The regulation of feeding and drinking in rats with lesions of the lateral hypothalamus made by N-methyl-D-aspartate

Time to drink: Activating lateral hypothalamic area neurotensin neurons promotes intake of fluid over food in a time-dependent manner

The lateral hypothalamic area (LHA) is essential for ingestive behavior but has primarily been studied in modulating feeding, with comparatively scant attention on drinking. This is partly because most LHA neurons simultaneously promote feeding and drinking, suggesting that ingestive behaviors track together. A notable exception are LHA neurons expressing neurotensin (LHANts neurons): activating these neurons promotes water intake but modestly restrains feeding. Here we investigated the connectivity of LHANts neurons, their necessity and sufficiency for drinking and feeding, and how timing and resource availability influence their modulation of these behaviors. LHANts neurons project broadly throughout the brain, including to the lateral preoptic area (LPO), a brain region implicated in modulating drinking behavior. LHANts neurons also receive inputs from brain regions implicated in sensing hydration and energy status. While activation of LHANts neurons is not required to maintain homeostatic water or food intake, it selectively promotes drinking during the light cycle, when ingestive drive is low. Activating LHANts neurons during this period also increases willingness to work for water or palatable fluids, regardless of their caloric content. By contrast, LHANts neuronal activation during the dark cycle does not promote drinking, but suppresses feeding during this time. Finally, we demonstrate that the activation of the LHANts → LPO projection is sufficient to mediate drinking behavior, but does not suppress feeding as observed after generally activating all LHANts neurons. Overall, our work suggests how and when LHANts neurons oppositely modulate ingestive behaviors.

Behavioural and neuronal activation after microinjections of AMPA and NMDA into the perifornical lateral hypothalamus in rats

The perifornical lateral hypothalamic area (PeFLH), which houses orexin/hypocretin (OX) neurons, is thought to play an important role in arousal, feeding, and locomotor activity. The present study examined behavioural effects of activating PeFLH neurons with microinjections of ionotropic glutamate receptor agonists. Three separate unilateral microinjections of either (1) AMPA (1 and 2mM in 0.1 μL artificial cerebrospinal fluid, ACSF) and ACSF, or (2) NMDA (1 and 10mM in 0.1 μL ACSF), and ACSF were made into the PeFLH of adult male rats. Following each injection, the rats were placed into an open field for behavioural scoring for 45 min. Rats were perfused after the third injection for immunohistochemistry for c-Fos and OX to assess the level of activation of OX neurons. Behavioural analyses showed that, as compared to ACSF conditions, AMPA injections produced a dose-dependent increase in locomotion and rearing that persisted throughout the 45 min recording period, and an increase in drinking. Injection of NMDA at 10mM, but not 1mM, induced a transient increase in locomotion and an increase in feeding. Histological analyses showed that while both agonists increased the number of neurons immunoreactive for c-Fos in the PeFLH, only AMPA increased the number of neurons immunoreactive for both c-Fos and OX. There were positive correlations between the number of c-Fos/OX-immunoreactive neurons and the amounts of locomotion, rearing, and drinking. These results support the role of ionotropic glutamate receptors on OX and other neurons in the PeFLH in the regulation of locomotor and ingestive behaviours.

The role of hypothalamic ingestive behavior controllers in generating dehydration anorexia: a Fos mapping study

Giving rats 2.5% saline to drink for 3-5 days simply and reliably generates anorexia. Despite having the neurochemical and hormonal markers of negative energy balance, dehydrated anorexic rats show a marked suppression of spontaneous food intake, as well as the feeding that is usually stimulated by overnight starvation or a 2-deoxy-d-glucose (2DG) challenge. These observations are consistent with a dehydration-dependent inhibition of the core circuitry that controls feeding. We hypothesize that this inhibition is directed at those neurons in the paraventricular nucleus and lateral hypothalamic area that constitute the hypothalamic "behavior controller" for feeding rather than their afferent inputs from the arcuate nucleus or hindbrain that convey critical feeding-related sensory information. To test this hypothesis, we mapped and quantified the Fos-immunoreactive response to 2DG in control and dehydrated rats drinking 2.5% saline. Our rationale was that regions showing an attenuated Fos response to 2DG in dehydrated animals would be strong candidates as the targets of dehydration-induced suppression of 2DG feeding. We found that the Fos response to combined dehydration and 2DG was attenuated only in the lateral hypothalamic area, with dehydration alone increasing Fos in the lateral part of the paraventricular nucleus. In the arcuate nucleus and those regions of the hindbrain that provide afferent inputs critical for the feeding response to 2DG, the Fos response to 2DG was unaffected by dehydration. Therefore, dehydration appears to target the lateral hypothalamic area and possibly the lateral part of the paraventricular nucleus to suppress the feeding response to 2DG.

