15 days, after most of the normal neuron death has occurred, males have significantly more neurons than females. This difference persists in the adult. The present study was undertaken to determine whether the presence of afferent input to the SCG is required for the development of this sex difference. Bilateral transection of the cervical sympathetic trunk, which deafferents the SCG neurons, or a sham operation was performed on male and female Sprague Dawley rats on the day of birth. Numbers of neurons were counted in SCGs of animals sacrificed on either postnatal day 4 or 15, before or after normal development of the SCG sex difference. At 4 days, the number of SCG neurons in sham-operated males and females were not different, but by 15 days, females had lost a significant number of neurons, whereas the males had not. Transection of the cervical sympathetic trunk led to a significant loss of over 6,000 SCG neurons by postnatal day 4 in both males and females. Whereas some of this loss is due to axotomy of caudally projecting SCG neurons, at least half of the neuron loss is due to removal of the afferent input. At 15 days, sham-operated males had significantly more SCG neurons than did sham-operated females, but the gender difference was not significant in animals with neonatally deafferented ganglia. Thus, the normal development of the gender difference in SCG neuron numbers requires an intact afferent input.

Depolarizing stimuli increase tyrosine hydroxylase in the mouse locus coeruleus in culture

The influence of membrane depolarization on the development and regulation of brain noradrenergic neurons was studied in explant cultures of the mouse locus coeruleus (l.c.). Exposure to the depolarizing agents veratridine or elevated K+ significantly increased the catalytic activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis. The effects of veratridine were prevented by tetrodotoxin, suggesting that transmembrane Na+ influx was necessary for the rise in TH. Morphometric analysis indicated that the rise in TH activity was not accompanied by altered TH-positive cell number or cell diameter. Rather, TH fluorescence intensity increased in each neuron, suggesting that depolarization increased TH per neuron. Immunoblot and densitometric analysis indicated that depolarization did, indeed, increase TH immunoreactive protein. Moreover, depolarization elevated enzyme activity in cultured neurons expressing the normal developmental increase in TH, as well as those in which plateau levels had already been attained. We conclude that depolarization and/or Na+ influx regulates a critical transmitter macromolecule in brain neurons, as in the periphery, by altering enzyme molecule number.

Embryologic development of rat adrenal medulla in transplants to the anterior chamber of the eye

The morphological development and plasticity of embryonic and postnatal rat adrenal medullary cells were studied in homologous adrenal grafts to the anterior chamber of the eye. The eyes of recipient rats were adrenergically denervated 10 days prior to grafting by extirpation of the superior cervical ganglion in order to increase levels of NGF and NGF-like activities in the iris. Grafts taken at the 15th day of embryonic development (E15), i.e., at the beginning of immigration of medullary progenitor cells into the adrenal cortical anlagen, contained no cortical or mature medullary cells after 2 weeks in oculo. Numerous sympathoblastic cells, however, were located at the anterior surface of the iris. E 16 and E 17 transplants showed abundant mature cortical tissue after 2 weeks. Small groups of medullary cells with the ultrastructural characteristics of mature pheochromoblasts or young chromaffin cells were interspersed among cortical cells without forming a discrete medulla. Neuronal cells were exclusively found outside the cortical cell mass. Sympathoblasts grew at the surface of the iris, while young sympathetic nerve cells, which were invested by Schwann cells and received synaptic axon terminals, were embedded into the stroma of the iris. Grafting of E 21 adrenals yielded very similar results except that, in a few instances, young chromaffin cells were located outside the cortex and sympathetic nerve cells were seen to be in close contact with cortical cells. In transplants of adult medullary cells typical mature adrenaline and noradrenaline cells were clearly distinguishable after 8 weeks even in the absence of cortical cells. The only indication of phenotypical changes in these cells was the formation by some of them, of neuritic processes which could be visualized in glyoxylic acid-treated whole mounts of irises. These results are compatible with the idea that embryonic adrenal medullary cells have the environmentally controlled potential to develop along the neuronal or endocrine line, but could also be interpreted in terms of a selection of a specific subpopulation with predetermined potentialities by a specific microenvironment. Moreover, these results suggest that increasing differentiation of medullary cells is accompanied by progressive restrictions in their genetic program, which eventually prevent full transdifferentiation of mature chromaffin into neuronal cells.

