Effects of lateral habenular lesions on local cerebral glucose utilization in the rat

Lateral Habenula and Its Potential Roles in Pain and Related Behaviors

The lateral habenula (LHb) is a tiny structure that acts as a hub, relaying signals from the limbic forebrain structures and basal ganglia to the brainstem modulatory area. Facilitated by updated knowledge and more precise manipulation of circuits, the progress in figuring out the neural circuits and functions of the LHb has increased dramatically over the past decade. Importantly, LHb is found to play an integrative role and has profound effects on a variety of behaviors associated with pain, including depression-like and anxiety-like behaviors, antireward or aversion, aggression, defensive behavior, and substance use disorder. Thus, LHb is a potential target for improving pain management and related disorders. In this review, we focused on the functions, related circuits, and neurotransmissions of the LHb in pain processing and related behaviors. A comprehensive understanding of the relationship between the LHb and pain will help to find new pain treatments.

Altered formalin-induced pain and Fos induction in the periaqueductal grey of preadolescent rats following neonatal LPS exposure

Animal and human studies have demonstrated that early pain experiences can produce alterations in the nociceptive systems later in life including increased sensitivity to mechanical, thermal, and chemical stimuli. However, less is known about the impact of neonatal immune challenge on future responses to noxious stimuli and the reactivity of neural substrates involved in analgesia. Here we demonstrate that rats exposed to Lipopolysaccharide (LPS; 0.05 mg/kg IP, Salmonella enteritidis) during postnatal day (PND) 3 and 5 displayed enhanced formalin-induced flinching but not licking following formalin injection at PND 22. This LPS-induced hyperalgesia was accompanied by distinct recruitment of supra-spinal regions involved in analgesia as indicated by significantly attenuated Fos-protein induction in the rostral dorsal periaqueductal grey (DPAG) as well as rostral and caudal axes of the ventrolateral PAG (VLPAG). Formalin injections were associated with increased Fos-protein labelling in lateral habenula (LHb) as compared to medial habenula (MHb), however the intensity of this labelling did not differ as a result of neonatal immune challenge. These data highlight the importance of neonatal immune priming in programming inflammatory pain sensitivity later in development and highlight the PAG as a possible mediator of this process.

Cerebral metabolic effects of serotonin drugs and neurotoxins

The functional effects of serotonin (5-HT) drugs and toxins on regional cerebral metabolic rates for glucose (rCMRglc) have been determined in rats with the in vivo, quantitative, autoradiographic [14C]2-deoxyglucose technique. Serotonin agents produced rCMRglc patterns different and more specific that one would predict from binding studies. At low doses 5-HT1 agonists reduced rCMRglc in limbic areas and at high doses increased rCMRglc in brain motor regions. The 5-HT2 agonists dose-dependently decreased rCMRglc in proencephalic areas and increased it in thalamic nuclei. 5-HT3 receptor antagonism resulted in rCMRglc decreases in limbic, auditory and visual areas and agents with 5-HT3 receptor activity increased rCMRglc in brain regions with high 5-HT3 receptor densities. Serotonin anxiolytics (e.g. azapirones) and antidepressants (e.g. tryciclic and non-tryciclic 5-HT reuptake inhibitors) reduced rCMRglc selectively in limbic areas and in brainstem monoaminergic nuclei. Dose, time from administration, receptor affinity, behavioral and neurochemical correlates, 5-HT system lesion and circulating glucocorticoid were all relevant factors in determining the rCMRglc effects of 5-HT drugs. Acutely neurotoxic amphetamines markedly increased rCMRglc in brain regions such as the nucleus accumbens that are thought to mediate amphetamine reinforcing properties; on the long term, toxic or electrolytic lesions or chronic treatment with 5-HT agonists produced minimal rCMRglc alterations in spite of marked and persistent changes in 5-HT function. In lesioned or chronically treated rats, acute challanges with 5-HT and non 5-HT agonists demonstrated specific deficits that were not detected in a resting state. Serotonin neuromodulation has been studied in humans by using positron emission tomography with 15O-water. Sequential measurements of regional cerebral blood flow (rCBF) were obtained during combined pharmacological challange with the 5-HT1A agonist buspirone and cognitive activation. Buspirone increased a memory related rCBF activation in task specific regions. This technique can provide a strong theoretical basis for the understanding of 5-HT drug mode of action in normal human brain and in neuropsychiatric diseases. Brain metabolism studies in animals will still be needed to elucidate the factors (e.g. pharmacokinetic and pharmacodynamic) relevant to the cerebral response to 5-HT drugs in humans.

