Blockade of amphetamine but not opiate-induced locomotion following antagonism of dopamine function in the rat

The effects of pharmacological blockade of dopamine (DA) receptors or 6-OHDA lesions of mesolimbic DA fibers on the locomotor-activating properties of systemic amphetamine (0.35 mg/kg) and heroin (0.5 mg/kg) were examined. Pharmacological blockade of DA receptors or lesions of mesolimbic DA neurons blocked amphetamine but not heroin-induced locomotion. These results show that the opiate receptors essential for opiate-induced locomotor activation are not located on mesolimbic DA neurons. It appears that DA plays a primary role in stimulant-induced locomotion, but may have only a secondary role in opiate locomotion.

Corticotropin-releasing factor potentiates acoustic startle in rats: blockade by chlordiazepoxide

A series of experiments was performed to investigate the effects of corticotropin-releasing factor (CRF) on the amplitude of the acoustic startle response (ASR) in rats. Intracerebroventricular (ICV) administration of 1 microgram rat CRF significantly potentiated acoustic startle amplitude; these effects were reversed in a dose-dependent manner by pretreatment with the benzodiazepine chlordiazepoxide (CDP). Doses of CDP that antagonized CRF-potentiated ASR did not lower startle baseline or antagonize amphetamine- or strychnine-potentiated ASR. These results suggest that CRF has "anxiogenic" properties and may serve as a neuroendocrine modulator of stress-enhanced behaviors.

The effects of verapamil on the locomotor-activating properties of corticotropin releasing factor (CRF) in the rat

CRF was administered to rats following intracerebroventricular administration of the calcium antagonist verapamil. Treatment with verapamil elicited biphasic dose-response effects on rat locomotor activity: doses equimolar to CRF treatment slightly increased CRF-stimulated locomotion, while doses of verapamil ten times greater slightly depressed CRF-stimulated locomotion. These results suggest that CRF-stimulated locomotion, unlike other effects of CRF, is not mediated in a simple way by processes dependent on neuronal calcium influx.

Central dopamine hyperactivity in rats mimics abnormal acoustic startle response in schizophrenics

Schizophrenic patients show loss of the normal inhibition of the acoustic startle response (ASR) when the startle stimulus is preceded by a weak prepulse stimulus. In rats experimentally induced to have locally supersensitive dopamine receptors within forebrain dopamine terminal regions, we find a similar deficit in "prepulse inhibition" of the ASR. Our results suggest that there are intriguing parallels between the aberrant sensory gating of the ASR demonstrated by schizophrenics and that demonstrated by rats during stimulation of supersensitive brain dopamine receptors. Thus, prepulse inhibition of the ASR provides a useful paradigm for investigating the bridge between mesolimbic dopamine overactivity and the specific time-dependent information processing-sensory gating disturbances that have been identified in schizophrenic patients.

Effects of chronic dietary lithium on behavioral indices of dopamine denervation supersensitivity in the rat

The present study was designed to test the hypothesis that chronic lithium (Li) treatment could alter the behavioral manifestations of dopamine (DA) denervation supersensitivity. Rats were maintained on a Li diet continuously for 4 weeks before and 4 weeks after bilateral chemical denervation of DA terminal fields in the nucleus accumbens (N.Acc.) and the caudate nucleus (CN). We examined the behavioral responses of these animals to apomorphine (0.1 mg/kg s.c.) treatment 2 and 4 weeks postdenervation. Animals exposed to similar chemical denervation but maintained on a control diet exhibited well-documented "supersensitive" behavioral responses to apomorphine: N.Acc. denervated animals demonstrated increases in ambulatory sniffing, rearing and locomotor activity; CN-denervated animals demonstrated increases in rearing and focused sniffing in one location but no increases in locomotor activity. Compared to these control-fed animals, Li-diet animals receiving either CN- or N.Acc-denervation demonstrated a significantly decreased "supersensitive" behavioral response to apomorphine. N.Acc.-denervated animals receiving Li showed less apomorphine-induced locomotor activity. CN-denervated animals receiving Li showed less focused behavior which became evident as actual increases in locomotor activity after apomorphine. These results suggest that chronic dietary Li modifies postsynaptic mechanisms to suppress elements of the behavioral manifestations of supersensitive mesolimbic and nigrostriatal DA activity.