Water intake and the neural correlates of the consciousness of thirst

Thirst and resultant water drinking can arise in response to deficits in both the intracellular and extracellular fluid compartments. Inhibitory influences mediating the satiation of thirst also are necessary to prevent overhydration. The brain regions that underpin the generation or inhibition of thirst in these circumstances can be categorized as sensory, integrative, or cortical effector sites. The anterior cingulate cortex and insula are activated in thirsty human beings as shown by functional brain-imaging techniques. It is postulated that these sites may be cortical effector regions for thirst. A major sensory site for generating thirst is the lamina terminalis in the forebrain. Osmoreceptors within the organum vasculosum of the lamina terminalis and subfornical organ detect systemic hypertonicity. The subfornical organ mediates the dipsogenic actions of circulating angiotensin II and relaxin. Major integrative sites are the nucleus of the tractus solitarius, the lateral parabrachial nucleus, the midbrain raphé nuclei, the median preoptic nucleus, and the septum. Despite these advances, most of the neural pathways and neurochemical mechanisms subserving the genesis of thirst remain to be elucidated.

Aldosterone-sensitive neurons in the nucleus of the solitary tract: Efferent projections

The nucleus of the solitary tract (NTS) contains a subpopulation of neurons that express the enzyme 11-beta-hydroxysteroid dehydrogenase type 2 (HSD2), which makes them uniquely sensitive to aldosterone. These neurons may drive sodium appetite, which is enhanced by aldosterone. Anterograde and retrograde neural tracing techniques were used to reveal the efferent projections of the HSD2 neurons in the rat. First, the anterograde tracer Phaseolus vulgaris leucoagglutinin was used to label axonal projections from the medial NTS. Then, NTS-innervated brain regions were injected with a retrograde tracer, cholera toxin beta subunit, to determine which sites are innervated by the HSD2 neurons. The HSD2 neurons project mainly to the ventrolateral bed nucleus of the stria terminalis (BSTvl), the pre-locus coeruleus (pre-LC), and the inner division of the external lateral parabrachial nucleus (PBel). They also send minor axonal projections to the midbrain ventral tegmental area, lateral and paraventricular hypothalamic nuclei, central nucleus of the amygdala, and periaqueductal gray matter. The HSD2 neurons do not innervate the ventrolateral medulla, a key brainstem autonomic site. Additionally, our tracing experiments confirmed that the BSTvl receives direct axonal projections from the neighboring A2 noradrenergic neurons in the NTS, and from the same pontine sites that receive major inputs from the HSD2 neurons (PBel and pre-LC). The efferent projections of the HSD2 neurons may provide new insights into the brain circuitry responsible for sodium appetite.

Water deprivation increases the expression of neuronal nitric oxide synthase (nNOS) but not orexin-A in the lateral hypothalamic area of the rat

The discovery that the lateral hypothalamic area (LHA) might be important in modulating drinking behavior and fluid balance has led to numerous studies aimed at identifying the key neurotransmitters/neuromodulators and pathways involved. While past studies have demonstrated the presence of neuronal nitric oxide synthase (nNOS) within the LHA, its role in the regulation of fluid homeostasis is not known. In light of this, and the mounting evidence suggesting a role for nitric oxide in osmotic regulation within the hypothalamus, this study sought to determine the effects of 24- and 72-hours of water deprivation on nNOS protein expression within the LHA of the rat with immunohistochemistry. In euhydrated control animals we observed nNOS-like immunoreactivity throughout all levels of the LHA. Following 24 hours of dehydration the number of nNOS-like immunopositive neurons was significantly increased in the rostral but not the caudal regions of LHA. Seventy-two hours of water deprivation lead to further increases in nNOS-like immunoreactivity at different levels of the LHA. Interestingly, however, we observed increased nNOS-like immunoreactivity in the caudal regions of the LHA that was not evident after 24 hours of water deprivation. Double-labeling immunofluorescence histochemistry revealed that the nNOS-like immunoreactive neurons were not colocalized with the orexin-A-containing neurons. These results suggest that an osmotic challenge leads to an upregulation of nNOS immunoreactivity within discrete areas of the LHA. This altered neurochemistry within the LHA further highlights the potential importance of nitric oxide and the LHA in central regulation of fluid homeostasis.

