Bilateral Injections Of 6-OHDA Research Articles

Impact of Serotonergic 5HT1A and 5HT2A Receptor Activation on the Respiratory Response to Hypercapnia in a Rat Model of Parkinson's Disease.

In Parkinson's disease (PD), along with typical motor dysfunction, abnormal breathing is present; the cause of which is not well understood. The study aimed to analyze the effects of stimulation of the serotonergic system with 5-HT1A and 5-HT2A agonists in a model of PD induced by injection of 6-hydroxydopamine (6-OHDA). To model PD, bilateral injection of 6-OHDA into both striata was performed in male Wistar rats. Respiratory disturbances in response to 7% hypercapnia (CO2 in O2) in the plethysmographic chamber before and after stimulation of the serotonergic system and the incidence of apnea were studied in awake rats 5 weeks after 6-OHDA or vehicle injection. Administration of 6-OHDA reduced the concentration of serotonin (5-HT), dopamine (DA) and norepinephrine (NA) in the striatum and the level of 5-HT in the brainstem of treated rats, which have been associated with decreased basal ventilation, impaired respiratory response to 7% CO2 and increased incidence of apnea compared to Sham-operated rats. Intraperitoneal (i.p.) injection of the 5-HT1AR agonist 8-OH-DPAT and 5-HT2AR agonist NBOH-2C-CN increased breathing during normocapnia and hypercapnia in both groups of rats. However, it restored reactivity to hypercapnia in 6-OHDA group to the level present in Sham rats. Another 5-HT2AR agonist TCB-2 was only effective in increasing normocapnic ventilation in 6-OHDA rats. Both the serotonergic agonists 8-OH-DPAT and NBOH-2C-CN had stronger stimulatory effects on respiration in PD rats, compensating for deficits in basal ventilation and hypercapnic respiration. We conclude that serotonergic stimulation may have a positive effect on respiratory impairments that occur in PD.

A Guide to the Generation of a 6-Hydroxydopamine Mouse Model of Parkinson's Disease for the Study of Non-Motor Symptoms.

In Parkinson’s disease (PD), a large number of symptoms affecting the peripheral and central nervous system precede, develop in parallel to, the cardinal motor symptoms of the disease. The study of these conditions, which are often refractory to and may even be exacerbated by standard dopamine replacement therapies, relies on the availability of appropriate animal models. Previous work in rodents showed that injection of the neurotoxin 6-hydroxydopamine (6-OHDA) in discrete brain regions reproduces several non-motor comorbidities commonly associated with PD, including cognitive deficits, depression, anxiety, as well as disruption of olfactory discrimination and circadian rhythm. However, the use of 6-OHDA is frequently associated with significant post-surgical mortality. Here, we describe the generation of a mouse model of PD based on bilateral injection of 6-OHDA in the dorsal striatum. We show that the survival rates of males and females subjected to this lesion differ significantly, with a much higher mortality among males, and provide a protocol of enhanced pre- and post-operative care, which nearly eliminates animal loss. We also briefly discuss the utility of this model for the study of non-motor comorbidities of PD.

Pre-treatment by Rosemary Extract or Cell Transplantation Improves Memory Deficits of Parkinson’s Disease: When Tradition Meets the Future

HIGHLIGHTS Spatial learning and memory impairment through bilateral injection of 6-OHDA (6-Hydroxydopamine) into the SNc which also leads to the destruction of neurons in the hippocampus. ADSCs (Adipose tissue-derived stem cells) transplantation had neuroprotective effect and recovered the memory deficit in 6-OHDA-induced rat model of Parkinson’s disease. Rosemary extract pre-treatment compensated the memory deficit in 6OHDA-induced rat model of Parkinson’s disease.

