Osmosensitive neurons in the rat's dorsal midbrain

Neuroleptics and operant behavior: The anhedonia hypothesis

Neuroleptic drugs disrupt the learning and performance of operant habits motivated by a variety of positive reinforcers, including food, water, brain stimulation, intravenous opiates, stimulants, and barbiturates. This disruption has been demonstrated in several kinds of experiments with doses that do not significantly limit normal response capacity. With continuous reinforcement neuroleptics gradually cause responding to cease, as in extinction or satiation. This pattern is not due to satiation, however, because it also occurs with nonsatiating reinforcement (such as saccharin or brain stimulation). Repeated tests with neuroleptics result in earlier and earlier response cessation reminiscent of the kind of decreased resistance to extinction caused by repeated tests without the expected reward. Indeed, withholding reward can have the same effect on responding under later neuroleptic treatment as prior experience with neuroleptics themselves; this suggests that there is a transfer of learning (really unlearning) from nonreward to neuroleptic conditions. These tests under continuous reinforcement schedules suggest that neuroleptics blunt the ability of reinforcers to sustain responding at doses which largely spare the ability of the animal to initiate responding. Animals trained under partial reinforcement, however, do not respond as well during neuroleptic testing as animals trained under continuous reinforcement. Thus, neuroleptics can also impair responding (though not response capacity) that is normally sustained by environmental stimuli (and associated expectancies) in the absence of the primary reinforcer. Neuroleptics also blunt the euphoric impact of amphetamine in humans. These data suggest that the most subtle and interesting effect of neuroleptics is a selective attenuation of motivational arousal which is (a) critical for goal-directed behavior, (b) normally induced by reinforcers and associated environmental stimuli, and (c) normally accompanied by the subjective experience of pleasure. Because these drugs are used to treat schizophrenia and because they cause parkinsonian-like side effects, this action has implications for a better understanding of human pathology as well as normal motivational processes.

Effects of intracerebroventricular angiotensin II and olfactory stimuli on multiple unit activity in preoptic and anterior hypothalamic areas: medial-lateral comparison

Under urethane anesthesia multiple unit activity (MUA) recordings were taken from medial and lateral preoptic and anterior hypothalamic sites in 21 rats during multiple dose intracerebroventricular (i.vt.) injections of angiotensin II (AII), using artificial CSF as control. Olfactory stimuli were also presented. Whilst lateral sites on average were significantly less responsive to AII than were medial sites, some of the former were very responsive. None of the 14 lateral sites that yielded an MUA response to AII failed to yield an MUA response to olfactory stimulation. On the other hand, 11 of 12 medial sites that yielded an MUA response to AII failed to yield an MUA response to olfactory stimulation. On the basis of these data it is suggested that the medial and lateral regions of the basal forebrain serve different functions, the former more related to internal sensing and the latter more related to integration of internal and external sensing. The findings are discussed in relation to the dual olfactory system and to theories of motivation.

Mediobasal hypothalamic neurons are excited by the iontophoretic application of sodium

In the course of studies on the responsiveness of mediobasal hypothalamic neurons to the iontophoretic application of cortisol, it was found that positive currents applied to a sodium chloride (1 M) barrel alone, but not to a choline chloride (1 M) barrel, frequently increased the firing of these neurons. Subsequently, systematic examination demonstrated that out of 102 MBH neurons 52 (51%) increased their firing by at least 30% with application of NaCl, using currents no greater than 10 nA. No such effect was obtained in response to Na application from a dilute solution (0.05 or 0.1 M). When glutamate was absent from the electrodes, the incidence of Na+ sensitivity fell to 17%, despite the routine use of backing currents to the glutamate barrel. K+ ions were more active than Na+ ions in producing excitation. When Na+ sensitivity was found, however, Na+ effects were produced by currents greater than K+ currents producing equivalent excitation. Like glutamate, K+ ions were capable of greatly enhancing responses to Na+. Comparison was made between cortisol and Na+ sensitivity in 70 MBH neurons; 28 cells responded to both, and 24 of them were inhibited by cortisol. Thus Na+ sensitivity is a frequent characteristic of MBH neurons inhibited by cortisol, and was present in 83% of cortisol-sensitive cells in this region. Iontophoresis of Na+ is commonly used as a control in pharmacological studies of the nervous system. Even more common is the case of concentrated NaCl solutions for recording. These procedures may not be as inert as previously thought, particularly in the hypothalamus.

