Electron microscopic study of GABAergic synaptic innervation of neurotensin-immunoreactive neurons in the dorsal raphe nucleus

A double immunocytochemical method combining the preembedding avidin-biotin-peroxidase-complex technique and the postembedding immunogold technique was used to examine synaptic interactions between GABAergic and neurotensin-containing neurons in the same tissue sections of the dorsal raphe nucleus of the rat. Whereas the neurotensin-like immunoreactive perikarya rarely received synapses from GABA-like immunostaining axon terminals, the neurotensin-like immunoreactive dendrites frequently received synapses from GABA-like immunoreactive neurons. These results suggest that GABAergic neurons could modulate neurotensinergic neurons in the dorsal raphe nucleus through synaptic relations. The immunocytochemically identified local synaptic circuit in the dorsal raphe was discussed.     

Enkephalinergic innervation of GABAergic neurons in the dorsal raphe nucleus of the rat

The preembedding double immunoreaction method was used to study interrelations of enkephalinergic and GABAergic neuronal elements in the dorsal raphe nucleus of the Wistar albino rat. The enkephalin-like neuronal elements were immunoreacted by the peroxidase-antiperoxidase method and silver-gold intensified, which showed strongly and was specific. The GABA-like immunoreactive neurons were immunoreacted by the peroxidase-antiperoxidase method only. GABA-like neural somata were postsynaptic to both the enkephalin-like immunoreactive and the non-immunoreactive axon terminals. The enkephalin-like immunoreactive axon terminals were also found to synapse GABA-like immunoreactive dendrites. The GABA-like immunoreactive neuronal elements were also found to receive synapses from other non-immunoreactive as well as GABA-like immunoreactive axon terminals. Almost all of the synapses appeared to be asymmetrical. Possible functional activity of interactions among the enkephalinergic, GABAergic, and serotonergic neuronal elements in the dorsal raphe nucleus are discussed.     

Electron microscopic study of GABAergic synaptic innervation of nitric oxide synthase immunoreactive neurons in the dorsal raphe nucleus in the rat

A double immunocytochemical method combining the preembedding avidin biotin peroxidase complex technique and the postembedding immunogold technique was used to examine synaptic interactions between GABAergic and nitric oxide synthase containing neurons in the same tissue sections of the dorsal raphe nucleus of the Wistar white rat. Although a large number of immunogold stained GABAergic axon terminals were found to be presynaptic to dendrites containing nitric oxide synthase-like immunoreaction product, synapses between GABA-like immunoreactive axon terminals and nitric oxide synthase-like immunoreactive perikarya were rare. The labeled boutons were found to make symmetrical and asymmetrical synapses. No axo-axonic synapse was found. These results suggest that GABAergic neurons could modulate nitric oxide producing neurons in the dorsal raphe nucleus through direct synaptic relations. Synapse 25:24–29, 1997. © 1997 Wiley-Liss, Inc.     

Regulation of the release of serotonin in the dorsal raphe nucleus by alpha1 and alpha2 adrenoceptors

To investigate the modulation of serotonin release in the dorsal raphe nucleus (DRN) by alpha(1) and alpha(2) adrenoceptors, dual-probe microdialysis was performed in conscious rats. The specific alpha(1) and alpha(2) adrenoceptor agonists and antagonists were locally infused into the DRN via retrograde microdialysis. The release of serotonin was simultaneously sampled from the DRN and prefrontal cortex (PFC). Infusion of the alpha(1) adrenoceptor agonist cirazoline into the DRN (100 microM) produced an increase in the release of serotonin in the DRN to 200% of the basal levels, but no effect was seen in the PFC. After infusion of the alpha(1) adrenoceptor antagonist prazosin into the DRN (100 microM) the release of serotonin decreased in the DRN and PFC to about 40% and 65% of the basal levels, respectively. Infusion of the alpha(2) adrenoceptor agonist clonidine into the DRN (100 microM) slightly but significantly decreased the level of serotonin in the DRN as well as in the PFC to about 70% of the basal levels. Infusion of the alpha(2A) adrenoceptor antagonist BRL 44408 into the DRN (100 microM) caused an increase of serotonin release in the DRN to 270% of the basal levels, but at the same time no changes were seen in the extracellular levels of serotonin in the PFC. The present study demonstrates that alpha(1) as well as alpha(2) adrenoceptors in the DRN modulate the release of serotonin in the DRN, and that alpha(1) adrenoceptors in the DRN are maximally stimulated during resting conditions.     