Age-dependent cerebral metabolic effects of unilateral nucleus basalis magnocellularis ablation in rats

To investigate the age-dependent functional importance of cholinergic neocortical inputs, and to explore whether cortical cholinergic denervation in aged animals might better model the cerebral metabolic changes of Alzheimer's disease, the effects of unilateral ablation of the nucleus basalis magnocellularis (NBM) on cerebral glucose metabolism were studied in young and aged rats. Regional cerebral metabolic rates for glucose (rCMRglc) were determined, using the [14C]deoxyglucose method, in 48 brain regions of 3- and 24-month old Fischer-344 rats at 3, 7, 14 and 28 days after stereotaxic injection of ibotenate into the right NBM, and in sham-operated animals at 3 and 14 days later. For both ages the peak effect of unilateral NBM ablation occurred 3 days later: in young rats, rCMRglc was significantly reduced (compared to the contralateral side) in all 24 anterior cortical areas examined (mean decline 20%), whereas in aged animals, only 9 of 24 areas showed a significant decline in glucose utilization, and the magnitude of rCMRglc reduction (9%) was smaller. Near complete recovery of rCMRglc occurred by 7 days in young and old rats. We conclude that the basalocortical cholinergic projection plays a smaller role in neocortical function of aged rats, possibly because its tonic activity is reduced. Both young and aged rats undergo cortical metabolic normalization after unilateral NBM ablation; hence the NBM-lesioned aged rat is not a better model of the progressive decline in rCMRglc that occurs in Alzheimer's disease.

Relevance of the habenular complex to neuropsychiatry: a review and hypothesis

Since the initial observation by Brown (1914) that electrical stimulation applied to the habenular efferent bundle in the chimpanzee evoked a pattern of respiration which closely resembled the act of laughter, the habenular complex has remained a mysterious structure. The anatomy of the habenular complex is well delineated (Jones, 1985) forming a major component of the dorsal diencephalic conduction system. Data derived mainly from animal experimentation over the past decade point to the fact that the habenular complex functions as an important link between the limbic forebrain and the midbrain-extrapyramidal motor system. The elucidation of the functions of the habenular complex may thus significantly increase the current insight into the understanding of the interaction between behavioral and motor functions. Clearly, such information would be of great relevance for further understanding of neuropsychiatric disorders such as schizophrenia, Parkinson's disease, Tardive dyskinesia, and Tourette's syndrome in which behavioral and motor impairments are interfaced. This review summarizes anatomical, functional, and pharmacological aspects of the habenular complex and discusses its potential contribution to the pathophysiology of selected neuropsychiatric and movement disorders.

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.

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.

Habenula as a relay in the descending pathway from nucleus accumbens to periaqueductal grey subserving antinociception