Raphe Pallidus is Not Important to Central Chemoreception in a Rat Model of Parkinson’s Disease

Central chemoreceptors are primarily sensitive to changes in CO2/H+, and such changes lead to intense breathing activity. Medullary raphe and retrotrapezoid nucleus (RTN) neurons are candidates for central chemoreceptors because they are unusually pH sensitive. The pathophysiology of Parkinson’s disease (PD) is related to the reduction of neurons in the substantia nigra pars compacta (SNpc) that express dopamine, although other neurons can also be degenerated in this pathology. In rodent models of PD, we showed an impairment of the hypercapnia ventilatory response due to a reduction in the number of RTN chemosensitive neurons. Here, we aimed to investigate if serotonine-expressing neurons in the Raphe pallidus/parapyramidal region (RPa/PPy) are also involved in the modulation of breathing during central chemoreception activation in a PD animal model. PD was induced in male Wistar rats with bilateral injection of 6-OHDA (6-hydroxydopamine; 24 µg/µl) into the striatum, which leads to a reduction in the catecholaminergic neurons of the SNpc by 89%. In PD animals, we noticed a reduction in the number of RPa neurons that project to the RTN, without a change in the number of hypercapnia-activated (7% CO2) raphe neurons. The PD animals that received injection of the toxin saporin anti-SERT into the RPA/PPy region did not show a further reduction of respiratory frequency (fR) or ventilation (VE) at rest or during hypercapnia challenge. These experiments demonstrate that serotonergic neurons of RPa/PPy are not involved in the breathing responses induced by central chemoreceptor activation in a PD animal model.

Decreased synaptic plasticity in the medial prefrontal cortex underlies short-term memory deficits in 6-OHDA-lesioned rats

Parkinson’s disease (PD) is characterized by motor dysfunction associated with dopaminergic degeneration in the dorsolateral striatum (DLS). However, motor symptoms in PD are often preceded by short-term memory deficits, which have been argued to involve deregulation of medial prefrontal cortex (mPFC). We now used a 6-hydroxydopamine (6-OHDA) rat PD model to explore if alterations of synaptic plasticity in DLS and mPFC underlie short-term memory impairments in PD prodrome. The bilateral injection of 6-OHDA (20μg/hemisphere) in the DLS caused a marked loss of dopaminergic neurons in the substantia nigra (>80%) and decreased monoamine levels in the striatum and PFC, accompanied by motor deficits evaluated after 21 days in the open field and accelerated rotarod. A lower dose of 6-OHDA (10μg/hemisphere) only induced a partial degeneration (about 60%) of dopaminergic neurons in the substantia nigra with no gross motor impairments, thus mimicking an early premotor stage of PD. Notably, 6-OHDA (10μg)—lesioned rats displayed decreased monoamine levels in the PFC as well as short-term memory deficits evaluated in the novel object discrimination and in the modified Y-maze tasks; this was accompanied by a selective decrease in the amplitude of long-term potentiation in the mPFC, but not in DLS, without changes of synaptic transmission in either brain regions. These results indicate that the short-term memory dysfunction predating the motor alterations in the 6-OHDA model of PD is associated with selective changes of information processing in PFC circuits, typified by persistent changes of synaptic plasticity.

Ventral tegmental area dopaminergic lesion-induced depressive phenotype in the rat is reversed by deep brain stimulation of the medial forebrain bundle

DBS of the medial forebrain bundle (MFB) has been investigated clinically in major depressive disorder patients with rapid and long-term reduction of symptoms. In the context of chronic bilateral high frequency deep brain stimulation (DBS) of the MFB, the current study looked at the impact of lesioning the ascending dopaminergic pathway at the level of the ventral tegmental area (VTA). Sprague-Dawley female rats were given bilateral injection of 6-OHDA into the VTA (VTA-lx group) or were left unlesioned (control group). Later, all animals received bilateral microelectrode implantation into the MFB followed by chronic continuous stimulation for 3 weeks. Behavioral tests were performed as baseline and following MFB-DBS, along with histological analysis. Pre-stimulation baseline testing of the VTA-lx animals indicated depressive-like phenotype in comparison with controls. Response to MFB-DBS varied according to (i) the degree of dopaminergic depletion: animals with severe mesocorticolimbic dopamine depletion did not, whilst those with mild dopamine loss responded well to stimulation; (ii) environmental conditions and the nature of the behavioral tests, e.g., stressful vs non-stressful situations. Neuromodulation-induced c-fos expression in the prelimbic frontal cortex and nucleus accumbens was also dependent upon integrity of the dopaminergic ascending projections.Our results confirm a potential role for dopamine in symptom relief observed in clinical MFB-DBS. Although mechanisms are not fully understood, the data suggests that the rescue of depressive phenotype in rodents can work via both dopamine-dependent and independent mechanisms. Further investigations concerning the network of depression using neuromodulation platforms in animal models might give insight into genesis and treatment of major depression disorder.