Electrophysiological evidence for convergence of inputs from the medial prefrontal cortex and lateral habenula on single neurons in the dorsal raphe nucleus

Neuronal projections to the dorsal raphe nucleus (DRN) from the medial prefrontal cortex (mPFC) and lateral habenula nucleus (LHb) provide the two key routes by which information processed by mood regulatory, cortico-limbic-striatal circuits input into the 5-HT system. These two projections may converge as it appears that both activate local GABAergic neurons to inhibit 5-HT neurons in the DRN. Here we have tested this hypothesis by measuring the effect of stimulation of the mPFC and LHb on the activity of 5-HT and non-5-HT, putative gamma-amino butyric acid (GABA) neurons in the DRN using extracellular recordings in anaesthetized rats. A total of 119 5-HT neurons (regular, slow firing, broad spike width) and 21 non-5-HT, putative GABA neurons (fast-firing, narrow spike width) were tested. Electrical stimulation of the mPFC or LHb caused a poststimulus inhibition (30 ms latency) of 101/119 5-HT neurons, of which 61 (60%) were inhibited by both the mPFC and LHb. Electrical stimulation of the mPFC or LHb also caused a short latency (12-20 ms) poststimulus facilitation of 10/21 non-5-HT neurons, of which 5 (50%) were activated by both the mPFC and LHb. These data indicate that a significant number of 5-HT neurons and non-5-HT neurons in the DRN are influenced by both the mPFC and LHb. Moreover, the data are compatible with the hypothesis and that there is a convergence of mPFC and LHb inputs on local circuit GABAergic neurons in the DRN which in turn inhibit the activity of 5-HT neurons.     

Immunocytochemical evidence for the existence of GABAergic neurons in the nucleus raphe dorsalis. possible existence of neurons containing serotonin and GABA

It has been established that nerve cell bodies of the nucleus raphe dorsalis (NRD) belong to ascending 5-hydroxytryptamine systems. These neurons could be modulated by GABAergic interneurons or interposed GABA neurons. A high glutamate decar☐ylase (GAD) activity in the NRD and a specific high-affinity uptake mechanism for GABA suggest the presence of GABA synthesizing elements in the NRD. Anti-GAD antibodies were used by an immunocytochemical procedure to demonstrate the presence of GABAergic elements. Anti-GAD antibodies were previously tested in the cerebellum and substantia nigra. Large amounts of GAD-positive reaction product were observed in the cytoplasm of some neurons (fusiform, ovoid or multipolar) or appeared as punctate deposits apposed to dendrites, soma and dispersed in the neuropil of the NRD. At the electron microscopic level, GAD-positive reaction product was observed within the cytoplasm of numerous somata in sections from colchicine-treated rats. GAD-positive staining was observed in numerous fibers or axonal terminals and two types of morphologically different fibers could be distinguished. The first displays small clear vesicles and few large granular vesicles (LGV) (80–100 nm), the second displays only clear round vesicles (40–60 nm). After 5,7-dihydroxytryptamine treatment (a neurotoxic for 5-HT terminals), the immunocytochemical labeling is much decreased. Some reactive neurons are still dispersed in the nucleus but the fibers containing LGV are no longer observed. These results strongly suggest that some neuronal elements in the NRD are morphologically, pharmacologically and anatomically similar to 5-HT neurons described at this level. Such cell elements could possess a double GABA and 5-HT potentiality. If this is not the case, a population of GABA neurons could be sensitive to 5,7-DHT and so have the capacity to take up 5-HT. The other reactive elements, insensitive to 5,7-DHT, could represent the GABAergic interneurons postulated at this level. Numerous GAD positive fibers or axon terminals were observed in synaptic contact with dendrites, axons or soma of other neurons. The chemical nature of the neuronal postsynaptic elements remains unknown. These findings strongly support the hypothesis for GABA-mediated inhibition in the NRD.     

