Lateral Hypothalamic Neurons Research Articles

Activation of lateral hypothalamic neurons stimulates brown adipose tissue thermogenesis

The lateral hypothalamic area, containing orexin neurons, is involved in several aspects of autonomic regulation, including thermoregulation and energy expenditure. To determine if activation of lateral hypothalamic area neurons influences sympathetically-regulated thermogenesis in brown adipose tissue, we microinjected bicuculline (120pmol, 60nl, unilateral) into the lateral hypothalamic area in urethane/chloralose-anesthetized, artificially-ventilated rats. Disinhibition of neurons in lateral hypothalamic area evoked a significant increase (+1309%) in brown adipose tissue sympathetic nerve activity accompanied by parallel increases in brown adipose tissue temperature (+2.0°C), in expired CO 2 (+0.6%), in heart rate (+88 bpm) and in mean arterial pressure (+11 mm Hg). Subsequent microinjections of glycine (30nmol, 60nl) to inhibit local neurons in raphe pallidus or in dorsomedial hypothalamus or of glutamate receptor antagonists into dorsomedial hypothalamus promptly reversed the increases in brown adipose tissue sympathetic nerve activity, brown adipose tissue temperature and heart rate evoked by disinhibition of neurons in lateral hypothalamic area. We conclude that neurons in the lateral hypothalamic area can influence brown adipose tissue sympathetic nerve activity, brown adipose tissue thermogenesis and heart rate through pathways that are dependent on the activation of neurons in dorsomedial hypothalamus and raphe pallidus.

Collateral axonal projections from hypothalamic hypocretin neurons to cardiovascular sites in nucleus ambiguus and nucleus tractus solitarius

Hypocretin-1 (hcrt-1)-containing axons have been shown to have an extensive distribution within the central nervous system, although the total number of hypothalamic hcrt-1 neurons has been shown to be small. This suggests that hcrt-1 neurons may innervate central structures with similar function through collateral axonal projections. Retrograde tract-tracing techniques combined with immunohistochemistry were used in this study to investigate whether hypothalamic hcrt-1-containing neurons send collateral axonal projections to cardiovascular sites in the nucleus of the solitary tract (NTS) and in the nucleus ambiguus (Amb) in the rat. Fluorogold- (FG) and/or rhodamine (Rd)-labeled latex microspheres were microinjected into either the NTS or Amb at sites that elicited bardycardia responses ( l-glutamate; 0.25 M; 10 nl). After a survival period of 10–15 days, the rats were sacrificed and tissue sections of the hypothalamus were processed immunohistochemically for the identification of hcrt-1-containing cell bodies. After injection of the tract-tracers into the NTS or Amb, retrogradely labeled neurons were observed within several hypothalamic regions; the paraventricular hypothalamic nucleus, lateral hypothalamic area, perifornical hypothalamic area, and posterior hypothalamus, bilaterally, but with an ipsilateral predominance. In addition, after NTS injections, retrogradely labeled neurons were found within the ipsilateral caudal arcuate nucleus. Of the total number (1107±97) of hcrt-1-immunoreactive neurons found bilaterally within the lateral and perifornical hypothalamic nuclei, 7.9±1.4% were found to be retrogradely labeled from the NTS, 16.4±1.8% from the Amb, and 3.1±0.5% from both medullary sites. Hcrt-1 neurons projecting to the NTS were found mainly in and around the perifornical hypothalamic region, with a smaller number in the caudal lateral hypothalamic area. On the other hand, those innervating the Amb were primarily observed within the caudal lateral hypothalamic area, with a smaller number in the perifornical hypothalamic area. Neurons with collateral axonal projections to NTS and Amb were observed within two specific hypothalamic areas: one group of neurons was found in the perifornical hypothalamic area, and the other was observed in the lateral hypothalamic region just dorsal to the retrochiasmatic component of the supraoptic nucleus. These data indicate that axons from hcrt-1 neurons bifurcate to innervate functionally similar cardiovascular-responsive sites in the NTS and Amb. Although the function of these hcrt-1-containing hypothalamic–medullary pathways is not known, they likely represent the anatomical substrate by which the lateral hypothalamic hcrt-1 neurons simultaneously coordinate autonomic–cardiovascular responses to different behaviors.