Respiratory deficits in a rat model of Parkinson’s disease

Parkinson’s disease (PD) is a neurodegenerative disease characterized by loss of the dopaminergic nigrostriatal pathway. In addition to deficits in voluntary movement, PD involves a disturbance of breathing regulation. However, the cause and nature of this disturbance are not well understood. Here, we investigated breathing at rest and in response to hypercapnia (7% CO2) or hypoxia (8% O2), as well as neuroanatomical changes in brainstem regions essential for breathing, in a 6-hydroxydopamine (6-OHDA) rat model of PD. Bilateral injections of 6-OHDA (24μg/μl) into the striatum decreased tyrosine hydroxylase (TH+)-neurons in the substantia nigra pars compacta (SNpc), transcription factor phox2b-expressing neurons in the retrotrapezoid nucleus and neurokinin-1 receptors in the ventral respiratory column. In 6-OHDA-lesioned rats, respiratory rate was reduced at rest, leading to a reduction in minute ventilation. These animals also showed a reduction in the tachypneic response to hypercapnia, but not to hypoxia challenge. These results suggest that the degeneration of TH+ neurons in the SNpc leads to impairment of breathing at rest and in hypercapnic conditions. Our data indicate that respiratory deficits in a 6-OHDA rat model of PD are related to downregulation of neural systems involved in respiratory rhythm generation. The present study suggests a new avenue to better understand the respiratory deficits observed in chronic stages of PD.

Comparative efficacy of intravenous injection of bone marrow mesenchymal stem cells in rats with model of Parkinson-like syndrome

The purpose: to study efficacy of intravenous injection of human and rat bone marrow mesenchymal stem cells (BMMSC) in rats with Parkinson-like syndrome (PS), modeled by chemical degradation of substantia nigra (SN).Materials and methods. Animals were divided into 5 groups: I — intact (n=7); II — PS, bilateral injection of 6-OHDA in SN (n=15); III — PS, intravenous injection of human BMMSC (0.5x106 cells per 1 rat) (n=9); IV — PS, intravenous injection of rat BMMSC (1x106) (n=9); V — PS, intravenous injection of rat BMMSC (2x106) (n=9). Cells were injected 2 weeks after PS modelling. Efficacy of intravenous injection of stem cells was estimated by recovery of movements, changes of dopamine level in blood and brain frontal lobe of the rats and morphological changes of SN.Results. Intravenous injection of human BMMSC at PS helps to restore movement for 14–15 days, normalize DA level in the blood and tissue of frontal lobe of the rats on the 10th day and increase the number of neurons in damaged SN area.Intravenous injection of rats BMMSC (in doze 1x106) was inefficient, partial restoration of movement and DA level normalization in the blood and in tissue of frontal lobe was observed only to 20–21 day. Intravenous injection of rat BMMSC (in doze 2x106) at PS helps to restore movement to 9–10 day, normalize DA level in the blood and tissue of frontal lobe on 10th day, active neurons in SA are observed.Conclusions. Intravenous injections of rat and human BMMSC are effective for treatment of PS symptoms in rats, the results depend on the amount of injected cells.

Evaluation of Metabolic State in Striatal Rat Model of Parkinson’s Disease: before and after Deep Brain Stimulation

Weight loss occurs during the natural history of Parkinson’s disease (PD). This non-motor manifestation of the disease is reversed by deep brain stimulation of the sub thalamic nucleus (DBS-STN) therapeutics which is often associated with weight gain. Although it has been proposed that PD is associated with alterations in central energy metabolism the mechanisms responsible for this weight variation remain unknown. This study evaluates the weight profile and nutritional state of the 6-hydroxydopamine (6-OHDA) rat model of PD subjected to DBS-STN. Rats were rendered parkinsonian by bilateral injections of 6-OHDA into the striatum and electrodes were implanted bilaterally at the level of the STN. Rats were placed in metabolic cages for evaluation of weight, food and liquid intake and urine and fecal volume before and 2 and 4 weeks after the beginning of stimulation. Before stimulation began (2 weeks after 6-OHDA lesion), weight and metabolic parameters were similar between parkinsonian rats with and without electrodes and matched control rats. Two weeks after stimulation began (4 weeks after 6-OHDA lesion) and at the end of the study, 4 weeks after stimulation began (6 weeks after 6-OHDA lesion), body weight and the metabolic parameters evaluated remained unaltered between animal groups. Furthermore, at the end of the study, no statistically significant differences were found in efficiency of eating (change in weight/amount of food eaten) or weight gain between groups. In conclusion, in the rat model of PD with striatal dopaminergic neurodegeneration neither induction of PD or DBS-STN influenced weight variation or metabolic state. Additional mechanisms may be required to induce the altered metabolic state observed in PD patients before and after STN-DBS.

