Effects of ventral tegmental area stimulation and microiontophoretic application of dopamine and norepinephrine on hypothalamic neurons

Clinical review: Regulation of food intake, energy balance, and body fat mass: implications for the pathogenesis and treatment of obesity

CONTEXT

Obesity has emerged as one of the leading medical challenges of the 21st century. The resistance of this disorder to effective, long-term treatment can be traced to the fact that body fat stores are subject to homeostatic regulation in obese individuals, just as in lean individuals. Because the growing obesity epidemic is linked to a substantial increase in daily energy intake, a key priority is to delineate how mechanisms governing food intake and body fat content are altered in an obesogenic environment.

EVIDENCE ACQUISITION

We considered all relevant published research and cited references that represented the highest quality evidence available. Where space permitted, primary references were cited.

EVIDENCE SYNTHESIS

The increase of energy intake that has fueled the U.S. obesity epidemic is linked to greater availability of highly rewarding/palatable and energy-dense food. Obesity occurs in genetically susceptible individuals and involves the biological defense of an elevated body fat mass, which may result in part from interactions between brain reward and homeostatic circuits. Inflammatory signaling, accumulation of lipid metabolites, or other mechanisms that impair hypothalamic neurons may also contribute to the development of obesity and offer a plausible mechanism to explain the biological defense of elevated body fat mass.

CONCLUSIONS

Despite steady research progress, mechanisms underlying the resistance to fat loss once obesity is established remain incompletely understood. Breakthroughs in this area may be required for the development of effective new obesity prevention and treatment strategies.

Dopamine D1 and D2 Receptor Antagonism in the Preoptic Area Produces Different Effects on Maternal Behavior in Lactating Rats

The preoptic area (POA) is critical for maternal behavior in rats but little is known about what neurotransmitters released here influence maternal responding. POA infusion of 10 microg (but not 2 microg) of the dopamine D1 receptor antagonist SCH-23390 greatly impaired retrieval and licking of pups but not other maternal or nonmaternal behaviors in lactating rats. In contrast, POA infusion of 10 microg (but not 2 microg) of the D2 receptor antagonist raclopride facilitated nursing but did not affect oral maternal behaviors. SCH-23390 in the medial hypothalamus tended to impair licking but not retrieval. Raclopride in the medial hypothalamus had no effects. Therefore, D1 and D2 receptor activity, particularly in the POA, is important for regulating different maternal behaviors.

Altered accumbens neural response to prediction of reward associated with place in dopamine D2 receptor knockout mice

Midbrain dopaminergic activity seems to be important in forming the prediction of future events such as rewards. The nucleus accumbens (NAc) plays an important role in the integration of reward with motor function, and it receives dense dopamine innervation and extensive limbic and cortical afferents. Here, we examined the specific role of the dopamine D2 receptor (D2R) in mediating associative learning, locomotor activity, and regulating NAc neural responses by using D2R-knockout (KO) mice and their wild-type littermates. D2R-KO mice displayed reduced locomotor activity and slower acquisition of a place-learning task. D2R-KO eliminated the prereward inhibitory response of neurons in the NAc. In contrast, an increased number of neurons in D2R-KO mice displayed place-related activity. These results provide evidence that D2R in the NAc participates in coding for a specific type of neural response to incentive contingencies and partly in spatial learning.

Neuronal activity in female rat preoptic area associated with sexually motivated behavior

Single unit activities were recorded from 31 neurons in the preoptic area (POA) of female rats engaging in sexual interactions. Concurrent videotape recordings were used to establish a relationship between neuronal activity and particular behavioral events. In 14 of the 31 neurons, the firing rate changed in association with bouts of sexual activity. The remaining 17 fired with more variability regardless of episodes of sexual interactions. Peri-event histograms identified four types of neurons: type 1 (n=4) increased their firing rate when the female rats initiated proceptive behavior; type 2 (n=4) showed a brief activation when the male mounted; type 3 (n=4) fired in response to intromission, and type 4 (n=2) were inhibited prior to and throughout the display of lordosis reflex. Type 1 neurons fired at significantly higher rates during the solicitatory period, from the initiation of solicitatory locomotion to the male mounts. Their activity was suppressed when the males mounted successfully with intromission. Types 1-3 neurons were recorded from the transitional region between the medial and lateral POAs. Type 4 neurons were located more medially in the medial POA. Systemic injection of pimozide, a dopamine receptor blocker, diminished firing in type 1 neurons and abolished proceptivity. The firing pattern in type 1 neurons appeared to embody the motivational state of the animal with an implication for a consummatory value of penile intromission. Visceral or somatosensory inputs may be responsible for short bursts in types 2 and 3 neurons. Type 4 neurons behaved exactly as if they inhibit the execution of the lordosis reflex. The results showed separate sets of POA neurons each specifically associated with proceptive and receptive components of female rat sexual behavior.