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.

Indomethacin attenuates hyperthermia produced by anterior coronal lateral hypothalamic knife cuts

Electrolytic lesions of the lateral hypothalamus (LH) and coronal knife cuts of fibers anterior to the LH produce an elevation in core body temperature, or hyperthermia. Prostaglandin has been shown to mediate hyperthermia produced by electrolytic LH lesions. The present study characterizes the time course and the role of prostaglandin in mediating knife-cut-induced hyperthermia. Results show that the prostaglandin synthesis inhibitor indomethacin significantly attenuates hyperthermia produced by the knife cuts, suggesting that prostaglandin is involved in mediating this temperature increase. A disruption of axonal fibers that project from the LH to the preoptic area is postulated to be responsible for the temperature increase. There was no effect of knife cuts on food intake and body weight loss, which were also measured, suggesting that this fiber system is not involved in feeding behavior.

State-dependent activity of neurons in the perifornical hypothalamic area during sleep and waking

Neurons containing orexins are located in the perifornical hypothalamic area and are considered to have a role in sleep-wake regulation. To examine how this area is involved in the regulation of sleep and wakefulness, we recorded neuronal activity in undrugged, head-restrained rats across sleep-waking cycles. Recordings were made in the perifornical hypothalamic area where orexin-immunoreactive neurons are distributed (PFH), and in the area dorsal to the PFH, including the zona incerta and subincertal nucleus (collectively referred to as ZI). The 40 neurons recorded from in the PFH were divided into five groups: (1) neurons most active during paradoxical sleep (PS, n=14, 35%), (2) neurons active during both waking (W) and PS (n=12, 30%), (3) neurons most active during W (n=7, 18%), (4) neurons most active during slow-wave sleep (SWS, n=3, 7.5%), and (5) neurons whose activity had no correlation with sleep-waking states (n=4, 10%). Of 30 neurons recorded from in the ZI, the corresponding numbers were 13 (43%), seven (23%), six (20%), three (10%), and one (3.3%). In both areas, neuronal activity fluctuated more during PS than during W. Waking-specific neurons (group 3) in the PFH generated action potentials with longer durations than those produced by other types of neurons. About half of the neurons in the PFH that were classified in groups 1, 2, and 3 increased their firing rate after the transition from one state to another, while higher percentages of neurons of groups 1 and 2 in the ZI than those in the PFH increased their firing rate prior to the state shift from SWS to PS. In these ZI neurons, however, the firing rate varied considerably at the state shift. These results suggest that the PFH and ZI are involved in the regulation of PS or W, especially the regulation of phasic events during PS or the maintenance of W. The ZI appears to be more closely involved than the PFH in the induction of PS or some phasic phenomena associated with PS.

Reverse microdialysis of N-methyl-D-aspartic acid into the lateral hypothalamus of rats: effects on feeding and other behaviors

The effects of reverse microdialysis of N-methyl-D-aspartic acid (NMDA) into the lateral hypothalamus (LH) on feeding and other behaviors were examined. Consistent with similar studies utilizing central microinjections, NMDA reverse microdialysed into the rat LH rapidly elicited a strong concentration-dependent stimulation of feeding. The minimum perfusate concentration of NMDA needed to elicit feeding with reverse microdialysis was 660 microM, a concentration 1/50 of that needed with pressure injections. Further, eating responses could be consistently elicited in sequential tests separated by 2-4 h in the same subject, and the magnitude of the eating in the first and second tests was highly correlated (r=0.87). Behavioral analysis revealed that the main response to NMDA consisted of eating without the concomitant hyperactivity produced by central microinjections of this agonist. The other behaviors exhibited during NMDA administration were those that normally occur during spontaneous feeding in rats. Also, rats precisely compensated for the increased food intake elicited by NMDA by reducing spontaneous feeding during the subsequent nocturnal phase, so as to maintain normal daily intakes. In contrast, N-methyl-L-aspartate (NMLA) reverse microdialysed in to the LH (660 microM) did not elicit feeding nor affect any other behaviors we examined. These data support a role for LH glutamate and NMDA receptors in the control of feeding.

