Tegmental pedunculopontine nucleus lesions do not block cocaine reward

The mesopontine tegmentum in reward and aversion: From cellular heterogeneity to behaviour

The mesopontine tegmentum, comprising the pedunculopontine tegmentum (PPN) and the laterodorsal tegmentum (LDT), is intricately connected to various regions of the basal ganglia, motor systems, and limbic systems. The PPN and LDT can regulate the activity of different brain regions of these target systems, and in this way are in a privileged position to modulate motivated behaviours. Despite recent findings, the PPN and LDT have been largely overlooked in discussions about the neural circuits associated with reward and aversion. This review aims to provide a timely and comprehensive resource on past and current research, highlighting the PPN and LDT's connectivity and influence on basal ganglia and limbic, and motor systems. Seminal studies, including lesion, pharmacological, and optogenetic/chemogenetic approaches, demonstrate their critical roles in modulating reward/aversive behaviours. The review emphasizes the need for further investigation into the associated cellular mechanisms, in order to clarify their role in behaviour and contribution for different neuropsychiatric disorders.

c-Fos marking of identified midbrain neurons coactive after nicotine administration in-vivo

Despite the reduced life expectancy and staggering financial burden of medical treatment associated with tobacco smoking, the molecular, cellular, and ensemble adaptations associated with chronic nicotine consumption remain poorly understood. Complex circuitry interconnecting dopaminergic and cholinergic regions of the midbrain and mesopontine tegmentum are critical for nicotine associated reward. Yet our knowledge of the nicotine activation of these regions is incomplete, in part due to their cell type diversity. We performed double immunohistochemistry for the immediate early gene and surrogate activity sensor, c-Fos, and markers for either cholinergic, dopaminergic or GABAergic cell types in mice treated with nicotine. Both acute (0.5 mg/kg) and chronic (0.5 mg/kg/day for 7 days) nicotine strongly activated GABAergic neurons of the interpeduncular nucleus and medial terminal nucleus of the accessory optic tract (MT). Acute but not chronic nicotine also activated small percentages of dopaminergic and other neurons in the ventral tegmental area (VTA) as well as noncholinergic neurons in the pedunculotegmental and laterodorsal tegmental nuclei (PTg/LDTg). Twenty four hours of nicotine withdrawal after chronic nicotine treatment suppressed c-Fos activation in the MT. In comparison to nicotine, a single dose of cocaine caused a similar activation in the PTg/LDTg but not the VTA where GABAergic cells were strongly activated but dopaminergic neurons were not affected. These results indicate the existence of drug of abuse specific ensembles. The loss of ensemble activation in the VTA and PTg/LDTg after chronic nicotine represents a molecular and cellular tolerance which may have implications for the mechanisms underlying nicotine dependence.

Selective lesions of the cholinergic neurons within the posterior pedunculopontine do not alter operant learning or nicotine sensitization

Cholinergic neurons within the pedunculopontine tegmental nucleus have been implicated in a range of functions, including behavioral state control, attention, and modulation of midbrain and basal ganglia systems. Previous experiments with excitotoxic lesions have found persistent learning impairment and altered response to nicotine following lesion of the posterior component of the PPTg (pPPTg). These effects have been attributed to disrupted input to midbrain dopamine systems, particularly the ventral tegmental area. The pPPTg contains a dense collection of cholinergic neurons and also large numbers of glutamatergic and GABAergic neurons. Because these interdigitated populations of neurons are all susceptible to excitotoxins, the effects of such lesions cannot be attributed to one neuronal population. We wished to assess whether the learning impairments and altered responses to nicotine in excitotoxic PPTg-lesioned rats were due to loss of cholinergic neurons within the pPPTg. Selective depletion of cholinergic pPPTg neurons is achievable with the fusion toxin Dtx-UII, which targets UII receptors expressed only by cholinergic neurons in this region. Rats bearing bilateral lesions of cholinergic pPPTg neurons (>90% ChAT+ neuronal loss) displayed no deficits in the learning or performance of fixed and variable ratio schedules of reinforcement for pellet reward. Separate rats with the same lesions had a normal locomotor response to nicotine and furthermore sensitized to repeated administration of nicotine at the same rate as sham controls. Previously seen changes in these behaviors following excitotoxic pPPTg lesions cannot be attributed solely to loss of cholinergic neurons. These findings indicate that non-cholinergic neurons within the pPPTg are responsible for the learning deficits and altered responses to nicotine seen after excitotoxic lesions. The functions of cholinergic neurons may be related to behavioral state control and attention rather than learning.

