Evidence for dual serotonergic projections to neocortex: axons from the dorsal and median raphe nuclei are differentially vulnerable to the neurotoxin p-chloroamphetamine (PCA)

Involvement of serotonin receptors in methionine sulfoximine-induced hypothermia in the rat

L-Methionine-D,L-sulfoximine (MSO), intraperitoneally (i.p.) or intracerebroventricularly (i.c.v.) (third ventricle) injected at a convulsant dose, induced a centrally mediated body hypothermia in the restrained rat maintained at an ambient temperature of 23 degrees C. Pretreatment with (+/-)-pindolol (1.5-3 mg/kg s.c.) significantly attenuated MSO-induced hypothermia, but at a dose of 6 mg/kg s.c. hypothermia developed without any modification of its characteristics. Pretreatment with (-)-propranolol (16-25 mg/kg i.p.) potentiated MSO-induced hypothermia, but pretreatment of MSO-treated rats with ketanserin (0.7-4 mg/kg i.p.) did not significantly modify hypothermia. Selective antagonists for beta-adrenoceptors were used and their effects on MSO-induced hypothermia were compared with those of pindolol and propranolol. Pretreatment with betaxolol (1.5-4 mg/kg s.c.) did not modify the hypothermia following administration of MSO, but potentiation of hypothermia was recorded in rats pretreated with ICI 118,551 (2.26 mg/kg i.p.) then i.p. injected with MSO. These findings favour a control exerted by 5-HT1 receptors in the central development of MSO-induced hypothermia in the restrained rat.

Effects of p-chloroamphetamine on brain serotonin neurons

p-Chloroamphetamine (PCA) is a useful pharmacologic tool for selectively increasing brain serotonin function acutely by release of serotonin into the synaptic cleft. PCA produces behavioral, neurochemical and neuroendocrine effects believed due to serotonin release after doses in the range of 0.5-5 mg/kg. At higher doses and at longer times, PCA causes depletion of brain serotonin. The mechanisms of this depletion are not well understood but require the serotonin uptake carrier. Antagonism of PCA-induced depletion of brain serotonin is a useful means of assessing the ability of a compound to block the serotonin uptake carrier on brain serotonin neurons. PCA can also be used as a neurotoxic agent to deplete brain serotonin in functional studies, apparently by destroying some serotonergic nerve terminals. Used in this way, PCA has an advantage over 5,6- and 5,7-dihydroxytryptamines in being effective by systemic injection, and it affects brain serotonergic projections with a different neuroanatomic specificity than the dihydroxytryptamines.

Effects of para-chloroamphetamine upon the serotonergic innervation of the rat hippocampus

The ability of hippocampal serotonergic (5-HT) axons to proliferate in response to damage by para-chloroamphetamine (PCA) was examined in this study. Synaptosomal uptake of 5-HT in the hippocampal formation was decreased to 40% of control 3 days after systemic administration of PCA. Six weeks after PCA, uptake values were 44% of control. Retrograde tracing combined with 5-HT immunocytochemistry showed a significant reduction (18% of control) in the number of 5-HT raphe neurons projecting to the hippocampus 3 days after PCA. The number of 5-HT neurons projecting to the hippocampal formation increased to 69% of control by 6 weeks. The dorsal raphe nucleus was not retrogradely labeled after PCA; the increase in labeled neurons was observed in the median raphe nucleus. PHA-L, injections of the median raphe nucleus demonstrated a reduction of raphe axons in the hippocampal formation after PCA. In rats treated with PCA, raphe axons labeled with PHA-L also appeared to have fewer boutons than raphe axons labeled in control cases. The density of PHA-L containing axons in the hippocampal formation of rats injected 3 days and 6 weeks after PCA was less than control but there was no difference between the experimental groups. Based upon the results from synaptosomal uptake and anterograde tracing experiments, we feel that compensatory proliferation of 5-HT axons does not occur within 6 weeks of PCA-induced damage to the 5-HT plexus of the hippocampal formation. The data derived from the retrograde tracing experiment are thought to reflect reduced uptake and transport of WGA-HRP as an acute effect of PCA.

The molecular mechanism of "ecstasy" [3,4-methylenedioxy-methamphetamine (MDMA)]: serotonin transporters are targets for MDMA-induced serotonin release

MDMA ("ecstasy") has been widely reported as a drug of abuse and as a neurotoxin. This report describes the mechanism of MDMA action at serotonin transporters from plasma membranes and secretory vesicles. MDMA stimulates serotonin efflux from both types of membrane vesicle. In plasma membrane vesicles isolated from human platelets, MDMA inhibits serotonin transport and [3H]imipramine binding by direct interaction with the Na(+)-dependent serotonin transporter. MDMA stimulates radiolabel efflux from plasma membrane vesicles preloaded with [3H]serotonin in a stereo-specific, Na(+)-dependent, and imipramine-sensitive manner characteristic of transporter-mediated exchange. In membrane vesicles isolated from bovine adrenal chromaffin granules, which contain the vesicular biogenic amine transporter, MDMA inhibits ATP-dependent [3H]serotonin accumulation and stimulates efflux of previously accumulated [3H]serotonin. Stimulation of vesicular [3H]serotonin efflux is due to dissipation of the transmembrane pH difference generated by ATP hydrolysis and to direct interaction with the vesicular amine transporter.

Differential development of the dual serotoninergic fiber system in the cerebral cortex of the cat

The changes in distribution and density of the serotoninergic innervation of the cerebral cortex were studied in kittens from birth (PO) to 60 days of age (P59). Three cortical areas were sampled: prefrontal, primary auditory, and primary visual areas. Two systems of serotoninergic axons were demonstrated by immunocytochemical techniques: the fine axon system characterized by small fusiform varicosities up to 1 micron in diameter, and the beaded axon system, the fibers of which have round varicosities up to 5 microns in diameter. The density of the two types of fibers across the cortical layers at different ages was measured with a semiautomatic computerized system. In all three areas, the density of fine axons increased steadily from birth, although the pattern of innervation changed from an even distribution at PO to a distinct concentration of the fibers in layers I-III by week 2 in the prefrontal cortex and by week 3 in auditory and visual cortices. By contrast, the beaded axons first appeared in the cortex at week 2 for the prefrontal cortex, at week 3 in auditory and visual areas. Initially, these fibers were distributed throughout all cortical layers and were of much lower density than the fine axons. At later ages the beaded axons became confined to layers I-III where they gradually increased in number, and from week 4, they formed pericellular arrays which were only observed in the prefrontal and auditory cortices, not in visual cortex. These findings provide further evidence for the existence of two parallel subsystems of serotoninergic axons which are different not only in their morphology and nuclear origin, but also in their development. Our finding that the two serotoninergic fiber systems arrive in the cortex in two different stages suggests that they have differential roles in development. The late formation of the pericellular arrays indicates that the formation of the specific connections made by the beaded fibers could be dependent on a certain degree of maturity of the target neurons.

Aging of the serotonergic system in the rat forebrain: An immunocytochemical and neurochemical study

Age-related changes in both morphological and neurochemical parameters of indol- and catecholaminergic system in the rat brain were examined. A qualitative histochemical survey of the occurrence of aberrant serotonergic fibers in the aged rat brain suggests region-specificity in the process of degeneration. Forebrain areas, such as the caudate-putamen complex, globus pallidus, prefrontal and frontoparietal cortices were consistently affected, whereas serotonergic fibers were only infrequently affected in other areas like septal and amygdaloid nuclei. Neurochemical data similarly revealed regional differences. 5-Hydroxytryptamine levels were increased in the frontoparietal cortex, hippocampus, hypothalamus and the mesencephalic raphe region but remained unchanged in the caudate-putamen complex. 5-Hydroxyindolacetic acid levels were also enhanced in all these areas. Examination of brains of 12-, 18- and 24-month-old rats revealed that aberrant serotonergic fibers were already present at the age of 12 months and their incidence increase with age. There was no difference in the number of serotonergic cells in the dorsal raphe nucleus of young and aged rats. Aberrant tyrosine hydroxylase-immunoreactive fibers were observed only infrequently. Their occurrence showed no overlap with the areas containing aberrant serotonergic fibers. Neurochemical estimates of the levels of catecholamines in young versus aged rat brain areas similarly revealed regional and neurotransmitter specific differences to occur during the process of aging.