Expression of mu, kappa, and delta opioid receptor messenger RNA in the human CNS: a 33P in situ hybridization study

The existence of at least three opioid receptor types, referred to as mu, kappa, and delta, is well established. Complementary DNAs corresponding to the pharmacologically defined mu, kappa, and delta opioid receptors have been isolated in various species including man. The expression patterns of opioid receptor transcripts in human brain has not been established with a cellular resolution, in part because of the low apparent abundance of opioid receptor messenger RNAs in human brain. To visualize opioid receptor messenger RNAs we developed a sensitive in situ hybridization histochemistry method using 33P-labelled RNA probes. In the present study we report the regional and cellular expression of mu, kappa, and delta opioid receptor messenger RNAs in selected areas of the human brain. Hybridization of the different opioid receptor probes resulted in distinct labelling patterns. For the mu and kappa opioid receptor probes, the most intense regional signals were observed in striatum, thalamus, hypothalamus, cerebral cortex, cerebellum and certain brainstem areas as well as the spinal cord. The most intense signals for the delta opioid receptor probe were found in cerebral cortex. Expression of opioid receptor transcripts was restricted to subpopulations of neurons within most regions studied demonstrating differences in the cellular expression patterns of mu, kappa, and delta opioid receptor messenger RNAs in numerous brain regions. The messenger RNA distribution patterns for each opioid receptor corresponded in general to the distribution of opioid receptor binding sites as visualized by receptor autoradiography. However, some mismatches, for instance between mu opioid receptor receptor binding and mu opioid receptor messenger RNA expression in the anterior striatum, were observed. A comparison of the distribution patterns of opioid receptor messenger RNAs in the human brain and that reported for the rat suggests a homologous expression pattern in many regions. However, in the human brain, kappa opioid receptor messenger RNA expression was more widely distributed than in rodents. The differential and region specific expression of opioid receptors may help to identify targets for receptor specific compounds in neuronal circuits involved in a variety of physiological functions including pain perception, neuroendocrine regulation, motor control and reward.

Mediation of dehydration-induced peptidergic gene expression in the rat lateral hypothalamic area by forebrain afferent projections

We have previously shown in dehydrated rats that cellular levels of the mRNAs encoding the precursor peptides for corticotropin-releasing hormone and neurotensin/neuromedin N significantly increase in a restricted region of the lateral hypothalamic area (Watts, 1992, Brain Res. 581:208-216). The experiments reported here address the role that forebrain osmosensitive cells groups or regions associated with autonomic regulation play in developing this mRNA response. The first experiment showed that unilateral knife cuts placed between the rostral forebrain and the lateral hypothalamic area (LHA) will unilaterally attenuate the mRNA response in the LHA to dehydration. In a second experiment, small injections of the retrograde tracer Fluorogold into the region of the LHA containing these mRNAs revealed a direct input from the osmosensitive median preoptic nucleus and subfornical organ and from the fusiform nucleus of the bed nuclei of the stria terminalis, which is part of a complex of cell groups associated with autonomic regulation. We found that at least 30% of the neurons in the median preoptic nucleus and subfornical organ and 14% of the neurons in the fusiform nucleus of the bed nuclei of the stria terminalis that project to the LHA responded to a rapid increase in plasma osmolality with increased c-fos mRNA levels. In the final experiment, injections of Fluorogold into the LHA were made simultaneously with ipsilateral rostral knife cuts. Here the numbers of neurons accumulating Fluorogold in the median preoptic nucleus, subfornical organ, and the fusiform nucleus were all significantly decreased concomitantly with attenuated mRNA responses in the LHA to dehydration. We conclude that the LHA receives direct and functional projections from the median preoptic nucleus, subfornical organ, and the fusiform nucleus. These projections appear capable of mediating a substantial part of the response of peptidergic mRNAs in the LHA to dehydration.