High‐frequency stimulation of the subthalamic nucleus restores neural and behavioral functions during reaction time task in a rat model of Parkinson's disease

Deep brain stimulation (DBS) has been used in the clinic to treat Parkinson's disease (PD) and other neuropsychiatric disorders. Our previous work has shown that DBS in the subthalamic nucleus (STN) can improve major motor deficits, and induce a variety of neural responses in rats with unilateral dopamine (DA) lesions. In the present study, we examined the effect of STN DBS on reaction time (RT) performance and parallel changes in neural activity in the cortico-basal ganglia regions of partially bilateral DA- lesioned rats. We recorded neural activity with a multiple-channel single-unit electrode system in the primary motor cortex (MI), the STN, and the substantia nigra pars reticulata (SNr) during RT test. RT performance was severely impaired following bilateral injection of 6-OHDA into the dorsolateral part of the striatum. In parallel with such behavioral impairments, the number of responsive neurons to different behavioral events was remarkably decreased after DA lesion. Bilateral STN DBS improved RT performance in 6-OHDA lesioned rats, and restored operational behavior-related neural responses in cortico-basal ganglia regions. These behavioral and electrophysiological effects of DBS lasted nearly an hour after DBS termination. These results demonstrate that a partial DA lesion-induced impairment of RT performance is associated with changes in neural activity in the cortico-basal ganglia circuit. Furthermore, STN DBS can reverse changes in behavior and neural activity caused by partial DA depletion. The observed long-lasting beneficial effect of STN DBS suggests the involvement of the mechanism of neural plasticity in modulating cortico-basal ganglia circuits.

Central noradrenergic blockade prevents autotomy in rat: implication for pharmacological prevention of postdenervation pain syndrome

Following transection of peripheral nerve, rats exhibit autotomy, which is considered to be the animal model of postdenervation pain syndrome. It has been suggested that phantom limb pain is a result of peripheral denervation leading to reorganization of somatosensory pathways, particularly in the cerebral cortex, which is shown to depend upon central noradrenergic activity. In this study, sciatic and saphenous nerves were sectioned in the left hindpaw of 30 adult rats resulting in complete loss of pain sensation in the hindpaw. A group of rats received normal saline, compared to another group which received N-(2-) Chloroethyl-N-ethyl-2-bromobenzylamine (DSP4) injection 24 h prior to transection. The latter group was also compared to a third group whose central noradrenergic system were also blocked by bilateral injection of 6-OHDA into the ascending noradrenergic bundle 1 week prior to transection. A fourth group received contralateral cortical ablation in addition to peripheral nerve transection and was compared to the first group whose cortex remained intact. The animals were observed daily for 60 days and autotomy was scored in accordance to the system of Wall et al. [42]. After 1 week, control animals began to exhibit autotomy. In contrast, autotomy was absent in rats treated with DSP4, similar to rats which received 6-OHDA. Rats which had contralateral cortical ablation showed a considerably delayed onset of autotomy and a reduction in final autotomy scores. We conclude that autotomy, as a model of postdenervation pain syndrome, can be prevented by blockade of noradrenergically mediated cortical reorganization. The clinical implications of this finding are discussed.

