Serotonergic and nonserotonergic projections from the rat interpeduncular nucleus to the septum, hippocampal formation and raphe: a combined immunocytochemical and fluorescent retrograde labelling study of neurons in the apical subnucleus

Temporary inactivation of interpeduncular nucleus impairs long but not short term plasticity in the perforant-path dentate gyrus synapses in rats

The interconnectivity of the hippocampus, interpeduncular nucleus (IPN) and several brain structures which are involved in modulating hippocampal theta rhythm activity makes a complicated dynamic network of interconnected regions and highlights the role of IPN in the hippocampal dependent learning and memory. In the present study we aimed to address whether IPN is involved in the perforant path-dentate gyrus (PPDG) short term and long term synaptic plasticity in rats. To silent IPN transiently, lidocaine was injected through the implanted cannula above the IPN. To evaluate short term plasticity, paired pulses stimulation of PPDG synapses were used upon IPN temporary inactivation. Furthermore, long term plasticity was investigated by measuring the induction and maintenance of PPDG synapses long term potentiation (LTP) after high frequency stimulation (HFS) of the mentioned pathway following to IPN inactivation. The results showed that IPN reversible inactivation had no effect on short term plasticity of PPDG synapses. However, IPN inactivation before the PPDG high frequency stimulation could significantly suppress both the population spike (PS) and fEPSP-LTP induction compared to the saline group. Conversely, IPN inactivation had no significant effect on maintenance of both PS-LTP and fEPSP-LTP. All together our study suggests the contribution of IPN in the PPDG synaptic plasticity and excitability of DG granule cells which could be through direct and/or indirect pathways from IPN to the hippocampus.

Habenular connections with the dopaminergic and serotonergic system and their role in stress-related psychiatric disorders

The habenula (Hb) is a phylogenetically old epithalamic structure differentiated into two nuclear complexes, the medial (MHb) and lateral habenula (LHb). After decades of search for a great unifying function, interest in the Hb resurged when it was demonstrated that LHb plays a major role in the encoding of aversive stimuli ranging from noxious stimuli to the loss of predicted rewards. Consistent with a role as an anti-reward center, aberrant LHb activity has now been identified as a key factor in the pathogenesis of major depressive disorder. Moreover, both MHb and LHb emerged as new players in the reward circuitry by primarily mediating the aversive properties of distinct drugs of abuse. Anatomically, the Hb serves as a bridge that links basal forebrain structures with monoaminergic nuclei in the mid- and hindbrain. So far, research on Hb has focused on the role of the LHb in regulating midbrain dopamine release. However, LHb/MHb are also interconnected with the dorsal (DR) and median (MnR) raphe nucleus. Hence, it is conceivable that some of the habenular functions are at least partly mediated by the complex network that links MHb/LHb with pontomesencephalic monoaminergic nuclei. Here, we summarize research about the topography and transmitter phenotype of the reciprocal connections between the LHb and ventral tegmental area-nigra complex, as well as those between the LHb and DR/MnR. Indirect MHb outputs via interpeduncular nucleus to state-setting neuromodulatory networks will also be commented. Finally, we discuss the role of specific LHb-VTA and LHb/MHb-raphe circuits in anxiety and depression.

Cbln2 and Cbln4 are expressed in distinct medial habenula-interpeduncular projections and contribute to different behavioral outputs

Cerebellins are important neurexin ligands that remain incompletely understood. Two critical questions in particular remain unanswered: do different cerebellins perform distinct functions, and do these functions act in the initial establishment of synapses or in rendering nascent synapses capable of normal synaptic transmission? Here we show that in mice, Cbln2 and Cbln4 are expressed in the medial habenula (MHb) nucleus in different types of neurons that project to distinct target neurons in the interpeduncular nucleus. Conditional genetic deletion of Cbln2 in the MHb impaired synaptic transmission at Cbln2⁺ synapses in the interpeduncular neurons within 3 wk, but decreased synapse numbers only after 3 mo, suggesting a functional, but not a structural, requirement for Cbln2 in synapses formed by Cbln2-expressing neurons. In contrast, genetic deletions of Cbln4 in the MHb had no major effect on synaptic transmission or synapse numbers in interpeduncular target neurons. Nevertheless, MHb ablation of both Cbln2 and Cbln4 significantly impaired behavioral responses in mice, but affected different types of behaviors. Specifically, Cbln2 MHb deletions decreased spatial learning, as measured in the water T-maze, whereas Cbln4 MHb deletions increased anxiety levels, as monitored in the open field test and elevated plus maze. Thus, Cbln2 and Cbln4 are expressed in distinct MHb neurons that contribute to different behaviors.