Regulation of enzymes responsible for neurotransmitter synthesis and degradation in cultured rat sympathetic neurons. III. Effects of sodium butyrate

The effects of Na butyrate on the differentiation of newborn rat sympathetic neurons in primary cultures have been studied. Butyrate did not affect the long-term survival of these neurons in the presence of optimal concentrations of nerve growth factor, but decreased in a dose-dependent manner their protein content. In the range, 0.5-20 mM, butyrate did not modify the specific activity of lactate dehydrogenase in these cultures. Choline acetyltransferase activity developed at a 4.5- to 12-fold higher rate in cultures grown with 1-5 mM butyrate than in its absence. Concomitantly, tyrosine hydroxylase, dopa decarboxylase, dopamine-beta-hydroxylase, and acetylcholinesterase were depressed in cultures grown with butyrate. The deficit in acetylcholinesterase total activity was accompanied by an inhibition of the development of the asymmetric 16 S form of the enzyme. The deficit in tyrosine hydroxylase activity did not result from either a modification of the app Km for the enzyme's cofactor or a modification of its state of cAMP-dependent phosphorylation, but from a decrease in the number of immunoprecipitable enzyme molecules. A similar result was obtained with acetylcholinesterase. Butyrate thus reproduced in a qualitative manner the effects of a macromolecular factor purified from muscle conditioned medium on these neurons (J. P. Swerts, A. LeVan Thaï, A. Vigny, and M. J. Weber (1983) Dev. Biol. 100, 1-11; J. P. Swerts, Le Van Thai, and M. J. Weber (1984) 103, 230-234), raising the hypothesis of a common pathway in the regulation of neurotransmitter phenotype by these two agents.

Olfactory bulb dopamine neurons survive deafferentation-induced loss of tyrosine hydroxylase

Peripheral deafferentation of the rodent olfactory bulb results in loss of dopamine content, tyrosine hydroxylase activity and immunocytochemical staining for tyrosine hydroxylase in juxtaglomerular dopamine neurons. Reinnervation of the bulb by afferent neurons results in the return of all parameters to control levels suggesting that the dopamine neurons did not degenerate but that the expression of tyrosine hydroxylase enzyme was transneuronally regulated in a static population of juxtaglomerular cells. To evaluate this possibility, we determined the activity and immunocytochemical localization of the second enzyme in the dopamine biosynthetic pathway, DOPA decarboxylase. At a time when tyrosine hydroxylase activity was reduced to 25% of control values, DOPA decarboxylase activity in the lesioned bulb was maintained at about 65% of that in the unlesioned bulb. Immunocytochemical staining with antibodies to both enzymes, performed sequentially in the same sections, demonstrated that in the unlesioned bulb tyrosine hydroxylase and DOPA decarboxylase are co-localized in the same population of juxtaglomerular neurons. Similar results were obtained in adjacent sections each stained with one of the two antibodies. In contrast, in the deafferented bulb, about three times as many neurons were stained with DOPA decarboxylase as with tyrosine hydroxylase antibodies. The DOPA decarboxylase activity measurements and immunocytochemistry argue for the continued presence, in the lesioned olfactory bulb, of a population of tyrosine hydroxylase deficient dopamine neurons. The data suggest that olfactory receptor cell innervation transneuronally regulates the expression of tyrosine hydroxylase by mechanisms separate from those controlling the levels of DOPA decarboxylase.

Developmental neuron death in the rat superior cervical sympathetic ganglion: cell counts and ultrastructure

Counts of neurons of the rat superior cervical ganglion (SCG) were made at two days before birth and at several postnatal ages. There is a significant decline in the number of apparently normal neurons over the first postnatal week, with the number falling from 39 500 at 3 days to 26 500 at 7 days. Cell numbers then remained constant up to day 60 when the number of neurons was 27 500. The incidence of degenerating neurons, identified by light and electron microscopy, was correlated temporally with the loss of normal neurons. The early manifestations of the neuron degeneration were chromatin clumping and the presence of free monoribosomes. Later stages were characterized by increased chromatin clumping, dense aggregations of monoribosomes, numerous intracytoplasmic vacuoles, and only short segments of rough endoplasmic reticulum. The ultrastructure of the majority of these dying neurons is similar to the 'nuclear' types of degeneration described by Pilar & Landmesser (1976) and Chu-Wang & Oppenheim (1978). Based on the presence of degenerating neurons coincident with the reduction in neuron numbers, we conclude that neuron death is an important aspect of early postnatal development in the rat SCG.

