Disruption of food hoarding by injections of procaine into mediodorsal thalamus, GABA into subpallidal region and haloperidol into accumbens.

The contributions of accumbens-subpallido-mediodorsal thalamus (MD) projections to food hoarding were investigated. The number of food pellets hoarded was reduced by bilateral injections of haloperidol into the accumbens, by bilateral injections of GABA into the subpallidal region and by bilateral injections of procaine into the mediodorsal thalamus. Food hoarding was not reduced by bilateral injections of procaine into the pedunculopontine nucleus. It appears that subpallido-mediodorsal thalamus projections are associated with hoarding behavior but not subpallido-pedunculopontine projections.

Differential effects on locomotor activity of injections of procaine into mediodorsal thalamus and pedunculopontine nucleus.

A comparison was made of the contributions to locomotor activity of output projections of the subpallidal region to the mediodorsal thalamus (MD) and to the pedunculopontine nucleus (PPN). Locomotor activity elicited by injections of picrotoxin into the subpallidal region was reduced by the administration of procaine to the pedunculopontine nucleus but not by the administration of procaine to mediodorsal thalamus. Since the pedunculopontine nucleus is part of the mesencephalic locomotor region (MLR) it appears that subpallido-pedunculopontine projections contribute to the locomotor component of adaptive behaviors associated with limbic integrative activities.

Subpallidal projections to the mesencephalic locomotor region investigated with a combination of behavioral and electrophysiological recording techniques.

Locomotor activity was increased as a function of current intensity in a series of rats with chronic stimulating electrodes in the tegmental pedunculopontine nucleus. Single pulse stimulation of these same mesencephalic locomotor region (MLR) sites antidromically activated more than one-quarter of the neurons sampled in the subpallidal area. In a second series of rats locomotor activity was increased by chronic stimulation of the zona incerta as well as by chronic stimulation of MLR. Approximately one-quarter of subpallidal neurons sampled were antidromically activated by single pulse stimulation of zona incerta and/or MLR. Thirty-one of the subpallidal neurons were antidromically activated by stimulation of both zona incerta and MLR and, according to the results of the reciprocal collision test, about half this number had branching projections to the zona incerta and MLR. These observations provide additional evidence that subpallidal-MLR neurons are associated with locomotion.

Electrophysiological evidence that the mediodorsal nucleus of the thalamus is a relay between the ventral pallidum and the medial prefrontal cortex in the rat.

The neural connections from the ventral pallidum (VP) through the mediodorsal nucleus of the thalamus (MD) to the medial prefrontal cortex (MPC) were investigated. Extracellular recordings were made from 219 neurons in the medial and lateral portions of the MD and the VP and the MPC were stimulated. The most frequent response to VP stimulation was inhibition and inhibition preceded by excitation. Also, the most frequent response of MD units to MPC stimulation was inhibition and inhibition preceded by excitation. Nineteen of 26 MD units, activated antidromically by MPC stimulation, responded orthodromically to VP stimulation. The most frequent orthodromic response of these MD output neurons was inhibition and inhibition preceded by excitation. GABA iontophorized onto MD neurons reduced their rate of discharge. GABA and picrotoxin iontophorized onto MD neurons did not influence the inhibitory or excitatory responses to VP stimulation. These electrophysiological results support previous anatomical findings of connections between the VP and the MPC by way of the MD. MD output neurons to the MPC receive mostly inhibitory inputs from VP afferents. A high proportion of MD neurons respond orthodromically to both VP and MPC stimulation, suggesting the convergence of synaptic inputs from these structures to the same MD units.

A comparison of the effects of electrical stimulation of the lateral and ventromedial hypothalamus on the activity of neurons in the ventral tegmental area and substantia nigra.