Reversible inactivation of interpeduncular nucleus impairs memory consolidation and retrieval but not learning in rats: A behavioral and molecular study

The Interpedundular nucleus (IPN) is a small midbrain structure located deeply between the two cerebral peduncles. The strategic placement of this nucleus makes it a possible relay between structures involved in the modulation of hippocampal theta rhythm activity. In this study we aimed to investigate how reversible inactivation of IPN could affect the acquisition, consolidation and retrieval phases of memory in passive avoidance (PA) and Morris water maze (MWM) tasks. To support our data, molecular studies were performed in order to detect possible changes in the expression of proteins related to learning and memory in the hippocampus. To address this issue rats' IPN was reversibly inactivated by microinjection of lidocaine hydrochloride (4%). After the behavioral studies, the phosphorylation of CREB and P70, and c-fos expression levels in the hippocampus were determined using western blotting and immunohistochemistry respectively. Our results in the PA and MWM tasks showed that IPN reversible inactivation could impair immediate post training consolidation and retrieval while it had no effect on the acquisition phase. In addition, there was a deficit in the retention of the MWM working memory. Our data showed the ratio of pCREB/CREB, pP70/P70 and c-fos expression in the hippocampus significantly decreased after IPN reversible inactivation. Collectively, the results show that behaviorally defined changes could be due to what happens molecularly in the hippocampus after IPN reversible inactivation. It is concluded that IPN not only makes part of a network involved in the modulation of hippocampal theta rhythm activity, but also is actively engaged in hippocampal memory formation.

Specific connections of the interpeduncular subnuclei reveal distinct components of the habenulopeduncular pathway

The habenulopeduncular pathway consists of the medial habenula (MHb), its output tract, the fasciculus retroflexus, and its principal target, the interpeduncular nucleus (IP). Several IP subnuclei have been described, but their specific projections and relationship to habenula inputs are not well understood. Here we have used viral, transgenic, and conventional anterograde and retrograde tract-tracing methods to better define the relationship between the dorsal and ventral MHb, the IP, and the secondary efferent targets of this system. Although prior studies have reported that the IP has ascending projections to ventral forebrain structures, we find that these projections originate almost entirely in the apical subnucleus, which may be more appropriately described as part of the median raphe system. The laterodorsal tegmental nucleus receives inhibitory inputs from the contralateral dorsolateral IP, and mainly excitatory inputs from the ipsilateral rostrolateral IP subnucleus. The midline central gray of the pons and nucleus incertus receive input from the rostral IP, which contains a mix of inhibitory and excitatory neurons, and the dorsomedial IP, which is exclusively inhibitory. The lateral central gray of the pons receives bilateral input from the lateral IP, which in turn receives bilateral input from the dorsal MHb. Taken together with prior studies of IP projections to the raphe, these results form an emerging map of the habenulopeduncular system that has significant implications for the proposed function of the IP in a variety of behaviors, including models of mood disorders and behavioral responses to nicotine.

Alterations in alpha5* nicotinic acetylcholine receptors result in midbrain- and hippocampus-dependent behavioural and neural impairments

RATIONALE

Evidence links alterations in α5-containing nicotinic receptors (α5*-nAChRs) to nicotine addiction. Notably, the rs16969968 polymorphism in the α5 gene (α5SNP) increases the risk for heavy smoking and impairs nicotine-rewarding properties in mice. Additional work is needed to understand how native and polymorphic α5*-nAChRs contribute to processes associated with the risk for nicotine addiction.

OBJECTIVES

We aimed at understanding the contribution of α5*-nAChRs to endophenotypes like increased responses to novelty and anxiety, known to promote vulnerability to addiction, and to the response of the dopamine and serotonin systems to nicotine.

METHODS

Behavioural phenotypes were investigated in mice lacking the α5 gene (α5(-/-)). Nicotine injections were performed to test the consequences of nicotine exposure on the phenotypes identified. Dopamine and serotonin signalling were assessed using in vivo microdialysis and electrophysiology. We used lentiviral vectors to compare the consequences of re-expressing either the α5 wild-type allele or the α5SNP in specific brain areas of α5(-/-) mice.

RESULTS

α5(-/-) mice did not exhibit high responses to novelty but showed decreased novelty-induced rearing behaviour together with high anxiety. Exposure to high doses of nicotine rescued these phenotypes. We identified altered spontaneous and nicotine-elicited serotonin and dopamine activity in α5(-/-) mice. Re-expression of α5 in the ventral tegmental area and hippocampus rescued rearing and anxiety levels in α5(-/-) mice, respectively. When expressing the α5SNP instead, this resulted in a knockout-like phenotype for both behaviours.

CONCLUSIONS

We propose that altered α5*-nAChR cholinergic signalling contributes to emotional/behavioural impairments that may be alleviated by nicotine consumption.

Brain Systems Underlying Susceptibility to Helplessness and Depression

There has been a relative lack of research into the neurobiological predispositions that confer vulnerability to depression. This article reviews functional brain mappings from a genetic animal model, the congenitally helpless rat, which is predisposed to develop learned helplessness. Neurometabolic findings from this model are integrated with the neuroscientific literature from other animal models of depression as well as depressed humans. Changes in four major brain systems are suggested to underlie susceptibility to helplessness and possibly depression: (a) an unbalanced prefrontal-cingulate cortical system, (b) a dissociated hypothalamic-pituitary-adrenal axis, (c) a dissociated septal-hippocampal system, and (d) a hypoactive brain reward system, as exemplified by a hypermetabolic habenula-interpeduncular nucleus pathway and a hypometabolic ventral tegmental area-striatum pathway. Functional interconnections and causal relationships among these systems are considered and further experiments are suggested, with theoretical attention to how an abnormality in any one system could affect the others.

