Christian Heidbreder

The medial prefrontal cortex in the rat: evidence for a dorso-ventral distinction based upon functional and anatomical characteristics

The prefrontal cortex in rats can be distinguished anatomically from other frontal cortical areas both in terms of cytoarchitectonic characteristics and neural connectivity, and it can be further subdivided into subterritories on the basis of such criteria. Functionally, the prefrontal cortex of rats has been implicated in working memory, attention, response initiation and management of autonomic control and emotion. In humans, dysfunction of prefrontal cortical areas with which the medial prefrontal cortex of the rat is most likely comparable is related to psychopathology including schizophrenia, sociopathy, obsessive-compulsive disorder, depression, and drug abuse. Recent literature points to the relevance of conducting a functional analysis of prefrontal subregions and supports the idea that the area of the medial prefrontal cortex in rats is characterized by its own functional heterogeneity, which may be related to neuroanatomical and neurochemical dissociations. The present review covers recent findings with the intent of correlating these distinct functional differences in the dorso-ventral axis of the rat medial prefrontal cortex with anatomical and neurochemical patterns.

Current perspectives on selective dopamine D3 receptor antagonists as pharmacotherapeutics for addictions and related disorders

Repeated exposure to drugs of abuse produces long‐term molecular and neurochemical changes that may explain the core features of addiction, such as the compulsive seeking and taking of the drug, as well as the risk of relapse. A growing number of new molecular and cellular targets of addictive drugs have been identified, and rapid advances are being made in relating those targets to specific behavioral phenotypes in animal models of addiction. In this context, the pattern of expression of the dopamine (DA) D3 receptor in the rodent and human brain and changes in this pattern in response to drugs of abuse have contributed primarily to direct research efforts toward the development of selective DA D3 receptor antagonists. Growing preclinical evidence indicates that these compounds may actually regulate the motivation to self‐administer drugs and disrupt drug‐associated cue‐induced craving. This report will be divided into three parts. First, preclinical evidence in support of the efficacy of selective DA D3 receptor antagonists in animal models of drug addiction will be reviewed. The effects of mixed DA D2/D3 receptor antagonists will not be discussed here because most of these compounds have low selectivity at the D3 versus D2 receptor, and their efficacy profile is related primarily to functional antagonism at D2 receptors and possibly interactions with other neurotransmitter systems. Second, major advances in medicinal chemistry for the identification and optimization of selective DA D3 receptor antagonists and partial agonists will be analyzed. Third, translational research from preclinical efficacy studies to so‐called proof‐of‐concept studies for drug addiction indications will be discussed.

Differential role of the medial and lateral prefrontal cortices in fear and anxiety.

In the rat, both the medial and lateral prefrontal cortices (PFC; mPFC and lPFC, respectively) have direct connections with limbic structures that are important in the expression of fear and anxiety. The present study investigated the behavioral effects of excitotoxic lesions of either the mPFC or the lPFC on conditioned and unconditioned fear paradigms. In both unconditioned fear paradigms (open field, elevated plus-maze), lesions of the mPFC decreased anxiety. In fear conditioning, lPFC lesions substantially increased freezing throughout the different phases of the experiment, whereas mPFC lesions increased freezing to contextual cues and showed reduced freezing to discrete cues. These results support the functional role of the PFC in mediating or modulating central states of fear and anxiety and suggest a functional dissociation between the lPFC and mPFC in their role in fear and anxiety.

Isolation rearing induces recognition memory deficits accompanied by cytoskeletal alterations in rat hippocampus.

Social isolation from weaning affects hippocampal structure and function in the rat. The intrinsic dynamic instability of the cytoskeletal microtubular system is essential for neuronal development and organization. Accordingly, the present paper investigated the effects of social isolation on hippocampal levels of α‐tubulin isoforms associated with microtubule dynamics, the dendritic marker MAP‐2 and alterations in locomotor activity and recognition memory. Male Lister Hooded rats (postnatal day 25–28) were housed either in groups or singly (isolated animals) for 30 days. Locomotor activity in a novel arena and novel object recognition were monitored in activity boxes. The hippocampus was dissected out 18 h after the novel object recognition task. Levels of α‐tubulin isoforms and MAP‐2 were analysed using Western blots. The experiments were conducted in duplicate, using two batches of rats obtained from different suppliers. Isolated animals were hyperactive and showed recognition memory deficits in the novel object recognition task. These behavioural alterations were accompanied by specific alterations in hippocampal α‐tubulin isoforms and decreased MAP‐2 expression. The results confirm that rearing rats in isolation produces hyperactivity and cognitive deficits. The behavioural alterations were accompanied by hippocampal cytoskeletal changes consistent with microtubule stabilization, and by decreased MAP‐2 expression. These findings are indicative of an abnormal development of synaptic connections and/or reductions in neuronal cell number. The developmental structural abnormalities in the hippocampus may contribute to the cognitive impairments which result from isolation rearing in rats.

