Dissociation between learning and remembering in rats with lesions in the lateral hypothalamus

Hypothalamic MCH Neurons: From Feeding to Cognitive Control

Modern neuroscience is progressively elucidating that the classic view positing distinct brain regions responsible for survival, emotion, and cognitive functions is outdated. The hypothalamus demonstrates the interdependence of these roles, as it is traditionally known for fundamental survival functions like energy and electrolyte balance, but is now recognized to also play a crucial role in emotional and cognitive processes. This review focuses on lateral hypothalamic melanin-concentrating hormone (MCH) neurons, producing the neuropeptide MCH-a relatively understudied neuronal population with integrative functions related to homeostatic regulation and motivated behaviors, with widespread inputs and outputs throughout the entire central nervous system. Here, we review early findings and recent literature outlining their role in the regulation of energy balance, sleep, learning, and memory processes.

Neuronal circuitry for recognition memory of object and place in rodent models

Rats and mice are used for studying neuronal circuits underlying recognition memory due to their ability to spontaneously remember the occurrence of an object, its place and an association of the object and place in a particular environment. A joint employment of lesions, pharmacological interventions, optogenetics and chemogenetics is constantly expanding our knowledge of the neural basis for recognition memory of object, place, and their association. In this review, we summarize current studies on recognition memory in rodents with a focus on the novel object preference, novel location preference and object-in-place paradigms. The evidence suggests that the medial prefrontal cortex- and hippocampus-connected circuits contribute to recognition memory for object and place. Under certain conditions, the striatum, medial septum, amygdala, locus coeruleus and cerebellum are also involved. We propose that the neuronal circuitry for recognition memory of object and place is hierarchically connected and constructed by different cortical (perirhinal, entorhinal and retrosplenial cortices), thalamic (nucleus reuniens, mediodorsal and anterior thalamic nuclei) and primeval (hypothalamus and interpeduncular nucleus) modules interacting with the medial prefrontal cortex and hippocampus.

Orexin/Hypocretin and MCH Neurons: Cognitive and Motor Roles Beyond Arousal

The lateral hypothalamus (LH) is classically implicated in sleep-wake control. It is the main source of orexin/hypocretin and melanin-concentrating hormone (MCH) neuropeptides in the brain, which have been both implicated in arousal state switching. These neuropeptides are produced by non-overlapping LH neurons, which both project widely throughout the brain, where release of orexin and MCH activates specific postsynaptic G-protein-coupled receptors. Optogenetic manipulations of orexin and MCH neurons during sleep indicate that they promote awakening and REM sleep, respectively. However, recordings from orexin and MCH neurons in awake, moving animals suggest that they also act outside sleep/wake switching. Here, we review recent studies showing that both orexin and MCH neurons can rapidly (sub-second-timescale) change their firing when awake animals experience external stimuli, or during self-paced exploration of objects and places. However, the sensory-behavioral correlates of orexin and MCH neural activation can be quite different. Orexin neurons are generally more dynamic, with about 2/3rds of them activated before and during self-initiated running, and most activated by sensory stimulation across sensory modalities. MCH neurons are activated in a more select manner, for example upon self-paced investigation of novel objects and by certain other novel stimuli. We discuss optogenetic and chemogenetic manipulations of orexin and MCH neurons, which combined with pharmacological blockade of orexin and MCH receptors, imply that these rapid LH dynamics shape fundamental cognitive and motor processes due to orexin and MCH neuropeptide actions in the awake brain. Finally, we contemplate whether the awake control of psychomotor brain functions by orexin and MCH are distinct from their “arousal” effects.