Naltrexone Suppresses Ethanol Intake in 6‐Hydroxydopamine–Treated Rats

The suppressive effect of opioid antagonists, such as naltrexone, on ethanol intake has been suggested to be based on the interference with ethanol-induced stimulation of dopamine release in the nucleus accumbens. The aim of this study was to determine whether reduction of dopamine innervation to the nucleus accumbens with the neurotoxin 6-hydroxydopamine (6-OHDA) alters naltrexone-induced suppression of ethanol consumption. Because the mesolimbic dopaminergic neurons have also been implicated in ethanol reinforcement, the effects of 6-OHDA on the maintenance and acquisition of ethanol intake were also studied. To damage accumbal terminals of the mesolimbic dopamine neurons, alcohol-preferring Alko Alcohol (AA) rats were given bilateral injections of 6-OHDA or vehicle into the nucleus accumbens after pretreatment with desipramine and pargyline. The effect of the lesion on the acquisition or maintenance of ethanol self-administration was studied in animals having continual access to ethanol solution (10% v/v) and water. Subsequently the effect of naltrexone on ethanol consumption was determined. Naltrexone (0.03-3.0 mg/kg subcutaneously) suppressed ethanol consumption in a dose-dependent manner both in 6-OHDA-treated and control animals given a daily 90-min access to ethanol solution. When the rats had continual access to ethanol, there was a clear day-to-day decline in ethanol intake during the first 5 days of the 7-day naltrexone treatment (10 mg/kg subcutaneously). 6-OHDA treatment had no effect on either the acquisition or maintenance of ethanol self-administration. Postmortem analysis of the brain dopamine content revealed approximately 92% depletion of dopamine in the nucleus accumbens of the 6-OHDA-treated rats. The suppressive effect of naltrexone does not depend on naltrexone's interaction with dopaminergic terminals in the nucleus accumbens. Furthermore, the role of the mesolimbic dopamine pathway is probably not central either in the acquisition or maintenance of ethanol self-administration in alcohol-preferring AA rats.

125I-[Tyr 0,D-Trp 8]somatostatin-14 binding sites in the locus coeruleus of the rat are located on both ascending and descending projecting noradrenergic cells

Radioautographic determinations of 125I-[Tyr 0,D-Trp 8]somatostatin-14 ( 125I-SRIF) binding sites were performed on frozen serial sections of the locus coeruleus (LC) of control rats and of rats subjected to either bilateral microinjections of 6 hydroxydopamine (6-OHDA) into the LC or unilateral microinjection into the ascending noradrenergic bundles. These experiments were performed in order to determine whether 125I-SRIF binding was localized to noradrenergic-containing cells and in which regions the cells which contain the binding sites are projecting. The extent of the lesions was assessed by measuring norepinephrine (NE) levels in the hippocampus (88% decrease as compared to sham-operated animals) for bilateral LC lesions and in the frontal cortex (87% reduction vs. contralateral side) for unilateral bundle lesions. In control rats, 125I-SRIF binding sites were restricted to the boundaries of the LC and followed closely the distribution of tyrosine hydroxylase-labeled cells. Three weeks after bilateral injections of 6-OHDA, 125I-SRIF binding decreased by 79% in all regions of the LC. In contrast, unilateral destruction of the ascending noradrenergic bundles resulted in a moderate decrease only in the middle part of the LC with a more important effect in the dorsal (55%) than in the ventral (24%) portion of the nucleus. These data demonstrate that: 1) most SRIF receptors in the LC are located in the vicinity of NE-containing cell bodies and 2) NE-containing cells bearing SRIF receptors project to the forebrain as well as to other terminal areas located more caudally in the brain. These data suggest a general role for SRIF in the control of the multiple functions of the LC.

Non-DA prefronto-cortical efferents modulate D 1 receptors in the nucleus accumbens

Bilateral injections of 6-OHDA in the ventral mesencephalic tegmentum (VMT) destroy the DA afferents of the nucleus accumbens but do not induce any denervation supersensitivity of the D1 receptors in the nucleus accumbens. Because other non-DA afferent fibers might also regulate D1 receptors in that structure, bilateral ablations of the prefrontal cortex were performed. This lesion induced a 55% decrease of [3H]glutamic acid high-affinity uptake activity in the nucleus accumbens. When the prefronto-cortical ablation was performed simultaneously with the bilateral injection of 6-OHDA in the VMT, a marked hypersensitivity of DA-sensitive adenylate cyclase activity was observed in the nucleus accumbens (+52% increase of Vmax and a two-fold decrease of the Kapp for DA) while the ablation of the prefrontal cortex alone induced only a +14% (P less than 0.01) increase of D1 receptors in that structure. These results indicate that the regulation of D1 receptors in the nucleus accumbens is not solely dependent on the presynaptic DA innervation and that other non-DA fibers, such as those of the cortico-nucleus accumbens pathway, might contribute to it.

