Autoradiographic experiments to examine uptake, anterograde and retrograde transport of tritiated serotonin in the mammalian brain

Subdural engraftment of serotonergic neurons following spinal hemisection restores spinal serotonin, downregulates serotonin transporter, and increases BDNF tissue content in rat

Spinal hemisection injury at T13 results in development of permanent mechanical allodynia and thermal hyperalgesia due to interruption and subsequent loss of descending inhibitory modulators such as serotonin (5-HT) and its transporter (5-HT(T)). We hypothesize that lumbar transplantation of non-mitotic cells that tonically secrete 5-HT and brain-derived neurotrophic factor (BDNF) will restore alterations in 5-HT and 5-HT(T) systems within the spinal dorsal horn. We used an immortalized rat neuronal cell line derived from E13 raphe (RN46A-B14) which is shown to secrete 5-HT and BDNF in vitro and in vivo. Three groups (n=35) of 30 day old male Sprague-Dawley rats were spinally hemisected at T13 and 28 days later received either lumbar RN46A-V1 control empty-vector (n=15) or RN46A-B14 (n=15) intrathecal grafts, or no transplant. Twenty-eight days following transplantation, animals were perfused and tissue examined for changes in 5-HT, 5-HT(T), and BDNF at the site of transplantation or at lumbar enlargements (L5). Immunohistochemistry revealed that RN46A-B14, but not RN46A-V1 cells, increased 5-HT tissue staining at L5 in the dorsal white matter as well as in superficial dorsal horn laminae I and II on both ipsilateral and contralateral sides, results confirmed by ELISA. Transplantation of RN46A-B14 cells significantly reduced ipsilateral 5-HT(T), upregulated after injury. Significantly increased levels of BDNF were also observed after RN46A-B14 transplantation but were not localized to particular spinal laminae. These results are consistent with recovery of locomotor function and reductions in chronic pain behaviors observed behaviorally after RN46A-B14 transplantation and supports the pragmatic application of cell-based therapies in correcting damaged circuitry after spinal cord injury.

The effects of propofol in the area postrema of rats

Propofol has an antiemetic effect that may be mediated by gamma-aminobutyric acid (GABA) influences on the serotonin system, the mechanism of which is not known. We used three techniques, immunohistochemistry, High Performance Liquid Chromatography, and electrophysiology, to define propofol's effects on the rat's brainstem. Paired male Wistar rats received propofol, 20 mg/kg/hr, or Intralipid for 6 h. The brains were then subjected to immunohistochemical analysis of serotonin. In a separate experiment after a propofol or Intralipid infusion, cerebrospinal fluid (CSF) was extracted from the fourth ventricle and analyzed for the amount of serotonin and 5-hydroxyindoleacetic acid. Electrophysiological neuronal recordings were made in the area postrema (AP) in response to propofol with and without a GABA or serotonin antagonist. Results showed that immunohistochemical staining for serotonin in the propofol rats was significantly increased (28 +/- 12%) in the dorsal raphe and decreased in the AP (17 +/- 6%) compared with control. There were no significant changes in the isoflurane-anesthetized animals. Both serotonin and 5-hydroxyindoleacetic acid in the CSF of the fourth ventricle at the level of the AP were significantly reduced by 63% and 36%, respectively. Both propofol and pentobarbital injections reduce AP neuronal activity, but only the propofol response was blocked by bicuculline, a GABA antagonist. We conclude that the reduced levels of serotonin in the AP and the CSF may explain the antiemetic property of propofol. Propofol may also directly act on AP neurons via a GABA(A) receptor to reduce their activity.

IMPLICATIONS

Propofol may produce its antiemetic effect by depleting the area postrema of serotonin as well as by a direct gamma-aminobutyric acid-mediated inhibition.

Effect of hypothalamic 5,7-dihydroxytryptamine lesion on the anterograde transport of serotonin as measured with labeled alpha-methyl serotonin

The anterograde axonal transport of serotonin along the medial forebrain bundle (MFB) 5 days after unilateral dorsal hypothalamic 5,7-dihydroxytryptamine (5,7-DHT) lesion was examined in rat brain. Measurements were done using in vivo synthesized radioactively labeled alpha-methyl serotonin (used here as a serotonin tracer), from i.v. injected labeled alpha-methyl-L-tryptophan, a substitute neurotransmitter and analog of serotonin. Data indicated that after destruction of the presynaptic terminals with 5,7-DHT, the rate of axonal transport of serotonin and/or the rate of serotonin synthesis in the spared and/or damaged neurons was increased. The rate of serotonin anterograde transport on the lesioned side was above 25 mm/d compared to 12.31 +/- 0.49 mm/d on the contralateral side and 12.13 +/- 0.44 mm/d in control (sham injected) rats. The difference in rate between the lesioned and control rats was highly significant; P < 0.005 (two-tailed t-test). The amount of neurotransmitter anterogradely transported along the medial forebrain bundle and/or synthesized in the neurons was 18.8 +/- 4.1 times greater on the side of the 5,7-DHT hypothalamic lesion than that on the contralateral sides or that found in the saline-injected rats. There was no difference in the radioactivity found on the left or right side of the medial forebrain bundle in the rats 5 days after lesion in the rostal midbrain with 5,7-DHT. Five days after the lesion there was also no difference in the anterograde protein transport measured by stereotaxic injection of [3H]proline into the dorsal raphe.

