Mecamylamine selectively blocks nicotinic receptors on vasomotor sympathetic C neurons

Mecamylamine differentially blocked fast nicotinic transmission in two functional subsets of sympathetic neurons within lumbar paravertebral ganglia of the bullfrog. EC₅₀s for inhibition of postsynaptic compound action potentials were 27.3±2.5 μM in the secretomotor B system and 5.7±0.7 μM in the vasomotor C system. This 5.2:1 selectivity is 2.6 times greater than observed previously with

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-tubocurarine, a nonselective blocker of nicotinic receptors, and it indicates that mecamylamine preferentially interacts with nicotinic receptors on sympathetic C neurons. We tested this by analyzing the effect of mecamylamine upon synaptic currents. In both cell types, the drug produced a qualitatively similar picture of open-channel blockade. It reduced EPSC amplitude, speeded EPSC decay, and became more effective with membrane hyperpolarization and repetitive activity. Despite these similarities, 8 μM mecamylamine reduced EPSC amplitude to a greater extent in C neurons, and the rate constant for drug binding to open channels was 4.4 times greater in B cells, irrespective of membrane potential. This implies that the unblocking rate for mecamylamine is much slower in C cells than B cells, and it shows that the drug recognizes a structural difference between nicotinic receptors on these two populations of sympathetic neurons.     

Glutamate selectively increases the high-threshold Ca channel current in sensory and hippocampal neurons

Previous studies resulted in conflicting conclusions that glutamate application either decreases or increases the activity of Ca²⁺ channels in hippocampal neurons. We studied whole-cell Ca²⁺ currents (I_Ca) in chick dorsal root ganglion neurons and rat hippocampal cells. For both cell types glutamate (1–30 μM) increased high-threshold Ca²⁺ current. It was independent of the charge carriers, Ca²⁺ or Ba²⁺. Low-threshold Ca²⁺ channel current and the fast sodium current were not changed with glutamate application. The effect developed within 1–2 min and then further facilitated after washout of the agonist. A second application of glutamate produced no additional increase in _ICa. No changes in the time-course of whole-cell currents were observed, suggesting that glutamate recruits ‘sleepy’ Ca²⁺ channels. Whatever its mechanism, overlasting increase of I_Ca by glutamate may be important in neuronal plasticity.     

Acute treatment with antidepressant drugs selectively increases the expression of c-fos in the rat brain.

Rats were treated acutely, ip, with saline vehicle or an antidepressant: iprindole (15 mg/kg), nortriptyline (15 mg/kg), A75200 (10 mg/kg), fluoxetine (15 mg/kg), desipramine (10 mg/kg), bupropion (20 mg/kg) or tranylcypromine (7.5 mg/kg). Mapping the neuroanatomical distribution at 64 sites of the immediate early gene, c-fos revealed several patterns: first, increased counts of Fos-like neurons were found in all but one instance; second, drugs which had dopaminergic effects (bupropion and tranylcypromine) were more likely to potentiate c-fos reactivity than were the other drugs; third, Fos-like counts were more likely to be significantly elevated in structures bordering brain ventricles; fourth, only in the central amygdala were the Fos-like counts higher in all seven drug groups relative to the saline group. It remains to be seen whether or not this shared substrate is therapeutically significant.     

Age-related increase in c-fos expression in the sympathetic neurons of the rat superior cervical ganglion

C-fos expression was studied immunocytochemically in sympathetic neurons of the rat superior cervical ganglion (SCG). Fos-like immunoreactivity was confined to the principal neurons of the ganglion and was exclusively localized within their nuclei. In 2-month-old rats, the immunoreactivity was detected in 1.2% of the principal neurons with a density of 4.95 Fos-positive cells/mm2 of ganglion area. This proportion increased with age and reached a value which was 6.5-fold higher in the 26-month-old rats than that in the young adult. A density of 24.5 Fos-positive cells/mm2 of ganglion area was seen in the 26-month-old animals. The age-enhanced c-fos expression suggests that Fos may be involved in regulation of the genetic events associated with the adaptive changes in neuronal activity of the sympathetic ganglion during aging.     