Alteration of dynorphin and secretogranin II in the prolactin immunoreactive neurons of the rat lateral hypothalamus upon osmotic stimulation

The prolactin immunoreactive neurons of the rat lateral hypothalamus were previously reported to express the dynorphin and secretogranin II genes. In the present study, the response of these neurons to osmotic challenge was immunocytologically investigated by using prolactin, dynorphin, secretogranin II and c-Fos antisera. In addition, the mRNA levels for secretogranin II and dynorphin were compared by in situ hybridization in controls and salt-loaded rats. For this model of chronic hyperosmolality, the prolactin and c-Fos immunoreactivities were not stimulated by salt drinking, but dynorphin and secretogranin II immunoreactivities as well as mRNA levels for dynorphin and secretogranin II significantly increased in the lateral hypothalamus. We suggest that the prolactin-immunoreactive neurons may be involved in the regulation of water homeostasis.

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.

The pedunculopontine tegmental nucleus: a role in cognitive processes?

The cholinergic pedunculopontine tegmental nucleus, located in the brainstem and part of the reticular formation, has been traditionally linked to motor function, arousal and sleep. Its anatomical connections, however, raise the possibility that the pedunculopontine tegmental nucleus is also involved in other aspects of behaviour such as motivation, attention and mnemonic processes. This is of obvious importance, since the pedunculopontine tegmental nucleus undergoes degeneration in human neurodegenerative disorders also characterized by attentional and/or mnemonic deficits. Moreover, recent behavioural animal work suggests that cognitive processes may be represented in the pedunculopontine tegmental nucleus. The difficulty that faces research in this area, however is the possible influence of cognition by other processes, such as arousal state, motivation and motor function. Nevertheless, by reviewing the literature, the pedunculopontine tegmental nucleus seems to be involved in attentional and possibly also in learning processes. These processes could be mediated by influencing cortical function via the thalamus, basal forebrain and basal ganglia. The involvement of the pedunculopontine tegmental nucleus in mechanisms of memory, however, seems to be rather unlikely.

Efferent connections from the lateral hypothalamic region and the lateral preoptic area to the hypothalamic paraventricular nucleus of the rat

The lateral preoptic and lateral hypothalamic regions contain the majority of the cell groups embedded in the fibre trajectories of the medial forebrain bundle on its course through the hypothalamus. Recent studies have extended considerably the parcellation of the lateral hypothalamic region, and therefore, the need to emphasize new insights into the anatomical organisation of projections from the neurons of the lateral hypothalamic region. In the present study we describe the anatomical organisation of efferent projections from the lateral preoptic and lateral hypothalamic regions to the hypothalamic paraventricular nucleus (PVN) on the basis of retrograde- and anterograde-tracing techniques. Iontophoretic injections of the retrograde tracer, cholera toxin subunit B, into the PVN revealed that most hypothalamic nuclei project to the PVN. Within the lateral hypothalamic region, retrogradely labelled cells were concentrated in the intermediate hypothalamic area, the lateral hypothalamic area, and the perifornical nucleus, whereas fewer retrogradely labelled cells were found in the lateral preoptic area. To determine the distribution of terminating fibres in subnuclei of the heterogeneous PVN, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were delivered into distinct areas of the lateral hypothalamic region. Neurons of the intermediate hypothalamic area projected mainly to the PVN subnuclei, which contained parvicellular neuroendocrine cells. In contrast, neurons of the rostral and tuberal parts of the lateral hypothalamic area and the perifornical nucleus projected to the PVN subnuclei, which contained parvicellular neurons that send descending projections to preganglionic cell groups in the medulla and spinal cord. The perifornical nucleus was the only area within the lateral hypothalamic region that consistently innervated magnocellular perikarya of the PVN. Finally, all areas of the lateral hypothalamic region contributed substantially to fibres terminating in the perinuclear shell of the PVN. These results demonstrate that anatomically distinct areas of the lateral hypothalamic region have distinct projections to subnuclei of the PVN and further substantiate the view that the lateral hypothalamic region as well as the PVN constitute anatomically and functionally heterogeneous structures.