Developmental and neurochemical specificity of neuronal deficits produced by electrical impulse blockade in dissociated spinal cord cultures

Blockade of spontaneous electrical activity in dissociated fetal spinal cord cultures produced neuronal deficits as measured by biochemical and morphological techniques. Spinal cord cultures exhibited an age-dependent vulnerability to impulse blockade with tetrodotoxin (TTX) or xylocaine. Neuronal cell counts, [125I]tetanus toxin fixation and [125I]scorpion toxin binding indicated that TTX application produced neuronal deficits during the second or third week in culture. Application of TTX during the first or fourth week did not produce a difference in tetanus toxin fixation from controls. Radioautography of [125I]tetanus toxin revealed no obvious change in the label distribution after TTX treatment. Suppression of electrical activity during the first 6 days in culture had no effect on choline acetyltransferase (CAT) activity and no apparent effect on the appearance of the cultures. Application of TTX during the seventh day in culture decreased CAT activity to 68% of control. Chronic electrical blockade produced a progressively greater loss of CAT activity through 21 days in culture. GABAergic neurons, as indicated by high-affinity GABA uptake, glutamic acid decarboxylase activity and [3H]GABA radioautography, were not affected by electrical blockade. These data indicate that there is developmental and neurochemical specificity in the neuronal death produced by blocking spontaneous electrical activity in dissociated spinal cord cultures.

Development of catecholaminergic phenotypic characters in the mouse locus coeruleus in vivo and in culture

While abundant studies have begun to elucidate ontogeny of the peripheral nervous system, molecular mechanisms underlying brain development remain obscure. To approach this problem, we initiated parallel in vivo and in vitro studies of the mouse locus coeruleus (l.c.), a brainstem noradrenergic nucleus. The catecholaminergic enzymes tyrosine hydroxylase (TH) and dopamine-beta-hydroxylase (DBH) were used to monitor phenotype expression and development. TH catalytic activity and immunocytochemical reactivity were initially detectable on gestational Day 13 (E-13) in vivo, and adult levels of activity were approximately by the third postnatal week. Immunotitration studies indicated that the developmental increase was due to accumulation of enzyme molecules and not enzyme activation. The in vivo developmental profile of DBH approximated that of TH. To begin defining regulatory mechanisms, explants of embryonic brainstem were placed in culture. Explantation on E-12, prior to expression of TH or DBH, resulted in the de novo appearance of these phenotypic characters after 4 days. Explantation on E-18, after the enzymes are already expressed, was followed by a striking sixfold rise in TH activity. Immunotitration studies revealed that the increase in TH activity in E-18 cultures was attributable to increased molecule number, reproducing the in vivo results. Moreover, the E-18 explants, cultured for 3 weeks, attained higher plateau levels of TH activity than E-12 cultures, and this differences was due to increased molecule number. Morphometric analysis indicated that 3-week E-12 cultures actually had more l.c. cells than E-18 cultures, indicating that differences in TH were not due to increased cells in the E-18 l.c. Finally, systemic study revealed that the development of TH activity in culture increased progressively from E-11 to E-12 to E-13, suggesting that critical regulatory events occur at this time. Our studies suggest that the l.c. is an excellent model for the study of brain development in vivo and in vitro. Initial phenotypic expression and dramatic development occur in culture in the absence of normal targets, normal afferent innervation and, presumably, normal humoural milieu.

Presynaptic modulation of activity and molecular forms of acetylcholinesterase in the rat superior cervical ganglion during early postnatal development