Interleukin-17A increases neurite outgrowth from adult postganglionic sympathetic neurons

Inflammation can profoundly alter the structure and function of the nervous system. Interleukin (IL)-17 has been implicated in the pathogenesis of several inflammatory diseases associated with nervous system plasticity. However, the effects of IL-17 on the nervous system remain unexplored. Cell and explant culture techniques, immunohistochemistry, electrophysiology, and Ca2+ imaging were used to examine the impact of IL-17 on adult mouse sympathetic neurons. Receptors for IL-17 were present on postganglionic neurons from superior mesenteric ganglia (SMG). Supernatant from activated splenic T lymphocytes, which was abundant in IL-17, dramatically enhanced axonal length of SMG neurons. Importantly, IL-17-neutralizing antiserum abrogated the neurotrophic effect of splenocyte supernatant, and incubation of SMG neurons in IL-17 (1 ng/ml) significantly potentiated neurite outgrowth. The neurotrophic effect of IL-17 was accompanied by inhibition of voltage-dependent Ca2+ influx and was recapitulated by incubation of neurons in a blocker of N-type Ca2+ channels (ω-conotoxin GVIA; 30 nM). IL-17-induced neurite outgrowth in vitro appeared to be independent of glia, as treatment with a glial toxin (AraC; 5 μM) did not affect the outgrowth response to IL-17. Moreover, application of the cytokine to distal axons devoid of glial processes enhanced neurite extension. An inhibitor of the NF-κB pathway (SC-514; 20 μM) blocked the effects of IL-17. These data represent the first evidence that IL-17 can act on sympathetic somata and distal neurites to enhance neurite outgrowth, and identify a novel potential role for IL-17 in the neuroanatomical plasticity that accompanies inflammation.     

Psychological stress increases dopamine turnover selectively in mesoprefrontal dopamine neurons of rats: reversal by diazepam

The effects of psychological stress on catecholamine and indoleamine metabolism were examined in various brain regions of rats. Psychologically stressed rats were exposed to emotional responses of foot-shocked rats, but were themselves prevented from receiving foot-shock. Psychological stress for 30 min resulted in significant increases of both 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) levels in the medial prefrontal cortex (MPFC), but not in other dopamine (DA) terminal fields. The levels of noradrenaline (NA), serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were unaffected in all brain regions examined after 30 min of psychological stress. A small but significant increase of DOPAC levels in the ventral tegmental area (VTA) was observed after a shorter (10 min) duration of stress. Moreover, an increase of DOPAC levels in the MPFC 30 min after psychological stress was attenuated by diazepam (5 mg/kg), and this attenuating effect was antagonized by Ro 15-1788 (15 mg/kg). These results suggest that mesoprefrontal DA neurons are selectively activated by psychological stress, and that the activation of the A10 cell body site (VTA) may precede that of the terminal field (MPFC). Moreover, diazepam was found to possess an inhibitory effect on the activation of mesoprefrontal DA neurons induced by psychological stress, and this effect may be partly mediated by benzodiazepine (BZD) receptors and implicated in the specific anxiolytic action of BZDs.     

Depolarization increases vasoactive intestinal peptide- and substance P-like immunoreactivities in cultured neonatal and adult sympathetic neurons.

Depolarization has been shown to alter the biosynthesis of a number of neurotransmitters and neuromodulators. In the rat superior cervical ganglion (SCG), for example, depolarization has been reported to increase catecholamine biosynthesis and to decrease the level of substance P. We have recently found that, although the level of vasoactive intestinal peptide (VIP)-like immunoreactivity (IR) is normally low in the SCG, it increases significantly 48 hr after adult ganglia are deafferented in situ or placed in organ culture. Both manipulations decrease electrical activity of postganglionic neurons. To determine whether the increases in ganglionic VIP-IR could be a consequence of decreased depolarization of sympathetic neurons, the effect of depolarization on the expression of VIP-IR was examined in organ cultures of neonatal and adult SCG. Depolarization with elevated K+ (30 mM) or veratridine (1.5 microM) amplified, rather than blocked, the increases in VIP-IR content seen after 24 hr. Further, it increased the number of detectable VIP-IR neuronal cell bodies and processes. The stimulatory effects of veratridine were prevented by TTX. Since similar changes in expression of VIP-IR were evident in dissociated cell cultures of the SCG, cell-cell interactions requiring intact ganglionic architecture are not necessary for altered peptide expression. Elevating the concentration of Mg2+ blocked the ability of K+ and veratridine to increase VIP-IR in dissociated cell culture, raising the possibility that the effects of depolarization on VIP-IR are mediated by increased Ca2+ entry. The depolarizing conditions that increased VIP-IR also increased substance P-IR. While higher concentrations of veratridine (50 microM) blocked the elevation of both VIP- and substance P-IR induced by explantation, they produced significant neuronal death. Since depolarization with either 30 mM KCl or 1.5 microM veratridine increases expression of VIP-IR in neonatal and adult ganglia, decreased depolarization is unlikely to cause the increases in VIP- and substance P-IR that occur in culture. Furthermore, our data raise the possibility that sympathetic nerve activity in vivo can increase expression of these peptides.     

