Action of intrahypothalamic 6-hydroxydopamine on motivated responding for food and water in the rat

Hypothalamic monoamines associated with the cephalic phase insulin response

This study elucidated the hypothalamic monoamine systems associated with the cephalic phase insulin release. Male Wistar rats conditioned to drink a glucose solution were killed 2 min after the onset of their scheduled feeding; control rats were killed at the same time. The ventromedial (VMH) and lateral (LH) portions of the hypothalamus were analyzed for the monoamines and their principal metabolites. Serum was assayed for insulin and glucose. The results showed that the experimental animals had significantly higher serum insulin levels than did the control animals, although glucose levels were unchanged. In the LH, there were increased levels of norepinephrine, its metabolite dihydroxyphenylethylene glycol (DHPG), and serotonin [(5-hydroxytryptamine (5-HT)]. In the VMH, 5-HT, its metabolite 5-hydroxyindoleacetic acid, and the dopamine metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) were all higher in the conditioned animals. 5-HT levels in the LH and DOPAC levels in the VMH were closely associated with serum insulin (r = 0.80 and 0.71, respectively). This study has defined, in the normal animal, changes in monoamines in specific areas of the hypothalamus associated with the cephalic phase insulin release.

CCK and other peptides modulate hypothalamic norepinephrine release in the rat: dependence on hunger or satiety

The purpose of this investigation was to determine the functional relationship between putative satiety peptides and endogenous norepinephrine (NE) activity in the hypothalamus. Permanent guide cannulae for push-pull perfusion were implanted stereotaxically in Sprague-Dawley rats so as to rest above the medial or lateral hypothalamus (LH). Post-operatively, the animals were either satiated with food and water, both available ad lib, or fasted for 18-22 hr prior to an experiment. To perfuse a site in the LH, paraventricular (PVN) or ventromedial nucleus (VMN), a concentric 29-23 ga push-pull cannula system was lowered to a pre-determined site, in most cases after catecholamine stores had been pre-labeled with [3H]-NE. During control tests, an artificial CSF was perfused at a rate of 20-25 microliter/min for 5-8 min with a 5 min interval between each sample. The addition of cholecystokinin (CCK) in a concentration of 2.0-6.0 ng/microliter to the CSF perfused in PVN or VMN of the satiated rat enhanced the efflux of NE; however, in the fasted animal CCK often suppressed the catecholamine's release. Perfused in the LH, CCK exerted opposite effects, typically augmenting NE output when the rat was fasted but not affecting the amine's activity during the sated condition. Proglumide (1.2 micrograms/microliter) attenuated CCK's effect in releasing NE when the antagonist was perfused in the PVN of the satiated rat. Similar experiments in which neurotensin (NT) was perfused in the LH, PVN and VMN revealed virtually the same inverse effects on NE release in the fasted and satiated rat, which again were anatomically specific. Finally, insulin and 2-deoxy-D-glucose (2-DG) exerted similar state-dependent effects on the release of NE within LH and PVN. Overall, the results suggest that CCK or other neuroactive peptide could serve as a "neuromodulator" of the pre-synaptic release of NE within classical hypothalamic structures which are thought to underlie both hunger and satiety. The state-dependent nature of the peptides' activity on the noradrenergic feeding mechanism implies that these substances constitute a pivotal portion of the profile of factors which impinge functionally upon the hypothalamic neurons responsible for the feeding response and its cessation.

Amine accumulation in behavioural pathology

When nigro-striatal and meso-cortical neurons degenerate there is a loss of dopamine in the terminal fields and an accumulation of amines in the axons of these systems as they traverse the hypothalamus through the medial forebrain bundle. Traditional lines of thought have attributed the occurrence of motor and consummatory deficits which occur after dopamine neuron degeneration to the loss of functional dopamine neurotransmitter in the terminal fields. However, we have hypothesized that hypothalamic amine accumulation represents an area of brain tissue where processes such as neurotransmitter release, ephaptic transmission or local axon swelling may be affecting adjacent neurons and may thereby participate in the production of behavioural deficits. There is a considerable amount of evidence from studies on both peripheral and central catecholamine-containing neurons indicating that when their axons degenerate a release of functional neurotransmitter can occur. Information from neuropharmacological studies indicates that several drugs which facilitate behavioural recovery from dopamine-depleting lesions may do so by affecting amine release or receptor sensitivity near areas of accumulation rather than depleted terminal fields. We conclude that amine accumulation is a component of dopamine neuron degeneration which should be considered when assessing the role of the central catecholamine systems in the control of various behavioural and physiological processes.

A role for amine accumulation in the syndrome of ingestive deficits following lateral hypothalamic lesions

Lesions of the lateral hypothalamus produce ascending catecholamine neuron degeneration which results in terminal depletion and proximal accumulation above the lesions. The occurrence of deficits in ingestive behaviour has been attributed traditionally to the loss of functional dopamine neurotransmitter in the terminal fields. However, release of functional amines may occur in the lateral hypothalamus at areas of accumulation, to produce at least some of the behavioural symptoms characterizing the lateral hypothalamic syndrome. Recovery from behavioural deficits as a result of various pharmacological treatments, after dopamine-depleting lesions, may be mediated by changes in amine release or modified sensitivity of receptors affected by released amines. We conclude that amine accumulation should be considered when interpreting experiments implicating central catecholamine systems in the control of consumatory behaviour and the regulation of body weight.