Preganglionic nerve trunk of the rat superior cervical ganglion was transected shortly after birth in order to evaluate the influence of preganglionic nerves on the development of acetylcholinesterase and choline acetyltransferase in ganglionic neurons. In spite of an early decentralization, specific activity of acetylcholinesterase in the ganglion is increasing during the first 3 wk of life until it is about equal to the activity which remains in the superior cervical ganglion decentralized in an adult animal. Thus, the preganglionic nerves, which per se contribute the presynaptic fraction of total ganglionic AChE activity in normal innervated ganglia, apparently exert no significant regulatory effect on the specific activity of the fraction of acetylcholinesterase affiliated with the developing ganglionic cells. However, the absence of innervation during development is strongly reflected in the pattern of acetylcholinesterase molecular forms. The activity of the 16 S molecular form of AChE remains high in the developing superior cervical ganglion, decentralized at birth, in contrast to the substantial absolute and relative decrease of specific activity of this form during development of a normally innervated ganglion. A high proportion of 16 S AChE probably reflects a shift of decentralized immature ganglion nerve cells toward a cholinergic character. In accordance with this assumption, choline acetyltransferase activity in early decentralized ganglia is significantly higher than that in the ganglia decentralized in adult animals.

Biochemical and morphological characterization of sympathetic neurons grown in a chemically-defined medium

Previous studies have demonstrated that individual neurons from neonatal rat superior cervical ganglion express a mixed adrenergic-cholinergic phenotype when grown under certain tissue culture conditions. The expression of this phenotype is critically influenced by a number of undefined components present in the culture medium. In the present study, we have examined whether superior cervical ganglion neurons grown on a chemically defined serum-free medium similarly develop dual transmitter expression, or if under these conditions, neurons express only those properties characteristic of their adrenergic heritage. To address this issue, we established that superior cervical ganglion neurons could be maintained in culture for extended periods on the defined medium described by Bottenstein & Sato in the absence of supporting cells. We then studied the biochemical, immunocytochemical and ultrastructural characteristics of these neurons. We found that in defined medium, superior cervical ganglion neurons continued to express, in a modified form, certain of their expected adrenergic properties, including the development of tyrosine hydroxylase and dopamine-beta-hydroxylase activities, stores of endogenous norepinephrine, synaptic vesicles with dense cores and tyrosine hydroxylase-immunoreactive staining properties. Superior cervical ganglion neurons grown on a defined medium did not, however, acquire cholinergic traits in culture. In this paper we show that choline acetyltransferase activity did not reach detectable levels; the comparison paper documents that cholinergic synapses were not formed. We concluded that superior cervical ganglion neurons, grown under serum-free culture conditions, develop certain properties characteristic of adrenergic neurons and do not express a mixed adrenergic-cholinergic phenotype. A comparison paper describes the electrophysiological properties of these neurons and demonstrates the frequent occurrence of electrotonic synapses in these cultures.

An ultrastructural analysis of the development of foetal rat retina transplanted to the occipital cortex, a site lacking appropriate target neurons for optic fibres

Foetal retina was removed from donor rats at 15 days of gestation and transplanted to the occipital cortex of neonatal host rats. The purpose of this procedure was to examine the development of retinal neurons and photoreceptors, and document synaptic patterns during maturation of the transplanted retina in an environment lacking a normal target for optic axons. Host animals were sacrificed at 5, 10, 15, 20 and 30 days and samples of cortex containing the transplant were subjected to a light and electron microscopic analysis. During early stages of development, (5 days) the retina assumes a radial orientation with the scleral (outer) surface located centrally and the vitreal (inner) surface occupying the periphery. Numerous mitotic figures are found at the centre of the transplant and columns of primitive neuroblasts appear to radiate out from this zone. By 10 to 15 days after transplantation the retinal tissue contains numerous small rosettes each of which displays a histotypic organization with recognizable layers of sensory cells and their centrally-projecting processes, an outer limiting membrane, made up of a network of zonulae adherentes, and a rudimentary outer and inner plexiform layer which delineate the cells of the inner nuclear layer. Ultrastructural analysis of such rosettes confirmed the presence of typical bipolar, amacrine, horizontal and ganglion cells, but revealed that while the plexiform layers were occupied by numerous processes from these neurons, few if any, of these exhibited synaptic vesicles. By 20 to 30 days following transplantation sensory cells have completely differentiated, giving rise to prominent inner and outer segments which display typical cilia, centrioles and basal bodies, together with numerous stacked lamellae of photoreceptors which were contorted, presumably due to growth in an abnormal site. It should be further emphasized that these structures developed in the absence of pigment cells. Synaptic development ensues during this period to form characteristic dyads within the outer and inner plexiform layers. Additionally, clusters of amacrine to amacrine contacts occurred in the inner plexiform layer and were found to be increased relative to other types of junctions. In general, synaptogenesis takes place in the outer and inner plexiform layers and all categories of retinal synapses are established, but the process was found to be significantly delayed in comparison to normal retina at the same stage of development. Quantitative analysis revealed a reduced number of presumptive ganglion cells in proportion to the other categories of neurons. Optic fibres remained small and failed to myelinate. It is suggested that lack of an appropriate target for optic axons induced this alteration and may be indirectly related to the delay in the onset of synaptic development.