Gastric distension modulates hypothalamic neurons via a sympathetic afferent path through the mesencephalic periaqueductal gray

The effects of gastric distension on extracellularly recorded single neuronal activity in the lateral hypothalamuslateral preoptic area-medial forebrain bundle (LPA-LH-MFB), other areas of the hypothalamus, mesencephalic periaqueductal gray (PAG), and other areas associated with the mesencephalic reticular formation were determined in the anesthetized rat. Gastric distension was produced by filling a gastric balloon with water using a calibrated infusion pump. Experimental conditions were based on previous studies that simulated gastric distension during fluid consumption in the rat. The effects of stomach distension using water at body temperature and room temperature were compared. Neurons in both the hypothalamus and mesencephalon were modulated by gastric distension. Hypothalamic neurons exhibited responses associated with gastric distension and exhibited interactions between distension and temperature stimulation of the stomach. Neurons in the mesencephalic periaqueductal gray (PAG) and associated reticular formation also were modulated by these gastric stimuli. When the PAG was electrically stimulated, similar responses to gastric distension and PAG stimulation were observed for hypothalamic neurons. The effects of gastric distension on hypothalamic neurons were reduced or eliminated when the PAG stimulating electrode site was destroyed by electrocoagulation. In addition, the microiontophoretic application of horseradish peroxidase at hypothalamic neuronal recording sites where gastric distension effects were observed resulted in the retrograde labeling of neurons in the PAG. These gastric stimulation-induced effects on hypothalamic and mesencephalic neuronal activity were attenuated but were not permanently eliminated by bilateral cervical vagotomy. However, these effects were significantly reduced or eliminated by bilateral transection of the cervical sympathetic chain or spinal transection at the first cervical level. Because the filling of balloons placed into the abdominal cavity close to the stomach had no similar effects on neural activity, these results can be attributed primarily to the activation of gastric mechano-and temperature-sensitive receptors. These results indicate that the effects of gastric temperature/distension stimulation under these conditions are mediated to a large degree by sympathetic afferents. The PAG is clearly involved as one of the mesencephalic relays for gastric afferent input to the hypothalamus.

Reticular thalamic inhibitory input to lateral hypothalamic neurons: a functional and histochemical determination

Both the lateral hypothalamus (LH) and reticular thalamus (RT) have been implicated in the integration of a variety of vital functions. To determine relevant connections more specifically between cells of these two regions in the rat, functional neurophysiological and horseradish peroxidase (HRP) histochemical techniques were utilized. Single pulse electrical stimulation of the RT modified the discharge frequency of the majority of LH neurons tested. LH neuronal responses to RT stimulation included; inhibition (66.32%), excitation followed by inhibition (6.12%), and excitation (7.14%). Because RT stimulation resulted primarily in the inhibition of LH neuronal activity with a relative short mean latency of 1.87 +/- 0.23 ms, the existence of a possible direct inhibitory pathway from the RT to LH also was evaluated using HRP histochemistry. Retrogradely HPR labeled soma were identified in the RT after HRP was ejected extracellularly onto LH neurons, which exhibited a decrease in spontaneous activity in response to RT stimulation. These data demonstrate direct projections from the RT to the LH. In addition, HRP labeling of neurons and axons in the zona incerta-LH area following LH HRP ejections suggest this route as a synaptic pathway from RT to LH.