Nicotine Dependence Reveals Distinct Responses from Neurons and Their Resident Nicotinic Receptors in Medial Habenula

Nicotinic acetylcholine receptors (nAChRs) are the molecular target of nicotine. nAChRs in the medial habenula (MHb) have recently been shown to play a role in nicotine dependence, but it is not clear which nAChR subtypes or MHb neuron types are most important. To identify MHb nAChRs and/or cell types that play a role in nicotine dependence, we studied these receptors and cells with brain slice electrophysiology using both acute and chronic nicotine application. Cells in the ventroinferior (MHbVI) and ventrolateral MHb (MHbVL) subregions expressed functional nAChRs with different pharmacology. Further, application of nicotine to cells in these subregions led to different action potential firing patterns. The latter result was correlated with a differing ability of nicotine to induce nAChR desensitization. Chronic nicotine caused functional upregulation of nAChRs selectively in MHbVI cells, but did not change nAChR function in MHbVL. Importantly, firing responses were also differentially altered in these subregions following chronic nicotine. MHbVI neurons treated chronically with nicotine exhibited enhanced basal pacemaker firing but a blunted nicotine-induced firing response. MHbVL neurons did not change their firing properties in response to chronic nicotine. Together, these results suggest that acute and chronic nicotine differentially affect nAChR function and output of cells in MHb subregions. Because the MHb extensively innervates the interpeduncular nucleus, an area critical for both affective and somatic signs of withdrawal, these results could reflect some of the neurophysiological changes thought to occur in the MHb to the interpeduncular nucleus circuit in human smokers.

Median raphe stimulation-induced motor inhibition concurrent with suppression of type 1 and type 2 hippocampal theta

This study investigated behavioral, anatomical and electrophysiological effects produced by electrical stimulation of posterior hypothalamic (PH) or median raphe (MR) nuclei, independently and during combined stimulation of both PH and MR. These three stimulation conditions were applied during spontaneous behavior in an open field and during PH stimulation-induced wheel running, while simultaneously recording hippocampal (HPC) field activity. An additional objective was to determine the effects of MR stimulation on Type 1 movement related theta and Type 2 sensory processing related theta. To achieve the latter, when behavioral studies were completed we studied the same rats under urethane anesthesia and then during urethane anesthesia with the addition of atropine sulfate (ATSO4). Here we demonstrated that electrical stimulation of a localized region of the MR nucleus resulted in a profound inhibition of both spontaneously occurring theta related motor behaviors and the theta related motor behaviors induced by electrical stimulation of the PH nucleus. Furthermore, this motor inhibition occurred concurrently with strong suppression of hippocampal theta field oscillations in the freely moving rat, a condition where the theta recorded is Type 2 sensory processing theta occurring coincidently with Type 1 movement related theta (Bland, 1986). Our results indicate that motor inhibition resulted from stimulation of neurons located in the mid central region of the MR, while stimulation in adjacent regions produced variable responses, including movements and theta activity. The present study provided evidence that the pharmacological basis of the suppression of Type 2 sensory processing HPC theta was cholinergic. However, MR inhibition of PH-induced wheel running was not affected by cholinergic blockade, which blocks Type 2 theta, indicating that MR stimulation-induced motor inhibition also requires the suppression of Type 1 theta.

The habenulo-interpeduncular pathway in nicotine aversion and withdrawal

Progress has been made over the last decade in our understanding of the brain areas and circuits involved in nicotine reward and withdrawal, leading to models of addiction that assign different addictive behaviors to distinct, yet overlapping, neural circuits (Koob and Volkow, 2010; Lobo and Nestler, 2011; Tuesta et al., 2011; Volkow et al., 2011). Recently the habenulo-interpeduncular (Hb-IPN) midbrain pathway has re-emerged as a new critical crossroad that influences the brain response to nicotine. This brain area is particularly enriched in nicotinic acetylcholine receptor (nAChR) subunits α5, α3 and β4 encoded by the CHRNA5-A3-B4 gene cluster, which has been associated with vulnerability to tobacco dependence in human genetics studies. This finding, together with studies in mice involving deletion and replacement of nAChR subunits, and investigations of the circuitry, cell types and electrophysiological properties, have begun to identify the molecular mechanisms that take place in the MHb-IPN which underlie critical aspects of nicotine dependence. In the current review we describe the anatomical and functional connections of the MHb-IPN system, as well as the contribution of specific nAChRs subtypes in nicotine-mediated behaviors. Finally, we discuss the specific electrophysiological properties of MHb-IPN neuronal populations and how nicotine exposure alters their cellular physiology, highlighting the unique role of the MHb-IPN in the context of nicotine aversion and withdrawal. This article is part of the Special Issue entitled 'The Nicotinic Acetylcholine Receptor: From Molecular Biology to Cognition'.