Extracellular unit recordings were obtained from neurons in the ventral tegmental area (VTA), the substantia nigra, zona compacta (SNC) and zone reticulata (SNR) of adult female albino rats anaesthetized with urethane and chloral hydrate. Neurons were divided into two types based on their electrophysiological characteristics; Type I neurons had long duration action potentials (greater than 2.6 msec) and slow discharge rates and Type II neurons had shorter duration action potentials and a wider range of discharge rates. Both types of neurons were found in the VTA and SNC, but there were only Type II neurons in the SNR. The effects of single pulse stimuli delivered to the ipsilateral ventromedial (VMH) or lateral (LH) hypothalamic areas on activities of the two types of neurons were investigated. Only a small portion of neurons in the VTA and SNC responded to VMH stimulation, but in contrast a majority of the two types of neurons in the VTA and SNC responded to LH stimulation. Most neurons in the SNR did not respond to VMH and LH stimulation. Type iI neurons in the VTA and SNC were predominantly suppressed by LH stimulation with short onset latencies (less than 6 msec), indicating the possibility of monosynaptic mediation. However Type I neurons in the VTA and SNC were activated and suppressed and the onset latencies of these responses were relatively longer. The high proportion of neurons of VTA and SNC responding to electrical stimulation of LH is consistent with anatomical evidence. Suppression and activation of Type I neurons in VTA and SNC suggest that the LH exerts modulatory influences on these neurons of the midbrain.

Water intake elicited by injections of angiotensin II into preoptic area of rats.

Drinking in response to unilateral injections of angiotensin II (AII) into the preoptic area is relatively weak and a relatively high dose of AII is required. A comparison was made of the drinking elicited by bilateral and unilateral injections of AII. Reliable drinking was observed when 5 X 10(-12) mol of AII in 0.2 microliter was injected bilaterally into the preoptic area, and in some animals when the dose of AII was 0.5 X 10(-12) mol in 0.2 microliter. The volumes of water intake were significantly larger with bilateral injections compared to unilateral injections. By injection of tritiated AII to elicit drinking and subsequent autoradiographic analysis of brain sections, it was shown that the injections were confined to the preoptic region and did not reach the cerebral ventricles. The results suggest that 1) bilateral injections may be more appropriate for comparing the preoptic region with other putative angiotensin receptive sites, 2) there is a greater responsiveness of the preoptic region to bilateral as compared with unilateral injections, and 3) the AII receptive area is diffusely represented in the region of the medial preoptic nucleus.

An electrophysiological study of inputs to neurons of the ventral tegmental area from the nucleus accumbens and medial preoptic-anterior hypothalamic areas.

Extracellular recordings were obtained from neurons in the ventral tegmental area (VTA) of urethane-anesthetized rats. Neurons were divided into two types based on the latencies of antidromic activation following electrical stimulation of the nucleus accumbens (NAcc), and on the durations of action potentials. Type A neurons had longer latencies for antidromic activation (mean 15.9 msec) and longer durations of action potentials (> 2.6 msec), while type B neurons had shorter latencies (mean 4.5 msec) and shorter duration of action potentials (< 2.6 msec). Electrical stimulation of the medial preoptic-anterior hypothalamic areas (mPOA-AHA) and NAcc produced the following effects on the two types of VTA neurons: (i) the majority of both type A and B neurons were suppressed by mPOA-AHA stimulation with onset latencies of less than 10 msec; (ii) 42% of type B neurons were also suppressed by NAcc stimulation, with onset latencies of less than 10 msec; (iii) type A neurons were suppressed (33%) or activated (43% by NAcc stimulation, the onset latencies usually being longer than 10 msec; (iv) 71% of type A neurons tested had convergent inputs from the mPOA-AHA and NAcc, usually suppressed-suppressed or suppressed-activated, while 45% of type B neurons had convergent inputs from these two areas, usually suppressed-suppressed.

Oral motor deficits following lesions of the central nervous system in the rat.

The effects of lesions of the zona incerta (ZI), globus pallidus (GP), midlateral and far lateral hypothalamus (MLH and FLH), and the central amygdaloid complex (CAC) on oral motor deficits were investigated. Lesions of the ZI, GP, and CAC resulted in a significant reduction in tongue extension and lap volume. MLH lesions significantly reduced tongue extension whereas FLH lesions significantly reduced both tongue extension and lap volume. Injections of 6-hydroxydopamine into the GP, MLH, and CAC also significantly reduced tongue extension and lap volume. Water and/or food intakes were reduced in some of the lesioned groups but the oral motor deficits were observed both in the presence and in the absence of reduced water and food intakes. The possibility that oral motor deficits are associated with damage to striatal and non-striatal dopamine neurons needs further investigation.

Central neural pathways for angiotensin-induced thirst.