GABA inhibition of lateral hypothalamic neurons: role of reticular thalamic afferents

GABA and reticular thalamic (RT) stimulation induced inhibition of lateral hypothalamic (LH) neuronal activity was studied to determine if RT inhibitory input to the LH is mediated by stimulation of GABA receptors. Seven barrel electrodes were utilized to record simultaneously from the LH during microiontophoretic application of GABA, glycine, bicuculline, picrotoxin, and electrical stimulation of the RT. GABA produced an ejection current-related decrease in LH neuronal activity that was antagonized, in a dose-related manner, by the simultaneous administration of picrotoxin or bicuculline. LH neurons were much less sensitive to glycine inhibition which also was relatively insensitive to the GABA receptor antagonists. RT electrical stimulation provided a short latency inhibitory input to LH neurons that was mimicked by the microiontophoretic administration of GABA. In addition, the microiontophoretic application of picrotoxin or bicuculline blocked the RT stimulation induced inhibition of LH neuronal activity. These data indicate that the inhibitory effects of both GABA and RT synaptic inputs onto LH neurons are mediated by stimulation of GABAA receptors and substantiates the involvement of GABA as the inhibitory transmitter of thalamic to hypothalamic projections.

Complex attributes of lateral hypothalamic neurons in the regulation of feeding of alert rhesus monkeys

To elucidate the roles of glucose-sensitive (GS) and glucose-insensitive (GIS) cells of the lateral hypothalamic area (LHA), single neuron activity was recorded during 1) microelectrophoretic administration of chemicals, 2) a conditioned bar press feeding task, 3) gustatory, 4) olfactory, and 5) electrical brain stimulation. GS and GIS neurons showed different firing rate changes during phases of the task, and the responses were highly influenced by the palatability of the food and the motivational (hunger or satiety) state of the animal. The two groups of cells also differed in their responsiveness to gustatory and olfactory stimuli: GS neurons were more likely to respond to tastes and odors than GIS cells. Taste- and odor-responsive GS neurons were primarily suppressed by electrophoretically applied noradrenaline and were localized ventromedially within the LHA. The chemosensitive GIS cells, being organized along a dorsolateral axis, were especially excited by dopamine. The two sets of neurons had distinct connections with associative (orbitofrontal, prefrontal) cortical areas. GS and GIS cells, thus, appear to have differential and complex attributes in the control of feeding.

Prenatal development of mesencephalic and diencephalic dopaminergic systems in the male and female rat

Previous results had suggested mesencephalic dopaminergic neurons in cultures of gestational day (GD) 14 rat embryonic brains to be characterized by an early maturation and acquisition of sex-related differences in transmitter uptake. Therefore development of dopaminergic systems was reexamined in the rat in vivo with special emphasis on the prenatal period, mesencephalo-hypothalamic relationships, and possible sex differences. Perfusion-fixed brains of GD 14, 17, 20, 21 and newborn rats were sectioned or processed as whole-mounts and immunostained for tyrosine hydroxylase (TH). Total numbers of mesencephalic TH-immunoreactive cell bodies as assessed by a stereological method rose between GD 14 and 17 and fell again between GD 17 and 21. As early as GD 14, a prominent mesencephalo-hypothalamic projection was observed coming off the medial forebrain bundle and terminating in the retrochiasmatic region. Two additional TH-immunoreactive fiber bundles leaving the medial forebrain bundle, one rostral and one caudal to the former, and terminating in the paraventricular and premammillary region, respectively, were noticed on GD 17. Careful examination of developing TH-immunoreactive neurons in the lower brainstem ascertained that there was no interference from ascending catecholaminergic fibers other than dopaminergic of mesencephalic origin during this early prenatal period. All 3 mesencephalo-hypothalamic projections had largely disappeared by GD 20 and were no longer detectable as distinct fiber bundles thereafter. There were subtle sex differences in numbers and distribution of both mesencephalic and diencephalic TH-immunoreactive neurons present at GD 17, which thus occurred prior to manifestation of other well-known sexual dimorphisms of the rat brain.(ABSTRACT TRUNCATED AT 250 WORDS)

Olfactory coding in the monkey lateral hypothalamus: behavioral and neurochemical properties of odor-responding neurons

The activity of glucose-sensitive (GS) and glucose-insensitive (GIS) neurons was recorded in the lateral hypothalamic area (LHA) of monkeys during olfactory stimulation, a conditioned alimentary bar press task, and microelectrophoretic application of catecholamines. Olfactory stimuli evoked response of 88% of the GS neurons, and 52% of the GIS cells responded to odors. The GS neurons were more broadly tuned across odorants than the GIS cells, and their responses to various smells with distinct hedonic value were also differential. The odor-responding GS neurons were depressed during the bar press and reward periods of the task, and were mainly inhibited by dopamine. The odor-responding GIS cell activity increased in response to cue light and tone, and was facilitated by dopamine. Histological examinations disclosed topographic dissociation of the odor-responding GS and GIS cells: the former were located in more ventromedial regions than the latter. The results indicate that the GS neurons integrate multiple chemosensory inputs from both endogenous and exogenous sources in the regulation of feeding: whereas the GIS cells distinguish among fewer, more specific cues to control food acquisition behavior.