CNS cell groups projecting to pancreatic parasympathetic preganglionic neurons

The CNS cell groups that project to the pancreatic parasympathetic preganglionic neurons were identified by the viral retrograde transneuronal labeling method. Pseudorabies virus (PRV) was injected into the pancreas of C8 spinal rats and after 6 days survival, the animals were perfused and their brains processed for immunohistochemical detection of PRV. Parasympathetic preganglionic neurons of the dorsal vagal nucleus were retrogradely labeled with PRV. Several CNS cell groups consistently contained transneuronally labeled neurons. In the medulla oblongata, labeled neurons were found in the nucleus tractus solitarius, area postrema, paratrigeminal nucleus, lateral paragigantocellular reticular nucleus, raphe pallidus and obscurus nuclei, C3 region and scattered cells in the ventral medullary reticular formation. In the pons, the A5 cell group, Barrington's nucleus and the subcoeruleus region contained labeled neurons. Only an occasional labeled cell was identified in the parabrachial nucleus. In the midbrain, almost no labeling was found except for an occasional neuron in the central gray matter. In the diencephalon, labeling was found in the paraventricular hypothalamic nucleus (PVN) as well as in the lateral hypothalamic nucleus at two levels (one at the level of the PVN and the other at the level of the subthalamic nucleus). In addition, the perifornical and dorsal hypothalamic nuclei contained labeled neurons. A few cells were found in the peripheral part of the dorsomedial hypothalamic nucleus. No labeling was seen in the ventromedial hypothalamic nucleus. In the telencephalon, the central amygdaloid nucleus and the bed nucleus of the stria terminalis were labeled.

Differential regulation of melanin-concentrating hormone gene expression in distinct hypothalamic areas under osmotic stimulation in rat

Melanin-concentrating hormone and associated peptides represent a novel peptide neuronal system that may be involved in the control of water homeostasis in mammals. We have examined the effect of 24 h dehydration or salt-loading over a period of six days, on melanin-concentrating hormone messenger RNA levels in rat brains by using complementary methods of Northern blotting and in situ hybridization histochemistry. In response to one to six day salt-loading regimen, hypothalamic melanin-concentrating hormone messenger RNA content in male or female rats decreased by two to three-fold. Levels of melanin-concentrating hormone messenger RNA in the hypothalamus were also dramatically decreased following dehydration in female rats whereas contrasting responses were noted in male rats. In addition, no significant variation in the low levels of melanin-concentrating hormone gene transcripts in medulla pons and cortex was found after osmotic stimulus. In agreement with Northern blot data, in situ hybridization studies revealed that the majority of the melanin-concentrating hormone-expressing neurons in the anterior part of the lateral hypothalamus of dehydrated or salt-loaded rats expressed lower amounts of melanin-concentrating hormone messenger RNAs than those found in control rats. Interestingly, less variation was found in the posterior part of the lateral hypothalamus. Furthermore few clusters of cells, located in zona incerta and near the internal capsula and fornix, increased their contents in melanin-concentrating hormone messenger RNA in salt-loaded but not in dehydrated rats suggesting that melanin-concentrating hormone gene expression may be regulated differently by various osmotic stimuli. Finally, diurnal variations in melanin-concentrating hormone messenger RNA contents were observed in normal and dehydrated rats with highest levels around 22.00 h and lowest levels during daylight hours. However, the up-regulation of melanin-concentrating hormone gene activity at night was found lower in dehydrated rats than in control animals suggesting that osmotic stress may interfere with the generation of the diurnal pattern of melanin-concentrating hormone messenger RNA expression. Altogether, our results indicate that osmotic stimulations lead to a selective and conspicuous inhibition of melanin-concentrating hormone gene activity in the whole hypothalamus of rat. We suggest that the melanin-concentrating hormone neuronal system plays an important role in integration processes relative to nocturnal regulation of water homeostasis and drinking behavior.

Preprotachykinin A gene expression in distinct hypothalamic and brain stem regions of the rat is affected by a chronic osmotic stimulus: a combined immunohistochemical and in situ hybridization histochemistry study