Updating of action-outcome associations is prevented by inactivation of the posterior pedunculopontine tegmental nucleus

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The pedunculopontine tegmental nucleus is essential for action–outcome learning.

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Sensitivity to instrumental contingency degradation is blocked by PPTg inactivation.

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Inactivation of PPTg does not change performance of previously learnt operant tasks.

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This is the first demonstration of a role for brainstem in action–outcome learning.

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Learning functions of basal ganglia extend into the deepest parts of the circuitry.

The pedunculopontine tegmental nucleus (PPTg) is in a pivotal position between the basal ganglia and brainstem: it is able to influence and regulate all levels of basal ganglia and corticostriatal activity as well as being a key component of brainstem reticular and motor control circuitry. Consistent with its anatomical position, the PPTg has previously been shown to process rapid, salient sensory input, is a target for Parkinson’s disease treatments and has been implicated in associative learning. We explicitly investigated the role of the posterior pPPTg (pPPTg) in action–outcome processes, where actions are performed with the goal-directed aim of obtaining an anticipated outcome. We assessed rats’ sensitivity to degradation of the contingency between actions (lever pressing) and outcomes (food reward) during either inactivation of pPPTg by microinjection of the GABA agonist muscimol or control infusions of saline. In response to the degradation of contingency between lever press and food reward, saline treated rats rapidly reduced rates of lever pressing whereas muscimol treated rats (pPPTg inactivation) maintained previous lever pressing rates. In contrast, when the contingency between lever press and food reward was unchanged saline and muscimol treated rats maintained their previous rates of lever pressing. This shows that the pPPTg is critically required for updating associations between actions and outcomes, but not in the continued performance of previously learned associations. These results are consistent with a role for the PPTg in ‘higher-order’ associative learning and are the first to demonstrate a brainstem role in action–outcome learning.

Adenosine A1 and A2A receptors are not upstream of caffeine's dopamine D2 receptor-dependent aversive effects and dopamine-independent rewarding effects

Caffeine is widely consumed throughout the world, but little is known about the mechanisms underlying its rewarding and aversive properties. We show that pharmacological antagonism of dopamine not only blocks conditioned place aversion to caffeine, but also reveals dopamine blockade-induced conditioned place preferences. These aversive effects are mediated by the dopamine D(2) receptor, as knockout mice showed conditioned place preferences in response to doses of caffeine that C57Bl/6 mice found aversive. Furthermore, these aversive responses appear to be centrally mediated, as a quaternary analog of caffeine failed to produce conditioned place aversion. Although the adenosine A(2A) receptor is important for caffeine's physiological effects, this receptor seems only to modulate the appetitive and aversive effects of caffeine. A(2A) receptor knockout mice showed stronger dopamine-dependent aversive responses to caffeine than did C57Bl/6 mice, which partially obscured the dopamine-independent and A(2A) receptor-independent preferences. Additionally, the A(1) receptor, alone or in combination with the A(2A) receptor, does not seem to be important for caffeine's rewarding or aversive effects. Finally, excitotoxic lesions of the tegmental pedunculopontine nucleus revealed that this brain region is not involved in dopamine blockade-induced caffeine reward. These data provide surprising new information on the mechanism of action of caffeine, indicating that adenosine receptors do not mediate caffeine's appetitive and aversive effects. We show that caffeine has an atypical reward mechanism, independent of the dopaminergic system and the tegmental pedunculopontine nucleus, and provide additional evidence in support of a role for the dopaminergic system in aversive learning.