Ventral tegmental (A10) system: neurobiology. 1. Anatomy and connectivity

The VTA contains the A10 group of DA containing neurons. These neurons have been grouped into nuclei to be found on the floor of the midbrain tegmentum--Npn, Nif, Npbp and Nln rostralis and caudalis. The VTA is traversed by many blood vessels and nerve fibers. Close to its poorly defined borders are found DA (A8, A9, A11) and 5-HT containing neurons (B8). Efferent projections of the VTA can be divided into 5 subsystems. The mesorhombencephalic projects to other monoaminergic nuclei, the cerebellum and a fine projection descends to other tegmental nuclei as far as the inferior olive. Fibers to the spinal cord have not been demonstrated. The mesodiencephalic path projects to several thalamic and hypothalamic nuclei and possibly the median eminence. Functionally important examples are the anterior hypothalamic-preoptic area, N. medialis dorsalis and reuniens thalami. These two subsystems are largely non-dopaminergic. A minor mesostriatal projection is overshadowed by the large mesolimbic projection to the accumbens, tuberculum olfactorium, septum lateralis and n. interstitialis stria terminalis. There are also mesolimbic connections with several amygdaloid nuclei (especially centralis and basolateralis), the olfactory nuclei and entorhinal cortex. A minor projection to the hippocampus has been detected. The mesocortical pathway projects to sensory (e.g. visual), motor, limbic (e.g. retrosplenial) and polysensory association cortices (e.g. prefrontal). Prefrontal, orbitofrontal (insular) and cingulate cortices receive the most marked innervation from the VTA. A more widespread presence of DA in other cortices of rodents becomes progressively more evident in carnivores and primates. Most but not all projections are unilateral. Some neurons project to more than one area in mesodiencephalic, limbic and cortical systems. The majority of these fibers ascend in the MFB. Most areas receiving a projection from the VTA (DA or non-DA) project back to the VTA. The septohippocampal complex in particular and the limbic system in general provide quantitatively much less feedback than other areas. The role of the VTA as a mediator of dialogue with the frontostriatal and limbic/extrapyramidal system is discussed under the theme of circuit systems. The large convergence of afferents to certain VTA projection areas (prefrontal, entorhinal cortices, lateral septum, central amygdala, habenula and accumbens) is discussed under the theme of convergence systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of neurotransmitters and vagus nerve stimulation on diencephalic and mesencephalic neuronal activity

The effects of microiontophoretically applied neurotransmitters and cervical vagus nerve stimulation on neuronal discharge frequency was determined for cells located in the lateral hypothalamus-lateral preoptic area (LH-LPA), medial hypothalamus, thalamus-zona incerta area, and midbrain of anesthetized rats. Seven barrel electrodes were utilized to record simultaneously from and apply gamma aminobutyric acid (GABA), dopamine, glutamate, acetylcholine and norepinephrine to single neurons. Microiontophoretically applied GABA resulted in similar decreases in neuronal discharge frequency in all central areas tested. Thalamus-zona incerta cells were the most sensitive to GABA and required the lowest ejection current for threshold responses. Dopamine application resulted in increases and decreases in hypothalamic and mesencephalic neuronal discharge frequency. Thalamus-zona incerta neurons were the most sensitive to dopamine and exhibited only decreased activity during its application. Glutamate application resulted in a non-specific excitation of neurons. Acetylcholine induced increases and decreases in neuronal discharge frequency. Thalamus-zona incerta cells were the most sensitive to acetylcholine application. Hypothalamic neurons were the most sensitive to norepinephrine induced decreases in neuronal activity and effects in this area persisted for a prolonged period after norepinephrine was applied. Neuronal discharge frequency was significantly increased during vagus nerve stimulation in all central areas studied. Only hypothalamic neurons also exhibited decreased activity during vagus nerve stimulation. Results are discussed in terms of previous neurochemical and neurophysiological data and in terms of the importance of vagal afferents in the control of central neuronal activity.