[3H]paroxetine binding and serotonin content of rat cortical areas, hippocampus, neostriatum, ventral mesencephalic tegmentum, and midbrain raphe nuclei region following p-chlorophenylalanine and p-chloroamphetamine treatment

The agents p-chlorophenylalanine (PCPA) and p-chloroamphetamine (PCA) deplete brain serotonin (5-HT) levels by two different mechanisms; PCPA inhibits the enzyme tryptophan hydroxylase, whereas PCA has a neurotoxic action on certain 5-HT neurons. The parameters of [3H]paroxetine binding to homogenates prepared from the cerebral cortex of rats treated with PCPA, PCA, or saline; vehicle were investigated. The tissue concentrations of 5-HT and 5-hydroxyindole-3-acetic acid (5-HIAA) were also determined by HPLC in the same brain samples. After PCPA treatment, neither the maximum binding capacity (Bmax) nor the dissociation constant (KD) of [3H]paroxetine for the 5-HT uptake recognition site differed from controls despite a substantial reduction in the concentration of 5-HT and 5-HIAA. In contrast, significant changes in both the Bmax and KD values were observed in the cerebral cortex of rats treated with PCA. Furthermore, [3H]paroxetine binding and tissue concentrations of 5-HT and 5-HIAA were measured in the following different regions of the rat brain: cingulate, parietal, and visual cortical areas; dorsal and ventral hippocampus; rostral and caudal halves of neostriatum; ventral mesencephalic tegmentum; and midbrain raphe nuclei region after administration of PCPA, PCA, or saline vehicle. There was an excellent correlation between regional 5-HT levels and specific [3H]paroxetine binding in control and PCA-treated rats although this correlation was lost after PCPA treatment. Under these conditions, the 5-HT innervation remains unchanged whereas the concentration of 5-HT and 5-HIAA is greatly reduced. Thus, [3H]paroxetine binding appears to provide a reliable marker of 5-HT innervation density within the mammalian CNS.

Chronic treatment with meta-chlorophenylpiperazine (m-CPP) alters behavioral and cerebral metabolic responses to the serotonin agonists m-CPP and quipazine but not 8-hydroxy-2(di-N-propylamino)tetralin

The effects of the serotonin (5-HT) agonists meta-chlorophenylpiperazine (m-CPP), quipazine and 8-hydroxy-2(di-n-propylamino)tetralin (DPAT) on behavior and on regional cerebral metabolic rates for glucose (rCMRglc) were measured in control rats or in rats pretreated for 2 weeks with continuous infusion of saline or m-CPP (2.5 mg/kg/day, subcutaneously). rCMRglc was measured in 71 brain regions, using the quantitative autoradiographic [14C]2-deoxy-D-glucose technique, at 15 min after acute administration of m-CPP 2.5 mg/kg, 60 min after quipazine 20 mg/kg, or 10 min after DPAT 1 mg/kg. Behavioral effects were assessed for m-CPP with an activity monitor, for quipazine by counting head shakes and for DPAT by scoring the serotonin syndrome. Chronic m-CPP pretreatment produced tolerance to hypolocomotion induced by acute m-CPP and to head shakes caused by acute quipazine, but did not alter the serotonin syndrome produced by DPAT. m-CPP 2.5 mg/kg IP produced widespread rCMRglc reductions in control rats but failed to modify rCMRglc in any region after chronic m-CPP pretreatment. Quipazine increased rCMRglc in 4 regions in control rats, but reduced rCMRglc in 14 brain areas of chronically m-CPP-pretreated animals. DPAT altered rCMRglc to the same degree in control (25 regions affected) and in chronically m-CPP-pretreated rats (28 regions affected). Reduced behavioral and metabolic effects of acute m-CPP in chronically m-CPP-pretreated rats were not due to pharmacokinetic alterations. These results demonstrate that chronic administration of m-CPP produces behavioral and metabolic tolerance to acute administration of m-CPP, but not of DPAT.(ABSTRACT TRUNCATED AT 250 WORDS)

Lack of effect of high-dose cocaine on monoamine uptake sites in rat brain measured by quantitative autoradiography

There have been a number of claims that high-dose administration of cocaine to rats leads to neurotoxic effects on dopamine neurons. In this study possible neurotoxic effects on monoamine neurons were examined by measuring the effects of cocaine (35 mg/kg daily for 10 days) on the binding of radioligands to uptake sites for dopamine, serotonin and norepinephrine using qualitative autoradiography. No effects of cocaine on any of the binding sites were observed and therefore, it is concluded that cocaine, unlike amphetamine derivatives which have similar pharmacologic properties, does not produce neurotoxic effects on monoamine neurons.

Dual serotoninergic projections to forebrain in the rat: morphologically distinct 5-HT axon terminals exhibit differential vulnerability to neurotoxic amphetamine derivatives

The cerebral cortex of the rat and other mammals is innervated by two morphologically distinct classes of serotoninergic (5-HT) axon terminals: fine axons with minute varicosities and beaded axons characterized by large, spherical varicosities. Fine and beaded 5-HT axons exhibit different regional and laminar distributions in forebrain and arise from separate brainstem nuclei, the dorsal and median raphe nuclei, respectively. The present neuroanatomic study, based on immunocytochemical methods to visualize 5-HT axons, demonstrates that the two axon types differ markedly in their vulnerability to the neurotoxic amphetamine derivatives, methylenedioxyamphetamine (MDA), and p-chloroamphetamine (PCA). While both drugs cause extensive degeneration of fine 5-HT axons throughout forebrain, beaded 5-HT axons are consistently spared. Fine 5-HT axons, which richly innervate most regions of dorsal forebrain in control rats, are rarely seen 2 weeks after treatment with MDA or PCA; this loss of fine axons reflects a marked denervation that persists for months after drug administration. The serotoninergic axon terminals remaining after MDA or PCA administration are almost entirely of the beaded type and appear to be unaffected by both drugs. Over a wide range of doses (2.5-40 mg/kg PCA) and survival times (2 weeks to 2 months), these spared 5-HT axons with large, spherical varicosities cannot be distinguished from the normal, beaded 5-HT axons in control rats by morphologic criteria. Moreover, beaded 5-HT axons exhibit a highly characteristic regional distribution which is the same in control as in MDA- and PCA-treated rats: these axons innervate specific zones or layers within parietal and occipital cortex, hippocampus, cingulate cortex, entorhinal cortex, and the olfactory bulb, among other forebrain areas, and they form a dense plexus lining the ventricular system. Taken together, the results of this study demonstrate that fine 5-HT axons are highly vulnerable to the neurotoxic effects of the amphetamine derivatives MDA and PCA, while beaded 5-HT axons are markedly resistant. These findings are consistent with the hypothesis that there are two anatomically and functionally distinct sets of serotoninergic neurons projecting to forebrain. While both of these neuronal systems utilize 5-HT as a neurotransmitter, they differ in several features: 1) origin from separate nuclei in the brainstem (the dorsal and median raphe), 2) two types of morphologically distinct axon terminals, 3) markedly different distribution and innervation patterns in forebrain, and 4) dissimilar pharmacological properties. The results further suggest that psychotropic amphetamine derivatives have a selective action upon fine serotoninergic axons that arise from the dorsal raphe nucleus.

