Intrinsic neurons are involved in lateral hypothalamic self-stimulation

The Orexin/Receptor System: Molecular Mechanism and Therapeutic Potential for Neurological Diseases

Orexins, also known as hypocretins, are two neuropeptides secreted from orexin-containing neurons, mainly in the lateral hypothalamus (LH). Orexins orchestrate their effects by binding and activating two G-protein–coupled receptors (GPCRs), orexin receptor type 1 (OX1R) and type 2 (OX2R). Orexin/receptor pathways play vital regulatory roles in many physiological processes, especially feeding behavior, sleep–wake rhythm, reward and addiction and energy balance. Furthermore several reports showed that orexin/receptor pathways are involved in pathological processes of neurological diseases such as narcolepsy, depression, ischemic stroke, drug addiction and Alzheimer’s disease (AD). This review article summarizes the expression patterns, physiological functions and potential molecular mechanisms of the orexin/receptor system in neurological diseases, providing an overall framework for considering these pathways from the standpoints of basic research and clinical treatment of neurological diseases.

Interaction between orexins and the mesolimbic system for overriding satiety

In North American society, it is all too common for the intake of calories to outweigh an individual's energy demands. Such over-consumption where high-energy foods are readily available undoubtedly contributes to the growing problem of obesity. Palatable food stimulates brain circuits similar to those that mediate behavioral responses to drugs of abuse, which may underlie the continuation of food intake long after energy requirements are met. Among the brain areas implicated in reward and food intake, the lateral hypothalamus (LH) has long been recognized as a common region involved in both. It has been suggested that orexin neurons that are expressed exclusively within and adjacent to the LH comprise a major cellular substrate for the functioning of the LH. Here, we review the idea that the orexin neuropeptides play a key role in the rewarding aspects of food intake through interactions with both peripheral and central signals reflecting current energy stores as well as the classic reward pathway--the mesolimbic dopamine system. Furthermore, a possible heterogeneity of orexin neurons is discussed. Uncovering orexin's role in food reinforcement may provide insight into hyperphagia and obesity. In addition, the idea that food intake and substance abuse involve similar brain circuitry suggests potential for a single treatment aiding both obesity and addiction.

The central extended amygdala network as a proposed circuit underlying reward valuation

The phenomenon of medial forebrain bundle self-stimulation offers a powerful model of reward-based behavior. In particular, it appears to activate a neural system whose natural function is to compute the survival value or utility of present stimuli and to help orchestrate responses toward those inputs. Although the anatomical identity of this system is as yet unknown, recent descriptions of anatomical macrosystems within the basal forebrain lead to the proposal that it may be largely contained within the central extended amygdala network. This paper reviews decades' worth of behavioral and neurophysiological investigations of brain stimulation reward that support or are at least consistent with this idea. The proposed network circuitry underlying self-stimulation is also placed into the larger context of basal forebrain function, specifically, the role of the ventral striatopallidum in linking motivation to behavior, the role of the amygdala in detecting motivationally significant inputs, and the role of the magnocellular complex in communicating reward information to cortical and hippocampal targets.

Gene expression evidence for remodeling of lateral hypothalamic circuitry in cocaine addiction

By using high-density oligonucleotide arrays, we profiled gene expression in reward-related brain regions of rats that developed escalated cocaine intake after extended access to cocaine (6 h per day). Rats allowed restricted daily access to cocaine (only 1 h) that displayed a stable level of cocaine intake and cocaine naive rats were used for controls. Four analysis methods were compared: Affymetrix microarray suite 4 and microarray suite 5, which use perfect-match-minus-mismatch models, and dchip and rma, which use perfect-match-only models to generate expression values. Results were validated by RT-PCR in individual animals from an independent replication of the experiment. A small number of genes was associated with escalated cocaine intake (ESC genes). Unexpectedly, of the brain regions examined [prefrontal cortex, nucleus accumbens, septum, lateral hypothalamus (LH), amygdala, and ventral tegmental area], the LH was the most transcriptionally responsive in escalation of cocaine intake. Most of the ESC genes identified are also expressed during synaptogenesis and synaptic plasticity and include genes that code for several presynaptic and postsynaptic proteins involved in neurotransmission. These results suggest that LH intrinsic circuitry undergoes a structural reorganization during escalation of cocaine use. This remodeling of LH circuitry could contribute to the chronic deficit in reward function that has been hypothesized to drive the transition to drug addiction. Results also support the value of using multiple analysis strategies to identify the most robust changes in gene expression and to compensate for the biases that affect each strategy.