Serotonin-mediated inhibition from dorsal raphe nucleus of neurons in dorsal lateral geniculate and thalamic reticular nuclei

Electrophysiological studies using rats anesthetized with chloral hydrate were performed to determine whether or not serotonin originating in the dorsal raphe nucleus (DR) acts as an inhibitory transmitter or neuromodulator on neurons of the dorsal lateral geniculate nucleus (LGN) and neurons located in the thalamic reticular nucleus (TRN) immediately rostral to the dorsal LGN. In the LGN, conditioning stimuli applied to the DR preceding test stimulus to the optic tract and visual cortex inhibited orthodromic and antidromic spikes in about one-third of the relay neurons and in more than half of the intrageniculate interneurons. Conditioning stimulation of the DR also produced an inhibition of the spikes elicited by stimulation of the optic tract and visual cortex of at least three-quarters of the TRN neurons. Iontophoretic application of serotonin (25 nA) inhibited the orthodromic spikes of the LGN relay neuron and TRN neuron. A close correlation was observed between the effects of DR conditioning stimulation and iontophoretic serotonin in the same neurons. The inhibition with DR conditioning stimulation and iontophoretically applied serotonin was antagonized during iontophoretic application of methysergide (15-40 nA), a serotonin antagonist. These results strongly suggest that serotonin derived from the DR acts on the LGN and TRN neurons as an inhibitory transmitter or neuromodulator to inhibit transmission in these nuclei.     

Evidence for a tonic GABAergic control of serotonin neurons in the median raphe nucleus

We examined whether serotonin (5-HT)-containing neurons of the midbrain raphe nuclei are subject to an inhibitory control by GABA. We found that injection into the median raphe nucleus of the GABA antagonists picrotoxin and bicuculline and the GABA agonist muscimol increase and decrease, respectively, the 5-HT turnover and the steady-state content of 5-hydroxyindoleacetic acid. The results provide biochemical evidence of a tonic inhibition by GABA of 5-HT neuronal activity in the median raphe nucleus; this inhibitory effect is potentiated by benzodiazepines.     

Distribution and synaptic relations of NOS neurons in the dorsal raphe nucleus: A comparison to 5-HT neurons

Anti-nitric oxide synthase antibody was used to study the distribution, cytowchhecture, and synaptic relations of nitric oxide synthase-like immunoreactive neurons in the whole rostral-caudel length of the dorsal raphe nucleus of the rat and compared them with serotonergic neurons. Results showed that the distribution of the nitric oxide synthase in the dorsal raphe nucleus was similar to that of the serotonergic neurons at the rostral part of the dorsal raphe nucleus, including the mediodorsal and the medioventral cell groups, and changed at the middle and caudal parts of the dorsal raphe nucleus. The cytoarchitecture of the nitric oxide synthase-like immunoreactive neurons in the medioventrai cell group of the dorsal raphe nucleus was similar to that of the serotonergic neurons. Similar to the serotonergic neurons there, nitric oxide synthase-like immunoroactive neurons also received synapses from axon terminals that contained round, or flattened vesicles, or both kinds. Different to the serotonergic neurons, the few nitric oxide synthase-like immunoroactive axon terminals that were in this area formed synapses.     

Role of the dorsal raphe nucleus in morphine-induced immediate early gene expression in the rat striatum

Serotonin (5-HT) is thought to be involved in morphine action in the brain. To determine if the periaqueductal gray (PAG) and the dorsal raphe nucleus (DRN) are involved in morphine-induced c-Fos and JunB expression in the caudate-putamen (CPu), the mu receptor antagonist, beta-funaltrexamine (beta-FNA), was unilaterally infused into the PAG adjacent to DRN prior to morphine. Behaviorally, beta-FNA prevented morphine-induced loss of righting and Straub tail. In the CPu of beta-FNA treated rats, morphine-induced c-Fos and JunB were attenuated compared to vehicle-infused rats. These results suggest that morphine acts within the PAG-DRN to exert rapid behavioral effects and to induce c-Fos and JunB in the striatum.     

Effects of various frequency electrical stimulation of the dorsal raphe nucleus on spontaneous firing activities in the rat subthalamic nucleus★

BACKGROUND： Some investigations have demonstrated that exogenous 5-hydroxytryptamine increases the spontaneous firing rate of subthalamic nucleus （STN） neurons in the rat brain. OBJECTIVE： To validate the effect of electrical stimulation to the dorsal raphe nucleus （DRN） on the neuronal activities of the STN in rats, as well as analyze the differences in the effects of electrical stimulation at various frequencies. DESIGN, TIME AND SETTING： Experiments were performed from March 2007 to June 2007 in the Electrophysiology Laboratory of Liaoning Medical University with a randomized controlled animal study design. MATERIALS： Twenty-four healthy male Sprague-Dawley （SD） rats, weighing 250-350 g, were selected for this study. An A320R constant electrical stimulator was purchased from World Precision Instruments Company （USA）; a Spike 2 biological signal acquisition system was purchased from British CED Company. METHODS： Twenty-four SD rats were randomly assigned into a model group and a normal group, with 12 rats in each group. To mimic Parkinson＇s disease, rats in the model group were injected with 4μL of 6-hydroxydopamine into the right striatum, then received deep brain stimulation. Rats in the normal group received deep brain stimulation in same brain region without modeling. Electrical stimulation （width, 0.06 ms; intensity, 0.2-0.6 mA; frequency, 20-130 Hz; train duration, 5 seconds） was delivered to the DRN. MAIN OUTCOME MEASURES： The firing rates of STN neurons were observed by extracellular recording using a biological signal acquisition system. RESULTS： DRN-high-frequency stimulation （DRN-HFS） induced excitation in 59% of the STN neurons in the normal group and 50% of the STN neurons in the model group; mean firing rates increased significantly from （7.14±0.75） and （7.94 ± 0.61） Hz to （11.17 ±1.49） and （12.11 ± 1.05） Hz, respectively （P 〈 0.01）. Spontaneous firing rate increased significantly in 53% of neurons in normal rats in a frequency-dep     