Serotoninergic innervation of the ferret cerebral cortex. I. Adult pattern

We have investigated the serotoninergic innervation of the adult ferret cerebral cortex with immunohistochemical techniques. Distribution pattern of serotoninergic fibers in the ferret neocortex is characterized by a decrease in the density of fibers as one moves from the pial surface towards the white matter. Throughout the entire cerebral cortex, the serotoninergic fibers are very dense within the supragranular layers, especially within layer 1. In contrast, granular and infragranular layers exhibit only a sparse innervation. Although this general pattern of innervation is roughly the same in all cortical areas, significant variations in the fiber density are apparent in different regions. Areas 17, 1, 6, and 8 (primary visual cortex, presumptive somatosensory cortex, presumptive motor cortex, and prefrontal cortex, respectively) are described in more detail to illustrate the diversity of the serotoninergic innervation patterns. The density of innervation is highest in areas 1 and 6, intermediate in area 8, and lowest in area 17. It is noteworthy that while areas 1, 6, and 8 show a marked decrease in fiber density at the boundary between layer 3 and 4, the less strongly innervated area 17 shows a change in density in the transition from layer 2 to layer 3. The types of fibers found within the ferret cortex are similar to those described in other mammalian species. The bulk of the innervation is made by very fine fusiform axons with small ovoid varicosities. In addition to this fiber type, axons with thick round varicosities and some smooth nonvaricose axons were found. The latter types occur in very small numbers within the supragranular layers and mostly in more anterior cortical regions. While the general innervation pattern and the fiber types are similar to those described in the cat cerebral cortex, the pericellular baskets found in the cat cortex (Mulligan and Törk, J Comp Neurol 270:86-110, 1988) are not seen in the ferret.

Behavioral sensitization to cocaine is not associated with changes in serotonin (5-HT) fiber immunoreactivity in rat forebrain

We investigated whether cocaine-induced behavioral sensitization is associated with changes in serotonin (5-HT) immunoreactivity. Male Sprague-Dawley rats were injected with either cocaine (15 mg/kg, IP) or saline twice daily for seven days. Their behavior was observed and rated for locomotor activation and stereotypy. One day after the final injection, the brains were processed for 5-HT immunohistochemistry. The intensity of 5-HT immunoreactive staining of 5-HT axons and terminal varicosities was blindly rated in cocaine-sensitized rats and found not to differ from saline-treated rats. The results support the hypothesis that unlike some amphetamine derivatives, repeated cocaine administration which results in behavioral sensitization is not neurotoxic to 5-HT axons and terminals in the forebrain.

Serotonin neurotoxicity in rats after combined treatment with a dopaminergic agent followed by a nonneurotoxic 3,4-methylenedioxymethamphetamine (MDMA) analogue

There is increasing evidence linking dopamine (DA) to the long-term serotonergic (5-HT) neurotoxic effects of certain substituted amphetamines such as 3,4-methylenedioxymethamphetamine (MDMA). The present study was undertaken to examine the importance of DA metabolism, uptake inhibition and release in the long-term effects of these drugs by combining various dopaminergic agents with an analogue of MDMA that had low neurotoxic liability, namely 5,6-methylenedioxy-2-aminoindan (MDAI). Monoamine and metabolite levels and the number of 5-HT uptake sites (using [3H]paroxetine binding) were determined 3 hours or 1 week after treatments. Combining the monoamine oxidase inhibitors, clorgyline (MAOA selective) or deprenyl (MAOB selective) with MDAI did not result in any long-term reductions of serotonergic markers. Similarly, combining the DA uptake inhibitor GBR-12909 with MDAI did not result in any long-term changes in monoamine levels at 1 week. In contrast, a single pretreatment of posttreatment with the nonvesicular DA releaser S-amphetamine and MDAI resulted in small but significant long-term changes in monoamine levels. More importantly, if a subacute dosing regimen (every 12 hours for 4 days) was utilized, the combination of S-amphetamine with MDAI resulted in a marked long-term decrease in the levels of cortical, hippocampal and striatal 5-HT, 5-HIAA and the number of 5-HT uptake sites. The results are discussed in terms of the significance of DA and especially nonvesicular DA release in the long-term effects of MDMA-like drugs.

A PHA-L analysis of ascending projections of the dorsal raphe nucleus in the rat

Ascending projections from the dorsal raphe nucleus (DR) were examined in the rat by using the anterograde anatomical tracer, Phaseolus vulgaris leucoagglutinin (PHA-L). The majority of labeled fibers from the DR ascended through the forebrain within the medial forebrain bundle. DR fibers were found to terminate heavily in several subcortical as well as cortical sites. The following subcortical nuclei receive dense projections from the DR: ventral regions of the midbrain central gray including the 'supraoculomotor central gray' region, the ventral tegmental area, the substantia nigra-pars compacta, midline and intralaminar nuclei of the thalamus including the posterior paraventricular, the parafascicular, reuniens, rhomboid, intermediodorsal/mediodorsal, and central medial thalamic nuclei, the central, lateral and basolateral nuclei of the amygdala, posteromedial regions of the striatum, the bed nucleus of the stria terminalis, the lateral septal nucleus, the lateral preoptic area, the substantia innominata, the magnocellular preoptic nucleus, the endopiriform nucleus, and the ventral pallidum. The following subcortical nuclei receive moderately dense projections from the DR: the median raphe nucleus, the midbrain reticular formation, the cuneiform/pedunculopontine tegmental area, the retrorubral nucleus, the supramammillary nucleus, the lateral hypothalamus, the paracentral and central lateral intralaminar nuclei of the thalamus, the globus pallidus, the medial preoptic area, the vertical and horizontal limbs of the diagonal band nuclei, the claustrum, the nucleus accumbens, and the olfactory tubercle. The piriform, insular and frontal cortices receive dense projections from the DR; the occipital, entorhinal, perirhinal, frontal orbital, anterior cingulate, and infralimbic cortices, as well as the hippocampal formation, receive moderately dense projections from the DR. Some notable differences were observed in projections from the caudal DR and the rostral DR. For example, the hippocampal formation receives moderately dense projections from the caudal DR and essentially none from the rostral DR. On the other hand, virtually all neocortical regions receive significantly denser projections from the rostral than from the caudal DR. The present results demonstrate that dorsal raphe fibers project significantly throughout widespread regions of the midbrain and forebrain.

Investigations of origins of serotonergic projection to developing rat visual cortex: a combined retrograde tracing and immunohistochemical study

The present study investigated whether the raphe neurons which give rise to the transient serotonergic fibers in the visual cortex of neonatal rats persist or disappear as the rats mature. Three experiments were performed employing the WGA-apoHRP-Au retrograde transport technique in conjunction with 5-HT or WGA-HRP immunohistochemical staining. WGA-apoHRP-Au was injected into the primary visual cortex of all rats 9 days postnatally. In the first experiment, the animals were examined after 2 days; retrogradely labeled cells were observed in the dorsal raphe nucleus (DR), the median raphe nucleus (MR), and in the B9 and B6 cell groups; the majority (82.5%) of the cells was serotonergic. In the second experiment, the examinations took place following a survival time of 8 weeks: virtually all of the original raphe-visual cortical serotonergic neurons were found to the present. In the third experiment, also performed after 8 weeks relabeling the raphe-visual cortical neurons by WGA-HRP, it was found that 37.2% of the raphe neurons which had projected to the neonatal visual cortex no longer possessed such projections.

Comparison of the effects of repeated oral versus subcutaneous fenfluramine administration on rat brain monoamine neurons: pharmacokinetic and dose-response data

The importance of the route of drug administration (oral vs. subcutaneous) on the neurochemical effects and pharmacokinetics of repeated d,1-fenfluramine administration in rats (1-24 mg/kg b.i.d., i.e., 2-48 mg/kg/day for 4 days) was examined. Overall, comparable dose-dependent alterations in brain monoamine markers were observed following repeated oral (PO) and subcutaneous (SC) administration of fenfluramine. Doses of 1 and 2 mg/kg fenfluramine were without significant effects on the density of 3H-paroxetine-labeled serotonin (5-HT) uptake sites. Higher doses of fenfluramine (4, 12 and 24 mg/kg) produced dose-dependent decreases in 5-HT, 5-hydroxyindoleacetic acid and 5-HT uptake sites with maximal decreases (80-90%) occurring at the 12 mg/kg dose. Fenfluramine administration produced dose-dependent and biphasic effects on brain dopamine markers with increases in homovanillic acid (HVA) observed at 2 hours, whereas decreases in the levels of dopamine, HVA and dihydroxyphenylacetic acid were evident at 18 hours posttreatment. Norepinephrine levels were only decreased at the highest dose of fenfluramine. Significantly higher levels of brain fenfluramine were observed following SC than following PO administration of the drug. On the other hand, comparable levels of its active metabolite norfenfluramine were present in the brain following the two routes of fenfluramine administration. These data suggest the importance of norfenfluramine levels in the brain in determining the high-dose neurotoxic effects of fenfluramine on brain 5-HT neurons in rats.