Altered action of dopamine and cholecystokinin on lateral hypothalamic neurons in rats raised under different feeding conditions

Single-unit activity was recorded in the lateral hypothalamus (LH) of adult Wistar rats anaesthetized with urethane. The rats were differently nourished till weaning by raising in small (SL), control (CL) or large litters (LL). They gained significantly different body weight leading to overweight in SL (mean: 428.4 g on day 90) and underweight in LL rats (mean 399.5 g) compared to CLs (414.5 g). The mean basal firing rate of LH neurons differed, it was lowest in SL and highest in LL rats. The proportion of neurons changing their firing rate by more than 30% in response to iontophoretically administered dopamine (DA) was significantly greater in SL (76%) than LL rats (54%). Effects of DA were significantly more often blocked by a D 1 receptor antagonist in LL than CLs. The responsiveness to cholecystokinin (CCK) alone and coadministered with DA was also greater in SL than LL. Furthermore, the proportion of neurons inhibited by DA alone and in the presence of CCK was significantly greater in SL than LL rats. In conclusion, litter size and difference in nourishment during early postnatal development of rats seem to determine LH basal firing rate. The increased neuronal responsiveness to exogenous DA and CCK in neonatally overfed SL rats may indicate a decreased activity of these endogenous signals which normally contribute to limitation of energy intake.

Orexins induce increased excitability and synchronisation of rat sympathetic preganglionic neurones.

The neuropeptides orexin A and B are synthesised by perifornical and lateral hypothalamic (LH) neurones and exert a profound influence on autonomic sympathetic processes. LH neurones project to spinal areas containing sympathetic preganglionic neurones (SPNs) and therefore may directly modulate sympathetic output. In the present study we examined the possibility that orexinergic inputs from the LH influence SPN activity. Orexin-positive neurones in the LH were labelled with pseudorabies virus injected into the liver of parasympathetically denervated animals and orexin fibres were found adjacent to the soma and dendrites of SPNs. Orexin A or B (10-1000 nM) directly and reversibly depolarised SPNs in spinal cord slices. The response to orexin A was significantly reduced in the presence of the orexin receptor 1 (OX1R) antagonist SB334867A at concentrations of 1-10 micro M. Single cell reverse transcriptase-polymerase chain reaction revealed expression of mRNA for both OX1R and OX2R in the majority of orexin-sensitive SPNs. The orexin-induced depolarisation involved activation of pertussis toxin-sensitive G-proteins and closure of a K+ conductance via a protein kinase A (PKA)-dependent pathway that did not require an increase in intracellular Ca2+. Orexins also induced biphasic subthreshold membrane potential oscillations and synchronised activity between pairs of electrically coupled SPNs. Coupling coefficients and estimated junctional conductances between SPNs were not altered indicating synchronisation is due to activation of previously silent coupled neurones rather than modulation of gap junctions. These findings are consistent with a direct excitation and synchronisation of SPNs by orexinergic neurones that in vivo could increase the frequency and coherence of sympathetic nerve discharges and mediate LH effects on sympathetic components of energy homeostasis and cardiovascular control.

Food- and light-entrained circadian rhythms in rats with hypocretin-2-saporin ablations of the lateral hypothalamus