Nicotine aversion: Neurobiological mechanisms and relevance to tobacco dependence vulnerability

Nicotine stimulates brain reward circuitries, most prominently the mesocorticolimbic dopamine system, and this action plays a critical in establishing and maintaining the tobacco smoking habit. Compounds that attenuate nicotine reward are considered promising therapeutic candidates for tobacco dependence, but many of these agents have other actions that limit their potential utility. Nicotine is also highly noxious, particularly at higher doses, and aversive reactions to nicotine after initial exposure can decrease the likelihood of developing a tobacco habit in many first time smokers. Nevertheless, relatively little is known about the mechanisms of nicotine aversion. The purpose of this review is to present recent new insights into the neurobiological mechanisms that regulate avoidance of nicotine. First, the role of the mesocorticolimbic system, so often associated with nicotine reward, in regulating nicotine aversion is highlighted. Second, genetic variation that modifies noxious responses to nicotine and thereby influences vulnerability to tobacco dependence, in particular variation in the CHRNA5-CHRNA3-CHRNB4 nicotinic acetylcholine receptor (nAChR) subunit gene cluster, will be discussed. Third, the role of the habenular complex in nicotine aversion, primarily medial habenular projections to the interpeduncular nucleus (IPN) but also lateral habenular projections to rostromedial tegmental nucleus (RMTg) and ventral tegmental area (VTA) are reviewed. Forth, brain circuits that are enriched in nAChRs, but whose role in nicotine avoidance has not yet been assessed, will be identified. Finally, the feasibility of developing novel therapeutic agents for tobacco dependence that act not by blocking nicotine reward but by enhancing nicotine avoidance will be considered. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Organization of multisynaptic inputs to the dorsal and ventral dentate gyrus: retrograde trans-synaptic tracing with rabies virus vector in the rat

Behavioral, anatomical, and gene expression studies have shown functional dissociations between the dorsal and ventral hippocampus with regard to their involvement in spatial cognition, emotion, and stress. In this study we examined the difference of the multisynaptic inputs to the dorsal and ventral dentate gyrus (DG) in the rat by using retrograde trans-synaptic tracing of recombinant rabies virus vectors. Three days after the vectors were injected into the dorsal or ventral DG, monosynaptic neuronal labeling was present in the entorhinal cortex, medial septum, diagonal band, and supramammillary nucleus, each of which is known to project to the DG directly. As in previous tracing studies, topographical patterns related to the dorsal and ventral DG were seen in these regions. Five days after infection, more of the neurons in these regions were labeled and labeled neurons were also seen in cortical and subcortical regions, including the piriform and medial prefrontal cortices, the endopiriform nucleus, the claustrum, the cortical amygdala, the medial raphe nucleus, the medial habenular nucleus, the interpeduncular nucleus, and the lateral septum. As in the monosynaptically labeled regions, a topographical distribution of labeled neurons was evident in most of these disynaptically labeled regions. These data indicate that the cortical and subcortical inputs to the dorsal and ventral DG are conveyed through parallel disynaptic pathways. This second-order input difference in the dorsal and ventral DG is likely to contribute to the functional differentiation of the hippocampus along the dorsoventral axis.

Mesolimbic dopamine and habenulo-interpeduncular pathways in nicotine withdrawal

The majority of people who attempt to quit smoking without some assistance relapse within the first couple of weeks, indicating the increased vulnerability during the early withdrawal period. The habenula, which projects via the fasciculus retroflexus to the interpeduncular nucleus, plays an important role in the withdrawal syndrome. Particularly the α2, α5, and β4 subunits of the nicotinic acetylcholine receptor have critical roles in mediating the somatic manifestations of withdrawal. Furthermore, withdrawal from nicotine induces a hypodopaminergic state, but there is a relative increase in the sensitivity to phasic dopamine release that is caused by nicotine. Therefore, acute nicotine re-exposure causes a phasic DA response that more potently reinforces relapse to smoking during the withdrawal period.

Reward, addiction, withdrawal to nicotine

Nicotine is the principal addictive component that drives continued tobacco use despite users' knowledge of the harmful consequences. The initiation of addiction involves the mesocorticolimbic dopamine system, which contributes to the processing of rewarding sensory stimuli during the overall shaping of successful behaviors. Acting mainly through nicotinic receptors containing the α4 and β2 subunits, often in combination with the α6 subunit, nicotine increases the firing rate and the phasic bursts by midbrain dopamine neurons. Neuroadaptations arise during chronic exposure to nicotine, producing an altered brain condition that requires the continued presence of nicotine to be maintained. When nicotine is removed, a withdrawal syndrome develops. The expression of somatic withdrawal symptoms depends mainly on the α5, α2, and β4 (and likely α3) nicotinic subunits involving the epithalamic habenular complex and its targets. Thus, nicotine taps into diverse neural systems and an array of nicotinic acetylcholine receptor (nAChR) subtypes to influence reward, addiction, and withdrawal.

Functional topography of midbrain and pontine serotonergic systems: implications for synaptic regulation of serotonergic circuits

RATIONALE

Dysfunction of serotonergic systems is thought to play an important role in a number of neurological and psychiatric disorders. Recent studies suggest that there is anatomical and functional diversity among serotonergic systems innervating forebrain systems involved in the control of physiologic and behavioral responses, including the control of emotional states.

OBJECTIVE

Here, we highlight the methods that have been used to investigate the heterogeneity of serotonergic systems and review the evidence for the unique anatomical, hodological, and functional properties of topographically organized subpopulations of serotonergic neurons in the midbrain and pontine raphe complex.