This study explored the possibility of a relay at habenula for the descending neural pathway of antinociception. The latency of the escape response elicited by radiant heat on the snout of the rabbit was taken as index of nociception. (1) Microinjection of 20 micrograms of morphine into nucleus accumbens resulted in a one-fold increase in nociceptive threshold 20-40 min after the injection. This effect of morphine was markedly attenuated by naloxone or met-enkephalin antiserum administered to the nucleus habenula, suggesting that the release of met-enkephalin in habenula is essential for the antinociception induced by morphine injected into nucleus accumbens. (2) Injection of 10 micrograms of morphine into habenula produced a significant increase in escape response latency 20-40 min after the injection. This antinociceptive effect of morphine was attenuated by naloxone or muscimol, and enhanced by bicuculline methochloride administered to periaqueductal grey, suggesting that morphine may act on habenula to activate a descending neural pathway extending to periaqueductal grey to induce an antinociceptive effect, which seems to utilize endogenous opioid peptides and gamma-aminobutyric acid as its mediators. Taking together, the results suggest that habenula is an important relay in the descending neural pathway from nucleus accumbens to periaqueductal grey subserving antinociception.

The distribution and cellular localization of the serotonin 1C receptor mRNA in the rodent brain examined by in situ hybridization histochemistry. Comparison with receptor binding distribution

The regional distribution and cellular localization of mRNA coding for the serotonin 1C receptor were investigated in tissue sections of mouse and rat brain by in situ hybridization histochemistry. Several 32P-labelled riboprobes derived from mouse genomic clones were used. The serotonin 1C receptor binding sites were visualized autoradiographically and quantified using [3H]mesulergine as ligand, in the presence of spiperone to block serotonin 1C receptors. Strong hybridization signal was observed in the choroid plexus of all brain ventricles. High levels of hybridization were also seen in the anterior olfactory nucleus, pyriform cortex, amygdala, some thalamic nuclei, especially the lateral habenula, the CA3 area of the hippocampal formation, the cingulate cortex, some components of the basal ganglia and associated areas, particularly the nucleus subthalamicus and the substantia nigra. The midbrain and brainstem showed moderate levels of hybridization. The distribution of the serotonin 1C receptor mRNA corresponded well to that of the serotonin 1C receptors. The highest levels of serotonin 1C receptor binding were observed in the choroid plexus. In addition, significant levels of the serotonin 1C receptor binding were seen in the anterior olfactory nucleus, pyriform cortex, nucleus accumbens, ventral aspects of the striatum, paratenial and paracentral thalamic nuclei, amygdaloid body and substantia nigra pars reticulata. The cingulate and retrosplenial cortices as well as the caudal aspects of the hippocampus (CA3) were also labelled. Binding in brainstem and medulla was low and homogeneously distributed. No significant binding was seen in the habenular and subthalamic nuclei. Similar findings were obtained in rat brain. These results demonstrate that, in addition to their enrichment in the choroid plexus, the serotonin 1C receptor mRNA and binding sites are heterogeneously distributed in the rodent brain and thus could be involved in the regulation of many different brain functions. The combination of in situ hybridization histochemistry with receptor autoradiography opens the possibility of examining the regulation of the serotonin 1C receptor synthesis after pharmacological or physiological alterations.

Chronic effects of the selective serotoninergic neurotoxin, methylenedioxyamphetamine, upon cerebral function

The amphetamine derivative methylenedioxyamphetamine selectively destroys serotoninergic terminals in the brain. We have studied the effects of this toxin upon resting cerebral function, as reflected in rates of glucose utilization. Rats were injected subcutaneously with either 1 ml/kg saline (n = 5) or 20 mg/kg methylenedioxyamphetamine (n = 5) twice daily for four days. Local cerebral glucose utilization was measured between six and nine weeks after treatment using [14C]2-deoxyglucose quantitative autoradiography. Samples of frontal cortex taken from these animals for in vitro [3H]paroxetine binding showed a 64% reduction in 5-hydroxytryptamine uptake sites. In the majority of the 31 functionally diverse brain areas analysed, no significant changes were measured, but significant (P less than 0.05) increases in glucose use were found in neocortical regions e.g. anterior cingulate cortex (+16%) and sensorimotor cortex (+21%). However, the most profound increases were found in globus pallidus (+30%) and hippocampus molecular layer (+34%). It would appear, therefore, that treatment with methylenedioxyamphetamine results in long-lasting alterations in cerebral functional activity.