Limbic connections to the lateral preoptic area: a horseradish peroxidase study in the rat

Horseradish peroxidase, 13% Sigma Type VI, was administered iontophoretically to the lateral preoptic area (LPA) of male hooded rats. Animals were perfused intracardially on the following day and brains were removed and sliced in the coronal plane into 50 microns sections. Alternate sections were processed with DAB and BDH for the brown and blue reaction products and later examined by bright and dark field microscopy for the presence and location of retrogradely labeled neurons. Results indicate that there are a significant number of limbic efferent connections to the LPA. Afferents to the LPA originate in the prefrontal corex, nucleus accumbens, diagonal band and olfactory structures, lateral and medial septum, stria hypothalamic tract and stria terminalis, the magnocellular and medial preoptic nuclei, along the extent of the medial forebrain bundle in the LPA and LH, anterior and basolateral amygdala, ventromedial caudate-putamen, stria medullaris and lateral habenula, the stellatocellular-periventricular, ventromedial, arcuate and anterior hypothalamic nuclei, the perifornical area, zona incerta, ventral medial thalamic area, ventral tegmental area of Tsai, interpeduncular nucleus, reticular zone of the substantia nigra, mesencephalic periaqueductal gray and reticular formation, all aspects of the raphe nuclei and the locus coeruleus. Results are discussed in terms of known anatomical and neurophysiological data and the similar limbic inputs observed for lateral hypothalamic neurons which are found along the extent of the medial forebrain bundle.

Mesencephalic reticular formation stimulation effects on hypothalamic neuronal activity

The effects of mesencephalic reticular formation (RF) single pulse, 0.5 msec and 0-500 microA, stimulation on lateral preoptic-lateral hypothalamic (LPA-LH) neuronal activity were determined in anesthetized rats. In addition, the effects of LH stimulation on neural activity in the RF and periaqueductal gray (PAG) were evaluated. Recordings from 117 neurons indicate reciprocal connections between the LPA-LH and the mesencephalon. Stimulation of the RF affected 70% of the LPA-LH neurons tested. Short latency decreases in activity predominated indicating an inhibitory synaptic input from the RF to the LPA-LH. Short latency increases in discharge frequency were observed infrequently. Stimulation of the LH affected only 32% of the mesencephalic neurons tested. Short latency decreases in activity were usually observed indicating reciprocal inhibitory synaptic connections between the LPA-LH and the RF and periaqueductal gray. Antidromic responses verified these interconnections and revealed relatively slow conduction velocities of approximately 1.0 m/sec. Results are discussed in terms of the involvement of the LPA-LH and RF in sensorimotor functions, spinal motor excitability, and ingestive behavior.

Effects of periaqueductal gray stimulation of diencephalic neural activity

The effects of ipsilateral mesencephalic periaqueductal gray (PAG) stimulation on lateral hypothalamic (LH), lateral preoptic area (LPA), and ventral and subthalamic activity were determined in anesthetized rats. Recordings from 119 diencephalic neurons indicate that the PAG provides a predominantly inhibitory input to diencephalic neurons. Excitatory input occurred infrequently in the hypothalamus and was not observed in the thalamus. Following single rectangular pulse stimulation, 0.5 msec, 0-500 micro A, short latency decreases in activity occurred. Longer latency increases in discharge frequency were also observed. Dose response relations were established for 74% of the LH neurons, 68% of the LPA neurons, and for 72% of the ventral and subthalamic neurons following VTA stimulation. Decreases and, in a few hypothalamic neurons, increases in activity seemed to involve only one or two synapses. The effects of contralateral PAG stimulation on LPA-LH neuronal activity were alos determined. Dose response relations were established for 66% of the LPA-LH neurons following contralateral stimulation. However, results were different in that many more cells were increased with a shorter latency and at a lower threshold following contralateral stimulation. Antidromic responses verified PAG and diencephalic interconnections and revealed relatively slow conduction velocities, less than 1.0 m/sec. Results were discussed in terms of the anatomy of known PAG pathways, PAG neuronal activation vs. PAG fibers of passage, and the functions of midbrain-hypothalamic interconnections in the integration of somatic, visceral and nociceptive sensory inputs.