CONCLUSION

The emerging understanding of the topographically organized synaptic regulation of brainstem serotonergic systems, the topography of the efferent projections of these systems, and their functional properties, should enable identification of novel therapeutic approaches to treatment of neurological and psychiatric conditions that are associated with dysregulation of serotonergic systems.

Deficits in landmark navigation and path integration after lesions of the interpeduncular nucleus

Experiments were designed to determine the role of the interpeduncular nucleus (IPN) in 3 forms of navigation: beacon, landmark, and path integration. In beacon navigation, animals reach goals using cues directly associated with them, whereas in landmark navigation animals use external cues to determine a direction and distance to goals. Path integration refers to the use of self-movement cues to obtain a trajectory to a goal. IPN-lesioned rats were tested in a food-carrying task in which they searched for food in an open field, and returned to a refuge after finding the food. Landmark navigation was evaluated during trials performed under lighted conditions and path integration was tested under darkened conditions, thus eliminating external cues. We report that IPN lesions increased the number of errors and reduced heading accuracy under both lighted and darkened conditions. Tests using a Morris water maze procedure indicated that IPN lesions produced moderate impairments in the landmark version of the water task, but left beacon navigation intact. These findings suggest that the IPN plays a fundamental role in landmark navigation and path integration.

Nonserotonergic projection neurons in the midbrain raphe nuclei contain the vesicular glutamate transporter VGLUT3

The brainstem raphe nuclei are typically assigned a role in serotonergic brain function. However, numerous studies have reported that a large proportion of raphe projection cells are nonserotonergic. The identity of these projection cells is unknown. Recent studies have reported that the vesicular glutamate transporter VGLUT3 is found in both serotonergic and nonserotonergic neurons in both the median raphe (MR) and dorsal raphe (DR) nuclei. We injected the retrograde tracer cholera toxin subunit B into either the dorsal hippocampus or the medial septum (MS) and used triple labeled immunofluorescence to determine if nonserotonergic raphe cells projecting to these structures contained VGLUT3. Consistent with previous studies, only about half of retrogradely labeled MR neurons projecting to the hippocampus contained serotonin, whereas a majority of the retrogradely labeled nonserotonergic cells contained VGLUT3. Similar patterns were observed for MR cells projecting to the MS. About half of retrogradely labeled nonserotonergic neurons in the DR contained VGLUT3. Additionally, a large number of retrogradely labeled cells in the caudal linear and interpeduncular nuclei projecting to the MS were found to contain VGLUT3. These data suggest the enigmatic nonserotonergic projection from the MR to forebrain regions may be glutamatergic. In addition, these results demonstrate a dissociation between glutamatergic and serotonergic MR afferent inputs to the MS and hippocampus suggesting divergent and/or complementary roles of these pathways in modulating cellular activity within the septohippocampal network.

Influence of Neuronal Nicotinic Receptors over Nicotine Addiction and Withdrawal

Cigarette smoking represents an enormous, global public health threat. Nearly five million premature deaths during a single year are attributable to smoking. Despite the resounding message of risks associated with smoking and numerous public health initiatives, cigarette smoking remains the most common preventable cause of disease in the United States. Fortunately, even in an adult smoker, smoking cessation can reverse many of the potential harmful effects. The symptoms associated with nicotine withdrawal represent the major obstacle to smoking cessation. This minireview examines the roles of various nicotinic receptors in the mechanisms of nicotine dependence, discusses the potential role of the habenula-interpeduncular nucleus axis in nicotine withdrawal, and highlights nicotinic receptors containing the β4 subunit as a potential pharmacological target for smoking cessation strategies.

A conductor hidden in the orchestra? Role of the habenular complex in monoamine transmission and cognition

Influences of the habenular complex on electrophysiological and neurochemical aspects of brain functioning are well known. However, its role in cognition has been sparsely investigated until recently. The habenular complex, composed of medial and lateral subdivisions, is a node linking the forebrain with midbrain and hindbrain structures. The lateral habenula is the principal actor in this direct dialogue, while the medial habenula mostly conveys information to the interpeduncular nucleus before this modulates further regions. Here we describe neuroanatomical and physiological aspects of the habenular complex, and its role in cognitive processes, including new behavioral, electrophysiological and imaging findings. Habenular complex lesions result in deficits in learning, memory and attention, some of which decline during repeated testing, while others become worse, consistent with multiple roles in cognition. The habenular complex is particularly responsive to feedback about errors. Electrophysiological studies indicate a role in metaplasticity, the modulation of neuroplasticity. These studies thus reveal important roles of the habenular complex in learning, memory and attention.

Citalopram combined with WAY 100635 inhibits ejaculation and ejaculation-related Fos immunoreactivity

The role of 5-HT (5-hydroxytryptamine, 5-HT)(1A) receptor activation in the sexual side-effects, in particular delayed ejaculation, of selective serotonin reuptake inhibitors (SSRIs) was studied. Male Wistar rats were treated for 15 days with vehicle, the SSRI citalopram (10 mg/kg/day p.o.), the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-(2-pyridinyl) cyclohexane carboxamide 3HCL (WAY 100635, 0.1 mg/kg/ day s.c.), or both drugs combined. Sexual behavior was assessed weekly. One h after the last sexual behavior test, rat brains were processed for Fos-immunohistochemistry. Acute and chronic citalopram mildly inhibited ejaculation, which was strongly augmented by co-administration of WAY 100635. WAY 100635 alone did not alter sexual behavior. Brain sites associated with ejaculation showed reduced Fos-immunoreactivity in rats treated with both citalopram and WAY 100635. Citalopram reduced Fos-immunoreactivity in the arcuate hypothalamic nucleus, an area that might link serotonergic neurotransmission to ejaculation.