Involvement of the lateral hypothalamic peptide orexin in morphine dependence and withdrawal

The lateral hypothalamus (LH) is implicated in the behavioral actions of drugs of abuse, but the cellular and molecular basis of this role is unclear. Recent identification of neuropeptides localized in LH neurons has allowed for more specific studies of LH function. The LH-specific peptide orexin (hypocretin) has been shown to be important in arousal and sleep regulation. However, orexin cells of the LH project broadly throughout the brain such that orexin may influence other behaviors as well. In this study, we show that orexin neurons, and not nearby LH neurons expressing melanin-concentrating hormone (MCH), have mu-opioid receptors and respond to chronic morphine administration and opiate antagonist-precipitated morphine withdrawal. cAMP response element-mediated transcription is induced in a subset of orexin cells, but not MCH cells, after exposure to chronic morphine or induction of withdrawal. Additionally, c-Fos and the orexin gene itself are induced in orexin cells in the LH during morphine withdrawal. Finally, we show that orexin knock-out mice develop attenuated morphine dependence, as indicated by a less severe antagonist-precipitated withdrawal syndrome. Together, these studies support a role for the orexin system in molecular adaptations to morphine, and demonstrate dramatic differences in molecular responses among different populations of LH neurons.

Electrical self-stimulation in the central amygdaloid nucleus after ibotenic acid lesion of the lateral hypothalamus

This experiment was carried out in order to investigate the involvement of lateral hypothalamus (LH) in electrical self-stimulation of the central amygdaloid nucleus (CeA). Adult male Sprague-Dawley rats were bilaterally implanted with a guide cannula situated above each LH and with two electrodes in the CeA. Self-stimulation was subsequently obtained separately from both right and left electrodes. The LH was then lesioned unilaterally by ibotenic acid (IBO) injection. Eight days later, the effect of this unilateral lesion on self-stimulation of the ipsilateral and contralateral CeA was tested. Then the neurons of the remaining non-lesioned LH side were lesioned with IBO and self-stimulation was tested 15 days after the second lesion. Both unilateral as well as bilateral lesions of LH produced a significant decrease in CeA self-stimulation rates but had no significant effect on the reward effectiveness. The unilateral lesions did not produce any modification of the rate-intensity function in the contralateral CeA. This lesion-induced depression in performance was reversed by treatment with phenobarbital. These results provide clear evidence that the rewarding effects of CeA electrical stimulation do not result from the activation of the LH outputs and that the apparent decrease in CeA self-stimulation may result from the LH lesion-induced increase in the frequency of epileptiform manifestations that occur following amygdaloid stimulation.

The lateral hypothalamic area revisited: ingestive behavior

This article discusses the role of the lateral hypothalamic area (LHA) in feeding and drinking and draws on data obtained from lesion and stimulation studies and neurochemical and electrophysiological manipulations of the area. The LHA is involved in catecholaminergic and serotonergic feeding systems and plays a role in circadian feeding, sex differences in feeding and spontaneous activity. This article discusses the LHA regarding dietary self-selection, responses to high-protein diets, amino acid imbalances, liquid and cafeteria diets, placentophagia, "stress eating," finickiness, diet texture, consistency and taste, aversion learning, olfaction and the effects of post-operative period manipulations by hormonal and other means. Glucose-sensitive neurons have been identified in the LHA and their manipulation by insulin and 2-deoxy-D-glucose is discussed. The effects on feeding of numerous transmitters, hormones and appetite depressants are described, as is the role of the LHA in salivation, lacrimation, gastric motility and secretion, and sensorimotor deficits. The LHA is also illuminated as regards temperature and feeding, circumventricular organs and thirst and electrolyte dynamics. A discussion of its role in the ischymetric hypothesis as an integrative Gestalt concept concludes the review.