Effects of naloxone on heroin-, amphetamine- and caffeine-stimulated locomotor activity in the rat

We investigated the effects of naloxone on the locomotor activating properties of heroin (0.25 mg/kg SC), d-amphetamine (0.25 mg/kg SC) and caffeine (7.5 mg/kg SC). Naloxone eliminated heroin-stimulated locomotion at doses approximately six times lower than those that blocked amphetamine-stimulated locomotion. Caffeine-induced locomotor activation was insensitive to naloxone at all doses tested. These results suggest that central opiate systems are differentially involved in the behavioral activation produced by heroin, amphetamine and caffeine.

Separate neural substrates of the locomotor-activating properties of amphetamine, heroin, caffeine and corticotropin releasing factor (CRF) in the rat

Destruction of dopamine terminals within the nucleus accumbens (N.Acc.) with 6-hydroxydopamine blocked the locomotor activating properties of d-amphetamine, but not caffeine or corticotropin-releasing factor (CRF). Infusion of muscimol into the region of nucleus accumbens efferent terminals in the substantia innominata and lateral preoptic region (SI/LPO) blocked amphetamine-, but not caffeine- or CRF-stimulated locomotion. These muscimol infusions also blocked heroin-stimulated locomotion. These results suggest that amphetamine, acting through a process dependent on N.Acc. dopamine transmission, stimulates locomotion by decreasing GABAergic activity within this N.Acc.-SI/LPO projection; heroin, known to act through a process dependent on N.Acc. opiate receptor activation, also stimulates locomotion by decreasing GABAergic activity within the SI/LPO; caffeine and CRF produce their activation through different neural substrates.

The neural substrates of apomorphine-stimulated locomotor activity following denervation of the nucleus accumbens

Efferent fibers from the nucleus accumbens (N.Acc.) to the substantia innominata and lateral preoptic region (SI/LPO) are believed to be essential for the supersensitive locomotor response to apomorphine in rats following 60HDA-induced denervation of the N.Acc., since destruction of cells within the SI/LPO eliminates this supersensitive response. In the present study, the neurochemical properties of this efferent projection were investigated. Injection of muscimol, a GABA agonist, directly into the SI/LPO produced a dose-dependent decrease in supersensitive locomotor activity at doses that did not alter baseline locomotion. Higher doses of muscimol actually produced a prolonged locomotor activation in all animals. These results suggest that the locomotor activation following stimulation of supersensitive dopamine receptors within the N.Acc. may involve a decrease in GABAergic activity at the level of the SI/LPO.

Substantia innominata: critical link in the behavioral expression of mesolimbic dopamine stimulation in the rat

The 'supersensitive' locomotor response to apomorphine resulting from 6-hydroxydopamine (6-OHDA)-induced denervation of the nucleus accumbens (N.Acc.) was studied in animals following selective damage to cell bodies within the substantia innominata (SI) and lateral preoptic region (LPO). Ibotenic acid-induced lesions of the SI and LPO dramatically attenuated the locomotor response to apomorphine in 6-OHDA-infused animals. Thus, the locomotor response that follows dopaminergic stimulation within the N.Acc. depends to a great degree on the integrity of cells within the SI and LPO region.

Electrolytic lesions of the substantia innominata and lateral preoptic area attenuate the 'supersensitive' locomotor response to apomorphine resulting from denervation of the nucleus accumbens

Apomorphine-stimulated locomotion in the rat is greatly enhanced following destruction of dopamine terminals in the nucleus accumbens (NA) with 6-hydroxydopamine (6-OHDA). While this augmented response is ascribed to the action of the dopamine stimulant apomorphine on supersensitive receptors within the NA, little is known regarding the mechanisms by which increased receptor stimulation within the NA influences lower motor circuitry to produce changes in locomotion. In this study, we examined apomorphine-stimulated locomotion in 6-OHDA-infused rats following electrolytic damage to the terminal region of first-order NA efferent fibers within the substantia innominata and lateral preoptic area. This damage greatly diminished the locomotor response to apomorphine in 6-OHDA-infused animals, but did not diminish locomotion in vehicle-infused animals. Destruction of dopamine terminals within the NA has also been reported to enhance the place-preference response to apomorphine in rats. Damage to the substantia innominata and lateral preoptic area significantly decreased the place-preference for apomorphine-paired environments in 6-OHDA-infused animals, but did not alter place-preference responses in vehicle-infused animals. Our results indicate that the efferent pathway from the NA to the substantia innominata and lateral preoptic area serves as an important output of mesolimbic activity into motor circuitry involved in the expression of apomorphine-stimulated locomotion and place-preference.