Fos expression in serotonergic neurons in the rat brainstem following noxious stimuli: an immunohistochemical double-labelling study

In order to detect whether there were different expression patterns of Fos protein induced by somatic or visceral noxious stimulation in the serotonergic neurons in the rat brainstem, an immunohistochemical double-labelling technique for serotonin (5-HT) and Fos was employed after subcutaneous or stomach injection of formalin. The two stimuli were matched in pilot experiments to produce maximum Fos expression. The expression of Fos protein in 5-HT-containing neurons (5-HT/Fos co-localized neurons) could be observed in the ventrolateral subdivision of the midbrain periaqueductal grey, interpeduncular nucleus, paramedian raphe nucleus, all of the brainstem raphe nuclei, the alpha part of the gigantocellular reticular nucleus and the lateral paragigantocellular reticular nucleus. The locations of the 5-HT/Fos co-localized neurons in the brainstem of animals subjected to somatic noxious stimulation were similar to those subjected to visceral noxious stimulation. However, the number and proportion of the 5-HT/Fos co-localized neurons in the median raphe nucleus and nucleus raphe obscurus of the rat subjected to visceral noxious stimulation were statistically greater than those in rats subjected to somatic noxious stimulation. These results suggest that serotonergic neurons in median raphe nucleus and nucleus raphe obscurus have a tendency to higher neuronal activity after visceral noxious stimulation.

Longitudinal axis of the hippocampus: both septal and temporal poles of the hippocampus support water maze spatial learning depending on the training protocol

It has been suggested previously that 30% sparing of the hippocampus is enough to support spatial learning of a reference memory task in a water maze provided the spared tissue is located septally (Moser et al. 1995, Proc Natl Acad Sci USA 92:9697-9701). Therefore, the temporal hippocampus may not be involved in spatial memory. Place cells are also found in this part of the structure, and it has been suggested that these place cells have larger, less well-tuned place fields than are found in the septal hippocampus. We tested the possibility that the temporal hippocampus might be involved in spatial learning when the animals are required to distinguish between different contexts. Experiment 1 was a replication of the findings reported by Moser et al., using their protocol (8 trials/day, 6 days) and the groups with 20-40% hippocampus spared septally or temporally (volume assessed by quantitative volumetric techniques). In experiment 2, rats with also 20-40% sparing of the hippocampus either septally or temporally were trained in two water maze concurrently (four trials/day/water maze, 8 days). Rats with 20-40% hippocampus spared temporally were able to learn the two water maze tasks normally, and no difference was observed between rats with septal and temporal hippocampus spared across different measures of performance. In experiment 3, rats with 20-40% hippocampus spared septally or temporally were trained in one water maze as in experiment 1, but using a spaced training protocol similar to that of experiment 2 (four trials/day, 8 days). Rats with temporal hippocampus spared developed a preference for the training quadrant and acquired levels of performance indistinguishable from those of rats with septal hippocampus spared. The results suggest that the temporal hippocampus can support the learning of two, but also one, spatial water maze reference memory task, provided the training protocol is adequate.

The fasciculus retroflexus controls the integrity of REM sleep by supporting the generation of hippocampal theta rhythm and rapid eye movements in rats

The fasciculus retroflexus (FR) fiber bundle comprises the intense cholinergic projection from the medial division of the habenula nucleus (Hbn) of the epithalamus to the interpeduncular nucleus (IPN) of the limbic midbrain. Due to the widespread connections of the Hbn and IPN, it could be surmised that the FR is integrated in the processings of various subsystems that are known to be involved in the sleep-wake mechanisms; relevant sites include the limbic forebrain and midbrain areas and more caudal pontine structures. Consequently, the present study addressed the significance of the FR in the spontaneous sleep-wake stage-associated variations of the different activity patterns of frontal cortex and hippocampal electroencephalograms (EEGs), the electrooculogram, and body movements, in freely behaving rats that had been subjected to either bilateral electrolytic lesioning of the FR or control operations. The evolution of different state combinations was assessed by the combinatory analysis of different activity stages appearing on the 6-h records. As compared to the control-operated group, the FR lesioning substantially reduced the time spent in rapid eye movement (REM) sleep by 79%, moderately decreased the duration of the intermediate state of sleep by 29%, and quiet waking state by 44%, but had virtually no effects on the durations of different types of non-REM sleep (i.e., drowsiness that which involved quiet sleep or slow-wave sleep containing delta and spindle state components) or on the times of active waking behavior that corresponded to the body movements. Quantitative decomposition analyses revealed marked variations in the frontal cortex and hippocampal activity as well as REM during the course of the extracted sleep-wake stages described and there were also some group differences. Of those individual features that were used to determine different sleep-wake stages, the overall hippocampal theta time (41% decrease) and single REM frequency (71% reduction during the REM sleep) were most affected. In contrast, the various properties of desynchronization/synchronization patterns of frontal cortex EEGs were consistently hardly influenced by the FR lesioning. Therefore, the present data suggest the involvement of the FR in the REM sleep processes by establishing prominent associations with the limbic and REM control mechanisms that involve the hippocampus and plausibly pontine ocular activity networks.