Deficits in locomotor behaviour and motor performance after central 6-hydroxydopamine or peripheral L-DOPA

Intracerebral injections of 6-hydroxydopamine (6-OHDA) which produce deficits in locomotor behaviour and motor performance cause localized accumulation of monoamines as well as depletion of central catecholamines (CA). To determine whether the localized increases in monoamines could be contributing to the behavioural deficits seen after intracerebral 6-OHDA injection, the behavioural and biochemical effects of peripherally administered L-DOPA were compared to those which occurred after central injections of 6-OHDA. Forty-five minutes after the injection of 40 mg/kg of L-DOPA, in animals pretreated with 50 mg/kg of R04-4602, the ability to step down, to retract a limb and to ambulate were significantly impaired. Locomotor behaviour and body temperature of L-DOPA-injected animals were similar to those displayed by animals receiving intracerebral injections of 6-OHDA. Fluorescent histochemical and biochemical assessment of diencephalic and telencephalic CA revealed that depletion of CA was not related to the severity of the motor deficits which occurred, but that increased diencephalic CA were present in impaired animals injected with either 6-OHDA or L-DOPA. These findings support the contention that increased CA in degenerating hypothalamic neurones may contribute to behavioural deficits seen after the injection of 6-OHDA.

The behavioural effects of intrahypothalamic multistage versus single injections of 6-hydroxydopamine

16 micrograms of 6-hydroxydopamine were injected bilaterally into the lateral hypothalamus of male Sprague-Dawley rats in either a single injection or in a series of multistage injections over a period of 75 days. While the single injection produced behavioural deficits typical of those seen following catecholamine depletion of the forebrain, the behaviour of animals injected incrementally with the same dose was indistinguishable from that of controls. Fluorescent histochemical assessment revealed that the animals in the multistage injection group, the behaviourally unimpaired, suffered more severe depletion of forebrain catecholamines than those in the single-stage injection group. Accumulation of amines proximal to the site of drug injection was extensive only in those animals displaying behavioural deficits, that is, the group with the lesser amount of forebrain catecholamine depletion. It is suggested that the severity of deficits in consummatory and motor behaviour occurring after hypothalamic trauma is determined by a lesion's effectiveness in producing amine accumulation rather than catecholamine depletion.

The role of nerve-growth factor (NGF) in the central nervous system

NGF is a protein that stimulates growth and differentiation of sympathetic and sensory components of the peripheral nervous system. The purpose of this review is to examine the evidence that NGF has similar activity in the central nervous system. First, the primary mode of interaction of NGF with the nerve cell will be discussed, and the possibility that such an interaction takes place in the brain will be examined. Recent studies have demonstrated that NGF promotes regenerative sprouting of damaged catecholamine-containing neurons in the brain. The next part of the paper reviews this literature, and other findings that indicate or contraindicate a role of NGF in brain maturation of maintenance. The final part of this paper suggests specific avenues for future research in this area, and presents conclusions regarding the literatureon brain activity of NGF to date.

Impairment of thermoregulation, food and water intakes in the rat after hypothalamic injections of 5,6-dihydroxytryptamine

A bilateral microinjection into the anterior hypothalamus of 5,6-dihydroxytryptamin (5,6-DHT), a substance that lesions serotonin (5-HT)-containing neurons, caused a rise in the body temperature of the rat. The anatomical sites were the same as those at which 5-HT given in the same dose range (1.25-2.5 mug) evoked a similar hyperthermia. When exposed for one hour to a temperature of either 35 degrees C or 8 degrees C, the rats were not able to defend against the heat or cold, respectively. The magnitude of this thermoregulatory deficit depended upon the dose of 5,6-DHT given as well as the site of injection. A partial recovery from the warmth deficit was evident 13-17 days following the 5,6-DHT microinjection. Food and water intakes were also suppressed significantly and body weights declined concomitantly. These results provide additional evidence to support the view that a serotonergic mechanism in the hypothalamus is involved in both thermoregulation and the control of ingestive behavior.

Differential effects of intraventricular administration of 6-hydroxydopamine on behavior of rats in approach and avoidance procedures: Reversal of avoidance decrements by diazepam

The administration of 6-hydroxdopamine (6-HD) and pargyline to rats produced similar selective decreases in responding during the conditioned stimulus (CS) on a discriminated avoidance test where the unconditioned stimulus (US) was shock and on an analogous conditioned approach test where the US was water. Approach behavior during the CS generally recovered, however, while avoidance decrements in the same rats remained for the duration of testing. This suggested that 6-HD-induced avoidance decrement was a result of two independent but interacting processes: (1) a decrease in conditioned behavior as reflected by the similar decrease in responding on both tests; and (2) a hyper-reaction to aversive stimuli that resulted in a tendency to selectively suppress avoidance behavior after the animal received shock. In support of this hypothesis, it was found that 6-HD-induced avoidance decrements could be reversed (1) by treatment with diazpam, a drug that releases suppressed responses; or (2) by delaying avoidance testing until conditioned responding had recovered, thus minimizing the interaction of the two processes.