Daily feeding schedules can entrain circadian rhythms of food-anticipatory activity in mammals. The site of the circadian oscillators that drive food-entrained rhythms is unknown. Lateral hypothalamic (LH) neurons containing hypocretins (Hcrt1 and 2, also known as orexin A and B) regulate feeding, energy metabolism and arousal state, raising the possibility that they may also participate in the entrainment of activity rhythms by a daily mealtime. To examine this, Hcrt neurons in rats were ablated by LH injections of Hcrt2 conjugated to the ribosome-inactivating protein saporin. To assess photic entrainment and masking, drinking activity was recorded continuously in LD 12:12 for ∼6 weeks, in DD for 48 h, and in LD 2:2 for 24 h. To assess food-entrainment, drinking and food cup activity were recorded for 4–7 weeks during which food was restricted to a 3-h daily meal beginning 6 h after lights-on. Lesions were assessed by immunocytochemistry or inspection of Nissl stained sections. Hcrt cell depletion ranged from 0 to 100%. Lesions were associated with hypophagia, hypodypsia and weight loss. Despite reduced mean daily drinking, there was no significant effect on the shape or amplitude of the circadian waveforms in LD, LD 2:2 or DD at ∼6 weeks after surgery. All rats exhibited drinking or food cup activity in anticipation of the daily meal, indicative of circadian entrainment. These results indicate that the Hcrt system modulates ingestive behaviors but does not play a necessary role in the entrainment or expression of food-anticipatory circadian rhythms.

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.

Identification of neurons containing orexin-B (hypocretin-2) immunoreactivity in limbic structures

Orexins (hypocretins) are neuropeptides which have recently been identified exclusively within lateral hypothalamic and perifornical neurons, and these orexin (ox) containing neurons appear to have extensive projections to all levels of the neuraxis. In this study, we report the identification of two distinct clusters of neurons containing ox-B-like immunoreactivity within the amygdaloid complex of the rat. A cluster of small to medium size ovoid shaped neurons containing ox-B-like immunoreactivity was found predominantly within the lateral division of the central nucleus of the amygdala (ACe). A second distinct, but smaller group of ox-B labelled neurons with similar shapes and sizes to those in ACe was also identified in the anterior lateral subnucleus of the bed nucleus of the stria terminalis (BST) immediately adjacent the internal capsule, and in an area just ventral to the lateral ventricle. Neurons containing ox-A-like immunoreactivity were not observed in either structure. However, both structures contained ox-A- and ox-B labelled varicose fibers. Unilateral electrolytic lesions of the lateral hypothalamic area that contained ox-A and ox-B neurons did not alter the labelling of either ACe or BST ox-B pericarya. As both the ACe and BST are known to be involved in integrating complex homeostatic mechanisms associated with behaviours, these data suggest that a specific subset of ox-B neurons within the amygdaloid complex may serve as a component of neuronal circuits coordinating these responses.

Cholinergic modulation of purinergic and GABAergic co-transmission at in vitro hypothalamic synapses.

The lateral hypothalamus (LH) is an important center for the integration of autonomic and limbic information and is implicated in the modulation of visceral motor and sensory pathways, including those underlying feeding and arousal behaviors. LH neurons in vitro release both ATP and GABA. The control of ATP and GABA co-transmission in LH may underlie the participation of LH in basic aspects of arousal and reinforcement. LH neurons receive cholinergic input from the pedunculopontine and laterodorsal tegmental nuclei as well as from cholinergic interneurons within the LH per se. This study presents evidence for nicotinic acetylcholine receptor (nAChR)-mediated enhancement of GABAergic, but not of purinergic, transmission despite the co-transmission of ATP and GABA at LH synapses in vitro. Facilitation of GABAergic transmission by nicotine is inhibited by antagonists of (alphabeta)*-containing nAChRs, but is unaffected by an alpha7-selective antagonist, consistent with a nAChR-mediated enhancement of GABA release mediated by non-alpha7-containing nAChRs. Activation of muscarinic ACh receptors enhances the release of ATP while concomitantly depressing GABAergic transmission. The independent modulation of ATP/GABAergic transmission may provide a new level of synaptic flexibility in which individual neurons utilize more than one neurotransmitter but retain independent control over their synaptic activity.

Melanin-concentrating hormone depresses L-, N-, and P/Q-type voltage-dependent calcium channels in rat lateral hypothalamic neurons.