Interhemispheric links in brain stimulation reward

The MFB substrate of self-stimulation (SS) has generally been viewed as a unilateral system. We re-examined this belief with pairs of moveable SS electrodes placed bilaterally in the MFB. Rats barpressed for trains of single or twin cathodal pulses of fixed intensity and width and of variable frequency. The first (C) and second (T) pulse of each pair was delivered through the left and right electrode or inversely. C-T intervals ranging from 0.2 to 5.0 ms were tested. The frequency of C pulses required for criterial bar-pressing was used to plot the stimulation efficacy (SE), as a function of the C-T interval and pulse presentation order. The electrodes were subsequently moved and the same procedure repeated for more ventral sites. With some pairs of contralateral hypothalamic (H) sites, the SE was independent of the C-T interval. However, with other pairs of contralateral H sites, the SE increased with C-T interval in a manner resembling a collision effect, with the important exception that no conduction time (CT) was apparent in the data. The absence of CT excludes the presence of a genuine collision effect. When one pulse was sent to the H and another to the contralateral ventral tegmentum (VT), the H-VT curve rose always earlier than the VT-H curve, thus resembling a transynaptic collision effect. However, the C-T interval at which the VT-H curve began rising (always 1.0 ms or less) fails to support the contention that the electrodes activated fibers separated by a synapse. Finally, a typical collision effect was noted with ipsilateral H-VT electrode placements, confirming the presence of direct linkage between ipsilateral MFB sites. Computer-generated data based on two parsimonious assumptions were found to match the empirical results. These assumptions were that each electrode activated a different branch of the same reward neuron and that conduction failure occurred at the branchpoint. The model, which posits that a large number of MFB reward neurons send branches to the other hemisphere, is testable and makes clear-cut predictions about the effects of lesions. In a preliminary test, we recorded the H and contralateral VT threshold frequencies before and after lesioning the H. The H threshold increased more when using small pulse current and remained constant throughout the 4-week testing period. The VT threshold was elevated more for intermediate pulse current and kept increasing with time.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effect of self-stimulation on dopamine release and metabolism in the rat medial frontal cortex, nucleus accumbens and striatum studied by in vivo microdialysis

Changes in the extracellular levels of dopamine (DA) and its metabolites in the dopaminergic terminal regions, the medial frontal cortex (MFC), nucleus accumbens (NAC), and striatum (STR), were measured by microdialysis during self-stimulation of the medial forebrain bundle (MFB) in rats pretreated with the DA uptake inhibitor, nomifensine (1 mg/kg, i.p.). Self-stimulation of the MFB in nomifensine-pretreated rats caused an increase in the extracellular DA level in the MFC and NAC but not in the STR. Self-stimulation also increased the extracellular concentrations of the main DA metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) to a similar extent in the MFC and NAC and to a lesser extent in the STR. Thus, there was a regional difference in the neurochemical changes following self-stimulation with either the MFC or the NAC showing larger extracellular levels of DA, DOPAC, and HVA than the STR. Furthermore, these changes were observed on both hemispheres ipsilateral and contralateral to the stimulation. The results indicate that self-stimulation of the MFB preferentially activates the mesocorticolimbic DA systems, thereby bilateral increases in the release of DA and its metabolism being produced in their terminal regions, the MFC and NAC.

The effects of excitotoxin lesions of the lateral hypothalamus on self-stimulation reward

Unilateral microinjection into rat lateral hypothalamus (LH) of the excitotoxins ibotenic acid (IBO) and N-methyl-D-aspartic acid (NMDA) produced a local zone of neuronal death but also produced a zone of demyelination. The size of this demyelination zone was related to excitotoxin dose and was smaller than the zone of neuron killing. In behavioral testing, MFB self-stimulation reward and performance were measured with a rate-frequency curve-shift method before and after IBO or NMDA lesions of the LH. Excitotoxin lesions were made anterior or posterior to the LH electrode so that the zone of neuronal death, but not demyelination, extended to the electrode tip. These lesions produced small, temporary LH stimulation reward deficits, leading to the conclusion that intrinsic LH neurons are not a major substrate of MFB stimulation reward.