A test of the opponent-process theory of motivation using lesions that selectively block morphine reward

The opponent-process theory of motivation postulates that motivational stimuli activate a rewarding process that is followed by an opposed aversive process in a homeostatic control mechanism. Thus, an acute injection of morphine in nondependent animals should evoke an acute rewarding response, followed by a later aversive response. Indeed, the tegmental pedunculopontine nucleus (TPP) mediates the rewarding effects of opiates in previously morphine-naive animals, but not other unconditioned effects of opiates, or learning ability. The aversive opponent process for acute morphine reward was revealed using a place-conditioning paradigm. The conditioned place aversion induced by 16-h spontaneous morphine withdrawal from an acute morphine injection in nondependent rats was abolished by TPP lesions performed prior to drug experience. However, TPP-lesioned rats did show conditioned aversions for an environment paired with the acute administration of the opioid antagonist naloxone, which blocks endogenous opioids. The results show that blocking the rewarding effects of morphine with TPP lesions also blocked the opponent aversive effects of acute morphine withdrawal in nondependent animals. Thus, this spontaneous withdrawal aversion (the opponent process) is induced by the acute rewarding effects of morphine and not by other unconditioned effects of morphine, the pharmacological effects of morphine or endogenous opioids being displaced from opiate receptors.

How best to consider the structure and function of the pedunculopontine tegmental nucleus: Evidence from animal studies

This review presents the hypothesis that the best way to consider the pedunculopontine tegmental nucleus is by analogy with the substantia nigra. The substantia nigra contains two main compartments: the pars compacta and the pars reticulata. The former contains dopamine neurons that project widely within the basal ganglia while the latter is in receipt of corticostriatal output. Similarly, the PPTg contains the Ch5 acetylcholine containing neurons that project to the thalamus and corticostriatal systems (notably the pars compacta of substantia nigra and the subthalamic nucleus) while the non-cholinergic neurons of the pedunculopontine are in receipt of corticostriatal output. Assessment of the location, composition and connections of the pedunculopontine tegmental nucleus is made to support the hypothesis that it has structural similarities with substantia nigra. Assessment of the motor, sensory and cognitive functions of the pedunculopontine is also made, suggesting functional similarities exist also. Having a clear model of pedunculopontine structure and function is a matter of some importance. It is clearly involved in Parkinson's disease and could potentially be a target for therapeutic intervention. If this is to be realized it will be best to have as clear an understanding as possible of pedunculopontine structure and function in order to maximize positive benefits.

An examination of d-amphetamine self-administration in pedunculopontine tegmental nucleus-lesioned rats

The pedunculopontine tegmental nucleus (PPTg) has long been suggested to have a role in reward-related behaviour, and there is particular interest in its possible role in drug reward systems. Previous work found increased i.v. self-administration (IVSA) of d-amphetamine following PPTg lesions when training had included both operant pre-training and priming injections. The present study examined the effect of excitotoxin lesions of the PPTg on d-amphetamine IVSA under three training conditions. Naive: no previous experience of d-amphetamine or operant responding. Pre-trained: given operant training with food before lesion surgery took place. Primed: given single non-contingent d-amphetamine infusion (0.1 mg/0.l ml) at the start of each session. Rats in all conditions were given either ibotenate or phosphate buffer control lesions of the PPTg before d-amphetamine (0.1 mg/0.1 ml infusion) IVSA training took place. Rats received eight sessions of training under a fixed ratio (FR2) schedule of d-amphetamine IVSA, followed by four sessions under a progressive ratio (PR5) schedule. In the naive condition, PPTg-lesioned rats were attenuated in their responding under FR2, and took significantly fewer infusions under PR5 than the control group. Under FR2 in the pre-trained condition, there was no difference between PPTg excitotoxin and control lesioned rats; however, PPTg-lesioned rats took significantly fewer infusions under the PR5 schedule. In the primed condition, there were no differences between PPTg-lesioned and control rats under either FR2 or PR5 schedules. These data demonstrate that operant training prior to PPTg lesion surgery corrects some, but not all, of the deficits seen in the naive condition. PPTg-lesioned rats in both naive and pre-trained conditions showed reduced responding for d-amphetamine under a PR5 schedule. These deficits are overcome by priming with d-amphetamine. We suggest that alterations in striatal dopamine activity following PPTg lesions underlie these effects.