Melanin-concentrating hormone (MCH), a cyclic 19-amino-acid peptide, is synthesized exclusively by neurons in the lateral hypothalamic (LH) area. It is involved in a number of brain functions and recently has raised interest because of its role in energy homeostasis. MCH axons and receptors are found throughout the brain. Previous reports set the foundation for understanding the cellular actions of MCH by using non-neuronal cells transfected with the MCH receptor gene; these cells exhibited an increase in cytoplasmic calcium in response to MCH, suggesting an excitatory action for the peptide. In the study presented here, we have used whole-cell recording in 117 neurons from LH cultures and brain slices to examine the actions of MCH. MCH decreased the amplitude of voltage-dependent calcium currents in almost all tested neurons. The inhibition desensitized rapidly (18 s to half maximum at 100 nM concentration) and was dose-dependent (IC(50) = 7.8 nM) when activated with a pulse from -80 mV to 0 mV. A priori activation of G-proteins with GTPgammaS completely eliminated the MCH-induced effect at low MCH concentrations and reduced the MCH-induced effect at high MCH concentrations. Inhibition of G-proteins with pertussis toxin (PTX) blocked the MCH-induced inhibitory effect at high MCH concentrations. Pre-pulse depolarization resulted in an attenuation of the MCH-induced inhibition of calcium currents in most neurons. These data suggest that MCH exerts an inhibitory effect on calcium currents via PTX-sensitive G-protein pathways, probably the G(i)/G(o) pathway, in LH neurons. L-, N- and P/Q-type calcium channels were identified in LH neurons, with L- and N-type channels accounting for most of the voltage-activated current (about 40 % each); MCH attenuated each of the three types (mean 50 % depression), with the greatest inhibition found for N-type currents. In contrast to previous data on non-neuronal cells showing an MHC-evoked increase in calcium, our data suggest that the reverse occurs in LH neurons. The attenuation of calcium currents is consistent with an inhibitory action for the peptide in neurons.

Coordinate Release of ATP and GABA at In VitroSynapses of Lateral Hypothalamic Neurons

Autonomic and limbic information is integrated within the lateral hypothalamus (LH), and excitability of LH neurons is important in the control of feeding and behavioral arousal. Despite the prominent expression of P2X-type ATP receptors throughout the hypothalamus, the role of ATP in LH excitability is not known. Perforated-patch-clamp recordings of synaptically coupled neurons from both embryonic chick and postnatal mouse lateral hypothalamus in vitro reveal robust stimulus-evoked purinergic synaptic transmission. Suprathreshold activation elicits reliable and concurrent release of ATP with GABA. Tetrodotoxin-resistant P2X receptor-mediated events are readily observed at LH synapses from the embryonic chick, whereas GABA miniature postsynaptic currents (mPSCs) are recorded in innervated LH neurons from either embryonic chicks or postnatal mice. Two distinct mPSCs are recorded at ATP-GABA cosynapses; one has a monoexponential decay phase and is modulated by flunitrazepam, and the other has a decay phase that is best fit by a sum of two exponential functions (tau(fast) and tau(slow)), and only the tau(slow) component is affected by flunitrazepam. Bicuculline does not completely inhibit all mPSCs. The remaining bicuculline-resistant mPSCs are blocked by suramin, and their decay phase is briefer than that of GABAergic mPSCs. Furthermore, at a holding potential intermediate for the reversal potentials of GABA(A) and P2X receptors, little or no current is observed, consistent with concomitant release (and detection) of GABA and ATP. Together, our data suggest that a subset of spontaneous and evoked PSCs arise from the concurrent activation of both GABA(A) and P2X receptors.