Control of fear extinction by hypothalamic melanin-concentrating hormone-expressing neurons

Learning to fear danger is essential for survival. However, overactive, relapsing fear behavior in the absence of danger is a hallmark of disabling anxiety disorders that affect millions of people. Its suppression is thus of great interest, but the necessary brain components remain incompletely identified. We studied fear suppression through a procedure in which, after acquiring fear of aversive events (fear learning), subjects were exposed to fear-eliciting cues without aversive events (safety learning), leading to suppression of fear behavior (fear extinction). Here we show that inappropriate, learning-resistant fear behavior results from disruption of brain components not previously implicated in this disorder: hypothalamic melanin-concentrating hormone-expressing neurons (MNs). Using real-time recordings of MNs across fear learning and extinction, we provide evidence that fear-inducing aversive events elevate MN activity. We find that optogenetic disruption of this MN activity profoundly impairs safety learning, abnormally slowing down fear extinction and exacerbating fear relapse. Importantly, we demonstrate that the MN disruption impairs neither fear learning nor related sensory responses, indicating that MNs differentially control safety and fear learning. Thus, we identify a neural substrate for inhibition of excessive fear behavior.

Natural hypothalamic circuit dynamics underlying object memorization

Brain signals that govern memory formation remain incompletely identified. The hypothalamus is implicated in memory disorders, but how its rapidly changing activity shapes memorization is unknown. During encounters with objects, hypothalamic melanin-concentrating hormone (MCH) neurons emit brief signals that reflect object novelty. Here we show that targeted optogenetic silencing of these signals, performed selectively during the initial object encounters (i.e. memory acquisition), prevents future recognition of the objects. We identify an upstream inhibitory microcircuit from hypothalamic GAD65 neurons to MCH neurons, which constrains the memory-promoting MCH cell bursts. Finally, we demonstrate that silencing the GAD65 cells during object memory acquisition improves future object recognition through MCH-receptor-dependent pathways. These results provide causal evidence that object-associated signals in genetically distinct but interconnected hypothalamic neurons differentially control whether the brain forms object memories. This gating of memory formation by hypothalamic activity establishes appropriate behavioral responses to novel and familiar objects.

Somatostatin and corticotrophin releasing hormone cell types are a major source of descending input from the forebrain to the parabrachial nucleus in mice

The pontine parabrachial nucleus (PBN) receives substantial descending input from higher order forebrain regions that exerts inhibitory and excitatory influences on taste-evoked responses. Somatostatin (Sst) and corticotrophin releasing hormone (Crh) reporter mice were used in conjunction with injection of the retrograde tracer CTb-488 into the caudal PBN to determine the extent to which Sst and Crh cell types contribute to the descending pathways originating in the lateral hypothalamus (LH), central nucleus of the amygdala (CeA), bed nucleus of the stria terminalis (BNST), and insular cortex (IC). Five to 7 days following injections, the animals were euthanized and tissue sections prepared for confocal microscopy. Crh cell types in each forebrain site except IC project to the PBN with the greatest percentage originating in the BNST. For Sst cell types, the largest percentage of double-labeled cells was found in the CeA followed by the BNST. Few retrogradely labeled cells in the LH coexpressed Sst, whereas no double-labeled cells were observed in IC. The present results suggest that Sst and Crh cell types are a substantial component of the descending pathways from the amygdala and/or BNST to the PBN and are positioned to exert neuromodulatory effects on central taste processing.

Parabrachial-hypothalamic interactions are required for normal conditioned taste aversions

Rats with bilateral excitotoxic lesions of the parabrachial nuclei (PBN) fail to acquire a conditioned taste aversion (CTA), yet they retain the ability to express a CTA learned prior to incurring the damage. Rats with bilateral electrolytic lesions of the lateral hypothalamus (LH) also have CTA learning deficits. The PBN have reciprocal neural connections with the LH. This suggests that these CTA deficits may be functionally related. Electrolytic lesions damage fibers of passage, as well as intrinsic neurons. Thus, these LH lesions might also interrupt reciprocal connections between the PBN and other ventral forebrain areas, such as the amygdala and bed nucleus of the stria terminalis. To distinguish the source of the LH-lesion deficit, we tested for CTA first after bilateral excitotoxic lesions of LH and subsequently with a second set of animals that had asymmetric excitotoxic PBN and LH lesions. The rats with bilateral excitotoxic LH lesions showed deficits when acquiring a postlesion CTA. The asymmetrical PBN-LH lesions not only slowed acquisition of a CTA but also sped up extinction. This implies that interaction between the two structures, at minimum, facilitates CTA learning and may have a role in its consolidation.