Neuroanatomic specificity and time course of alterations in rat brain serotonergic pathways induced by MDMA (3,4-methylenedioxymethamphetamine): assessment using quantitative autoradiography

The widely abused "designer" drug MDMA (3,4-methylenedioxymethamphetamine) has been shown to cause marked and long-lasting changes in brain serotonergic systems. The present study uses quantitative in vitro autoradiography of 3H-paroxetine labeled 5-HT uptake sites to assess the time-dependent effects of MDMA on 5-HT neurons in specific neuroanatomic loci. Following treatment with MDMA (20 mg/kg, b.i.d. for 4 days), marked decreases in 5-HT uptake sites were observed in a number of brain regions known to receive projections of 5-HT neurons. These regions included cerebral cortex, caudate nucleus, hippocampus, nucleus accumbens, olfactory tubercle, superior and inferior colliculi, geniculate nuclei, and most thalamic nuclei. In contrast, other areas such as the septal nuclei and some thalamic nuclei which also receive 5-HT projections were not substantially affected by this drug. In most regions, decreases in 5-HT uptake sites occurred within 24 hours of the last dose of MDMA and persisted at the 2 week time point. Some regions such as dorsal striatum exhibited a time-dependent reduction with greater reductions occurring at 2 weeks rather than immediately following the MDMA treatment regimen. The density of 5-HT uptake sites in other regions such as endopiriform nucleus and substantia nigra at the 2 week versus 18 hour time point indicated some degree of region-specific recovery. Regions which demonstrated no significant reduction in 5-HT uptake sites included the dorsal and median raphe nuclei, ventral tegmental area, central grey, interpeduncular nucleus, locus coerulus, pontine reticular formation and cerebellum. Likewise, regions containing 5-HT axons of passage (e.g., indusium griseum and lateral hypothalamus) appeared to be insensitive to the neurotoxic effects of MDMA on 5-HT neurons. Furthermore, the neurotoxic effects of MDMA showed specificity in that the catecholamine neurons labeled by 3H-mazindol were unaffected by the treatment regimen. These data indicate that the preferential degeneration of serotonergic neurons by MDMA is mediated primarily at 5-HT terminal regions, whereas regions containing 5-HT perikarya and axons of passage remain relatively unaffected. In addition, the observed time-dependent reductions and recovery of 5-HT uptake sites which were detected within 2 weeks of the treatment regimen in certain brain regions suggest region-specific differences in recovery of 5-HT systems from MDMA-induced lesion.

[3H]monoamine releasing and uptake inhibition properties of 3,4-methylenedioxymethamphetamine and p-chloroamphetamine analogues

The ability of several 3,4-methylenedioxymethamphetamine (MDMA) analogues to inhibit the uptake of [3H]serotonin (5-HT), dopamine (DA) and norepinephrine (NE) into synaptosomes was examined. In addition, the ability of the compounds to inhibit the uptake of [3H]5-HT and DA into synaptosomes from rats pretreated with reserpine (5 mg/kg i.p., 16 h pretreatment) was compared to control experiments. All of the test compounds were found to be potent releasers of non-vesicular 5-HT (the reserpine IC50 was significantly smaller than the control IC50). The range of 5-HT inhibitory activity corresponds well to the small range of ED50 values of the test compounds to substitute in drug discrimination experiments with animals trained to discriminate MDMA or S-(+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-aminobutane (S-MBDB) from saline. In contrast, there was a wide range of potency for the inhibition of NE and DA uptake. In addition, several of the analogues appeared to be pure uptake inhibitors of DA while others were found to be releasers of non-vesicular DA. Several of the compounds were very selective for 5-HT over DA or NE uptake inhibition, including 3-methoxy-4-methylamphetamine (MMA) and 5-methoxy-6-methyl-2-aminoindan (MMAI). A correlation was noted between the 5-HT neurotoxic potential of some of the test compounds and their relative ability to induce a release of non-vesicular DA. The potential catechol metabolites of the methylenedioxy-substituted compounds also showed potent monoamine releasing properties, suggesting that metabolism may play a role in the neurotoxic actions of some of these drugs. The present data support the hypothesis that drug-stimulated non-vesicular 5-HT release is primarily responsible for the discriminative cue of MDMA.

Parachloroamphetamine selectively alters regional cerebral metabolic responses to the serotonergic agonist metachlorophenylpiperazine in rats

To determine if reported reductions of regional cerebral metabolic rates for glucose (rCMRglc) induced by the 5-HT agent metachlorophenylpiperazine (MCPP) (2.5 mg/kg) are due to a presynaptic action, 3-month old Fischer-344 rats were given parachloroamphetamine (PCA), a serotonin neurotoxin, and rCMRglc was measured 1 or 3 weeks later with the quantitative autoradiographic [14C]2-deoxyglucose procedure in 74 brain regions after administering saline, MCPP or other drugs. PCA alone increased rCMRglc significantly only in the raphe nuclei and in visual structures (visual cortex, lateral geniculate, superior colliculus). MCPP alone reduced rCMRglc in 75% of the regions studied. In PCA-lesioned rats, metabolic responses to MCPP 2.5 mg/kg were virtually abolished and rCMRglc was increased in interanteromedial and centrolateral thalamic nuclei. rCMRglc responses to quipazine, a postsynaptic serotonin agonist, and to arecoline and bromocriptine, cholinergic and dopaminergic agonists, were unchanged by PCA-pretreatment. Selective abolition by PCA of the metabolic response to MCPP confirms that MCPP, at the dose studied, reduces rCMRglc in the forebrain via a presynaptic mechanism and that postsynaptic serotonergic function is not altered by PCA.

Ultrastructural features of the serotonin innervation in adult rat hippocampus: an immunocytochemical description in single and serial thin sections

This study was aimed at characterizing the fine-structural features of the normal serotonin (5-HT) innervation in adult rat hippocampus, by means of electron microscopic immunocytochemistry with a polyclonal antiserum against 5-HT-glutaraldehyde-protein conjugate (donated by Michel Geffard, Bordeaux). Two hippocampal sectors were examined, at mid-level along the septo-temporal axis: CA3-a of Ammon's horn and crest of the dentate gyrus (DG-c). A large number of axonal varicosities (terminals) were sampled in single ultrathin sections, to achieve a statistically significant comparison of their size and of their relative frequency of synaptic specialization, junctional targets and juxtaposed elements, between the oriens and the radiatum layer of CA3-a, and the molecular and the polymorph layer of DG-c. In both CA3-a layers, the microenvironment of the immunostained terminals was also compared to that of a population of unlabeled varicosities randomly selected from the same micrographs. Moreover, 57 varicosities from the oriens and the radiatum layer of CA3-a were visualized in a long series of thin sections, allowing for their examination from end to end in 43 instances. As measured in single sections, hippocampal 5-HT varicosities were of comparable diameter (0.57 microns on the average) in the two anatomical sectors and four neuropil layers examined. As extrapolated stereologically to whole varicosities, the proportion making a synaptic membrane specialization (synaptic incidence) ranged from 18 to 33% (average of 24%), without statistically significant differences between the two sectors and four layers. The synaptic incidence determined directly from serial sections of CA3-a (18%) was nearly identical to that extrapolated from single sections (18.1% in the oriens and 19.5% in the radiatum layer). In both CA3-a and DG-c, the 5-HT varicosities showing a junctional complex were slightly larger than their non-junctional counterparts. In CA3-a, only dendritic shafts were targeted by synaptic 5-HT varicosities, whereas in DG-c there were also a few axo-spinous synapses. The microenvironment of CA3-a 5-HT varicosities differed markedly from that of randomly selected unlabeled varicosities, due to its much lower frequency of synaptic targets and higher frequency of juxtaposed axonal varicosities, at least in the radiatum layer. In all four layers examined, other axonal varicosities were indeed the most frequently encountered neuronal element in the immediate vicinity of immunostained 5-HT varicosities. Neurites and dendritic shafts were also common, but dendritic spines (4%) were relatively infrequent.(ABSTRACT TRUNCATED AT 400 WORDS)