Excitotoxic lesions of the tegmental pedunculopontine nucleus impair copulation in naive male rats and block the rewarding effects of copulation in experienced male rats

The tegmental pedunculopontine nucleus (TPP) of the brainstem mediates food reward in food-sated animals and opiate reward in drug-naive animals. In the present study, we examine the effect of excitotoxic lesions of the TPP on sexual behaviour in naive and experienced male rats. Male, Long-Evans rats received either 0.25 micro L injections of NMDA (4.2 micro g/side) or vehicle (shams) into the TPP. In sexually naive males, complete bilateral TPP lesions decreased all measure of copulation (i.e. mounts, intromissions and ejaculations), prevented acquisition of conditioned sexual excitement, decreased approach preference for a receptive female over a non-receptive one, and decreased non-contact erections; unilateral or bilateral posterior-sparing TPP lesions did not affect any of these measures. Conversely, in sexually experienced males, lesions not only failed to disrupt copulation, but also increased conditioned sexual excitement, decreased post-ejaculatory interval and blocked the effect of prolonged copulation on conditioned sexual excitement. Following differential pairing of distinctive environments with and without copulation, sham males with sexual experience displayed a significant preference for the environment paired with copulation, whereas the lesion males with sexual experience displayed a significant aversion for the environment paired with copulation. These findings indicate that the TPP is critical for the acquisition of copulation in naive males and mediates the rewarding consequences of copulation in experienced males. Together these findings demonstrate that the TPP mediates sexual reward, but that sexual experience is not sufficient to produce a deprivation state.

Neurokinin-1 receptor-expressing neurons in the amygdala modulate morphine reward and anxiety behaviors in the mouse

Mice lacking the neurokinin-1 (NK1) receptor, the preferred receptor for the neuropeptide substance P (SP), do not show many of the behaviors associated with morphine reward. To identify the areas of the brain that might contribute to this effect, we assessed the behavioral effects of ablation of neurons expressing the NK1 receptor in specific regions of the mouse brain using the neurotoxin substance P-saporin. In a preliminary investigation, bilateral ablation of these neurons from the amygdala, but not the nucleus accumbens and dorsomedial caudate putamen, brought about reductions in morphine reward behavior. Subsequently, the effect of ablation of these neurons in the amygdala on anxiety behavior was assessed using the elevated plus maze (EPM), before conditioned place preference (CPP), and locomotor responses to morphine were measured. Loss of NK1 receptor-expressing neurons in the amygdala caused an increase in anxiety-like behavior on the EPM. It also brought about a reduction in morphine CPP scores and the stimulant effect of acute morphine administration relative to saline controls, without affecting CPP to cocaine. NK1 receptor-expressing neurons in the mouse amygdala therefore modulate morphine reward behaviors. These observations mirror those observed in NK1 receptor knock-out (NK1-/-) mice and suggest that the amygdala is an important area for the effects of SP and the NK1 receptor in the motivational properties of opiates, as well as the control of behaviors related to anxiety.