Regulation of striatal serotonin release by the lateral habenula-dorsal raphe pathway in the rat as demonstrated by in vivo microdialysis: role of excitatory amino acids and GABA

Striatal extracellular levels of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) were monitored with the microdialysis technique during electrical stimulation of the lateral habenula-dorsal raphe (LHb-NRD) pathway in halothane anaesthetized rats. A new double-loop probe, with an improved recovery factor, was implanted into the head of the caudate-putamen and perfused with Ringer solution containing 1 microM of the 5-HT uptake blocker indalpine. Samples were collected every 15 min and analyzed with HPLC coupled to fluorimetric detection. Low frequency stimulation of the LHb (1.5 and 3 Hz, 0.5 mA) produced no detectable changes in striatal indole levels, whereas 15 Hz stimulation induced a 70% increase in 5-HT release. This effect was most likely mediated by a direct LHb-NRD link, since it persisted after ibotenic acid lesions of the interpeduncular nucleus (which is the major projection area for the medial habenular nucleus), but was completely abolished after transection of the fasciculus retroflexus, which carries the axons of the LHb-NRD pathway. The possible identity of the transmitter operating in the LHb-NRD pathway was investigated by NRD injections of kynurenic acid, a potent blocker of excitatory amino acid transmission, and by NRD injections of the GABA antagonist bicuculline. Kynurenic acid (300 nl, 50 mM) did not by itself induce any detectable changes in spontaneous indole output, but completely blocked the effect of LHb stimulation. Injection of bicuculline (300 nl, 2 mM) increased the striatal 5-HT output by about 70%, and potentiated the effect of LHb stimulation by a further 50%. In none of the experiments performed in this study were there any significant changes in striatal 5-HIAA output. These data are compatible with the idea that excitatory amino acids in the LHb-NRD pathway are involved in the regulation of striatal 5-HT release, and that this influence is modulated by GABAergic synaptic activity at the level of the NRD.

Influence of ascending serotonergic pathways on glucose use in the conscious rat brain. I. Effects of electrolytic or neurotoxic lesions of the dorsal and/or median raphé nucleus

The regional cerebral metabolic effects of manipulations of the central serotonergic pathways are largely unknown. To address this topic, we have examined the consequences of electrolytic lesions of the rostral (median and/or dorsal) raphé nuclei on local cerebral glucose utilization (CMRglu) in the unanaesthetized rat brain. These studies were complemented by comparing control rats to rats that received prior intraventricular administration of the serotonergic neurotoxin, 5,7-dihydroxytryptamine (5,7-DHT). CMRglu was determined in 56 neuroanatomically defined regions of the central nervous system in lightly restrained rats, by the quantitative autoradiographic 2-deoxyglucose technique. In all, 6 groups of rats were studied: sham-lesioned rats, rats with electrolytic lesion of the median, dorsal, or both these raphé nuclei; sham-injected and 5,7-DHT pretreated rats. The efficacy of both electrolytic and neurotoxic lesions was verified, in each animal, by neurochemical microassay of 5-hydroxytryptamine and its metabolite in samples of striatum, hippocampus and prefrontal cortex. Chronic interruption of serotonergic transmission was remarkable for the lack of resultant change in CMRglu. In rats that were subjected to electrolytic lesions of both median and dorsal raphé nuclei, discrete and significant decreases in CMRglu were observed in the red nucleus, substantia nigra and inferior olivary nucleus only. The rats subjected to 5,7-DHT treatment displayed no significant changes in CMRglu in all the brain regions analyzed, despite an 80% decrease in the concentrations of endogenous 5-hydroxytryptamine. Thus, it would appear that a viable serotonergic transmission is not a major determinant of integrated functional activity, even in those brain structures that receive rich raphé projections. Two hypotheses are advanced for this lack of change: firstly, the chronic reduction of 5-hydroxytryptamine levels is accompanied by compensatory changes in this or other neurotransmitter systems; secondly, serotonergic neurones may exert a phasic--rather than tonic--influence on glucose use in the mammalian brain.