The organization of serotonergic projections to cerebral cortex in primates: retrograde transport studies

Retrograde axonal transport and immunocytochemical methods were utilized to determine the origin of serotonergic afferents to selected primary projection and association areas of cerebral cortex in macaque monkeys. After injections of Fast Blue or Diamidino Yellow in primary motor, somatosensory, or visual cortex, retrogradely labeled neurons are found in both the dorsal and median raphe nuclei. The sets of dorsal raphe neurons which innervate these cortical areas differ in their spatial distributions along the rostrocaudal axis of the brainstem; a coarse rostrocaudal topographic relationship is found between these groups of dorsal raphe neurons and their cortical targets. In contrast, neurons in the median raphe which innervate these primary projection areas are not differentially distributed along the rostrocaudal axis. However, in both the median and dorsal raphe nuclei, most neurons projecting to primary visual cortex are situated lateral to the cells which project to motor and somatosensory areas; many of these visually projecting neurons lie among the fascicles of the medial longitudinal fasciculus. For comparison with the serotonergic innervation of primary projection areas, the locations of raphe cells projecting to three areas of association cortex were examined: dorsolateral prefrontal cortex, area 5 and area 7b. Neurons projecting to each of these association areas are found throughout the dorsal and median raphe nuclei. Their distributions are similar to one another; however, more cells projecting to dorsolateral prefrontal cortex are in the rostral part of the dorsal raphe. The dorsal and median raphe neurons projecting to these association areas are intermingled with neurons projecting to motor and somatosensory cortex, but are medial to most of those projecting to visual cortex. Thus, separate cortical areas are innervated by different sets of raphe neurons; these sets partially overlap, yet differ in their rostrocaudal and mediolateral distributions. Ascending serotonergic projections to cerebral cortex form a widely distributed system which exhibits a highly intricate anatomic organization. The present observations support the hypothesis that the dorsal raphe nucleus is comprised of distinct sets of neurons whose output is distributed to multiple, interconnected cortical areas; these serotonergic projections may play a role in the coordination of excitability in functionally related areas of cortex. In contrast, the serotonergic projections arising from the median raphe appear to be more divergent and are likely to have a global influence on cortical activity. Since these individual raphe nuclei have different projection patterns, they are likely to have distinct functional roles.

5-Iodo-2-aminoindan, a nonneurotoxic analogue of p-iodoamphetamine

A rigid analogue, 5-iodo-2-aminoindan (5-IAI), of the serotonin neurotoxic halogenated amphetamine p-iodoamphetamine (PIA) was pharmacologically evaluated for production of serotonin neurotoxicity. A comparison was also made between 5-IAI and PIA in the two-lever drug discrimination paradigm in rats trained to discriminate saline from 3,4-methylenedioxymethamphetamine (MDMA) or saline from the alpha-ethyl homologue of MDMA, MBDB. PIA and 5-IAI were both behaviorally active, and fully substituted in both groups of animals, but were considerably less potent than p-chloroamphetamine (PCA). PIA had about twice the potency of PCA as an inhibitor of [3H]-5-HT uptake in rat brain cortical synaptosomes, while 5-IAI was only about 75% as potent as PCA in this assay. A single 40 mg/kg dose of PIA resulted in a 40% reduction of 5-HT and 5-HIAA levels and in the number of 5-HT uptake sites in rat cortex at one week sacrifice. The same dose of 5-IAI with one week sacrifice led to about a 15% decrease in 5-HIAA levels and number of 5-HT uptake sites, but only the latter was statistically significant. In rat hippocampus, PIA gave significant decreases in all serotonin markers examined, while 5-IAI slightly but significantly decreased only 5-HT levels. Neither compound produced any change in catecholamine or catecholamine metabolite levels. The results confirm earlier reports of the selective serotonin neurotoxicity of PIA, which is less severe than that of PCA, and also demonstrate that its rigid analogue 5-IAI does not appear to cause significant serotonin deficits in the rat.

The organization of serotonergic projections to cerebral cortex in primates: regional distribution of axon terminals

Serotonergic axons are widely distributed in the primate forebrain and represent the most abundant ascending projection from the reticular formation. Immunocytochemical methods have been utilized to examine the density, laminar distribution and morphology of serotonergic axons in both primary projection (motor, somatosensory) and association areas (dorsolateral prefrontal, area 5) as well as in the hippocampus and in cingulate cortex of rhesus and cynomolgus macaques. Serotonergic axons are present in all areas of cortex examined, and all cortical layers receive serotonergic afferents. However, the intracortical distribution of serotonergic axon terminals is not uniform; rather, different regions of cortex exhibit dissimilarities in both the density and laminar distribution of serotonergic axons. Thus, there are local patterns of serotonin innervation that are characteristic of each cortical area. Highly diverse patterns of serotonin innervation are found in heterotypical areas of cortex; more subtle variations are present among homotypical areas. Two morphologic types of serotonergic axon terminals, fine and beaded, are present in all cortical areas, and they typically exhibit different laminar distributions: in most areas of neocortex, beaded axons predominate in layer I while fine axons predominate in layers II-VI. However, exceptions to this pattern were observed in primary visual cortex and in the hippocampal formation. The distinctive local patterns of serotonin innervation observed in this study indicate that raphe-cortical projections are likely to have differential influences on particular cytoarchitectonic areas of cerebral cortex in the primate. Moreover, the discrete laminar distribution of serotonin axons suggests that serotonergic projections selectively innervate particular neuronal elements in cerebral cortex. The present findings suggest that the two classes of serotonergic axons, fine and beaded, which have different patterns of termination, affect different sets of cortical neurons. In addition, these two serotonergic projections may be associated with different sets of serotonergic receptors and thus produce selective effects on cortical function.

The source of the transient serotoninergic input to the developing visual and somatosensory cortices in rat

For approximately the first two weeks of life, dense serotonin immunoreactivity closely matches the pattern of thalamocortical axons innervating both the granular portion of the primary somatosensory cortex and area 17 in rodents [D'Amato et al. (1987) Proc. natn. Acad. Sci. 84, 4322-4326; Fujimiya et al. (1986) J. comp. Neurol. (1986) 246, 191-201; Rhoades et al. (1990) J. comp. Neurol. 293, 190-207]. This serotonin immunoreactivity is not contained in thalamocortical axons [Rhoades et al. (1990) 293, 190-207] but its source has never been demonstrated. In the present study, a variety of approaches were used to address this issue. The combination of electron microscopy and immunocytochemistry showed that all serotonin immunoreactivity in the developing cerebral cortex was contained in axons and that the terminals of many of these fibers made synapses with the dendrites of cortical cells. Treatment with fluoxetine, a specific inhibitor of serotonin uptake, did not result in a loss of the cortical pattern of serotonin immunoreactivity, indicating that immunoreactive fibers were not labeled solely as a result of serotonin uptake. The combination of retrograde tracing from the primary somatosensory cortex and area 17 with immunocytochemistry demonstrated numerous double-labeled cells in nucleus raphe dorsalis and the median raphe nucleus. Smaller numbers of double-labeled neurons were located in the B9 cell group and the region of the lateral midbrain tegmentum. Large electrolytic lesions that included most of the nucleus raphe dorsalis and median raphe nucleus, but which left the B9 group and more caudal serotoninergic cells undamaged, caused either a substantial reduction in density or complete disappearance of the serotonin pattern in both hemispheres. Unilateral electrolytic lesions of the medial forebrain bundle resulted in a loss of the pattern only on the side of the damage. Injection of the neurotoxin 5,7-dihydroxytryptamine directly into the mesencephalon either abolished or substantially reduced the density of the cortical serotonin immunoreactivity. Injections that produced substantial cell loss in the median raphe nucleus, but only minor cell loss in the nucleus raphe dorsalis had little effect upon the cortical pattern of serotonin immunoreactivity. These results indicate that the dense serotonin immunoreactivity which appears transiently in the visual and somatosensory cortices of perinatal rodents is contained in serotoninergic axons that arise from cells in the nucleus raphe dorsalis and perhaps also the median raphe nucleus.