Selective antagonism at dopamine D3 receptors prevents nicotine-triggered relapse to nicotine-seeking behavior.

Drugs of abuse, including, nicotine have been shown to enhance brain reward functions in the mesocortico-limbic dopamine (DA) system in general, and the nucleus accumbens in particular. The latter occupies a prominent position in the ventral striatum and expresses a high density of DA D3 receptors. As such, the present study aimed at investigating the effect of the selective D3 receptor antagonist SB-277011-A on both the stable maintenance of intravenous nicotine self-administration and nicotine-triggered relapse to nicotine-seeking behavior in the rat. SB-277011-A (3–10 mg/kg i.p.) significantly reduced reinstatement of nicotine-seeking behavior without affecting nicotine self-administration per se. These results suggest that DA D3 receptors are involved in the reinstatement of nicotine-seeking behavior independently of any interaction with the primary reinforcing effects of nicotine itself. These findings point toward the potential use of selective DA D3 receptor antagonists for the pharmacotherapeutic management of relapse to drug-seeking behaviors.

High-performance liquid chromatography/tandem mass spectrometric assay for the simultaneous measurement of dopamine, norepinephrine, 5-hydroxytryptamine and cocaine in biological samples

A rapid liquid chromatography/tandem mass spectrometry (LC-MS/MS) method has been developed for the measurement of dopamine (DA), 5-hydroxytryptamine (5HT) and norepinephrine (NE) in brain microdialysates. The assay has also been utilised for the simultaneous measurement of these neurotransmitters and cocaine in brain dialysates. The neurotransmitters and cocaine were resolved in a single 4-min run using a binary gradient elution profile. The analytes were detected using tandem mass spectrometry in the positive ion electrospray mode. The limits of detection for DA, NE, 5HT and cocaine were 200, 1000, 900 pM and 1 pg ml(-1), respectively.

Selective dopamine D3 receptor antagonism by SB-277011A attenuates cocaine reinforcement as assessed by progressive-ratio and variable-cost-variable-payoff fixed-ratio cocaine self-administration in rats.

In rats, acute administration of SB‐277011A, a highly selective dopamine (DA) D3 receptor antagonist, blocks cocaine‐enhanced brain stimulation reward, cocaine‐seeking behaviour and reinstatement of cocaine‐seeking behaviour. Here, we investigated whether SB‐277011A attenuates cocaine reinforcement as assessed by cocaine self‐administration under variable‐cost–variable‐payoff fixed‐ratio (FR) and progressive‐ratio (PR) reinforcement schedules. Acute i.p. administration of SB‐277011A (3–24 mg/kg) did not significantly alter cocaine (0.75 mg/kg/infusion) self‐administration reinforced under FR1 (one lever press for one cocaine infusion) conditions. However, acute administration of SB‐277011A (24 mg/kg, i.p.) progressively attenuated cocaine self‐administration when: (a) the unit dose of self‐administered cocaine was lowered from 0.75 to 0.125–0.5 mg/kg, and (b) the work demand for cocaine reinforcement was increased from FR1 to FR10. Under PR (increasing number of lever presses for each successive cocaine infusion) cocaine reinforcement, acute administration of SB‐277011A (6–24 mg/kg i.p.) lowered the PR break point for cocaine self‐administration in a dose‐dependent manner. The reduction in the cocaine (0.25–1.0 mg/kg) dose–response break‐point curve produced by 24 mg/kg SB‐277011A is consistent with a reduction in cocaines reinforcing efficacy. When substituted for cocaine, SB‐277011A alone did not sustain self‐administration behaviour. In contrast with the mixed DA D2/D3 receptor antagonist haloperidol (1 mg/kg), SB‐277011A (3, 12 or 24 mg/kg) failed to impede locomotor activity, failed to impair rearing behaviour, failed to produce catalepsy and failed to impair rotarod performance. These results show that SB‐277011A significantly inhibits acute cocaine‐induced reinforcement except at high cocaine doses and low work requirement for cocaine. If these results extrapolate to humans, SB‐277011A or similar selective DA D3 receptor antagonists may be useful in the treatment of cocaine addiction.