Progesterone selectively increases amygdala reactivity in women

The acute neural effects of progesterone are mediated by its neuroactive metabolites allopregnanolone and pregnanolone. These neurosteroids potentiate the inhibitory actions of gamma-aminobutyric acid (GABA). Progesterone is known to produce anxiolytic effects in animals, but recent animal studies suggest that pregnanolone increases anxiety after a period of low allopregnanolone concentration. This effect is potentially mediated by the amygdala and related to the negative mood symptoms in humans that are observed during increased allopregnanolone levels. Therefore, we investigated with functional magnetic resonance imaging (MRI) whether a single progesterone administration to healthy young women in their follicular phase modulates the amygdala response to salient, biologically relevant stimuli. The progesterone administration increased the plasma concentrations of progesterone and allopregnanolone to levels that are reached during the luteal phase and early pregnancy. The imaging results show that progesterone selectively increased amygdala reactivity. Furthermore, functional connectivity analyses indicate that progesterone modulated functional coupling of the amygdala with distant brain regions. These results reveal a neural mechanism by which progesterone may mediate adverse effects on anxiety and mood.     

BMAA selectively injures motor neurons via AMPA/kainate receptor activation

The toxin beta-methylamino-l-alanine (BMAA) has been proposed to contribute to amyotrophic lateral sclerosis-Parkinsonism Dementia Complex of Guam (ALS/PDC) based on its ability to induce a similar disease phenotype in primates and its presence in cycad seeds, which constituted a dietary item in afflicted populations. Concerns about the apparent low potency of this toxin in relation to estimated levels of human ingestion led to a slowing of BMAA research. However, recent reports identifying potential new routes of exposure compel a re-examination of the BMAA/cycad hypothesis. BMAA was found to induce selective motor neuron (MN) loss in dissociated mixed spinal cord cultures at concentrations (∼ 30 μM) significantly lower than those previously found to induce widespread neuronal degeneration. The glutamate receptor antagonist NBQX prevented BMAA-induced death, implicating excitotoxic activation of AMPA/kainate receptors. Using microfluorimetric techniques, we further found that BMAA induced preferential [Ca²⁺]_i rises and selective reactive oxygen species (ROS) generation in MNs with minimal effect on other spinal neurons. Cycad seed extracts also triggered preferential AMPA/kainate-receptor-dependent MN injury, consistent with the idea that BMAA is a crucial toxic component in this plant. Present findings support the hypothesis that BMAA may contribute to the selective MN loss in ALS/PDC.     