Chronic osmotic stimulation influences the hypothalamoadenohypophysial axis by inhibiting the synthesis of corticotrophin releasing factor (CRF-41) in the parvocellular subdivision of the paraventricular nucleus (PVN) and, subsequently, the secretion of adrenocorticotrophin (ACTH) from the adenohypophysis. Using quantitative in situ hybridization histochemistry, we have investigated the effect of chronic osmotic stimulation on preprotachykinin A (PPT-A) mRNA levels in a number of brain areas known to send substance P-containing projections to the medial parvocellular part of the PVN. Chronic osmotic stimulation increased PPT-A gene expression in the lateral hypothalamic area, the arcuate nucleus, the catecholaminergic brain stem areas A2, C1, and C2, although PPT-A mRNA levels in the bed nucleus of the stria terminalis, the medial preoptic nucleus, the caudate-putamen, and the A1 were unaffected by chronic osmotic stimulation. In addition, immunohistochemical staining of substance P-immunoreactive elements contained within the same areas was carried out on colchicine-treated animals. Generally, those areas responding to the osmotic stimulus with increased PPT-A mRNA synthesis showed increased numbers of substance P-immunoreactive perikarya, suggesting that increased levels of mRNA are associated with increased peptide synthesis. These results provide evidence that central endogenous substance P contained in brain regions projecting to the paraventricular nucleus could have an inhibitory influence over the synthesis of CRF-41 during a chronic osmotic stimulus.

NMDA lesions of lateral hypothalamus enhance the acquisition of schedule-induced polydipsia

Schedule-induced polydipsia (SIP) is affected by damage to various limbic structures that have connections with the lateral hypothalamus. The present experiment sought to determine whether or not SIP could be induced in rats bearing NMDA-induced lesions of the lateral hypothalamus. Following surgery, lesioned rats lost weight and were hypophagic and hypodipsic. Drinking, in response to systemic injection of hypertonic saline, was impaired in lesioned rats. Prior to testing for SIP, all rats were placed on a food-restriction regime to maintain body weight at 85% of normal. There was no statistically significant difference in mean body weight between lesioned and control groups before deprivation began, though lesioned rats were hypodipsic in their home cages. The lateral hypothalamic-lesioned rats acquired SIP significantly more rapidly than controls over the first six sessions, but over four following sessions no differences were present. The enhanced acquisition of SIP by lateral hypothalamic-lesioned rats cannot be accounted for by postoperative recovery of body weight or by hypodipsia in the home cage, neither of which correlated with SIP. It is suggested that the lateral hypothalamus has a role in cueing appropriate and inhibiting inappropriate behavior in conditions of motivational excitement. SIP is suggested to have two CNS components--one excitatory and one inhibitory.

N-methyl-D-aspartate lesions of the lateral hypothalamus do not reduce amphetamine or fenfluramine anorexia but enhance the acquisition of eating in response to tail pinch in the rat

These experiments examine the acquisition of tail pinch-induced eating and responses to the anorectic agents d-amphetamine and d,l-fenfluramine by rats bearing N-methyl-D-aspartate (NMDA) lesions of the lateral hypothalamus. Lesioned rats lost weight following surgery but had no significant eating or drinking difficulties in the home cage (Clark et al. 1990). The acquisition of eating in response to tail pinch was enhanced in lateral hypothalamic-lesioned rats: they ate on earlier test sessions than controls and less pressure was required to elicit eating. Home cage food intake over the period when tail pinch was being examined was not affected by the lateral hypothalamic lesions. There were no significant differences between lateral hypothalamic-lesioned and control rats in terms of their anorectic responses to either d-amphetamine or d,l-fenfluramine, though the lesioned rats had a lower baseline intake. These data suggest that the lateral hypothalamus is not an important site for the mediation of amphetamine or fenfluramine anorexia but is involved in the acquisition of tail pinch-induced eating. The disinhibition of responding to tail pinch by lateral hypothalamic lesions is discussed in terms of the possible role the lateral hypothalamus plays in regulating cortical activity. The role of the medial hypothalamus and non-hypothalamic systems in the response to anorectic drugs and tail pinch is discussed.

Neuroendocrine and behavioural responses to hyperosmolality in rats with lesions of the lateral hypothalamus made by N-methyl-D-aspartate

Excitotoxic lesions of rat lateral hypothalamus produce impairments in eating and drinking, but not motor deficits. However, it has not been established what causes these eating and drinking impairments. In the present experiments, drinking, plasma osmolality and arginine-vasopressin concentration were measured in lateral hypothalamic-lesioned and control rats following systemic injection of hypertonic saline. In response to hyperosmolality, N-methyl-D-aspartate-lesioned rats drank significantly less than controls but showed normal increases in plasma osmolality and arginine-vasopressin concentration. This dissociation of neuroendocrine and behavioural responses suggests that the impairment of rats with excitotoxic lesions of the lateral hypothalamus is unrelated to physiological (as opposed to behavioural) mechanisms of homeostasis.