Forebrain neurons driven by rewarding stimulation of the medial forebrain bundle in the rat: comparison of psychophysical and electrophysiological estimates of refractory periods

Psychophysically derived estimates of recovery from refractoriness were obtained at self-stimulation sites in the lateral hypothalamus and ventral tegmental area. The refractory periods of single units driven by the same stimulation electrodes and stimulation fields were then measured electrophysiologically. Antidromically driven units with refractory periods longer than those of the neurons responsible for the rewarding effect were concentrated in the septal complex. Units with refractory periods that overlapped the estimates for the reward-related neurons were found in this region as well but were also encountered in neighboring structures lateral, ventral, and/or caudal to the septal nuclei. It is argued that this latter class of units should be considered as possible constituents of the directly stimulated substrate for the rewarding effect because they are driven by rewarding stimulation, have refractory periods similar to those of the reward-related neurons and arise in or near regions in which lesions have been effective in decreasing the rewarding effect of stimulating the medial forebrain bundle.

Opposite effects of ibotenic acid and 6-hydroxydopamine lesions of the lateral hypothalamus on intracranial self-stimulation and stimulation-induced locomotion

The purpose of the present study was to test the respective roles of the intrinsic neurons and of the catecholaminergic fibers in two behaviors elicited by electrical stimulation of the lateral hypothalamus, intracranial self-stimulation and the increase in locomotor activity produced by noncontingent stimulation. One group of rats was unilaterally injected in the middle lateral hypothalamus with a dose of ibotenic acid known to significantly decrease self-stimulation (4 micrograms/0.5 microliter). Two other groups received, in the same area, an injection of a small dose of 6-hydroxydopamine (2 micrograms/0.5 microliter). The rats of one of these groups were pre-treated with desmethylimipramine. Two other groups of rats were respectively injected with the vehicle of each neurotoxin. Eight days later all rats were bilaterally implanted with stimulation electrodes, one in the lesioned area, the other in the contralateral region. Each electrode of each animal was tested first for self-stimulation, then for locomotor activation measured in the open field produced by non-contingent stimulation. Whatever the lesion or the behavior tested, the response of the lateral hypothalamus contralateral to the lesioned area was normal. Self-stimulation was disturbed only with stimulation of the lateral hypothalamus lesioned by ibotenic acid. Self-stimulation in the lateral hypothalamus lesioned by 6-hydroxydopamine was normal. However, a significant loss of noradrenaline in the hippocampus and of dopamine in the striatum was observed. Furthermore, the brains of two rats unilaterally injected with the usual dose of 6-hydroxydopamine were processed for tyrosine hydroxylase immunocytochemistry.(ABSTRACT TRUNCATED AT 250 WORDS)

Basal forebrain knife cuts and medial forebrain bundle self-stimulation

Current autoradiographic and electrophysiological data suggest that fibers coursing from the diagonal band/medial septum and lateral preoptic area through the medial forebrain bundle (MFB) to the midbrain may carry the reward signals generated by lateral hypothalamic stimulation. To test this hypothesis, 40 rats were given a unilateral lateral hypothalamic stimulating electrode and an ipsilateral guide cannula for knife cut transection. In baseline self-stimulation testing, both the animal's capacity to respond for the stimulation and the reward efficacy of the stimulation were measured. A coronal plane knife cut transection was given following stabilization of baseline behavior, and any changes in response capacity and stimulation reward efficacy were observed for up to two weeks, beginning 24 h after transection. Cuts to the diagonal band/medial septal region or the outflow therefrom did not permanently or significantly alter stimulation reward effectiveness. Cuts in the lateral preoptic area or in the MFB just anterior to the stimulating electrode decreased stimulation reward effects only if considerable concomitant rostrocaudal tissue damage was apparent around the knife cut. Even in these cases, reward degradation was rarely permanent. These results suggest that the majority of reward-relevant fibers probably do not arise in forebrain nuclei rostral to the stimulating electrode. A possible role of neurons endemic to the lateral hypothalamus in stimulation reward effects is discussed.