Experiments on the neuropsychology of thirst

We report the results of a series of experiments whose main objectives are: (a) the identification of neural receptors for thirst; and (b) other neural structures of critical importance for thirst and drinking behavior. We have used results from hypertonic challenges during acute unit and multiple-unit recording experiments to identify responsive brain areas for behavioral study in chronic experiments. Results include the following. Single cells in the lateral preoptic area (LPOA) responded to injections of hypertonic saline into the carotid artery in a dose-related manner. Multiple unit activity (MUA) reactions were invariably facilitatory to challenge, and were much greater in the LPOA than in the medial preoptic area (MPOA). In unit and MUA recording we found extremely osmosensitive sites in the dorsal midbrain. Comparing the effects of NaCl vs sucrose as challenges via the intracarotid artery we found that LPOA MUA responses to sucrose were at least as strong and latencies as short as those to NaCl. These results support osmoreceptor theory, as revised by Epstein and his co-workers, and they are opposed to Andersson's sodium receptor theory. Hypertonic NaCl and sucrose solutions (but not artificial CSF controls) injected into the lateral ventricle were effective in producing strong MUA reactions, which typically were inhibition followed by facilitation. These and other findings support the following conclusions. (a) The LPOA appears to be a vital part of the neural mechanism for thirst and it is a probable site of osmoreceptors. (b) In addition, the LPOA seems to be a receiving area connected with putative periventricular neural receptors. (c) Intraventricular (i.v.t.) angiotensin II seems to be a much stronger dipsogen than hypertonic NaCl i.v.t., which in higher concentrations elicited stereotyped running.

Distribution of angiotensinogen in Brattleboro rat brain

Angiotensinogen, the precursor of angiotensin II, was quantitated in 46 brain regions of Brattleboro rats, which lack antidiuretic hormone, and Long-Evans control rats. The regional distribution of angiotensinogen in the two strains was similar except for a small number of areas which in the Brattleboro rats displayed significant decreases; namely, lateral preoptic area, medial basal hypothalamus, medial dorsal hypothalamus, lateral hypothalamus, lateral mammillary bodies, periaquaductal gray and substantia nigra. Additionally, angiotensinogen in the posterior pituitary was significantly elevated in the Brattleboro strain. These results indicate that angiotensinogen is present in the Brattleboro rat brain and that hereditary deficiency of the ability to synthesize antidiuretic hormone may be associated with a localized alteration in angiotensinogen concentration.

Ionic mechanism for the osmotically-induced depolarization in neurones of the guinea-pig supraoptic nucleus in vitro

1. Effects of hypertonic solutions prepared by adding NaCl or sucrose to Krebs solution on intracellular potentials were observed in neurones of the supraoptic nucleus using brain slices of the guinea-pig hypothalamus. 2. Hypertonic solutions (306-488 m-osmole/kg) depolarized the membrane, increased the input resistance and augmented the spontaneous firing rate in supraoptic neurones, whereas cells in the hippocampus and anterior or ventromedial hypothalamus were not affected by the hypertonic solutions. 3. The excitatory action of hypertonic solutions on supraoptic neurones was retained in the medium containing 0 mM-Ca2+ and 12 mM-Mg2+. 4. Amplitude of the depolarization induced by superfusion of hypertonic solutions was voltage-dependent. The reversal potential for the depolarization was about -90 mV. 5. The reversal potential for the depolarization induced by hypertonic solution shifted as a function of [K+]0. 6. These results suggest that the supraoptic neurones are themselves osmosensitive and that the local osmotic-related information is transduced to neural signals in these cells by alteration in the membrane ionic permeability, probably due to suppression of K+ conductance.

Responses of lateral preoptic neurons and behavioral reactions to angiotensin II and hypertonic NaCl and sucrose administered into the rat cerebral ventricle

In 18 acutely prepared rats and in 3 chronically implanted rats, multiple-unit responses of lateral preoptic area (LPOA) neurons to angiotensin II (A II) and hypertonic NaCl and sucrose injected into the lateral ventricle were recorded. In addition, behavioral reactions were observed in the chronically prepared rats. Results showed that i.vt. injections of hypertonic solutions or A II affect neural activity in the LPOA, indicating that the LPOA has connections with periventricular structures that are sensitive to these substances. Experiments with chronically implanted animals demonstrated the relevance of i.vt. injections for drinking. Some of the same solutions that produced changes in LPOA MUA in the acute series (or in anesthetized chronically implanted rats) also elicited drinking. A II was a more effective dipsogen than hypertonic NaCl, and in the strongest concentration used, NaCl produced stereotyped running, reduced head movement during quiescence, and caused a progressive fall in baseline MUA. Drinking was elicited by intermediate concentrations of i.vt. NaCl. Implications for the neuropsychology of thirst are discussed.