The effects of eliminating impulse activity on the development of the retinotectal projection in salamanders

The California newt Taricha torosa manufactures tetrodotoxin, a blocker of voltage-sensitive sodium channels and therefore of action potentials.The newt's own nervous system is insensitive to this toxin. Grafting an embryonic eye to the newt from a tetrodotoxin-sensitive species, the Mexican axolotl, blocks action potentials in the retinal ganglion cells of the transplanted eye. Neuroanatomical and electrophysical techniques demonstrate that while such ganglion cells are incapable of firing impulses, they develop normally, grow axons to the host tectum, terminate in the appropriate neuropil layers, form synapses, and project to the tectum retinotopically. Furthermore, they develop these apparently normal projections even in competition with electrically active axons from a host eye.

Acetylcholinesterase mutants in Drosophila and their effects on the structure and function of the central nervous system

Mutations that eliminate acetylcholinesterase (AChE) activity were used to study the effects of disrupted acetylcholine metabolism on the form and function of the central nervous system in Drosophila melanogaster. Mutants in the Ace gene, which have no AChE activity, usually die in early development, but the postembryonic effects of this lesion can be studied in genetic mosaics, or with conditional mutants. Adult mosaics, which expressed Ace mutations in part of their CNS, exhibited morphological defects in any ganglionic neuropile whose cells were mutant. The defects included reduction in ganglionic volume, a condensed appearance, and for a very large clone, degeneration. Examination of many such mosaics indicated that small clones restricted to one side of the CNS were not usually lethal. However, mosaics with large clones, with clones on either side of the posterior slope of the protocerebrum, or with clones encompassing symmetrical structures on both sides of the CNS rarely survived to adulthood. Mosaics with AChE-null tissue on either side of the optic lobes or the posterior-inferior protocerebrum had marked deficits in optomotor behavior, although they were outwardly normal in their movement and posture. Mosaics with Ace mutant tissue in the first-order optic lobe, the lamina, lacked a synaptic component of the electroretinogram, the "off" transient. Tests of courtship behavior revealed that AChE mosaics with mutant clones in the superior protocerebrum were often capable of demonstrating male courtship. However, their behavior was quantitatively and perhaps qualitatively deficient. In order to study critical periods for the effects of mutant AChE, temperature-sensitive mutations of the Ace gene were isolated. Flies bearing certain of these new mutations produced AchE activity that was thermolabile in vivo and in vitro. The critical period during which the mutants were most susceptible to conditional lethality was late in embryogenesis.

Target organ regulation of sympathetic neuron development

The role of target organs in the morphological and biochemical development of sympathetic neurons was examined in the neonatal rat. The superior cervical ganglion (SCG) and its end organs, the salivary glands and iris were employed as a model system. Unilateral sialectomy and iridectomy prevented the normal developmental increase in ipsilateral ganglion tyrosine hydroxylase (T-OH) activity, a marker for adrenergic maturation. Enzyme activity remained depressed by approximately 30% for at least 6 months, the longest time tested. Ganglion morphometry was performed to investigate the basis of the abnormal biochemical ontogeny. Target organ removal significantly decreased the number of adrenergic neurons in the Scg by approximately 30%. Total ganglion volume was reduced in a parallel fashion. Thus, end organ extirpation may prevent the biochemical maturation of the SCG by decreasing adrenergic neuron survival. Sialectomy without iridectomy prevented the normal postnatal increase in ganglion T-OH activity, but did not alter iris activity. These observations suggest that target removal prevents the development of only those neurons destined to innervate that organ. In addition to preventing normal adrenergic neuron ontogeny, target extirpation also prevented the normal development of presynaptic choline acetyltransferase activity. Presynaptic ganglion terminal may have failed to mature normally secondary to adrenergic destruction, or may have responded in some other manner to target organ extirpation.