Gustatory preference-aversion thresholds are increased by ibotenic acid lesion of the lateral hypothalamus in the rat

The main purpose of this study was to quantitate possible changes in the rewarding and aversive values of certain gustatory stimuli produced by bilateral ibotenic acid lesions of the lateral hypothalamus in the rat. Non-operated rats served as controls. Thirteen days after the operation, rats were placed on a water-deprivation schedule during 5 consecutive days. Rats were then given the choice of one of 5 concentrations of saccharin solution, using a two-bottle procedure. Fluid intake across concentrations generated a preference-aversion curve. The same type of procedure was used to obtain the aversion curve for increasing concentrations of quinine solution. The lesioned rats as well as the control animals showed a clear preference-aversion response to saccharin solutions and an aversive response to quinine solutions. However, the highest preference score of the lesioned rats was obtained with a saccharin concentration 3 times higher than the concentration preferred by the control rats. Moreover, unlike control rats operated animals did not show aversion to the highest concentrations of saccharin solutions. Finally in the lesioned rats the aversion threshold to quinine solutions was obtained with concentration 5 times higher than the concentration inducing aversion in the control rats. At the end of these experiments, rats used as controls were submitted, in turn, to bilateral lesion of the lateral hypothalamus. The change in the preference-aversion threshold of these rats in the saccharin choice procedure was the same as that observed with naive rats. Taken together, these results suggest that in the normal rat the palatability of certain gustatory stimuli is modulated by the intrinsic neurons of the lateral hypothalamus.

Electrical self-stimulation in the parabrachial area is depressed after ibotenic acid lesion of the lateral hypothalamus

The involvement of lateral hypothalamic intrinsic neurons on electrical self-stimulation of the parabrachial area was analyzed. Rats were bilaterally implanted in the parabrachial area and with a guide cannula located above each lateral hypothalamus. They were subsequently tested for intracranial self-stimulation. Then, the lateral hypothalamus on one side of the brain was injected with ibotenic acid. The effect of the induced lesion was tested 8 days later on self-stimulation of the ipsilateral and contralateral parabrachial areas. The intrinsic neurons of the non-lesioned lateral hypothalamus were then destroyed with ibotenic acid. Self-stimulation was then tested 8, 12 and 30 days later. The unilateral lesion produced a significant decrease of self-stimulation using the electrode ipsilateral to the lesion, without any modification of the stimulation using the contralateral electrode. After bilateral lesion, self-stimulation was greatly reduced bilaterally. The results suggest that the main effect of the lesion was to increase the self-stimulation threshold. Given that the parabrachial area is a relay station for the gustatory inputs and that the intrinsic neurons of the lateral hypothalamus project back to the parabrachial area, the present results are tentatively interpreted as an indication that self-stimulation in this pontine area results from the activation of feedback loops between the lateral hypothalamus and the parabrachial area.

The role of intrinsic neurons in lateral hypothalamic self-stimulation

In this brief review, we summarize some of our recent work concerning the effect of a specific lesion of the intrinsic neurons located in the middle part of the lateral hypothalamus on electrical self-stimulation of this structure by electrodes implanted along the medial forebrain bundle. In a first experiment the neurons of the lateral hypothalamus were destroyed unilaterally by local injection of ibotenic acid (4 micrograms in 0.5 microliter). The contralateral side served as the sham-lesion control. Between 10 and 20 days later, electrodes were bilaterally implanted, one in the lesioned area, the other in the contralateral hypothalamus. Intracranial self-stimulation (ICSS) was obtained separately for each electrode, at various current intensities, using a nose-poke response. ICSS from electrodes implanted in the lesioned area was decreased in all cases, whereas ICSS of the sham-lesioned side was normal. In a second experiment, two groups of rats lesioned and implanted as above, received two additional electrodes either in the anterior hypothalamus or in the posterior hypothalamus. In rats with electrodes in the anterior hypothalamus, the lesion produced a large deficit in self-stimulation when stimulation was applied to the anterior electrode ipsilateral to the lesion. Only 3 of 6 rats showed a decrease in ICSS with stimulation of the posterior hypothalamic electrode ipsilateral to the lesion. These results suggest that ICSS in the anterior part of the medial forebrain bundle is sustained by long fibers originating in the middle part of the lateral hypothalamus, while ICSS in the posterior part of the lateral hypothalamus may not depend on the neurons located in the lesioned area.