Evidence that serotonergic neurons in the dorsal raphe nucleus exert a stimulatory effect on the secretion of renin but not of corticosterone

Data from previous experiments in rats indicate that release of serotonin in the central nervous system increases renin and corticosterone secretion. To determine which serotonergic neurons are involved, lesions of the dorsal or median raphe nuclei were produced by local injections of 5,7-dihydroxytryptamine (5,7-DHT) in rats, and 2 weeks later, the renin responses to parachloroamphetamine (PCA) were determined. Plasma corticosterone was also measured. PCA produced significant increases in plasma renin activity and plasma corticosterone in sham-lesioned animals and animals with median raphe lesions. The plasma corticosterone response to PCA was also normal in rats with dorsal raphe lesions but the renin response was significantly reduced. The data support the hypothesis that serotonergic neurons in the dorsal raphe nucleus are part of a neural pathway mediating increased renin secretion, and that the stimulatory effect of serotonin on corticosterone secretion is mediated by a different pathway.     

Innervation of the nucleus of the solitary tract and the dorsal vagal nucleus by thyrotropin-releasing hormone-containing raphe neurons

The nucleus of the solitary tract and the dorsal vagal nucleus are richly innervated by thyrotropin-releasing hormone (TRH)-containing fibers arising from the caudal raphe nuclei. After transection of vertically oriented fibers by a horizontal knife-cut in the medulla oblongata, TRH-staining disappeared from the vagal nuclei while it increased in transected nerve fibers ventral to the knife-cut. TRH-containing cells are mainly located in the nucleus raphe pallidus and raphe obscurus. TRH-containing fibers run dorsally within the raphe and enter the dorsal vagal complex at its rostral tip. Then they turn caudally and send branches laterally. Immediately caudal to the level of the obex, several TRH-containing fibers cross over the central canal. Cells in regions other than the raphe (hypothalamus or other rostral areas, ventrolateral medulla, cranial nerves) must contribute little to the TRH innervation of the nucleus of the solitary tract and dorsal vagal nucleus, since various knife-cuts transecting all above possible connections did not alter the TRH innervation pattern or TRH concentrations of these vagal nuclei.     

GABAergic subsensitivity of dorsal raphe neurons in vitro after chronic benzodiazepine treatment in vivo

Previous studies have shown that chronic benzodiazepine treatment reduces the in vivo sensitivity of dorsal raphe neurons (DRN) to microiontophoretically applied gamma-aminobutyric acid (GABA). We have examined sensitivity of DRN in vitro using a modified midbrain slice technique which allows side-by-side analysis of slices from control and chronic diazepam-treated rats. GABA sensitivity of raphe neurons was reduced in slices from rats treated for 3 weeks with diazepam, compared to control sensitivity. Thus, GABA subsensitivity following chronic diazepam treatment appears to be dependent on changes intrinsic to the midbrain area.     

Control of serotonergic neurons in the dorsal raphe nucleus by the lateral hypothalamus

Anatomical evidence indicates the presence of projections from the lateral hypothalamus to serotonergic (5-hydroxytryptamine, 5-HT) neurons of the dorsal raphe nucleus (DR). Using dual probe microdialysis and extracellular recordings in the DR, we show that the application of GABAergic agents in the lateral hypothalamus modulates the activity of 5-HT neurons in the DR. GABA and bicuculline or baclofen, applied in the lateral hypothalamus significantly reduced and increased, respectively, the 5-HT output in the DR. Likewise, the intrahypothalamic application of GABA and bicuculline reduced (14/20 neurons) and increased (8/12 neurons), respectively, the firing rate of 5-HT neurons in the DR. A smaller percentage of neurons, however, were excited by GABA (3/20) and inhibited by bicuculline (1/12). Application of tetrodotoxin in the lateral hypothalamus suppressed the local 5-HT output and reduced that in the DR. The 5-HT output in the DR increased transiently soon after darkness. The hypothalamic application of GABA attenuated and that of bicuculline potentiated this spontaneous change with an efficacy similar to that seen in light conditions. These results indicate that the lateral hypothalamus is involved in the control of 5-HT activity in the DR, possibly through excitatory (major) and inhibitory (minor) inputs.     