Organization of human hypothalamus in fetal development

The organization of the human hypothalamus was studied in 33 brains aged from 9 weeks of gestation (w.g.) to newborn, using immunohistochemistry for parvalbumin, calbindin, calretinin, neuropeptide Y, neurophysin, growth-associated protein (GAP)-43, synaptophysin, and the glycoconjugate 3-fucosyl- N-acetyl-lactosamine. Developmental stages are described in relation to obstetric trimesters. The first trimester (morphogenetic periods 9-10 w.g. and 11-14 w.g.) is characterized by differentiating structures of the lateral hypothalamic zone, which give rise to the lateral hypothalamus (LH) and posterior hypothalamus. The PeF differentiates at 18 w.g. from LH neurons, which remain anchored in the perifornical position, whereas most of the LH cells are displaced laterally. A transient supramamillary nucleus was apparent at 14 w.g. but not after 16 w.g. As the ventromedial nucleus differentiated at 13-16 w.g., three principal parts, the ventrolateral part, the dorsomedial part, and the shell, were revealed by distribution of calbindin, calretinin, and GAP43 immunoreactivity. The second trimester (morphogenetic periods 15-17 w.g., 18-23 w.g., and 24-33 w.g.) is characterized by differentiation of the hypothalamic core, in which calbindin- positive neurons revealed the medial preoptic nucleus at 16 w.g. abutted laterally by the intermediate nucleus. The dorsomedial nucleus was clearly defined at 10 w.g. and consisted of compact and diffuse parts, an organization that was lost after 15 w.g. Differentiation of the medial mamillary body into lateral and medial was seen at 13-16 w.g. Late second trimester was marked by differentiation of periventricular zone structures, including suprachiasmatic, arcuate, and paraventricular nuclei. The subnuclear differentiation of these nuclei extends into the third trimester. The use of chemoarchitecture in the human fetus permitted the identification of interspecies nuclei homologies, which otherwise remain concealed in the cytoarchitecture.

Lowering glucose concentrations increases cytosolic Ca 2+ in orexin neurons of the rat lateral hypothalamus

Orexin neurons are specifically localized in and around the lateral hypothalamus (LH), a feeding center. Intracerebroventricular administration of orexin-A and -B stimulates feeding as well as arousal. However, little is known regarding the regulators of the orexin neuron activity. The neurons that are activated under low glucose conditions, glucose-sensitive neurons, are located in the LH and have been implicated in the control of feeding. The present study investigated the effect of glucose on the single orexin neurons isolated from the rat LH, by measuring cytosolic Ca 2+ concentration ([Ca 2+] i) by fura-2 microfluorometry followed by immunocytochemical staining with anti-orexin antiserum. A shift of glucose concentration form 8.3 to 2.8 mM in the superfusion solution increased [Ca 2+] i in 13 out of 32 orexin-immunoreactive LH neurons. The results demonstrate that glucose-sensitive orexin neurons are present in the LH and that these neurons may play a role in linking the metabolic state in the body to the orexigenic, and could also, awakening signaling in the brain.

Microinjection of carbachol in the lateral hypothalamus produces opposing actions on nociception mediated by α 1- and α 2-adrenoceptors

Electrical stimulation of the lateral hypothalamus (LH) produces antinociception partially blocked by intrathecal α-adrenergic antagonists, but the mechanism underlying this effect is not clear. Evidence from immunological studies demonstrates that substance P-immunoreactive neurons in the LH project near the A7 catecholamine cell group, a group of noradrenergic neurons in the pons known to effect antinociception in the spinal cord dorsal horn. Such evidence suggests that LH neurons may activate A7 neurons to produce antinociception. To test this hypothesis, the cholinergic agonist carbachol was microinjected into the LH at doses of 63, 125 and 250 nmol and the resulting effects on tail-flick and nociceptive foot-withdrawal latencies were measured. All three doses significantly increased response latencies on both tests, with the 125-nmol dose providing the optimal effect. Intrathecal injection of the opioid antagonist naltrexone (97 nmol) partially reversed antinociception, but neither the α 2-adrenoceptor antagonist yohimbine nor the α 1-adrenoceptor antagonist WB4101 altered latencies. However, two sequential doses of yohimbine blocked LH-induced antinociception on both tests. In contrast, two sequential doses of WB4101 increased nociceptive responses on both the tail-flick and foot-withdrawal tests. These findings, and those of published reports, suggest that neurons in the LH activate spinally projecting methionine enkephalin neurons, as well as two populations of A7 noradrenergic neurons that exert a bidirectional effect on nociception. One of these populations increases nociception through the action of α 1-adrenoceptors and the other inhibits nociception through the action of α 2-adrenoceptors in the spinal cord dorsal horn.