The effect of excitotoxic lesions of the pedunculopontine tegmental nucleus on performance of a progressive ratio schedule of reinforcement

The pedunculopontine tegmental nucleus has connections with sites in both dorsal and ventral striatum, and a number of studies have suggested that it has a role in reward-related behaviour. The present experiment aimed to investigate the perception of reward in pedunculopontine tegmental nucleus-lesioned rats responding for food under a progressive ratio schedule, which measures willingness to work for a given reward. Rats were trained on a progressive ratio-5 schedule for food reward, then given ibotenic acid or sham lesions of the pedunculopontine tegmental nucleus. Their performance under this schedule was examined again following recovery from surgery. Compared with sham-lesioned rats, those with lesions of the pedunculopontine tegmental nucleus showed significantly reduced breaking points and significantly longer post-reinforcement pauses. However, there was no difference between the groups in their latency to collect food pellets once earned, suggesting that pedunculopontine tegmental nucleus excitotoxin and sham-lesioned rats were equally motivated by the presence of food. Excitotoxin-lesioned rats made significantly more responses on the control lever and more entries to the food hopper as progressive ratio increment increased, but did not differ from controls when the schedule requirement was low. These results are interpreted as indicating no global loss of motivation, since lesioned rats performed normally at low schedule requirements, and were as fast as controls to collect pellets. But as the schedule requirement increased, excitotoxin-lesioned rats showed reductions in responding on the active lever (that is, a reduction in breaking point) and an increase in inappropriate responses towards the food hopper and the control lever.We consider these data to indicate that the behavioural deficits in pedunculopontine-lesioned rats arise not from a sensory or hedonic change, but from alteration in the control of motor output.

An examination of the effects of bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus on responding to sucrose reward

The effects of bilateral excitotoxic lesions of the pedunculopontine tegmental nucleus (PPTg) on sucrose intake were examined in three experiments. First, in tests of conditioned place preference using 20% sucrose as the reinforcer, it was shown that lesioned rats, regardless of whether they were food deprived or non-deprived, formed normal place preferences and showed normal amounts of locomotion. However, consumption of 20% sucrose in the pairing trials was increased in the deprived PPTg lesioned rats compared to their matched controls. A second experiment showed that sucrose consumption in the home cage was increased in both deprived and non-deprived PPTg lesioned rats, but only when the concentration of sucrose was greater than 12%: below this there were no differences in intake between the lesioned and control rats. In a third home cage experiment, it was again shown that non-deprived PPTg lesioned rats increased their consumption of 20% sucrose compared to controls. PPTg lesioned rats concomitantly reduced their intake of lab chow such that overall energy intake remained the same as that of control rats. These data are taken to suggest (i) that bilateral excitotoxic lesions of the PPTg increase consumption of sucrose selectively in conditions of high motivational excitement; (ii) that the perception of the rewarding value of 20% sucrose, as judged by place preference, is not affected by these lesions; and (iii) that PPTg lesioned rats are able to adjust their energy intake to accommodate increased sucrose loads. These data are consistent with the hypothesis that bilateral excitotoxic lesions of the PPTg do not affect energy balance regulation or judgment of the hedonic value of sucrose, but that they do affect the control of responding in the face of high levels of motivational excitement.

The effects of excitotoxic lesions of the pedunculopontine tegmental nucleus on conditioned place preference to 4%, 12% and 20% sucrose solutions

A number of studies have suggested that the pedunculopontine tegmental nucleus (PPTg) may play a role in reward-related behaviour. The present study was intended to investigate this further using conditioned place preference. In conditioned place preference paradigms the amount of time spent in a preferred environment is proportional to the value of the reinforcement present, until a maximum is reached. In the present experiments we aimed to determine whether this relationship was affected by lesions of the PPTg by examining the formation of a conditioned place preference to either 4%, 12% or 20% sucrose solutions in food-deprived PPTg lesioned rats. The conditioned place preference apparatus had two compartments different in colour, smell and floor texture. During conditioning, rats were restricted to one compartment or the other, one of which was paired with sucrose. This was carried out during 30 min sessions, alternating conditioned or nonconditioned trials for 14 days. On the test day, rats were given access to both compartments through a connecting chamber, and were scored for side preference over 15 min. Both PPTg and sham lesioned rats showed a conditioned place preference to 12% and 20% sucrose, but no place preference was formed by either group to 4% sucrose. There was no significant difference between the groups in the place preference shown. Consumption of 4% sucrose was not affected by excitotoxic lesions of the PPTg, but PPTg lesioned rats consumed significantly more 12% and 20% sucrose than sham controls. This suggests that perception of reward value, as judged by CPP formation, is unchanged by excitotoxic lesions of the PPTg. The increased consumption of 12% and 20% sucrose shown by rats bearing such lesions is therefore not likely to be a product of altered reward perception.