Developmental changes in heterogeneous patterns of neurotransmitter receptor binding in the human interpeduncular nucleus

The interpeduncular nucleus (IPN) exhibits many complex features, including multiple subnuclei, widespread projections with the forebrain and brainstem, and neurotransmitter heterogeneity. Despite the putative importance of this nucleus, very little is known about its neurochemical development in the human. The human IPN is cytoarchitectonically simple, unlike the rat IPN, which displays considerable heterogeneity. In the following study, we hypothesized that the developing human IPN is neurochemically heterogeneous despite its cytological simplicity. The chemoarchitecture in this study was defined by neurotransmitter receptor binding patterns by using quantitative tissue autoradiography for the muscarinic, nicotinic, serotoninergic, opioid, and kainate receptors. We examined neurotransmitter receptor binding in the developing human IPN in a total of 15 cases. The midbrains of five midgestational fetuses (19-26 gestational weeks) and six infants (38-74 postconceptional weeks) were examined. The midbrain of one child (4 years) and three adults (20-68 years) were analyzed as indices of maturity. At all ages examined, high muscarinic binding was localized to the lateral subdivision of the IPN, high serotoninergic binding was localized to the dorsal IPN, and high opioid receptor binding was localized to the medial IPN. The developmental profile was unique for each radioligand. We report a heterogenous distribution of neurotransmitter receptor binding in the developing human IPN, which supports a complex role for it in human brain function.

Excitatory actions of serotonin on GABAergic neurons of the medial septum and diagonal band of Broca

The physiological and pharmacological actions of serotonin (5-HT) on neurons in the medial septum and diagonal band of Broca (MSDB) were examined using extracellular and intracellular recording techniques in an in vitro rat brain-slice preparation. In addition to previously described inhibitory effects, novel excitatory actions of 5-HT on GABA-type cells were observed. In intracellular recordings with KCl-containing electrodes, bath-applied 5-HT induced a bicuculline and tetrodotoxin-sensitive increase in the number of reverse IPSPs in both cholinergic- and noncholinergic-type neurons (presumably GABAergic). In brain slices where all structures neighboring the MSDB, including the lateral septum, had been excised, a similar increase in 5-HT-induced IPSPs occurred, indicating that 5-HT-induced IPSPs in both cholinergic- and noncholinergic-type neurons originate from GABAergic neurons within the MSDB itself. Accordingly, GABA-type neurons in the MSDB were found to be directly excited by 5-HT. MDL 100,907, a selective 5-HT2A antagonist, blocked 5-HT-induced excitations in a majority of neurons (58%). ICS 205-930, a 5-HT3/5-HT4 antagonist, or mianserin, a nonselective 5-HT antagonist, blocked most MDL-resistant responses, indicating a role for multiple 5-HT receptor subtypes. This study also provides the first electrophysiological evidence for synaptic interactions between 5-HT-activated GABAergic neurons and cholinergic neurons and amongst GABAergic neurons in the MSDB. The implications of the findings vis-à-vis intraseptal circuitry and septohippocampal circuitry are discussed.

Serotonin-containing terminals synapse on septohippocampal neurons in the rat

Serotonin [5-hydroxytryptamine (5-HT)] is thought to be involved in mnemonic functions and dysfunctions possibly by directly contacting neurons in the medial septal and diagonal band nuclei (i.e., the septal complex) that project to the hippocampal formation. However, there is no cellular substrate for this modulation. Thus, we examined the ultrastructure and synaptic associations of 5-HT-containing terminals in relation to septohippocampal neurons in the septal complex of the rat brain. Projection neurons were identified by retrograde transport of wheat germ agglutinated apo-horseradish peroxidase conjugated to colloidal gold particles (WAHG) following an injection into the ventral hippocampal formation of anesthetized adult rats. After a 1 day survival, sections through the septal complex were labeled with antibodies to 5-HT. By light microscopy, numerous processes with 5-HT immunoreactivity (5-HT-I) were observed in close proximity to neurons containing retrogradely transported WAHG. By electron microscopy, 5-HT-I was found exclusively in axons and axon terminals. Axons were primarily unmyelinated. Terminals with 5-HT-I were 0.35-1.2 microns in diameter and contained numerous small, clear vesicles and 0-4 large, dense-core vesicles. The 5-HT-labeled terminals: 1) contacted perikarya and dendrites (220 of 349); 2) were closely apposed to other terminals (25 of 349); or 3) had no neuronal contacts in the plane of section analyzed (104 of 349). The 5-HT-labeled terminals formed exclusively symmetric synapses on perikarya; some of these perikarya as well as some large dendrites similarly contacted by the 5-HT-labeled terminals also contained WAHG affiliated with lysosomes and multivesicular and "sequestration" bodies in the cytoplasm. However, the majority of terminals with 5-HT-I formed contacts on the shafts of small unlabeled dendrites (69% of 220); most of these were characterized as either asymmetric synapses or appositions not separated by astrocytes in the plane of section analyzed. We conclude that 5-HT-containing terminals in the rat septal complex: 1) directly modulate septohippocampal and other neurons through symmetric (potentially inhibitory) synapses on soma and proximal dendrites; and 2) form primarily asymmetric (potentially excitatory) synapses with distal (small) dendrites from neurons of unidentified origin. These findings suggest that serotonin may affect learning and memory through modulation of septal efferents to the hippocampal formation and may have direct relevance to the neuropathological basis for Alzheimer's disease.