The restructuring of dopamine receptor subtype gene transcripts in c-fos KO mice

Although c-Fos protein is one of the principal molecules in intracellular signaling, c-fos gene disruption is associated with alterations in neuronal functions that do not correspond to its importance in function. The aim of the study was to evaluate the changes of dopaminergic system together with acetylcholinesterase (AChE) in c-fos disruption (KO). KO male mice showed an increase in D₁-like receptor (279% of WT) and D₂-like receptor (345% of WT) binding sites in the cortex. On the gene expression level (assessed by real-time PCR), lower quantities of D₁R-mRNA (0.64) and D₅R-mRNA (0.6) were found in females when compared to males in the frontal cortex, higher D₂R-mRNA in the parietal (1.43) and temporal (2.64) cortex and lower AChE-mRNA (0.67). On the contrary, female striatum contained higher level of D₂R-mRNA (1.62) and AChE-mRNA (1.57) but lower level of D₃R-mRNA (0.73). Hypothalamic D₁R-mRNA, D₂R-mRNA and D₄R-mRNA were higher in females (1.38, 1.63, and 1.68, respectively). Disruption of c-fos increased selectively D₅R-mRNA (1.31) in male parietal cortex, D₂R-mRNA (1.72) in male temporal cortex, and cerebellar D₂R-mRNA in both males (1.43) and females (1.42), respectively. In females, we found rather decrease in DR-mRNA. Multiple correlations in mRNA quantities (in WT mice) were found, which changed considerably upon c-fos KO. Main interactions in WT were inter-regional, CNS of KO underwent an extensive restructuring comprising intraregional interactions in the frontal cortex, hypothalamus, and cerebellum. These changes in DR (between others) could be considered as one of the adaptive mechanisms in c-fos KO mice.     

Maturational increases in c-fos expression in the ascending dopamine systems

The unique maturational period of adolescence is replete with numerous changes in anatomy and function that may yield clues as to why drug abuse emerges at this stage. The behavioral effects of amphetamine are diminished during periadolescence (35 days) relative to younger (21 days) and older (>60 days) rats, prompting us to examine amphetamine effects on neuronal activation with the immediate early gene, c-fos. Amphetamine (1 and 5 mg/kg, i.p.) increased c-fos immunoreactivity in rats 21, 35, and 60 days of age in a dose-dependent manner. When expressed as a percentage of vehicle for each age, amphetamine-induced effects on c-fos immunoreactivity were higher at 21 days of age compared with the effects at 35 and 60 days of age in the nucleus accumbens core and shell, striatum, and prefrontal cortex. These data provide a possible reason as to why stimulants produce dysphoria in children, before transitioning to euphoria during adolescence. Implications of these results are discussed for stimulant use in a pediatric population and the development of drug abuse.     

Single acetylcholine channel currents in sympathetic neurons

Single acetylcholine (ACh) channel currents were studied by the gigaohm patch-clamp technique in cultured sympathetic neurons of the bullfrog, Rana catesbeiana. Recordings were made at 22C oncell-attached and excised membrane patches. When ACh (0.5-1 μM) was present in the pipette, a single class of inward currents was observed with a chrod conductance of 30 pS and reversal potential of -2 mV. The mean channel open time was 11.6 ms at -65 mV and showed little or no voltage-dependence over the range -85 to -45 mV. These channels appear to mediate the fast nicotinic excitatory postsynaptic current.     

Electrical activity increases growth cone calcium but fails to inhibit neurite outgrowth from rat sympathetic neurons.

Previous studies have shown that the growth of axons from both mouse dorsal root ganglion neurons and Helisoma neurons is arrested when the cells are electrically stimulated (Cohan and Kater, 1986; Fields et al., 1990a). Furthermore, in the case of Helisoma neurons, this arrest has been attributed to a rise in the calcium concentration in the growth cones (Cohan et al., 1987). To test the generality of these results, we examined the response of cultured rat superior cervical ganglion (SCG) neurons to electrical stimulation and changes in cytoplasmic calcium. Suprathreshold electrical stimulation of SCG neurons at 10 Hz by extracellular patch electrodes for periods of up to 1 hr had no measurable effect on their rate of growth. In agreement with previous studies, electrical stimulation was accompanied by a rise in the internal calcium concentration: when measured by the fluorescence of fura-2, growth cone calcium levels rose from about 100 nM to greater than 500 nM and then settled to a plateau value of about 350 nM. Despite this increase, however, growth of SCG neurons' processes continued. Our results show that electrical activity is not a universal signal for neurons to stop growing and that a rise in internal calcium does not always arrest the migration of growth cones.     

Development of neurotransmitter phenotypes in sympathetic neurons

This review summarizes the current understanding of neurotransmitter phenotype specification of postganglionic sympathetic neurons, focusing, in particular, on the cellular processes of induction versus trans-differentiation. The emerging evidence is discussed that the noradrenergic and cholinergic neurotransmitter phenotypes are co-induced during early development and that the mature phenotypes develop by positive and negative selection of cellular properties in initially bimodal neurons, depending on extracellular signals during migration and after target contact.