The effect of a single dose of reserpine administered prior to incubation on the development of tyrosine hydroxylase activity in chick sympathetic ganglia

A single dose of reserpine administered into the yolk sac of chicken eggs prior to incubation produces two distinct periods of significant increase in tyrosine hydroxylase (TH) activity over controls. The first period is 21 days of incubation (55%) and the second is between day 14 and 30 after hatching (a.h.) (69%). Cholineacetyltransferase (ChAc) and dopadecarboxylase (DDC) are not modified in the two periods of increased TH activity. Reserpine had no effect on cholinergic parasympathetic synapses and neurons in the ciliary ganglion, as judged by ChAc activity. When reserpine was acutely administered in three different posthatching periods only the injection at the latest period (days 26 and 27) caused a significant (38%) increase in TH activity at day 30. Postsynaptic nicotinic receptors were blocked selectively by injecting chlorisondamine in the chick starting at hatching for one week. The administration of chlorisondamine almost completely abolished the reserpine induced increase of TH activity at day 15 a.h. The present results support the view that the development of enzyme activities specifically related to neurotransmitter biosynthesis in chick autonomic ganglia is regulated not only by transsynaptic influences but also by regulatory inputs originating in the periphery.

Electrotonic processing of information by brain cells

In contrast to well-studied through-protection neurons that propagate information from one region to another in the central nervous system, short-axon or axonless neurons form local circuits, transmitting signals through synapses and electrical junctions between their dendrites. Interaction in this dendritic network proceeds without spike action potentials. Interaction is mediated by graded electrotonic changes of potential and is transmitted through high sensitivity (submillivolt threshold) synapses rather than by lower sensitivity (20 to 100-mv threshold) synapses typical of projection neurons. A crucial feature of local circuits is their high degree of interaction both through specialized junctional structures and through the extracellular fields generated by local and more distant brain regions. The anatomical evidence for the nature and distribution of neuronal local circuits in the nervous system is surveyed. Bioelectric mechanisms are discussed in relation to the special properties of local circuits, including dendrodendritic synapses, synaptic sensitivity, electrotonic coupling, and field effects. Intraneuronal and interneuronal transport of various types of substances suggests that the biochemical and the bioelectrical parameters are functionally interwoven. Through such interactions neuronal local circuits, with their distinctive properties, may play an essential role in higher brain function.

The effect of taget organ removal on the development of sympathetic neurons

The role of target organs in the maturation of adrenergic neurons was studied in the neonatal rat. The superior cervical ganglion (SCG) and its end organs, the salivary glands and iris were employed as a model system. Unilateral sialectomy and iridectomy in 3-day-old animals prevented the normal development of ganglion tyrosine hydroxylase (T-OH) and DOPA decarboxylase activities. These enzymes are highly localized to adrenergic neurons in the SCG, and were used to monitor maturation of these cells. Enzyme activity remained depressed for at least two months, the longest time tested. In contrast, total ganglion protein, a measure of ganglion growth as a whole, initially developed normally. Six weeks after surgery, however, protein content was significantly lower in ganglia deprived of the normal field of innervation. Failure of normal enzyme maturation was apparently dependent on removal of ipsilateral end organs only, since bilateral sialectomy exerted no greater effect than unilateral sialectomy. In adults, unilateral sialectomy and iridectomy did not significantly alter ganglion T-OH activity or protein in rats followed up to one month after surgery.

Ontogeny of the induction of tyrosine hydroxylase by reserpine in the superior cervical ganglion, nucleus locus coeruleus and adrenal gland

The ontogeny of the induction of tyrosine hydroxylase by reserpine has been studied in the superior cervical ganglion, adrenal gland and nucleus locus coeruleus of the rat. The inductive response developed gradually over a period of days in all 3 areas. However, the onset of induction occurred at markedly different times in these regions, being present from day 2 of life, the earilest time tested, in the adrenal, day 6 in the locus coeruleus and day 24 in the ganglion. In the ganglion even extremely high, toxic doses of reserpine failed to induce the enzyme during the first 3 weeks of life. Decentralization studies indicated that the ganglion was functionally innervated at this time. Moreover, the onset of induction was not time-locked to a specific phase of the postnatal development of tyrosine hydroxylase activity in the areas examined. It is probable that the development of inducibility reflects maturation of mechanisms intrinsic to the adrenergic cell, and this timetable is different for cells in different areas.