Diazepam modifies the effect of pedunculopontine lesions on morphine but not on amphetamine conditioned place preference

We have previously shown that T-maze learning impairments caused by lesions to the pedunculopontine tegmental nucleus (PPTg) can be reversed by the anxiolytic diazepam. We now report that diazepam also reverses the effect of PPTg lesions on conditioned place preference (CPP) to morphine but not to amphetamine. Rats with bilateral sham or N-methyl-D-aspartate lesions (0.1 or 0.05 M) to the PPTg were trained in a unbiased CPP paradigm with 2 mg/kg morphine or 2 mg/kg D-amphetamine associated with one compartment of the apparatus and vehicle injections in the alternative compartment. After three drug/saline-compartment pairings, the preference of the animals was assessed by allowing them to explore the entire apparatus for 20 min. In contrast to sham-lesioned subjects, the rats with PPTg lesions did not show a preference for the compartment paired with morphine or amphetamine. In two experiments the expression of a morphine CPP was restored by injecting the lesioned animals with 1 mg/kg of diazepam 30 min before the test session. Diazepam pre-treatment did not restore the expression of amphetamine CPP.

Electrolytic lesions of the pedunculopontine nucleus disrupt concurrent learned aversion induced by NaCl

Bilateral electrolytic lesions in the pedunculopontine nucleus (PPN) impair acquisition of short-term, or concurrent, Taste Aversion Learning (TAL) in rats. This type of TAL is characterized by the daily presentation of two different flavor stimuli at the same time, one associated with simultaneous intragastric administration of an aversive product (hypertonic NaCl) and the other with physiological saline. Sham-lesioned control animals learn this taste discrimination task, but both lesioned animals and control animals learn a long-term, or delayed, TAL task in which each gustatory stimulus is presented individually every other day and the intragastric products, LiCl (0.15 M) and physiological saline, are administered after a 15-min delay. These results are analyzed in the context of the cerebellar circuits involved in learning and in relation to the two TAL modalities described above.

Frontal syndrome as a consequence of lesions in the pedunculopontine tegmental nucleus: a short theoretical review

In this review, it is argued that the consequence of bilateral damage to the pedunculopontine tegmental nucleus (PPTg) in experimental animals is the production of a form of frontal syndrome. Frontal syndrome is a term used to describe the behavioural consequences of damage to the frontal lobes in human patients. These behavioural changes can be classified as disinhibition of behaviour (a release of behavioural control), the production of inappropriate behaviour (which in patients can be either inappropriate actions or verbal behaviour), and the production of perseverative behaviour (the maintenance of an action beyond the point at which it should have been terminated). The psychological changes which underlie these behavioural changes are thought to involve executive functions, which include such things as the prospective planning of sequences of actions, attentional shifting and working memory. In this review, I attempt to demonstrate two things: first, that there are significant anatomical connections from frontostriatal systems to the PPTg. The motor cortex projects directly to the PPTg while the prefrontal cortex contacts it via striatal circuitry, forming clear routes by which the frontal lobes can communicate with the PPTg. Second, having established the existence of connections between frontostriatal systems and the PPTg, behavioural data are described. Experimental animals bearing bilateral lesions of the PPTg have been examined in a wide variety of tasks. Animals bearing such lesions are not impaired in basic processes of feeding, drinking, locomotion, or grooming and simple observation of lesioned rats' normal behaviour reveals no obvious gross impairment in function. However, the results of more subtle tests reveal a wide variety of deficits in various tasks. The outcome of these experiments are in many ways contradictory, but in the vast majority of cases, the changes can be described as involving disinhibition of behaviour, the release of inappropriate behaviour, and the production of perseverative behaviour. Anatomical and behavioural data support the conclusion that there are functional connections between frontal systems and the PPTg. This review also discusses what psychological processes might be served by such connections.