Behavioral, biochemical and neurotoxicological actions of the alpha-ethyl homologue of p-chloroamphetamine

The present set of experiments was designed to examine the effects of extension of the alpha-methyl of p-chloroamphetamine (PCA) to an alpha-ethyl. Therefore, the alpha-ethyl homologue of PCA, 1-(4-chlorophenyl)-2-aminobutane (CAB), was compared to PCA in a number of pharmacological assays. CAB was 2-fold less potent than PCA at inhibiting synaptosomal uptake of [3H]5-hydroxytryptamine ([3H]5-HT), and 5-fold less potent at inhibiting uptake of [3H]dopamine ([3H]DA). In drug discrimination assays, CAB was approximately 3-fold less potent than PCA in animals trained to discriminate 3,4-methylenedioxymethamphetamine (MDMA) or its alpha-ethyl homologue, S-(+)-N-methyl-1-(1,3-benzodioxol-5-yl)-2-butanamine (S-(+)-MBDB), from saline. Monitoring with in vivo microdialysis, 10 mg/kg of PCA caused a large increase in extracellular DA and a significant decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) in the striatum. In contrast, 11 mg/kg CAB caused no increase and 22 mg/kg CAB caused only a slight increase in extracellular DA. Both doses of CAB caused a decrease in extracellular DOPAC. The potential 5-HT neurotoxicity of CAB was examined by measuring monoamine and metabolite levels and [3H]paroxetine binding at one week following acute doses. A 10 mg/kg dose of PCA caused an 80% decrease in cortical and hippocampal serotonergic markers, while an equimolar dose of CAB decreased only hippocampal 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels. However, 22 mg/kg of CAB produced a 20-40% decrease in all serotonergic markers. Thus, extension of the alpha-alkyl significantly decreases the dopaminergic effects of PCA. The similar decrease in relative 5-HT neurotoxicity and the decreased ability to alter dopaminergic systems in vivo and in vitro supports the involvement of DA in the neurotoxicity of PCA.

Methylenedioxymethamphetamine-induced hyperthermia and neurotoxicity are independently mediated by 5-HT2 receptors

Methylenedioxymethamphetamine (MDMA) produced a significant hyperthermia in rats which was antagonized in a competitive manner by the selective 5-HT2 antagonist, MDL 11,939. The 5-HT antagonist also blocked MDMA-induced neurotoxicity as assessed by the decline in regional 5-HT concentrations observed 1 week later. These two effects of MDL 11,939 were dissociated at higher doses of MDMA where the antagonist still provided virtually complete protection against the neurochemical deficits but only partially attenuated the hyperthermic response. In contrast to the effect of the 5-HT2 antagonist, haloperidol did not alter MDMA-induced hyperthermia but did antagonize its long-term neurochemical effects. Similarly, coadministration of the selective 5-HT uptake inhibitor, MDL 27,777, did not affect the hyperthermia produced by a high dose of MDMA but completely prevented the depletion of 5-HT. When the MDMA-induced hyperthermia was prevented by temporarily maintaining animals at reduced ambient temperature, the neurochemical changes normally observed 1 week later were also blocked. Although these results demonstrate that the drugs tested do not antagonize MDMA-induced neurotoxicity by interfering with its effect on body temperature, they do indicate that MDMA-induced hyperthermia may contribute to the development of the drug's long-term neurochemical effects.

Neurotoxicity of MDMA and related compounds: anatomic studies

The cytotoxic effects of amphetamine derivatives were studied by immunocytochemistry to identify the cellular compartments affected by these drugs, to obtain morphologic evidence of neuronal degeneration, and to assess the potential for regeneration. The substituted amphetamines, MDA, MDMA, PCA, and fenfluramine, all release serotonin and cause acute depletion of 5-HT from most axon terminals in forebrain. (1) Unequivocal signs of axon degeneration were seen at 36-48 hour survivals: 5-HT axons exhibited increased caliber, huge, swollen varicosities, fragmentation, and dilated proximal axon stumps. (2) Fine 5-HT axon terminals were persistently lost after drug administration, while beaded axons and raphe cell bodies were spared. These two types of 5-HT axons, which arise from separate raphe nuclei and form distinct ascending projections, are differentially vulnerable to psychotropic drugs. (3) From 2-8 months after treatment, there was progressive serotonergic re-innervation of neocortex along a fronto-occipital gradient. Longitudinal 5-HT axons grew into layers I and VI from rostral to caudal, before sprouting into middle cortical layers; this bilaminar pattern of growth simulates perinatal development of 5-HT innervation. This study demonstrates differential vulnerability of 5-HT projections, evidence for axonal degeneration, and sprouting of 5-HT axons leading to re-innervation of forebrain. While the sprouting axons are anatomically similar to the type that was damaged, it is not known whether a normal pattern of innervation is re-established.

Increased tryptophan hydroxylase mRNA in raphe serotonergic neurons spared by 5,7-dihydroxytryptamine

Neurons expressing the tryptophan hydroxylase (TPH) mRNA within the raphe nuclei of control rats showed a distribution similar to that observed using an antibody for TPH. Numerous packed cells expressing the TPH mRNA were observed in the ventral and dorsal zone of the nucleus raphe dorsalis (NDR) and in the pars dorsalis of the nucleus centralis superior (NCS) whereas fewer and more scattered neurons were found in the pars medialis of NCS. Five days after the intracerebroventricular injection of 5,7-dihydroxytryptamine (5,7-DHT), which markedly reduced the serotonin (5-HT) content in the hippocampus, caudate putamen and cortex, the hybridization signal had completely disappeared in the dorsal region of the NDR. In the ventromedial region, above and between the medial longitudinal fasciculus (MLF), which includes the pars dorsalis of NCS, there was a partial decrease of cell number and a marked increase of the grain density over spared neurons. No significant change was noted in the number of TPH-positive cells and hybridization signal in individual neurons of the pars medialis of NCS. Consistent with previous evidence of increased TPH activity in the residual 5-HT terminals, the present study shows that synthesis of the TPH mRNA may be augmented in some neurons surviving the lesion.

Unlike systemic administration of p-chloroamphetamine, direct intracerebral injection does not produce degeneration of 5-HT axons

Systemic administration of the amphetamine derivative p-chloroamphetamine (PCA) causes degeneration of 5-HT axon terminals in rat brain. The present study was designed to determine whether PCA induces neurotoxic effects by a direct action on 5-HT axon terminals. PCA was administered by microinjection directly into the cerebral cortex of rats. Continuous intracerebral infusions were made over extended time periods (10 min-48 h) to explore whether the induction of neurotoxicity requires a prolonged exposure of axon terminals to the drug. Two weeks after drug administration, brain sections that passed through the injection site were processed for 5-HT immunohistochemistry. The 5-HT innervation of cerebral cortex in PCA-injected animals was compared with that after intracortical injection of saline or of 5,7-dihydroxytryptamine. The results demonstrate that, in the concentrations used, direct application of PCA into the neocortex does not elicit axonal degeneration, even after a continuous infusion for 2 days. This finding suggests that PCA itself is not directly toxic to 5-HT axons.