Amphetamine-induced neurochemical and locomotor responses are expressed differentially across the anteroposterior axis of the core and shell subterritories of the nucleus accumbens

The administration of psychostimulants increases dopamine (DA) release within the nucleus accumbens (NAC), a terminal projection site of mesolimbic DA neurons, originating in the ventral tegmental area (VTA). Recent evidence demonstrates that two subdivisions of the NAC, the dorsolateral core and the ventromedial shell, can be distinguished by morphological and immunohistochemical differences, as well as by their distinct anatomical connections. It has been suggested that these two subregions subserve different functions that are related to goal‐directed behaviors, stimulus‐reward associations, and reinforcement induced by addictive drugs. The shell region, in particular, modulates inputs from the limbic system, whereas the core is preferentially innervated by nuclei that process motor information. In the present study, we sought to investigate if (1) the direct infusion of d‐amphetamine (AMPH) by reverse microdialysis into either the core or shell of the NAC across its anteroposterior axis differentially affects dialysate DA and 5‐HT levels, and (2) these subterritories also subserve different behavioral functions. Following the determination of basal DA and 5‐HT levels, four increasing concentrations of AMPH (0.05, 0.10, 0.50, 1.00 μM) were substituted for the dialysis perfusate for 60 min each. Movement units were detected by an infrared sensor and were transmitted through a motion interface to an activity monitor analyzer. AMPH produced a dose‐dependent increase in locomotor activity after microinfusion into either the rostral shell, caudal shell or core of the NAC. The potency of the AMPH‐induced locomotor activating effect was significantly higher in the rostral shell of the NAC compared with the caudal shell and the core. The lowest concentrations of AMPH (0.05, 0.1 μM) produced an equipotent decrease in dialysate DA in either the rostral shell, caudal shell, or core. At 1.0 μM, however, AMPH selectively increased DA in the rostral shell, whereas DA reached baseline values both in the caudal shell and core. Basal dialysate DA levels were significantly higher in the core relative to both the rostral and caudal parts of the shell. The highest dose of AMPH significantly increased dialysate 5‐HT levels over baseline only in the caudal shell of the NAC. The basal dialysate 5‐HT levels did not significantly differ between the three subterritories of the NAC. These results emphasize the heterogeneity and functional compartmentalization within the NAC, the differential regulation of neurochemical and motor responses across the anteroposterior axis of the NAC, and the preferential effect of AMPH in the rostral shell subterritory of the NAC. Synapse 29:310–322, 1998.

Systemic administration of ghrelin increases extracellular dopamine in the shell but not the core subdivision of the nucleus accumbens.

The gut-hormone ghrelin endogenously binds to the ghrelin receptor (GHS-R) to promote foraging and feeding behaviours mainly via the hypothalamic arcuate nucleus (ARC). GHS-Rs are also expressed in midbrain dopaminergic neurons of the ventral tegmental area (VTA) suggesting that ghrelin may modulate the mesolimbic dopamine (DA) system. In support of this hypothesis, previous results have shown that intraventricular administration of ghrelin in rats increases DA levels in the nucleus accumbens (NAc). In the present study, the systemic doses of ghrelin capable of triggering central activation were first determined, and growth hormone (GH) levels were used as a marker of ghrelin-induced activation. Similar dose regimen was then used to measure ghrelin-induced effects on extracellular levels of monoamines in the shell and core subdivisions of the NAc using microdialysis in freely moving rats. We show that subcutaneous (s.c.) administration of ghrelin produced an increase in basal plasmatic ghrelin concentrations that was paralleled by enhanced GH secretion. In addition, a significant increase in extracellular levels of DA was observed specifically in the NAc shell, with no effect in the core subdivision. Extracellular serotonin (5-HT) levels were also affected in the shell subregion, but without reaching statistical significance. Increased extracellular DA levels in the NAc shell have been typically associated with the acute reinforcing effects of addictive drugs. The present findings therefore suggest that systemic ghrelin may modulate the valence of reinforcers such as food and drugs of abuse by interfering with mesolimbic DA activity.

Selective responding of nucleus accumbens core and shell dopamine to aversively conditioned contextual and discrete stimuli.

Dopamine transmission within the nucleus accumbens has been implicated as a neurochemical substrate of associative learning processes. It has been suggested that the acquisition of classically conditioned fear to a specific environment, or context, differs fundamentally from the development of conditioned fear to a discrete stimulus, such as a light or a tone. In this study, we assessed extracellular dopamine in the rat nucleus accumbens shell and core during the expression of a conditioned fear response. Animals were aversively conditioned to either a context or a tone and extracellular dopamine was measured in the nucleus accumbens shell and core by in vivo microdialysis over the next 2 days as animals were returned first to the conditioning chamber (day 1: context test), and subsequently as animals were again returned to the chamber and presented with the conditioned tone stimulus (day 2: tone test). Dopamine levels in the core were significantly higher in the Context-Shock group compared to the Tone-Shock group during the 30-min exposure to context while dopamine levels in the nucleus accumbens shell did not differ significantly during the context test between groups. In contrast, extracellular dopamine in the shell but not the core of Tone-Shock animals increased significantly during presentation of the tone. Dopamine in both the shell and core remained unchanged during the tone test in the Context-Shock groups.These data suggest distinct roles for shell and core dopamine transmission in the expression of a conditioned emotional response. While dopamine increased in the shell primarily during the presentation of a discrete tone conditioned stimulus, core dopamine responded more to a contextual conditioned stimulus. These results may reflect differences in either the type of information acquired or the salience of the learned associations which are formed to a context vs. a discrete tone cue.