Behavioural consequences following infusion of selective neurokinin agonists into the median raphe nucleus of the rat

The locomotor activity induced after infusion of selective neurokinin (NK) agonists into the median raphé nucleus of rats was investigated. In photocell cages, the NK-2-agonist, GR64349, and the NK-3 agonist, senktide, both increased motor activity in a dose-dependent manner. However, the NK-1 agonist, GR73632, had little effect over a range of doses. In the open field, the motor effect of all three NK agonists was identical to that observed in the photocell cages. In addition, senktide induced straub-tail, hind-limb splaying and various oral movements. Such effects were not noted with the other two agonists. These results suggest that activation of NK-2 or NK-3 receptors by the neurokinins, in the median raphe nucleus of the rat, leads to an increase in locomotor activity.

Differential effects of fasciculus retroflexus lesions on serotonin, glutamate and gamma-aminobutyrate content and choline acetyltransferase activity in the interpeduncular nucleus

After placing bilateral electrolytic lesions in the fasciculus retroflexus (FR) of the rat, the endogenous content of serotonin, glutamate and gamma-aminobutyrate (GABA) as well as choline acetyltransferase activity (ChAT) were measured in the interpeduncular nucleus (IPN) at the 7th, 28th and 120th survival days. Confirming earlier results, an almost total depletion of ChAT was obtained in the IPN following complete FR lesions at any survival day studied. In such cases, the following changes were observed; 1) the serotonin level increased consistently and roughly doubled at the 120th survival day, suggesting heterotypic sprouting of serotonergic fibers and/or enhanced serotonin synthesis in the serotonergic neurons in the IPN, 2) the glutamate level decreased by approximately one-half, while the activity of high affinity uptake of glutamate remained unaltered, at the 7th survival day, suggesting a lowered glutamate formation coupled with lowered glucose utilization in the IPN, and 3) the GABA level decreased at a slower rate and reached one-third of the control at the 120th survival day, for which either transsynaptic degeneration of GABA neurons in the IPN or a suppressed metabolic rate in the GABA shunt following the lowered glutamate formation is a possible explanation.

Regional changes in brain 5-HT1A serotonin receptors in the rat model of persistent spasmodic dyskinesias induced by iminodipropionitrile

Chronic injection of iminodipropionitrile (IDPN) to rats causes persistent motor abnormalities such as hyperactivity, lateral and vertical dyskinesia of the neck, and random circling. These behavioral changes are very similar to those observed after the acute administration of serotonin (5-HT) agonists in rodents. Moreover, some aspects of this syndrome are reproduced by stimulation of 5-HT1A receptors. The present quantitative autoradiographic study revealed a number of changes in 8-hydroxy-2-[di-n-propylamino-3H]tetralin (8-OH[3H]DPAT)-labeled 5-HT1A receptors in the brains of IDPN-treated rats. There were significant increases of 8-OH[3H]DPAT binding in the frontal cortex and in the caudate-putamen. In contrast, there were significant decreases in the interpeduncular nucleus, the pyramidal layer of the CA3 field of hippocampus, the superior colliculus and the pars reticulata of the substantia nigra. These results provide further evidence for the involvement of the 5-HT system in the development of the IDPN-induced dyskinetic syndrome.

Projections between the interpeduncular nucleus and basal forebrain in the rat as demonstrated by the anterograde and retrograde transport of WGA-HRP

The distribution of anterogradely-labeled fibers and retrogradely-labeled cell bodies in the interpeduncular nucleus (IPN) was mapped after injections of wheat-germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) into various structures of the basal forebrain in adult rats. WGA-HRP injections into the medial septum/vertical limb of the diagonal band nucleus resulted in: 1) dense anterograde labeling in the rostral, intermediate, and central subnuclei; and 2) retrograde labeling in the apical and central subnuclei. Injections into the lateral septum produced: 1) no anterograde labeling in the IPN; and 2) retrograde labeling which was dense in the apical subnucleus, moderate in the central and lateral subnuclei, and light in the intermediate subnucleus. Injections into the horizontal limb of the diagonal band nucleus resulted in: 1) anterograde labeling which was most pronounced in the central, rostral, and intermediate subnuclei; and 2) retrograde labeling which was strongest in the apical, central, and lateral subnuclei. After injections into the substantia innominata-magnocellular preoptic nucleus, there was: 1) dense anterograde labeling in the rostral and central subnuclei and moderate anterograde labeling in the intermediate subnucleus; and 2) essentially no retrograde cell labeling in the IPN. These findings demonstrate that the IPN receives inputs from, and projects to specific portions of the basal forebrain. The rostral and central subnuclei are the primary targets of inputs from the basal forebrain to the IPN, and the apical subnucleus is the primary source of IPN projections to the basal forebrain.