Toward a general psychobiological theory of emotions

Emotions seem to arise ultimately from hard-wired neural circuits in the visceral-limbic brain that facilitate diverse and adaptive behavioral and physiological responses to major classes of environmental challenges. Presumably these circuits developed early in mammalian brain evolution, and the underlying control mechanisms remain similar in humans and “lower” mammals. This would suggest that theoretically guided studies of the animal brain can reveal how primitive emotions are organized in the human brain. Conversely, granted this cross-species heritage, it is arguable that human introspective access to emotional states may provide direct information concerning operations of emotive circuits and thus be a primary source of hypotheses for animal brain research. In this article the possibility that emotions are elaborated by transhypothalamic executive (command) circuits that concurrently activate related behavior patterns is assessed. Current neurobehavioral evidence indicates that there are at least four executive circuits of this type – those which elaborate central states of expectancy, rage, fear, and panic. The manner in which learning and psychiatric disorders may arise from activities of such circuits is also discussed.

Comparison of somatostatin and corticotrophin-releasing hormone immunoreactivity in forebrain neurons projecting to taste-responsive and non-responsive regions of the parabrachial nucleus in rat

Several forebrain areas have been shown to project to the parabrachial nucleus (PBN) and exert inhibitory and excitatory influences on taste processing. The neurochemicals by which descending forebrain inputs modulate neural taste-evoked responses remain to be established. This study investigated the existence of somatostatin (SS) and corticotrophin-releasing factor (CRF) in forebrain neurons that project to caudal regions of the PBN responsive to chemical stimulation of the anterior tongue as well as more rostral unresponsive regions. Retrograde tracer was iontophoretically or pressure ejected from glass micropipettes, and 7 days later the animals were euthanized for subsequent immunohistochemical processing for co-localization of tracer with SS and CRF in tissue sections containing the lateral hypothalamus (LH), central nucleus of the amygdala (CeA), bed nucleus of the stria terminalis (BNST), and insular cortex (IC). In each forebrain site, robust labeling of cells with distinguishable nuclei and short processes was observed for SS and CRF. The results indicate that CRF neurons in each forebrain site send projections throughout the rostral caudal extent of the PBN with a greater percentage terminating in regions rostral to the anterior tongue-responsive area. For SS, the percentage of double-labeled neurons was more forebrain site specific in that only BNST and CeA exhibited significant numbers of double-labeled neurons. Few retrogradely labeled cells in LH co-expressed SS, while no double-labeled cells were observed in IC. Again, tracer injections into rostral PBN resulted in a greater percentage of double-labeled neurons in BNST and CeA compared to caudal injections. The present results suggest that some sources of descending forebrain input might utilize somatostatin and/or CRF to exert a broad influence on sensory information processing in the PBN.

Modulation of parabrachial taste neurons by electrical and chemical stimulation of the lateral hypothalamus and amygdala

The lateral hypothalamus (LH) and the central nucleus of the amygdala (CeA) exert an influence on ingestive behavior and are reciprocally connected to gustatory and viscerosensory areas, including the nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN). We investigated the effects of LH and CeA stimulation on the activity of 101 taste-responsive neurons in the hamster PbN. Eighty three of these neurons were antidromically activated by stimulation of these sites; 57 were antidromically driven by both. Of these 83 neurons, 21 were also orthodromically activated--8 by the CeA and 3 by the LH. Additional neurons were excited (n = 5) or inhibited (n = 8) by these forebrain nuclei but not antidromically activated. Taste stimuli were: 0.032 M sucrose, 0.032 M sodium chloride (NaCl), 0.032 M quinine hydrochloride (QHCl), and 0.0032 M citric acid. Among the 34 orthodromically activated neurons, more sucrose-best neurons were excited than inhibited, whereas the opposite occurred for citric-acid- and QHCl-best cells. Neurons inhibited by the forebrain responded significantly more strongly to citric acid and QHCl than cells excited by these sites. The effects of electrical stimulation were mimicked by microinjection of DL-homocysteic acid, indicating that cells at these forebrain sites were responsible for these effects. These data demonstrate that many individual PbN gustatory neurons project to both the LH and CeA and that these areas modulate the gustatory activity of a subset of PbN neurons. This neural substrate is likely involved in the modulation of taste activity by physiological and experiential factors.