Effects of ibotenic acid lesion in the basal forebrain on electrical self-stimulation in the middle part of the lateral hypothalamus

The aim of the present study was to analyse the involvement of neurons located in some basal forebrain areas in electrical self-stimulation elicited from the middle part of the lateral hypothalamus. The anterior hypothalamic region was unilaterally lesioned by local injection of ibotenic acid. Ten to 20 days later 4 electrodes were implanted bilaterally, two at the lesion level and two in the middle lateral hypothalamus. A small but significant self-stimulation deficit was observed in the lesioned area, while self-stimulation on the contralateral side was normal. At the middle lateral hypothalamic level, no deficit was observed when stimulation was applied either ipsilaterally or contralaterally to the anterior lesioned side. In most cases the lesion destroyed the dorsolateral part of the lateral preoptic area, the ventral pallidum-substantia innominata complex and the lateral part of the bed nucleus of the stria terminalis. Given the fact that these regions are known to project massively to the middle lateral hypothalamic area, our data suggest that descending influences originating in the damaged regions are not involved in self-stimulation elicited from the middle hypothalamus.

Evidence for a role of the preoptic area in lateral hypothalamic self-stimulation

We examined the effects of unilateral radiofrequency lesions in the preoptic area on lateral hypothalamic self-stimulation in 15 rats. The animals were tested for self-stimulation in the lateral hypothalamus at 3 different current intensities from electrodes placed in both hemispheres, and then received a unilateral lesion in the preoptic area. Four hours later they were again tested for self-stimulation at the 3 current intensities and then daily over the following 14 days, or until they recovered their presurgical rates of self-stimulation. Rate of self-stimulation decreased in the damaged hemisphere, and recovered to prelesion levels within 2 weeks in 6 of 9 rats. In the intact hemisphere rate of self-stimulation increased above the prelesion level during a period from 1 to 2 weeks after the lesion. These results suggest that the preoptic area is involved in lateral hypothalamic self-stimulation. The effects of D-amphetamine (1 mg/kg) and apomorphine (2 mg/kg) injections on turning behavior were also studied in an open field and in a rotometer. Apomorphine induced contraversive turning to the lesion side in the open field and D-amphetamine induced ipsiversive turning in the rotometer.

Unilateral lesion of the intrinsic cells in the medial forebrain bundle depresses self-stimulation but not stimulus-bound locomotor activity

The intrinsic neurons of the medial forebrain bundle were unilaterally destroyed, in rats, through local injection of ibotenic acid (4 micrograms in 0.5 microliter). Ten days later, electrodes were bilaterally implanted, one in the lesioned lateral hypothalamus, the other into the contralateral hypothalamus. Firstly, self-stimulation was studied with stimulation of each electrode separately. Later on, the effect of non-contingent electrical stimulation on evoked locomotor activity in the open-field was analysed for each electrode. While self-stimulation of the lesion area was greatly depressed in comparison with the unlesioned lateral hypothalamus, the increase in locomotor activity produced by stimulation, at the intensity applied, was the same whether the stimulated hypothalamus was lesioned or not.

Electrical self-stimulation deficits in the anterior and posterior parts of the medial forebrain bundle after ibotenic acid lesion of the middle lateral hypothalamus

The aim of the present study was to analyse the involvement of the intrinsic neurons located in the middle lateral hypothalamus in electrical self-stimulation measured with electrodes in the anterior and posterior parts of the medial forebrain bundle. In rats without hypothalamic lesions, self-stimulation rates from both anterior and posterior electrodes were similar on either side of the brain. For all rats with ibotenic acid-induced lesions in the lateral hypothalamus, self-stimulation rates were lower with electrodes in the area of the lesion, while self-stimulation on the contralateral side was normal. In rats with electrodes in the anterior hypothalamus, the lesion produced a large deficit when stimulation was applied to the anterior electrode ipsilateral to the lesion. Only three rats showed a decrease in self-stimulation with stimulation of the posterior hypothalamic electrode ipsilateral to the lesion; self-stimulation of the other three rats was normal. These results suggest that self-stimulation in the anterior part of the medial forebrain bundle is supported by long fibers originating in the middle part of the lateral hypothalamus, while self-stimulation in the posterior part of the lateral hypothalamus can be influenced by another system not involved in reward processes observed in the rostral part of the medial forebrain bundle.