Restrained rats learn amphetamine-conditioned locomotion, but not place preference

The relationship between the motor-activating and positive-reinforcing properties of d-amphetamine was examined in the place-preference paradigm. Two groups of animals were trained to associate one environment with amphetamine, and another environment with saline. Animals that were allowed to locomote in both environments during training later demonstrated a preference for the amphetamine-paired environment; animals in which hyperactivity was limited in both environments later failed to show any preference. However, both groups of animals demonstrated a conditioned locomotor activation to the amphetamine-associated environments. Our results suggest that a place-preference demonstrated for an amphetamine-paired environment depends on the ability of the drug to increase locomotor behavior.

Cholecystokinin produces conditioned place-aversions, not place-preferences, in food-deprived rats: evidence against involvement in satiety

The net reinforcing/aversive properties of cholecystokinin (CCK) were measured using a conditioned place-preference paradigm. Both sated and food-deprived rats showed a dose dependent aversion to an environment previously paired with CCK that was correlated with the effects of CCK on feeding. In contrast, all rats showed a conditioned preference for an environment previously paired with food. These results demonstrate the aversive properties of CCK and suggest that the decrease in feeding that follows peripheral administration of CCK results from a CCK-induced malaise.

Paradoxical reinforcing properties of apomorphine: effects of nucleus accumbens and area postrema lesions

Apomorphine (0.01-10.0 mg/kg, subcutaneously) paradoxically produced both dose-dependent aversive and positive reinforcing effects, as measured in conditioned taste aversion and place preference paradigms, respectively. The conditioned taste aversions produced by apomorphine were not modified in rats with bilateral 6-hydroxydopamine (6-OHDA) lesions of the nucleus accumbens (producing 92% depletion of dopamine in the nucleus accumbens) nor in rats with thermal lesions of the area postrema. Both types of lesions were behaviorally verified as effective in other paradigms; the 6-OHDA lesions potentiated the facilitatory effects to a novel flavor paired with scopolamine methylnitrate (1.0 mg/kg, intraperitoneally). However, 6-OHDA lesions of the nucleus accumbens did clearly potentiate the conditioned place preferences induced by apomorphine. These results suggest that both the positive reinforcing and locomotor effects of apomorphine may partially result from activation of post-synaptic dopamine receptors in the nucleus accumbens. Moreover, the dissociation of apomorphine's aversive and positive reinforcing properties revealed by the 6-OHDA lesions may provide the first step in attempts to pinpoint the different brain sites of action where apomorphine produces its opposite motivational effects.

The effect of tail pinch on the acoustic startle response in rats

A series of 3 experiments was carried out to evaluate the effect of tail pinch (TP) on the amplitude of the acoustic startle response (ASR) in rats. There is a consistent group of pharmacological findings which support the view that the amplitude of the ASR is facilitated by transmission in both dopaminergic and noradrenergic neural systems. It has recently been reported that TP increases cortical norepinephrine release and pars compacta unit activity. It might therefore be expected that TP would facilitate the amplitude of the ASR. This hypothesis was tested in the first experiment. Surprisingly, it was found that TP significantly depressed startle amplitude. In the second experiment if was found that this TP-induced depression in startle amplitude was reduced by damage to the nucleus accumbens and that the amount of reduction correlated with the extent of damage. In the third experiment the generality of the effect of TP on sensorimotor reactivity was evaluated by testing its effect on footshock threshold and airpuff-elicited startle response. TP also depressed responsiveness in these tests. These results are consistent with other observations that the nucleus accumbens plays a role in the modulation of sensorimotor reactivity.