GABAergic neurons of the cat dorsal raphe nucleus express c-fos during carbachol-induced active sleep

Serotonergic neurons of the dorsal raphe nucleus (DRN) cease firing during active sleep (AS, also called rapid-eye-movement sleep). This cessation of electrical activity is believed to play a ‘permissive’ role in the generation of AS. In the present study we explored the possibility that GABAergic cells in the DRN are involved in the suppression of serotonergic activity during AS. Accordingly, we examined whether immunocytochemically identified GABAergic neurons in the DRN were activated, as indicated by their expression of c-fos, during carbachol-induced AS (AS-carbachol). Three chronically-prepared cats were euthanized after prolonged episodes of AS that was induced by microinjections of carbachol into the nucleus pontis oralis. Another four cats (controls) were maintained 2 h in quiet wakefulness before being euthanized. Thereafter, immunocytochemical studies were performed on brainstem sections utilizing antibodies against Fos, GABA and serotonin. When compared with identically prepared tissue from awake cats, the number of Fos+ neurons was larger in the DRN during AS-carbachol (35.9±5.6 vs. 13.9±4.4, P<0.05). Furthermore, a larger number of GABA+ Fos+ neurons were observed during AS-carbachol than during wakefulness (24.8±3.3 vs. 4.0±1.0, P<0.001). These GABA+ Fos+ neurons were distributed asymmetrically with a larger number located ipsilaterally to the site of injection. There was no significant difference between control and experimental animals in the number of non-GABAergic neurons that expressed c-fos in the DRN. We therefore suggest that activated GABAergic neurons of the DRN are responsible for the inhibition of serotonergic neurons that occurs during natural AS.     

Glutamatergic afferent projections to the dorsal raphe nucleus of the rat

Based on WGA-apo-HRP-gold (WG) retrograde tracing, the present study revealed that different subdivisions of the dorsal raphe (DR) such as dorsomedial, ventromedial, lateral wing, and caudal regions receive unique, topographically organized afferent inputs, that are more restricted than previously reported. Phaseolus vulgaris leucoagglutinin anterograde tracing studies confirmed that the medial prefrontal cortex provides the major afferent input to each subdivision of the DR. Double-labeling studies combining WG tracing and glutamate immunostaining indicated that the medial prefrontal cortex, various hypothalamic nuclei including perifornical, lateral, and arcuate nuclei, and several medullary regions such as lateral and medial parabrachial nuclei, and laterodorsal tegmental nucleus provide the major glutamatergic input to each subregion of the DR. It should be noted that the degree of glutamatergic input from these afferent sites was specific for each DR subdivision. The present findings indicated that dorsomedial, ventromedial, lateral wing, and caudal subdivisions of the DR receive excitatory inputs from both cortical and subcortical sites which might be involved in regulation or modulation of a broad range of systems, including sensory and motor functions, arousal and sleep-wake cycle, biorhythmic, cognitive, and affective behaviors.     

GABAergic neurons are present in the dorsal column nuclei but not in the ventroposterior complex of rats

Neurons containing glutamatic acid decarboxylase (GAD) are known to exist in the spinal dorsal horn, dorsal column nuclei (DCN), n. ventralis posterior (VP), and somatosensory cortex of cats. Recent work suggested that species differences exist concerning the presence and/or density of GAD-positive neurons in VP. The present experiments demonstrate that, in contrast with carnivores and primates, the rat's VP contains virtually no GAD-positive neurons and that virtually all neurons in it project to the cortex. This conclusion is supported by the failure to find, in Golgi-impregnated material, neurons with characteristics commonly attributed to Golgi type II neurons in VP of cats. The lack of GAD-positive neurons in VP of rats contrasts also with the presence of such neurons in the DCN in the same species. As in cats, about one third of the neurons in the cuneate n. are GAD-positive; these have mostly small perikarya and they are present throughout the nucleus. It is likely that these are intrinsic neurons, i.e. non-projecting beyond the limits of the DCN since a comparable percentage of neurons are unlabeled by simultaneous injections of horseradish peroxidase in multiple targets of the DCN. Like GAD-positive neurons, neurons unlabeled by the retrograde transport of HRP have, for the most part, small perikarya. It is possible that inhibitory mechanisms necessary for basic transfer functions in VP of rats are sustained through projections to this nucleus from the n. reticularis thalami. Extrinsic source of GABAergic input to the DCN seem to be absent or very weak. From this and previous evidence it may be proposed that intrinsic inhibitory interneurons have gradually developed in VP of rabbits, carnivores, and primates in parallel with more elaborate levels of thalamic integration of somatosensation.     