Hypocretin (orexin) input to trigeminal and hypoglossal motoneurons in the cat: a double-labeling immunohistochemical study

In trigeminal and hypoglossal motor nuclei of adult cats, hypocretin immunoreactive fiber varicosities were observed in apposition to retrogradely labeled motoneuron somata and dendrites. Among those lateral hypothalamus neurons that project to the hypoglossal nucleus some were determined to be hypocretin immunoreactive and were located amongst the single-labeled hypocretinergic neurons. These data suggest that hypocretin may play a role in the synaptic control of these motoneurons.

Effect of LPS administration on the expression of POMC, NPY, galanin, CART and MCH mRNAs in the rat hypothalamus

Anorexia and weight loss are manifestations of inflammation seen both in patients and in experimental animal models such as the lipopolysaccharide (LPS)-treated rat. Using in situ hybridization, the levels of mRNAs encoding proopiomelanocortin (POMC), neuropeptide Y (NPY), galanin, melanin-concentrating hormone (MCH) and cocaine- and amphetamine-regulated transcript (CART) were investigated in the rat hypothalamus after a single intraperitoneal dose (125 μg/kg) of LPS. Four hours after LPS injection the food intake was significantly decreased. POMC and CART mRNA levels were increased in the arcuate nucleus, and MCH, CART and galanin mRNAs were all decreased in the lateral hypothalamic area in LPS-treated rats. Levels of mRNAs for NPY and galanin in the arcuate nucleus, and for MCH and CART in the zona incerta did not change significantly after LPS treatment. These findings support the hypothesis that LPS-induced factors mediate signalling to the POMC/CART neurons in the arcuate nucleus which could lead to reduced food intake by decreasing MCH, CART and galanin synthesis in target lateral hypothalamic neurons.

Melanin concentrating hormone depresses synaptic activity of glutamate and GABA neurons from rat lateral hypothalamus

The neuropeptide melanin concentrating hormone (MCH) is synthesised only by neurons of the lateral hypothalamic (LH) area in the CNS. MCH cells project widely throughout the brain. Despite the growing interest in this peptide, in part related to its role in feeding, little has been done to characterise its physiological effects in neurons. Using whole-cell recording with current and voltage clamp, we examined the cellular actions in neurons from the LH. MCH induced a consistent decrease in the frequency of action potentials and reduced synaptic activity. Most fast synaptic activity in the hypothalamus is mediated by GABA or glutamate. MCH inhibited the synaptic activity of both glutamatergic and GABAergic LH neurons, each tested independently. MCH reduced the amplitude of glutamate-evoked currents and reduced the amplitude of miniature excitatory currents, indicating an inhibitory modulation of postsynaptic glutamate receptors. In the presence of tetrodotoxin to block action potentials, MCH caused a depression in the frequency of miniature glutamate-mediated postsynaptic currents, suggesting a presynaptic site of receptor expression. In voltage clamp experiments, MCH depressed the amplitude of calcium currents, suggesting that a mechanism of inhibition may involve a reduced calcium-dependent release of amino acid transmitter. Previous reports have suggested that MCH activated potassium channels in non-neuronal cells transfected with the MCH receptor gene. We found no effect of MCH on voltage-dependent potassium channels in LH neurons. Baclofen, a GABAB receptor agonist, activated G-protein gated inwardly rectifying potassium (GIRK)-type channels; in the same neurons, MCH had no effect on GIRK channels. MCH showed no modulation of sodium currents. Blockade of the Gi/Go protein with pertussis toxin eliminated the actions of MCH. The inhibitory actions of MCH on both excitatory and inhibitory synaptic events, coupled with opposing excitatory actions of hypocretin, another LH peptide that projects to many of the same loci, suggest a substantial level of complexity in neuropeptide modulation of LH actions.