Distribution of two morphologically distinct subsets of serotoninergic axons in the cerebral cortex of the marmoset

The serotoninergic innervation of the marmoset (New World monkey, Callithrix jacchus) cerebral cortex has been analyzed by using immunocytochemistry. The use of a sensitive monoclonal antibody against serotonin allowed the visualization of the fine morphology of individual axons. Two types of terminal axons were demonstrated: one has sparse, small, ovoid varicosities (dia. less than 1 micron), and the other has large, spheroidal varicosities (up to 5 microns in dia.), which are more densely clustered. The first type of axon is distributed through all cortical layers, with a characteristic laminar distribution that varies from area to area. The second type of axons was distributed sparsely in all regions but was markedly denser in the frontal and anterior parietal lobes, and in the hippocampal formation. Axons with large varicosities typically surrounded certain cell bodies and proximal dendrites, forming pericellular arrays, or baskets. These morphological specializations were most frequent in the frontal and anterior parietal cortex, where they were found around stellate and horizontal cells in layer I and around stellate and bipolar cells in layer II and III. Similar baskets were also found in the hippocampal formation, mainly along the border between the hilus and the granule cell layer of the dentate gyrus, across the CA4 field, and at each side of the pyramidal cell layer of the CA3 regions. The distribution and cellular morphology of the cell surrounded by the 5-HT basket fibres were suggestive of a subpopulation of interneurons, possibly GABAergic and/or peptidergic. In agreement with previous reports on the innervation of the cerebral cortex of other mammalian species, the marmoset cerebral cortex is innervated by two separate subsystems of serotoninergic axons. One of these may have a strong and specific influence on the cortical inhibitory circuitry, via relay through cortical interneurons.

Comparative serotonin neurotoxicity of the stereoisomers of fenfluramine and norfenfluramine

The optical isomers of fenfluramine and norfenfluramine were administered to rats to examine their relative potency for destruction of serotonin neurons. Rats were sacrificed one week following a single 10 mg/kg SC injection of one of the four compounds and monoamine and metabolite levels in the frontal cortex and hippocampus brain regions were examined by HPLC-EC techniques. In addition, [3H]-paroxetine binding to homogenates of these brain regions was determined. With the exception of hippocampal 5-HT levels following d-fenfluramine treatment, there was a decrease in all the serotonergic markers assayed, following treatment with the d-enantiomers of fenfluramine and norfenfluramine. No decrease in any serotonergic marker was seen at this dose following treatment with the l-enantiomers of fenfluramine or norfenfluramine. Also, none of the drug treatments resulted in a significant decrease in catecholamines or their metabolites. With all the serotonergic markers examined, d-norfenfluramine was found to cause a significantly greater decrease than d-fenfluramine. The significance of these results is discussed in terms of the hypothesis that the long-term serotonergic deficits observed with d-fenfluramine may result from its metabolite, d-norfenfluramine.

alpha-Methyl-p-tyrosine partially attenuates p-chloroamphetamine-induced 5-hydroxytryptamine depletions in the rat brain

alpha-Methyl-p-tyrosine (AMT) partially attenuates the long-term p-chloroamphetamine (pCA)-induced 5-hydroxytryptamine (5-HT) depletions. Pretreatment of rats with the tyrosine hydroxylase inhibitor AMT before treatment with the serotonin neurotoxin pCA decreased the extent of 5-HT depletion in the two brain regions examined. In these experiments, rats were administered AMT (150 mg/kg) 1 and 5 hours prior to an injection of pCA (5, 10, or 15 mg/kg). AMT reduced the pCA-induced 5-HT depletions in the striatum and to a lesser extent in the hippocampus. Furthermore, the attenuation of neurotoxicity was dependent on dose of pCA, with greater AMT effects at higher doses of pCA. AMT-pretreated rats were still significantly depleted of brain 5-HT following all doses of pCA. However, at the higher doses of pCA, the AMT-pretreated rats were significantly less depleted than saline-pretreated, pCA-treated rats. These results suggest that the neurotoxic effects of high doses of pCA on 5-HT-containing nerve terminals may be in part dependent on the availability of newly synthesized dopamine (DA).

Effects of high-dose fenfluramine treatment on monoamine uptake sites in rat brain: assessment using quantitative autoradiography

Fenfluramine is an amphetamine derivative that in humans is used primarily as an anorectic agent in the treatment of obesity. In rats, subchronic high-dose d,l-fenfluramine treatment (24 mg/kg subcutaneously, twice daily for 4 days) causes long-lasting decreases in brain serotonin (5HT), its metabolite 5-hydroxyindoleacetic acid, and high-affinity 5HT uptake sites. Moreover, this high-dose treatment regimen causes both selective long-lasting decreases in fine-caliber 5HT-immunoreactive axons and appearance of other 5HT-immunoreactive axons with morphology characteristic of degenerating axons. Determination of the potential neurotoxic effects of fenfluramine treatment using immunohistochemistry is limited from the perspectives that staining is difficult to quantify and that it relies on presence of the antigen (in this case 5HT), and the 5HT-depleting effects of fenfluramine are well known. In the present study, we used quantitative in vitro autoradiography to assess, in detail, the density and regional distribution of [3H]paroxetine-labeled 5HT and [3H]mazindol-labeled catecholamine uptake sites in response to the high-dose fenfluramine treatment described above. Because monoamine uptake sites are concentrated on monoamine-containing nerve terminals, decreases in uptake site density would provide a quantitative assessment of potential neurotoxicity resulting from this fenfluramine treatment regimen. Marked decreases in densities of [3H]paroxetine-labeled 5HT uptake sites occurred in brain regions in which fenfluramine treatment decreased the density of 5HT-like immunostaining when compared to saline-treated control rats. These included cerebral cortex, caudate putamen, hippocampus, thalamus, and medial hypothalamus. Smaller, but nonetheless significant, decreases in density of [3H]paroxetine-labeled 5HT uptake sites were noted in brain regions in which partial sparing of 5HT-like immunoreactive fibers had been reported following fenfluramine treatment, specifically septum, lateral hypothalamus, and amygdala. In contrast, [3H]mazindol autoradiography revealed that total catecholamine (i.e., dopamine and norepinephrine) uptake sites in cerebral cortex, caudate putamen, and locus coeruleus, areas in which [3H]paroxetine-labeled 5HT uptake sites were significantly decreased, were unaffected by this fenfluramine treatment. These data support the hypothesis that subchronic, high-dose fenfluramine treatment causes selective degeneration of 5HT axons in rat brain. Since pharmacokinetic studies show that the dosing regimen used in this study exposes rat brain to concentrations of fenfluramine that are approximately 600 times greater than those resulting from the therapeutic oral dose, caution must be exercised in extrapolating these data to humans.

Drugs affecting serotonin neurons

Advances in serotonin pharmacology, the development of drugs that intervene at specific sites to modify serotonergic function, have accompanied advances in the understanding of physiologic roles of serotonin present in neurons and elsewhere and of serotonin receptors that are widely distributed in brain and many peripheral tissues. The pharmacologic advances have sometimes been stimulated by developments in serotonin physiology, such as the recognition of multiple serotonin receptor subtypes, and in other cases have been a major factor in providing new insights into physiologic roles of serotonin. Drugs that modify serotonin function have a variety of therapeutic applications currently and many more potential therapeutic uses to be explored in the future. Having drugs that act with high specificity or selectivity on particular enzymes in serotonin biosynthesis, on particular serotonin receptors, or at other sites such as uptake carriers for serotonin not only offers the hope of improved clinical therapy in diseases caused by abnormal serotonergic function or in which alteration of serotonergic function can alleviate symptoms, but also provides valuable pharmacologic tools for learning more about serotonin physiology and probing the functional status of serotonergic systems. The next few years promise to yield important new serotonergic drugs.