Immunohistochemistry of tryptophan hydroxylase in the rat brain

An antiserum raised against tryptophan tetrahydropterine oxygen oxidoreductase was used to examine in rat brain the immunohistochemical localization of this rate-limiting enzyme catalysing the biosynthesis of serotonin. Tryptophan tetrahydropterine oxygen oxidoreductase was detected in numerous nerve cell bodies, proximal dendrites and axon varicosities or terminals corresponding to those of serotonin neurons as judged by their anatomical distribution and concomitant immunoreactivity to an antiserum against serotonin. In hypothalamus, a serotonin-containing nerve cell group previously visualized in the pars ventralis of the nucleus dorsomedialis by radioautography after serotonin uptake, and by serotonin immunohistochemistry after tryptamine loading, remained tryptophan tetrahydropterine oxygen oxidoreductase-unreactive even in rats treated with colchicine. On the other hand, a small group of tryptophan tetrahydropterine oxygen oxidoreductase-positive cells was identified in the rostrolateral portion of nucleus dorsomedialis, which could play a part in the intrinsic serotonin innervation of hypothalamus. There was no overlap between tryptophan tetrahydropterine oxygen oxidoreductase immunostaining and the cellular distribution of N-acetyl serotonin as reported in earlier studies. It is therefore likely that the synthesis of N-acetyl serotonin from tryptophan does not take place in N-acetyl serotonin-containing neurons.

Selective retrograde labeling in some afferents to the rabbit lateral geniculate nucleus following injections of tritiated neurotransmitter-related compounds

Retrogradely labeled neurons were found in the visual cortex and superior colliculus following D-[3H]aspartate injections in the lateral geniculate nucleus (LGN). In these experiments labeling was also observed over the optic tract. [3H]dopamine and [3H]serotonin injections in the LGN caused weak labeling in a small number of superior colliculus neurons. Furthermore, in [3H]serotonin cases, labeled neurons were also found in the dorsal raphé nucleus. In contrast, when other amino acids or monoamines were injected, no retrograde labeling occurred in any of the afferents to the LGN. These results are largely consistent with the idea of D-[3H]aspartate being a useful marker for pathways using excitatory amino acids as neurotransmitters. The findings in [3H]dopamine and [3H]serotonin experiments indicate that these substances may induce retrograde labeling patterns, which are not related to the transmitter specificity of the pathways concerned.

Gamma-aminobutyric acid interneurons in the rat hippocampal region studied by retrograde transport of glutamic acid decarboxylase antibody after in vivo injections

The retrograde axonal labeling of hippocampal GABA-ergic neurons was studied after in vivo injections of a characterized antibody against glutamic acid decarboxylase (GAD) into different parts of the hippocampal region. Small injections (50 nl) of undiluted GAD antibody into the area dentata (AD) labelled fusiform and dentate pyramidal basket cells within the AD and fusiform and multipolar cells in subfields CA2/CA3 (a and b) of Ammon's horn. The labeled cells were characterized by intense immunoreactivity of the soma and proximal parts of the dendrites, while the nucleus contained little or none. The morphological appearance and laminar positions of these cells corresponded to hippocampal GAD-positive neurons, as shown previously (Ribak et al. 1978) with immunocytochemistry. Injections of anti-GAD into the medial entorhinal area, subiculum, and CA1 region labeled cells in strata oriens, pyramidale and radiatum of CA2 and CA3a, but injections of the antibody into these latter areas failed to label cells in the medial CA1 and subiculum, thus suggesting a preferential organization of hippocampal GABA neuronal projections in a lateral to medial direction. Injections of preimmune sera or antiserum preabsorbed with the pure enzyme antigen GAD failed to label cells in a manner similar to that described for the anti-GAD injections. These observations, taken together with the finding that injections of anti-GAD into the terminal field of non-GABA-ergic pathways never resulted in retrograde axonal transport of the antibody-antigen complex, suggest that the in vivo injection of GAD antibody is a useful method to study the organization of hippocampal GABA-ergic neurons and their projections.

Glial uptake of monoamines in primary cultures of rat median raphe nucleus and cerebellum. A combined monoamine fluorescence and glial fibrillary acidic protein immunofluorescence study

Glial uptake of serotonin and dopamine was studied in primary cultures of the median raphe nucleus and cerebellum by using consecutive demonstration of monoamine fluorescence and glial fibrillary acidic protein immunofluorescence. Most of the glial cells taking up monoamines were glial fibrillary acidic protein positive. Astrocytes with a strong immunoreactivity exhibited monoamine fluorescence only occasionally, although such cells did take up L-dopa readily. Glial fibrillary acidic protein negative cells--morphologically identified as astrocytes--were seen to exhibit monoamine fluorescence after exposure. Glial uptake of serotonin at a concentration of 10(-4) M was detected in cerebellar cultures but not in cultures from the median raphe nucleus. When the concentration was 10(-3) M uptake of serotonin took place in both the areas but was weaker in cultures from the median raphe nucleus. At concentrations greater than 10(-5) M glial uptake of dopamine was detected in cultures from both the regions studied. No region dependent differences in glial uptake of dopamine was demonstrated, however. Based on these observations astrocytes and astrocyte-like glial cells take up dopamine and serotonin. Also glial cells with a remarkably high content of the glial fibrillary acidic protein are more resistant to monoamine uptake than cells exhibiting less intense or no glial fibrillary acidic protein immunofluorescence. The existence of regional differences in uptake of serotonin between the median raphe nucleus and cerebellum suggests that glial uptake of monoamines is not an entirely passive mechanism but may be actively controlled by glial cells in a region dependent manner.