Behavioural and biochemical effects of dopamine and noradrenaline depletion within the medial prefrontal cortex of the rat

The effects of 6-hydroxydopamine (6-OHDA) lesions of catecholamine terminals within the medial prefrontal cortex on spontaneous motor activity, dopamine (DA)-dependent stereotyped behaviour and subcortical dopamine turnover were investigated in the rat. Two types of lesions were examined, bilateral injection of 6-OHDA into the medial prefrontal cortex of untreated rats (6-OHDA alone), and bilateral injection of 6-OHDA into the medial prefrontal cortex of animals pretreated with the noradrenaline (NA) uptake blocking agent desmethylimipramine (6-OHDA/-DMI). Ten days after surgery the 6-OHDA lesions produced no significant change in spontaneous motor activity and had no overall effects on stereotyped behaviour induced by apomorphine or (+)-amphetamine. This lesion caused gross depletion of NA within the medial prefrontal cortex and curiously, elevated DA concentrations within this site. No changes in DA concentration were recorded within subcortical sites, although concentrations of DA metabolites within striatum and nucleus accumbens were reduced. In contrast, the 6-OHDA/DMI lesion of the medial prefrontal cortex significantly enhanced spontaneous motor activity and amphetamine-induced stereotyped behaviour. Apomorphine-induced stereotypy, on the other hand, was significantly reduced. Biochemically the lesion caused a large depletion of DA with relatively little loss of NA within the medial prefrontal cortex. In addition, from this and another study (ref. 33), increases in DA and its metabolite concentrations were measured in striatum and nucleus accumbens, together with an apparent increase in DA turnover within these subcortical sites. It is thus apparent that in the absence of a substantial portion of the DA innervation of the medial prefrontal cortex, with a largely intact NA innervation, there is an increase in motor activity and amphetamine-induced stereotypy which may be related to functional changes in DA activity within subcortical telencephalic structures. Such a finding might suggest that DA within the frontal cortex has a behaviourally inhibitory role in the rat, although further work is required to substantiate this.

Influence of 6-hydroxydopamine injections in the substantia nigra on lateral hypothalamic reinforcement

Repeated unilateral injections of 6-hydroxydopamine (6-OHDA) into the substantia nigra of rats caused massive non-specific lesions and disrupted self-stimulation by lever-pressing equally from electrodes in the ipsilateral and contralateral hypothalamus. Repeated bilateral injections of 6-OHDA abolished self-stimulation when lever-pressing was the criteria, but not when simpler motor responses were used as operants. Hence, nigrostriatal dopamine is unlikely to be critically involved in operant learning and reward processes.

Feeding and drinking deficits after 6-hydroxydopamine administration in the rat: similarities to the lateral hypothalamic syndrome

Summary The role of catecholamines in the behavioral changes observed after lateral hypothalamic (LH) lesions was assessed by comparing LH lesioned animals with animals subjected to various treatments with 6-hydroxydopamine (6-OHDA). In agreement with previous reports, bilateral electrolytic lesions of the lateral hypothalamus (LH lesioned), intraventricular administration of 6-OHDA in monoamine oxidase inhibited animals (6-OHDA + tranylcypromine) or bilateral injections of 6-OHDA into the substantia nigra (nigral 6-OHDA) produced aphagia and adipsia. Recovery of food and water intake occurred in most animals within 10 days. Some of the nigral 6-OHDA injected animals did not, however, recover during the 3 months of the experiment. Intraventricular injections of 6-OHDA did not produce these effects unless monoamine oxidase was first inhibited. After recovery of food and water intake, the intraventricular 6-OHDA + tranylcypromine and the nigral 6-OHDA groups showed the same deficits in water regulation as were found in the LH lesioned animals: decreased daily water intake, prandial drinking, increased finickiness to quinine adulteration of drinking water, and a lack of drinking in response to hypertonic saline injections. Changes in regulation of food intake were also evident. Amphetamine anorexia was reduced in the LH lesioned and the nigral 6-OHDA groups. Amphetamine anorexia was abolished in the intraventricular 6-OHDA + MAOI animals. In contrast, the anorectic properties of fenfluramine were augmented in these groups. The results suggest that the lateral hypothalamic syndrome may to a large extent reflect the interruption of the dopaminergic nigro-striatal system. Based on tyrosine hydroxylase measures in the striatum, hypothalamus and midbrain, however, it is suggested that damage to this system cannot account for the magnitude of the behavioral changes seen after LH lesions and that other as yet unknown factors may also contribute to this syndrome.