The substrate for brain-stimulation reward in the lateral preoptic area: I. Anatomical mapping of its boundaries

Given the putative role of the lateral preoptic area as a primary contributor of the cell bodies of origin of the descending pathway linking a subset of lateral hypothalamic and ventral tegmental area reward neurons, the distribution of self-stimulation sites in this structure was mapped in 22 animals using moveable electrodes and threshold procedures. Ninety-seven electrode sites were evaluated with placements ranging from just rostral to the midline convergence of the anterior commissure back to the transition zone between the lateral preoptic and lateral hypothalamic areas; of these, roughly 2/3 supported self-stimulation which was widely observed throughout the lateral preoptic area and medial forebrain bundle. In general, self-stimulation thresholds obtained from lateral sites were most stable, and progressively so approaching more caudal regions. Examination of the slopes of the period/current trade-off functions revealed a tendency for higher values in lateral and caudal sites; in contrast, dorsoventral excursions did not influence these estimates. Taken together, these data provide support for the notion that the substrate for brain-stimulation reward in the lateral preoptic area has a relatively homogeneous distribution that is more diffusely organized than that found in reward sites activated further caudally in the medial forebrain bundle.

Effects of blockers of carbohydrate and lipid metabolism on expression of mRNA of some hypothalamic neuropeptides.

The effects of single injections of 2-deoxyglucose or 2-mercaptoacetate on the expression of mRNA of neuropeptide Y, pro-opiomelanocortin, and melanin-concentrating hormone in rat hypothalamus were studied by in situ hybridization in order to elucidate the role of these neuropeptides in the mechanisms of alimentary behavior caused by decreased levels of available fatty acids and glucose. The levels of neuropeptide Y mRNA in arcuate nuclei neurons are significantly increased under conditions of glucose deficiency, while the synthesis of melanin-concentrating hormone in the lateral hypothalamic neurons is increased in fatty acid deficiency. These data indicate that glyco- and lipodeprivation are different metabolic signals activating various neuropeptide systems responsible for alimentary behavior.

Evidence of a Functional Relationship between the Nucleus Accumbens Shell and Lateral Hypothalamus Subserving the Control of Feeding Behavior

Inhibition of neurons in the nucleus accumbens shell (AcbSh) with local injections of GABA agonists or glutamate antagonists elicits an intense, but specific, feeding response resembling that seen after stimulation of the lateral hypothalamus (LH). To help characterize the contribution of the LH to the expression of AcbSh-mediated feeding, we used the immunohistochemical detection of the nuclear protein Fos to determine whether inhibition of AcbSh cells results in an activation of LH neurons. Injections of the GABA(A) agonist muscimol into the AcbSh greatly increased the number of cells exhibiting Fos-like immunoreactivity in the LH, as well as in the lateral septum, paraventricular hypothalamic nucleus, ventral tegmental area, substantia nigra pars compacta, and nucleus of the solitary tract. Blocking activation of LH neurons with the selective NMDA receptor blocker D(-)-AP-5 is known to suppress deprivation-induced feeding. We found that injections of D(-)-AP5 into the LH also dose-dependently suppressed AcbSh-mediated feeding. It is likely that inhibition of GABAergic neurons in the AcbSh is responsible for eliciting this feeding. If a behaviorally relevant GABAergic projection terminates in the LH, we should be able to mimic the effects seen after inhibition of the projection neurons by applying a GABA receptor blocker to the area. However, injections of the GABA(A) receptor blocker bicuculline or the GABA(B) receptor blocker saclofen did not significantly affect food intake. Thus, it appears that the expression of the feeding response depends on an NMDA-preferring receptor-mediated activation of LH neurons and is not the result of disinhibiting LH cells by disrupting transmission at GABA synapses.

Hypothalamic aggression area under serotonergic control in mouse-killing behaviour of rats.

The serotonergic system participates in the modulation of aggressive behaviour. We examined the neuronal substrate underlying serotonergic control of aggressive behaviour in rats using a transplantation technique. Fetal serotonergic neurons transplanted into the hypothalamus restored inhibition of mouse-killing behaviour (muricide), in rats with raphe lesions induced by a serotonergic neurotoxin, 5,7-dihydroxytryptamine. Immunohistochemical and neurochemical studies indicated that the recovery of serotonergic innervation in the lateral hypothalamic area by the raphe grafts brought about the inhibition of muricide. The extension of serotonergic fibres into the lateral hypothalamic area from the grafted serotonergic neurons is strongly related to inhibition of muricide. These results indicate the possibility that serotonergic neurons regulate muricide through the inhibition of the activity of the lateral hypothalamic area neurons.