Evidence is reviewed implicating the preoptic region in angiotensin-induced thirst. The most responsive area according to results obtained with behavioral, electrophysiological, and autoradiographic mapping techniques is at the caudal border of the medial preoptic region and rostral border of the anterior hypothalamus. The neural pathway from this preoptic site for angiotensin-induced thirst extends along the medial forebrain bundle through the midlateral hypothalamus to the paramedial midbrain tegmentum and to an area ventrolateral to the central gray. Lesions of this pathway in the midlateral hypothalamus and rostral midbrain significantly attenuated drinking induced by microinjections of angiotensin II into the preoptic area but did not disrupt water intake induced by microinjections of angiotensin II into the subfornical organ or cerebral ventricles. Although the efferent pathways from angiotensin-receptive sites in the subfornical organ and cerebral ventricles are unknown, it appears from these observations that the medial forebrain bundle is not involved. Lesions of the medial forebrain bundle-lateral hypothalamus also do not disrupt drinking induced by microinjections of hypertonic saline into the preoptic region although lesions placed 1 mm further lateral do. Since fat lateral hypothalamic lesions are without effect on drinking induced by centrally administered angiotensin II, this suggests that intracellular and extracellular thirst signals are subserved by separate neural pathways in the hypothalamus.

Reversible hyperphagia and obesity following intracerebral microinjections of colchicine into the ventromedial hypothalamus of the rat.

Colchicine, a drug which produces a reversible inhibition of intraaxonal transport and synaptic transmission, was used as a reversible neural blocker to investigate the role of the ventromedial hypothalamus (VMH) in the control of ingestive behavior and body weight regulation. Male Sprague-Dawley rats received intracranial microinjections of colchicine into the VMH. Volume and concentration of the colchicine solution were varied to assess specificity of action and dose-response relationship. When colchicine (2 and 4 microgram) was microinjected bilaterally into the VMH, there was a dose-dependent increase in food and water intakes and body weight gain which lasted several days. The acute period of hyperphagia was followed by a marked depression in feeding which persisted until body weight was lowered to control levels. This suppression of feeding appeared to be a consequence of the preceding period of hyperphagia and obesity, since colchicine-treated rats which were pair-fed with controls to prevent obesity continued to maintain normal food intake and body weight gain when later fed ad libitum. The results of this study confirm the importance of the VMH in the long term regulation of feeding, and indicate that reversible neuronal blocking with colchicine is a useful technique for investigating the neural substrates of feeding and other behaviors.

Autoradiographic evidence for pathways from the medial preoptic area to the midbrain involved in the drinking response to angiotensin II.

The 3H-amino acid autoradiographic method was used to localize intracerebral sites from which angiotensin II (AII) elicits drinking and to identify their efferent neural pathways. Small injections (0.02-0.1 mul) of AII and 3H-amino acid mixtures were injected together or separately into widespread regions of the forebrain of adult rats in normal food and water balance. From an analysis of 39 positive and negative injection sites it was concluded that the caudal half of the medial preoptic area and the adjacent rostral part of the anterior hypothalamic area are sensitive to AII. Two anatomically defined pathways arising from neurons within this region were identified. One descends through the medial forebrain bundle and appears to terminate in the lateral hypothalamic area, the ventromedial nucleus, the mammillary body, and the ventral tegmental area. The other descends through the periventricular region and posterior hypothalamic area to end in the midbrain central gray. Additional widespread connections with the amygdala, septum, habenula, and pons appear to arise in the lateral preoptic area (Swanson, '76). Combined AII-3H-amino acid injections centered in the subfornical organ only elicited drinking in those cases in which injected label diffuse through the third ventricle to the medial preoptic area. No efferent pathways were identified in experiments in which a small injection (0.02 mul) heavily labeled cells strictly confined to the subfornical organ and there was no ventricular spread of label.

Sodium intake following destruction of the anterior hypothalamus in the rat.

Sodium intake and sodium output were measured in rats before and after lesioning of the anterior hypothalamus. Both parameters were lower than control rats following the lesioning procedure. When an amount of sodium was given orally to these lesioned rats to make up the deficit in sodium intake, no difference was found in sodium intake or output between the rats with lesions and normal rats. Rats with anterior hypothalamic lesions were able to increase their sodium intake following a period of sodium deprivation or adrenalectomy. When presented with concentrations of sodium chloride ranging between 0.9 and 2.5% rats with lesions decreased their intake of sodium in a manner similar to control rats except that the level of intake in the lesioned rats was lower for all concentrations. Control rats and those with lesions were not different in their intakes of KC1 solutions. These findings suggest that the anterior hypothalamus is important in setting the absolute intakes of sodium but does not interfere with the control mechanisms regulating the sodium intake.