Lateral hypothalamic self-stimulation persists in rats after destruction of lateral hypothalamic neurons by kainic acid or ibotenic acid

Bilateral injections of 1 microgram/microliter of kainic or ibotenic acid into the lateral hypothalamus (LH) of rats destroyed most of the cells in the LH. This treatment did not prevent electrical self-stimulation from electrodes placed in the LH, indicating that intrinsic neurons of the LH alone are not crucial elements of the neural system that mediates reinforcing hypothalamic stimulation.

Comparison of deficits in electrical self-stimulation after ibotenic acid lesion of the lateral hypothalamus and the medial prefrontal cortex

The aim of the present study was to compare the self-stimulation deficit produced by a unilateral injection of the neurotoxin, ibotenic acid, in the lateral hypothalamus (LH) to the deficit produced by the same unilateral injection in the medial prefrontal cortex (MPC). Four groups of adult male Sprague-Dawley rats were used: in two control groups, electrodes were bilaterally implanted in the LH (5 rats) or in the MPC (6 rats) and self-stimulation (ICSS) was obtained separately with the right and left electrodes. In the two experimental groups the intrinsic neurons of the LH (8 rats) or of the MPC (10 rats) were destroyed unilaterally by local injection of ibotenic acid (4 micrograms in 0.5 microliter); the other side served as the sham-lesioned control. Ten days later ICSS electrodes were implanted bilaterally, one in the lesioned area, the other in the contralateral region. As in the case of the control rats, ICSS was determined separately for each electrode, first by a rate dependent test (nose-poke) then by a 'rate-free' test (shuttle-box). In the LH and MPC control rats, ICSS responses were the same with stimulation on either side. In the LH-lesioned rats, the ICSS rates measured with the nose-poke test were significantly decreased with stimulation on the lesioned side, whereas rates with stimulation of the non-lesioned LH were normal. Likewise, while shuttle responses with stimulation of the non-lesioned LH were normal, the OFF-time was increased and the ON-time was decreased with stimulation of the lesioned LH. In the MPC-lesioned rats, ICSS (nose-poke) was totally suppressed and the shuttle responses were disorganized since neither the ON- nor the OFF-times changed in response to increasing current intensities. Nose-poke responses with stimulation of the non-lesioned MPC were just about normal. These results show that in the two brain regions studied local neurons are involved in ICSS. The difference in the magnitude of the deficit observed suggests, that the neuronal circuits involved in MPC self-stimulation are poorly represented whereas in the LH many neuronal circuits involved in these mechanisms overlap.

Selective neonatal depletion of dopamine has no effect on medial prefrontal cortex self-stimulation in the rat

The role of the dopaminergic input to the medial prefrontal cortex (MFC) on self-stimulation (SS) was investigated in adult rats injected neonatally with 6-hydroxydopamine (6-OHDA). Each subject on day 3 or 5 received bilateral intraventricular injection of 6-OHDA (total dose 200 micrograms, 50 micrograms/injection/2.5 microliters vehicle which contained 1 mg/ml ascorbic acid) or of the vehicle alone after pretreatment with desmethylimipramine (50 mg/kg i.p.) 30 min earlier. At 150 days of age, the animals were implanted with monopolar (100 microns) stainless steel electrodes in the MFC. One long (10 h) and 5 short (2 h) SS sessions resulted in similar percentages of responders for the brain reward in test and control subjects, and similar response rates in both groups. Biochemical assays of the levels of norepinephrine (NE) and dopamine (DA) in the frontal cortex showed depletion of DA 90% in the test animals, but no depletion of NE. Histochemical fluorescence visualization of the catecholamine input verified the biochemical results in the MFC. These results are viewed as negative evidence for the hypothesis that DA innervations in the MFC are critical neural substrates for SS, and suggest that activation of intrinsic neurons in the MFC are responsible for SS in the region.