The comparative effects of environmental enrichment with exercise and serotonin transporter blockade on serotonergic neurons in the dorsal raphe nucleus

We have previously reported that inhibition of the serotonin transporter (SERT) by selective serotonin reuptake inhibitor (SSRI) fluoxetine significantly reduces the number of tryptophan hydroxylase (TPH)-positive cells in the dorsal raphe nucleus (DRN). We have been interested in exploring whether this SSRI-induced change in TPH might be modified by housing in an enriched environment. Like SSRI antidepressants, environmental enrichment (EE) and physical exercise have been found to have efficacy in the prevention and alleviation of depression. We postulated that EE with exercise and SERT inhibition would similarly affect TPH regulation and that EE with exercise might modify the effect of fluoxetine on TPH. Three week old male Sprague-Dawley rats were housed in either a standard cage (SE) or an enriched environment (EE). SE animals were singly housed with no access to enrichment objects. EE animals were group housed and were provided with various enrichment objects (e.g. running wheel) that were changed and rearranged regularly. Nine weeks after the experiment began, the rats were randomly assigned to one of four treatment groups: (1) SE control; (2) SE fluoxetine; (3) EE control; or (4) EE fluoxetine. Fluoxetine (5 mg/kg/day) was placed in the drinking water. Sections of DRN were processed for TPH immunohistochemistry. The number of TPH-positive cells was determined by blinded, manual counting. Results were analyzed by analysis of variance (ANOVA) followed by post-hoc Tukey tests. Significance was set at P < 0.05. For animals housed in a standard environment, fluoxetine induced a significant 29% reduction in the number of TPH-immunoreactive cells in the DRN. A similar reduction in TPH immunoreactivity was observed in animals that were housed in an enriched environment but not exposed to fluoxetine (39%). The number of TPH-positive cells in the DRN for animals housed in an enriched environment and exposed to fluoxetine was not significantly different than animals housed in an enriched environment and not exposed to fluoxetine. The reduction of TPH immunoreactivity in the DRN by EE with exercise suggests that a modified housing environment and voluntary exercise affects regulation of TPH, possibly via a mechanism similar to that of SERT inhibitors. This downregulation of serotonin biosynthesis by fluoxetine and EE with exercise may ultimately play a role in the therapeutic action of both interventions.     

Increased mRNA expression of cytochrome oxidase in dorsal raphe nucleus of depressive suicide victims

Suicidal behavior is a problem with important social repercussions. Some groups of the population show a higher risk of suicide; for example, depression, alcoholism, psychosis or drug abuse frequently precedes suicidal behavior. However, the relationship between metabolic alterations in the brain and premorbid clinical symptoms of suicide remains uncertain. The serotonergic and noradrenergic systems have frequently been, implicated in suicidal behavior and the amount of serotonin in the brain and CSF of suicide victims has been found to be low compared with normal subjects. However, there are contradictory results regarding the role of noradrenergic neurons in the mediation of suicide attempts, possibly reflecting the heterogeneity of conditions that lead to a common outcome. In the present work we focus on the subgroup of suicide victims that share a common diagnosis of major depression. Based on post-mortem studies analyzing mRNA expression by in situ hybridization, serotonergic neurons from the dorsal raphe nucleus (DRN) from depressive suicide victims are seen to over-express cytochrome oxidase mRNA. However, no corresponding changes were found in the expression of tyrosine hydroxylase (TH) mRNA in the noradrenergic neurons of the Locus Coeruleus (LC). These results suggest that, despite of the low levels of serotonin described in suicide victims, the activity of DRN neurons could increase in the suicidally depressed, probably due to the over activation of serotonin re-uptake. No alteration was found in noradrenergic neurons, suggesting that they play no crucial role in the suicidal behavior of depressive patients.