Ultrastructural relationships of serotonin axon terminals in the cerebral cortex of the adult rat

PAP immunocytochemistry with an antiserum against serotonin (5-HT)-glutaraldehyde-protein conjugate (kindly donated by M. Geffard) was used to analyze the ultrastructural relationships of 5-HT axon terminals (varicosities) in the frontal (Fr1-Fr2), parietal (Par1), and occipital (Oc1M-Oc2) cortex of adult rats. One hundred-forty-five immunostained varicosities from Fr1-Fr2 (54 from layers I-II; 91 from layer VI) and 97 each from the upper layers (I-II) of Par1 and OcM1-Oc2 were examined in groups of serial thin sections (mean number of sections in series: 3.2 to 7.3). These terminals were of comparable shape and size in the 4 cortical sectors examined, and averaged 0.66 +/- 0.2 microns in mean diameter. The proportion of varicosities engaged in synaptic contact was evaluated by linear transformation of the relationship between the frequency of observed synaptic junctions and the number of thin sections available for examination. Reliability of the sampling was evidenced by a high coefficient of correlation (r greater than 0.95) in each cortical sector. The synaptic incidence extrapolated for whole varicosities ranged from 28% (layer VI of Fr1-Fr2) to 46% (Par1), without statistically significant differences between the 4 sectors examined. The interregional mean could thus be evaluated at 38%. The synaptic 5-HT terminals always made asymmetrical junctions, which were exclusively found on dendritic spines and shafts, and appeared more frequent on spines than shafts in the deep frontal and the upper occipital cortex. In all 4 sectors, dendritic shafts and spines and other axonal varicosities were frequently encountered in the immediate microenvironment of the immunostained varicosities. It is concluded that the cortical 5-HT innervation is predominantly nonjunctional throughout the neocortex of the adult rat, which reinforces earlier views of a highly divergent afferent system with particular functional properties and perhaps capable of widespread, global and/or sustained influences in this part of the brain.

Depletion of serotonin using p-chlorophenylalanine (PCPA) and reserpine protects against the neurotoxic effects of p-chloroamphetamine (PCA) in the brain

The present study attempts to determine whether the neurotoxicity of p-chloroamphetamine (PCA) is dependent on a releasable pool of serotonin (5-HT). Rats treated with PCA alone or with reserpine and PCA exhibit a profound loss of 5-HT innervation in cerebral cortex after a 2-week survival period. However, depletion of 5-HT by combined treatment with p-chlorophenylalanine (PCPA) and reserpine provides substantial protection against the neurotoxic effects of PCA. These results indicate that release of 5-HT is a necessary step in the neurotoxicity of PCA and that a peripheral source of 5-HT is involved. We suggest that 5-HT release from platelets into the peripheral circulation may result in the formation of a neurotoxic 5-HT metabolite.

Immunohistochemical analysis of the neurotoxic effects of DSP-4 identifies two populations of noradrenergic axon terminals

N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4) is a potent and highly selective neurotoxin which induces degeneration of noradrenergic axons. The effects of DSP-4 vary considerably in different brain regions: the drug produces nearly complete depletion of noradrenaline in neocortex, hippocampus, cerebellum and spinal cord, but only partial depletion in hypothalamus and brainstem. In this study we have employed an immunohistochemical method to assess the neurotoxic effects of DSP-4 on the structural integrity of central noradrenergic neurons in the rat, and to identify those noradrenergic axons that remain in the central nervous system 2-4 weeks after DSP-4 treatment. The staining results identified noradrenergic axon terminals as the principal site of action of DSP-4; noradrenergic cell bodies and preterminal axons were not noticeably affected. DSP-4 produced an almost all or none neurotoxic effect on noradrenergic axon terminals in different brain regions. Nearly all noradrenergic axon terminals were destroyed in the neocortex, hippocampus, olfactory bulb, thalamus, tectum, cerebellum and spinal cord dorsal horn. In contrast, most noradrenergic axons were unaffected in the basal forebrain, hypothalamus, reticular formation, brainstem motor nuclei and spinal cord ventral horn. These remaining noradrenergic axon terminals differed morphologically from sensitive axons by their thickness, size and spacing of their varicosities and their dense arborizations within terminal fields. The distribution of noradrenergic axons susceptible to DSP-4 correlates very closely with the distribution of locus coeruleus axons and possibly all regions in which noradrenergic terminals are unaffected by DSP-4 receive their major noradrenergic input from non-locus coeruleus neurons. This study provides the first direct evidence that DSP-4 destroys noradrenergic axon terminals from the locus coeruleus, but not those from non-locus coeruleus neurons. This profound differential sensitivity of noradrenergic axons to DSP-4 is matched by distinct differences in their morphology and their topographic projections. The results support the view that locus coeruleus and non-locus coeruleus noradrenergic neurons constitute two separate subsystems, which differ not only in their projections but also with respect to the pharmacological properties of their axon terminals.

Distinct morphologic classes of serotonergic axons in primates exhibit differential vulnerability to the psychotropic drug 3,4-methylenedioxymethamphetamine

Immunohistochemical methods were used to analyse the distribution and morphology of serotonergic axons in normal macaque monkeys and in monkeys given (+/-)3,4-methylenedioxymethamphetamine. In untreated monkeys, we observed two morphologic classes of serotonergic axon terminals, which differ in regional and laminar distribution. These two axon types, fine and beaded, correspond to the serotonergic axon types which have been described in the rat. In 3,4-methylenedioxymethamphetamine-treated monkeys, there is a profound loss of serotonergic axon terminals, yet some are consistently spared. The surviving axon terminals are nearly all of the beaded type; in contrast, fine serotonergic axons are markedly reduced in density. There are regional differences in the magnitude of denervation, which reflect differences in the distribution of these two types of serotonergic axons in controls. The present study demonstrates that 3,4-methylenedioxymethamphetamine has differential neurotoxic effects on fine and beaded serotonergic axons. These results indicate that in the primate there are two distinct classes of serotonergic axon terminals, which differ in morphology, distribution, and vulnerability to psychotropic drugs. We hypothesize that in the primate, as demonstrated in the rat, these two classes of serotonergic axon terminals may arise from different raphe nuclei. In both rodent and primate, the dorsal and median raphe nuclei give rise to parallel ascending serotonergic projections, which are likely to have different pharmacologic properties and functions.

Neurotoxic effects of p-chloroamphetamine on the serotoninergic innervation of the trigeminal motor nucleus: a retrograde transport study

The rat forebrain receives projections from both dorsal and median raphe nuclei. It has recently been shown that serotoninergic axons arising from the dorsal raphe nucleus, but not those from the median raphe nucleus, degenerate following systemic administration of p-chloroamphetamine (PCA). The present study was conducted to determine (i) whether the motor nucleus of the trigeminal nerve is innervated by overlapping projections from multiple serotonin cell groups and (ii) whether a particular subset of serotoninergic axon terminals in the trigeminal motor nucleus are sensitive to the neurotoxic effects of PCA. Retrograde transport was used in combination with immunofluorescence to identify the serotonin-positive cells that project to the trigeminal motor nucleus both in control rats and in rats previously treated with PCA. In untreated rats, an average of 95 retrogradely labeled serotonin-positive neurons were found in the dorsal raphe nucleus, 135 in the nucleus raphe obscurus, 132 in the nucleus raphe pallidus and 63 in the ventrolateral medulla. After treatment with PCA, there was a marked decrease (-77%) in the number of retrogradely labeled serotoninergic neurons in the dorsal raphe nucleus, whereas the number of labeled neurons was unchanged in the raphe obscurus and raphe pallidus. These results demonstrate that PCA selectively lesions serotonin axon terminals arising from the dorsal raphe nucleus, while sparing projections from the raphe obscurus and raphe pallidus to the trigeminal motor nucleus. This conclusion is in agreement with previous findings that in the forebrain only axons from the dorsal raphe are vulnerable to PCA. The data provide further evidence that serotoninergic axons originating in the dorsal raphe nucleus differ from other serotoninergic axons in their pharmacological properties and that the dorsal raphe may contain a functionally unique subset of serotonin neurons.