Ingestive behavior after intracerebral and intracerebroventricular infusions of glucose and 2-deoxy-D-glucose.

Direct infusions of 2-DG into the lateral hypothalamic area (LHA), the ventromedial hypothalamus (VMH), the dorsal hippocampus, the amygdaloid complex or the caudate nucleus were all ineffective in eliciting drinking or feeding in satiated rats. However, 2-DG (but not D-glucose) infused into the lateral ventricles of satiated rats elicited feeding preceded by an initial burst of drinking, which could be blocked by prior intraventricular infusion of the alpha-adrenergic antagonist, phentolamine, by the serotonergic blocker, methysergide, but only slightly by the dopaminergic receptor blocker, spiroperidol. The feeding response was totally blocked by phentolamine but not by methysergide or spiroperidol. These results show that 1) intraventricular administration of 2-DG can induce feeding, as well as drinking, supporting the hypothesis of cerebral glucoreceptors for the initiation of feeding behavior; and 2) local cytoglucopenia in the LHA, the VMH, or other limbic structures is not sufficient to initiate feeding. It is hypothesized that a specific pattern of cerebral glucoprivation rather than local glucoprivation, or an action of 2-DG on the presynaptic membrane of noradrenergic "feeding neurons" could be the mechanisms of 2-DG-induced feeding.

Activity of neurons in the region of the substantia nigra during feeding in the monkey.

The activity of single neurons in the region of the substantia nigra of the monkey was recorded during feeding to investigate their function in ingestive behavior. It was observed that some neurons in the substantia nigra and the adjacent tegmentum altered their activity during feeding, in relation to mouth movements. The activity of these neurons was related to mouth movements in that the responses of the units were similar when the monkeys drank fluids with different tastes as long as the same movements were made, in that equally good responses could be obtained when the monkey moved his mouth to non-food objects, and in that in some units opposite responses were obtained when ipsilateral as compared with contralateral mouth movements were made. It was also shown that the responses of these units associated with mouth movements were similar when the monkeys were hungry and when they were satiated. These findings suggest that the activity of some neurons in the region of the substantia nigra is related to the execution of movements which may be involved in feeding, and that the activity of these neurons is not related to the initiation of feeding. Self-stimulation through the recording microelectrodes could be obtained just dorsal to the substantia nigra, but the neural basis of this self-stimulation is not known.

Self-stimulation of the subfornical organ and lateral hypothalamus: differential effects of atropine and methysergide.

The effects of cholinergic blockade of neurons by atropine or serotonergic blockade by methysergide was investigated in rats responding for brain-stimulation reward. Bipolar stimulating electrodes were placed either in the subfornical organ (SFO) or the lateral hypothalamus (LH). Atropine sulphate and methysergide significantly suppressed self-stimulation of the SFO but not of the LH, suggesting that cholinergic and serotonergic neurons are involved in brain-stimulation reward associated with this site.

The role of mesencephalic structures in thirst induced by centrally administered angiotensin II.

(1) In 27 animals microinjection of 25--100 ng of angiotensin II through chronic cannulae implanted in the preoptic region initiated drinking and in subsequent acute experiments influenced the spontaneous discharge rate of single neurons in the ipsi-lateral mesencephalon. Of 148 neurons for which recordings were made, 52 (35%) increased their frequency of spike potentials following administration of angiotensin II, 2 (1%) showed inhibition and 94 (64%) showed no change in firing rate. (2) In another series of 44 animals, unilateral or bilateral lesions of the midbrain ventral tegmentum or reticular formation were found to have little or no effect on water intake elicited by the microinjection of angiotensin II into the preoptic region. (3) In contrast to the effects of tegmental and reticular lesions, unilateral lesions located dorsally and laterally to the mammillary peduncle, in the area of passage of the medial forebrain bundle, significantly attenuated the dipsogenic response to either contralateral or ipsilateral injections of angiotensin II into the preoptic region. With bilateral lesions this effect was permanent. (4) Since the more caudal lesions were relatively ineffective in disrupting the elicited drinking, it is suggested that signals from angiotensin II receptors in the preoptic region are transmitted along pathways which diverge in the midbrain. (5) The possibility of a forebrain-hypothalamus-midbrain circuit mediating thirst initiated by activation of the renin-angiotensin system is discussed.