d-Amphetamine and punished responding: the role of catecholamines and anorexia

Dysregulation of striatal dopamine signaling by amphetamine inhibits feeding by hungry mice

Amphetamine (AMPH) releases monoamines, transiently stimulates locomotion, and inhibits feeding. Using a genetic approach, we show that mice lacking dopamine (DA-deficient, or DD, mice) are resistant to the hypophagic effects of a moderate dose of AMPH (2 microg/g) but manifest normal AMPH-induced hypophagia after restoration of DA signaling in the caudate putamen by viral gene therapy. By contrast, AMPH-induced hypophagia in response to the same dose of AMPH is not blunted in mice lacking the ability to make norepinephrine and epinephrine (Dbh(-/-)), dopamine D(2) receptors (D2r(-/-)), dopamine D(1) receptors (D1r(-/-)), serotonin 2C receptors (Htr2c(-/Y)), neuropeptide Y (Npy(-/-)), and in mice with compromised melanocortin signaling (A(y)). We suggest that, at this moderate dose of AMPH, dysregulation of striatal DA is the primary cause of AMPH-induced hypophagia and that regulated striatal dopaminergic signaling may be necessary for normal feeding behaviors.

Amphetamine potentiation of anti-conflict action of chlordiazepoxide

A modified Geller-Seifter paradigm was employed to test in male albino rats the effects of subthreshold doses of amphetamine and chlordiazepoxide (CDP), administered separately or in combination, on shock induced suppression of food-reinforced lever-pressing. MK-801, a newly synthesized sympathomimetic with anxiolytic and anticonvulsant properties, was also tested. dl-Amphetamine in doses of 0.1, 0.2, 0.3, and 1.0 mg/kg had no anxiolytic nor anxiogenic effects, but at 1.0 mg/kg it increased non-conflict responding. CDP in doses of 0.1, 0.2, and 0.4 mg/kg had no significant effect on conflict and non-conflict responding. CDP in the dose of 0.8 mg/kg tended to increase conflict responding. Coadministration of amphetamine (0.2 mg/kg) and CDP (0.4 mg/kg) had a significant anti-conflict effect. MK-801 at 50 micrograms/kg and 100 micrograms/kg caused a significant increase in non-conflict responding. MK-801 at 50 micrograms/kg exerted also a significant anti-conflict effect. The disinhibitory effects of amphetamine coadministered with CDP were discussed in terms of a possible enhanced noradrenergic or dopaminergic activity and their interaction with GABA neurotransmission at GABA-benzodiazepine coupled sites.

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.

Role of 5-hydroxytryptamine in amphetamine effects on punished and unpunished behaviour

In order to assess the contribution of serotonergic (5-HT) mechanisms in the suppressant effect of amphetamine on punished responding, dose-effect curves of amphetamine on key-pecking behaviour of pigeons maintained by food presentation and punished by electric-shock were determined before and after pretreatment with methergoline, a potent and specific 5-HT receptor blocker in the central nervous system. A multiple fixed-interval 5 min, fixed-interval 5 min schedule of reinforcement in which every response, except the reinforced one, was punished in one of the two components (mult FI5 FI5-shock) was used. Effective doses of amphetamine decreased unpunished as well as punished FI response rates. However, the decreases in punished behaviour were more evident and dose-dependent. Methergoline markedly increased FI responding in the punished FI component but only slightly increased or decreased unpunished FI response rates. The most effective dose of methergoline for increasing punished responding was 0.56 mg/kg. Pretreatment with this dose of methergoline unmasked the facilitatory effects of amphetamine on unpunished responding, but did not antagonize its suppressant effect on punished responding. Therefore, although 5-HT seems to mediate punishment-induced response suppression and to inhibit the facilitatory effects of amphetamine on unpunished responding, it is not apparently involved in the suppressant effect of amphetamine on punished behaviour.

Drug alterations of punished responding after chlordiazepoxide: possible screen for agents useful in minimal brain dysfunction

In the present study, the effect of various stimulant drugs on the action of chlordiazepoxide to increase punished responding was studied. Drugs such as d-amphetamine, methylphenidate and imipramine that are effective in attentional deficit disorder (MBD) were found to reverse this benzodiazepine-induced increase in responding. Phenobarbital which worsens this condition enhanced the benzodiazepine effect. Since the impairment caused by chlordiazepoxide may be analogous to the lack of impulse control noted in MBD, the bupropion antagonism of this action of chlordiazepoxide suggests that bupropion may be useful in MBD.