Responses of supraoptic neurones in the intact and deafferented rat hypothalamus to injections of hypertonic sodium chloride

1. Recordings were made from a total of fifty-three neurones in the supraoptic nuclei of four groups of rats: intact rats, animals in which the hypothalamus had been partly denervated by anteriorly or posteriorly placed semicircular cuts, and rats with a totally deafferented hypothalamus. 2. When first encountered, cells from intact animals fired at a mean rate of 5.08 +/- 0.78 spikes/sec, those from posteriorly isolated hypothalami at 3.93 +/- 0.63 spikes/sec, those from the anteriorly isolated hypothalami at 2.05 +/- 0.83 spikes/sec, and those from totally isolated hypothalami at 0.99 +/- 0.46 spikes/sec. 3. When stimulated osmotically by an intraperitoneal injection of ml. 1.5 M-NaCl, eight out of eight cells in intact rats showed a significant increase in firing rate between 20 and 30 min after the injection. Six out of nine cells in posteriorly isolated hypothalami showed significant but smaller responses. No increase in firing rate could be detected in seven cells from totally isolated hypothalami or from eight cells in hypothalami partly isolated by anterior cuts. 4. The results imply that under the conditions of these experiments by the spontaneous activity of the supraoptic nucleus in intact animals was maintained by an extrahypothalamic excitatory input, that partial hypothalamic isolation reduced its intensity, possibly by unmasking an inhibitory input, and that total isolation reduced it to an even greater extent. Osmotic activation of supraoptic cells was only possible when the anterior connexions of the hypothalamus were intact. Thus the cerebral osmo-receptors for vasopressin release may be situated outside the supraoptic nuclei.

Responses of lateral preoptic neurons in the rat to hypertonic sucrose and NaCl

Multiple-unit recordings were taken from the lateral preoptic region during a series of hypertonic and isotonic NaCl and sucrose intracarotid injections. Subjects were 11 hooded rats (8 males and 3 ovariectomized females) under urethane anesthesia. The data showed that under favorable cannulation conditions there were strong multiple-unit responses to hypertonic sucrose injections, and that under these conditions NaCl injections were not significantly more effective than sucrose injections. The possible bearing of these findings on hypotheses concerning central receptors for thirst is discussed.

Spontaneous and osmotically-stimulated activity in slices of rat hypothalamus

Single unit activity was recorded from 400-500 mu m thick slices of rat hypothalamus, using either NaCl- or horseradish peroxidase-filled glass micropipettes. Spontaneous activity was present in the following hypothalamic loci: anterior hypothalamic-preoptic area, nucleus circularis, nucleus of the diagonal band of Broca, paraventricular accessory nucleus, paraventricular nucleus (all portions), periventricular regions of the anterior hypothalamus, and the suprachiasmatic nucleus. The supraoptic nucleus was the only major cell group studied to exhibit no spontaneous activity. Cells of the paraventricular and circularis nuclei were spontaneously active, displayed firing rates and patterns of activity similar to those recorded in vivo for magnocellular elements of the hypothalamus, and in some cases responded to increases in the osmolality of the bathing medium with altered firing rates and/or patterns of activity. Many cells in these preparations were characterized by phasic, bursting patterns of activity. Slow, irregular and regular, continuous activity was also frequently observed, as is typical in vivo. Median firing rates were in the range of 4-6 spikes/sec, somewhat faster than the rates usually reported for anesthetized in vivo preparations. These rates are more similar to those observed in unanesthetized monkeys or rats with diencephalic islands. Extracellular HRP marking provided a high degree of localization for many of the recorded cells. These results indicate that the hypothalamic slice preparation is useful for studies in which it is desirable to eliminate extrahypothalamic connections and in which it is necessary to exercise a fine degree of control over the extracellular environment of the cells.

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.

Thermosensitive midbrain neurons in the cat

Responses of single neurons in the midbrain of cats anesthetized with chloral hydrate were studied during manipulations of midbrain temperature produced with a bilateral water-perfused thermode. Temperatures of the thermodes and the anterior hypothalamus were monitored while single neuron activity was recorded between the thermodes and correlated with the midbrain temperature. Q10's and thermal coefficients were calculated from the estimated temperature at the neuron itself. A surprisingly high percentage (72%) of the 72 neurons recorded in the caudal paramedian midbrain of 11 cats were thermoresponsive. Most of these were heat sensitive and exhibited a variety of frequency/temperature curves. Explorations of more rostral regions of the midbrain in 9 cats yielded only 18% thermoresponsive units out of the 99 neurons sampled. We suggest that the concentrated pool of warm-sensitive neurons in the caudal midbrain is part of an extensive system of brain stem thermosensors which are involved in establishing and controlling normal brain temperature.

Movement-related forebrain and midbrain multiple unit activity in rats

In 16 rats multiple unit activity was recorded simultaneously from ventral forebrain and dorsal midbrain areas, with continuous monitoring by sensitive head movement and body movement recorders, along with careful observations of ongoing behaviors. Both forebrain and midbrain multiple unit activity were significantly correlated with head and body movements: recordings from both brain areas showed significant declines corresponding to decline in recordings head and body movement. The declines in midbrain multiple unit activity were significantly greater than those in forebrain multiple unit activity. The significant correlation of forebrain multiple unit activity with continuous recordings of bodily activity is, to the best of our knowledge, a new finding. The correlations between brain activity and bodily activity found in these experiments indicate the importance of monitoring brain recordings of awake animals with sensitive recordings of head and body movement. In addition, these findings represent a further demonstration of the need to analyze the operation of the brain in terms of its motor output.