Conditioned flavor preference and aversion: role of the lateral hypothalamus

Food-restricted rats with ibotenic acid lesions of the lateral hypothalamus (LH) and sham controls were trained to associate flavored solutions with positive or negative postingestive consequences. The LH rats learned to prefer a flavor that was paired with concurrent intragastric infusions of maltodextrin. Unlike controls, the LH rats failed to learn a preference for a flavor paired with delayed maltodextrin infusions and showed an attenuated preference for a flavor paired with concurrent fat infusions. The LH rats did not differ from controls in learning to avoid flavors paired with concurrent or delayed infusions of lithium chloride. These data indicate that the LH is not essential for all types of flavor-postingestive consequence conditioning but is critical for learning to associate flavors with delayed nutrient feedback.

The parabrachial nucleus and conditioned taste aversion

The parabrachial nucleus (PBN) surrounds the brachium conjunctivum in the dorsolateral pons. Although composed of numerous subnuclei, the PBN is typically organized into medial and lateral subdivisions according to their location relative to the brachium. In rodents, the medial PBN is part of the central gustatory system, whereas the lateral PBN is a component of the visceral sensory system. Lesions of the PBN disrupt conditioned taste aversion, a critically important learning mechanism that prevents the repeated ingestion of toxic food. Relevant neurobehavioral literature is reviewed to elucidate the role of the PBN in taste aversion learning.

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.

Gustatory function in the parabrachial nuclei: implications from lesion studies in rats

In rodents, third order gustatory neurons reside in the parabrachial nuclei of the dorsal pons. Lesions in this area of the brain have a variety of consequences on taste-related behaviors. Some behaviors are severely impaired, such as the expression of either conditioned taste aversion or depletion-induced sodium appetite. Other taste-based behaviors are less affected or not influenced at all. Although the lesion-behavior approach possesses serious methodological limitations, the constellation of findings from studies employing this experimental strategy in the PBN has promising implications. Foremost among these is the suggestion that the neural circuitry subserving performance in some of these taste-guided behavioral paradigms is dissociable. This paper critically reviews this body of behavioral research and discusses the conceptual ramifications of the results.

Tetrodotoxin blockade of amygdala overlapping with poisoning impairs acquisition of conditioned taste aversion in rats

The role of several forebrain structures in the association of the short-term gustatory memory (GSTM) of the conditioned stimulus (CS; 0.1% sodium saccharin) with the visceral unconditioned stimulus (US; 0.15 M LiCl, 2% b.wt.) in acquisition of conditioned taste aversion (CTA) was investigated. Experiment 1 examined the effects of bilateral reversible inactivation of amygdala (Amy), hippocampus (Hipp), gustatory cortex (GC), bed nucleus of stria terminalis (BNST), lateral hypothalamic area (LHA), ventral thalamus (VT) or LHA+VT, induced by intracerebral injection of tetrodotoxin (TTX; 10 ng/microliters per site) applied before i.p. injection of LiCl to rats anesthetized by pentobarbital (50 mg/kg) immediately after saccharin drinking. Amy blockade resulted in a complete disruption of learning, while the inactivation of the remaining areas examined produced mild or no impairments. The dose-related effects of TTX injection into Amy were investigated in Experiment 2. Doses of 3 and 1 ng TTX were as effective as the 10 ng dose used in Expt. 1. However, 0.3 ng or saline did not interfere with CTA acquisition. Analysis of the retrograde amnesic effect produced by transient amygdalectomy (Experiment 3), showed that TTX (10 ng) injected immediately or 1.5 h after LiCl application induced a marked learning disruption, whereas no amnesia was elicited at 6 and 24 h post-acquisition intervals. It is suggested that Amy plays an essential role in the associative phase of acquisition, but not in the consolidation of the permanent taste aversion engram.