Measuring reward with the conditioned place preference paradigm: a comprehensive review of drug effects, recent progress and new issues

This review gives an overview of recent findings and developments in research on brain mechanisms of reward and reinforcement from studies using the place preference conditioning paradigm, with emphasis on those studies that have been published within the last decade. Methodological issues of the paradigm (such as design of the conditioning apparatus, biased vs unbiased conditioning, state dependency effects) are discussed. Results from studies using systemic and local (intracranial) drug administration, natural reinforcers, and non-drug treatments and from studies examining the effects of lesions are presented. Papers reporting on conditioned place aversion (CPA) experiments are also included. A special emphasis is put on the issue of tolerance and sensitization to the rewarding properties of drugs. Transmitter systems that have been investigated with respect to their involvement in brain reward mechanisms include dopamine, opioids, acetylcholine, GABA, serotonin, glutamate, substance P, and cholecystokinin, the motivational significance of which has been examined either directly, by using respective agonist or antagonist drugs, or indirectly, by studying the effects of these drugs on the reward induced by other drugs. For a number of these transmitters, detailed studies have been conducted to delineate the receptor subtype(s) responsible for the mediation of the observed drug effects, particularly in the case of dopamine, the opioids, serotonin and glutamate. Brain sites that have been implicated in the mediation of drug-induced place conditioning include the 'traditional' brain reward sites, ventral tegmental area and nucleus accumbens, but the medial prefrontal cortex, ventral pallidum, amygdala and the pedunculopontine tegmental nucleus have also been shown to play important roles in the mediation of place conditioning induced by drugs or natural reinforcers. Thus, although the paradigm has also been criticized because of some inherent methodological problems, it is clear that during the past decade place preference conditioning has become a valuable and firmly established and very widely used tool in behavioural pharmacology and addiction research.

Continued trends in the conditioned place preference literature from 1992 to 1996, inclusive, with a cross-indexed bibliography

In light of the overwhelming response to the previous publication in Neuroscience and Biobehavioral Reviews (1993, 17, 21-41) regarding trends in place conditioning (either preference or aversion), the present work constitutes a five-year follow-up to review the empirical research in this behavioral paradigm from 1992 to 1996, inclusively. The behavioral technique has grown as indicated by the number of publications over the last five years which equals those authored over the 35 years covered by our last survey. The previous work used descriptive statistics to explore topical issues, whereas the present work discusses trends since that time and hopes to provide an exhaustive bibliography of the CPP literature, including articles, published abstracts, book chapters and reviews, as well as providing a cross-index of identified key words/drugs tested.

Neurobiological constraints on behavioral models of motivation

The application of neurobiological tools to behavioral questions has produced a number of working models of the mechanisms mediating the rewarding and aversive properties of stimuli. The authors review and compare three models that differ in the nature and number of the processes identified. The dopamine hypothesis, a single system model, posits that the neurotransmitter dopamine plays a fundamental role in mediating the rewarding properties of all classes of stimuli. In contrast, both nondeprived/deprived and saliency attribution models claim that separate systems make independent contributions to reward. The former identifies the psychological boundary defined by the two systems as being between states of nondeprivation (e.g. food sated) and deprivation (e.g. hunger). The latter identifies a boundary between liking and wanting systems. Neurobiological dissociations provide tests of and explanatory power for behavioral theories of goal-directed behavior.