Deafferentation elicits a transient decrease in Na+, K+-ATPase activity and ouabain binding in the olfactory tubercle

This work examines the effects of olfactory bulbectomy on Na+, K+-ATPase activity and ouabain binding in the olfactory tubercle. The activity and number of enzyme sites were reduced significantly in olfactory tubercle, but not in corpus striatum or hippocampus, 14 and 21 days after bulbectomy. Enzyme activity and ouabain binding returned to normal by 42 days after the lesions. The time of the reduction in Na+, K+-ATPase coincides with that observed earlier for dopaminergic sprouting and increased dopamine-sensitive adenylate cyclase activity.

Functional mapping of the effects of lesions of the habenular nuclei and their afferents in the rat

Through the use of the quantitative autoradiographic 2-[14C]deoxyglucose technique, we have investigated the functional significance of the habenular nuclei by the measurement of local cerebral glucose utilization (LCGU) in discrete brain areas of conscious rats following 3 kinds of lesioning. Bilateral electrolytic lesions of the habenular nuclei decreased LCGU in a limited number of well-defined brain areas (the interpeduncular nucleus, median and dorsal raphe, mammillary body and dorsal tegmental nucleus) at 7 and 14 days after lesions. These changes were also observed 180 days following lesioning except that of the dorsal tegmental nucleus. At 14 days after bilateral ibotenic acid-induced lesions of the lateral habenula, LCGU was significantly decreased in the median and dorsal raphe, mammillary body and interpeduncular nucleus. In further studies, bilateral electrolytic lesions of the stria medullaris (which conveys the major afferents to the habenula) decreased glucose use in the interpeduncular nucleus less than that observed after bilateral electrolytic lesions of the habenular nuclei. A highly significant positive correlation was observed between LCGU and choline acetyltransferase activity in the interpeduncular nucleus after all types of lesion. These results further support the view that the medial and the lateral habenula exert a major influence upon functional activity in the interpeduncular nucleus and the mesencephalic raphe nuclei, respectively.

Functional activity in the brain of socially deprivated rats produced by an active avoidance test after razobazam (Hoe 175) treatment: a 2-deoxyglucose study

The 2-deoxyglucose (2-DG) autoradiographic method was used to map metabolic activity in the brain of socially deprivated rats during an active avoidance test. The method investigated the effects of razobazam during this learning test. The animals were socially deprivated for 5 weeks. On the first experimental day the animals were trained to avoid a footshock by jumping onto a platform. During training and testing, the total number of avoidance responses was scored. On the second day during one 2-DG session of 40 min, razobazam increased the avoidance score by 18% as compared to controls. Autoradiographs were analyzed using a two-dimensional densitometric method. The analysis of the brain structures showed a 22% reduction of optical density in the nucleus habenularis lateralis, a 25% increase in the caudal part of the nucleus accumbens, and a 13% increase in the frontal cortex in rats treated with razobazam, but no change in the amygdala. These results provide a preliminary concept to explain how the new compound razobazam produced a better learning performance in socially deprivated rats.

Temporal effects of habenular lesions on glucose utilization in the anterior raphé nuclei of the rat

The effects of bilateral electrolytic lesions of both lateral and medial habenular nuclei on local cerebral glucose utilization have been examined in conscious rats by the use of the quantitative 2-deoxy[14C]glucose autoradiographic technique. Habenular lesions markedly reduced the use of glucose in the median raphé and, to a lesser extent, in the dorsal raphé nuclei (which receive major afferents from the lateral habenula) at 7, 14 and 180 days after the lesion. Habenular lesions failed to alter the use of glucose in the entopeduncular nucleus, lateral hypothalamic area and the nucleus of the diagonal band of Broca which project to the habenula. These data add further support to the view that the habenula exerts a major facilitatory influence upon functional activity in the anterior raphé nuclei.