Memory impairments following lesions to the mammillary region of the rat

The contribution of the mammillary region to learning and memory was investigated. It was demonstrated that lesions of this region impair performance on tasks that require memory for spatial information but that the deficit depends both on the amount of damage within the region and the difficulty of the task. A dissociation in the effect of such lesions on performance of comparable spatial and non-spatial memory tasks was shown. In contrast to the deficits observed on spatial memory tasks, the acquisition and retention of a complex non-spatial memory task was not impaired after extensive damage to the mammillary region. Such lesions also did not impair performance in a conditioned taste aversion task. These experiments suggest that the mammillary region may be selectively involved in spatial learning and memory.

Effects of conditioned taste aversion on extracellular serotonin in the lateral hypothalamus and hippocampus of freely moving rats

This study used microdialysis to monitor extracellular levels of 5-HT and its metabolite, 5-hydroxyindole acetic acid (5-HIAA) in the lateral hypothalamus (LH) and hippocampus of freely moving rats that had developed a CTA to a 2.5 mM saccharin solution (CS) following its pairing with illness induced by lithium chloride (US). Results showed that oral infusion of the saccharin CS significantly enhanced extracellular LH 5-HT in animals that had developed a taste aversion compared with control groups, including unconditioned (CS-no US) and pseudoconditioned (no CS-US) subjects. As an anatomical control, the hippocampus was identified based on previous research suggesting that it is not integrally involved in CTA learning or retrieval and that 5-HT in this brain site does not directly mediate feeding behavior but is closely correlated with arousal. In contrast with the results obtained in the LH, hippocampal 5-HT was not preferentially elevated in subjects in the CTA group but rather was increased to the same extend in both CTA and control groups after saccharin infusion. Moreover, the increase in LH 5-HT for the CTA group was nearly twice that observed in the hippocampus for any group. Acute administration of LiCl elevated extracellular 5-HT to similar levels in both sites, well above the changes observed following conditioning. 5-HIAA was unaffected in either brain site by oral infusion of saccharin solution or injection of LiCl.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Disruption of neophobia, conditioned odor aversion, and conditioned taste aversion in rats with hippocampal lesions

Previous studies have implicated the hippocampus in the acquisition of conditioned taste aversions. However, the effect of hippocampal (HPC) lesions on the acquisition of conditioned aversions to the distal olfactory cue has not been investigated. In this study rats with bilateral electrolytic hippocampal lesions were given access to an odor conditioned stimulus (CS) alone or a compound odor-taste CS, followed by an injection of LiCl or saline. The results indicated that HPC lesions attenuated the neophobic response to both CSs, and disrupted conditioned odor and taste aversions, relative to sham-operated controls. Furthermore, the disruption in conditioned odor aversions could not be attributed to attenuation of neophobia in lesioned subjects nor to prolonged neophobia in sham-operated controls. The results are consistent with pharmacological studies in suggesting that the hippocampus is involved in the formation of conditioned odor aversions.

Immunohistochemical localization of a melanin concentrating hormone-like peptide in the rat brain

Using antisera generated in rabbits against salmon melanin concentrating hormone (MCH) coupled to human thyroglobulin, the distribution of MCH-like immunoreactivity was mapped throughout the rat central nervous system. The distribution of MCH-like immunoreactivity in rat brain is unique and different from the distribution of other neuropeptides. MCH-like immunoreactive perikarya and fibers are predominant in the posterior hypothalamic area, mostly in the medial forebrain bundle-lateral hypothalamic area subzona incerta and the perifornical area. Cell bodies are located mainly in the medial forebrain bundle and in proximity to well defined hypothalamic nuclei. Fibers are seen throughout the rat brain in all neocortical areas, the neostriatum and the amygdala, in the diencephalon in most hypothalamic nuclei, the habenula, the mamillary body and very dense in the medial forebrain bundle and just ventral to the zona incerta ("subzona incerta"). In the mesencephalon there are fibers in the central gray; in the pons-medulla fibers are contained in the dorsal and ventral parabrachial nuclei; in the tegmental area ventral to the fourth ventricle; in the spinal trigeminal area, the substantia gelatinosa and the reticular nuclei. In the spinal cord there are more fibers in the dorsal than in the ventral horn. The posterior pituitary also contained few MCH-like fibers. It is suggested that a peptide similar, but not identical, to salmon MCH is present in the rat central nervous system.