Anthony G. Phillips

A YAC Mouse Model for Huntington’s Disease with Full-Length Mutant Huntingtin, Cytoplasmic Toxicity, and Selective Striatal Neurodegeneration

We have produced yeast artificial chromosome (YAC) transgenic mice expressing normal (YAC18) and mutant (YAC46 and YAC72) huntingtin (htt) in a developmental and tissue-specific manner identical to that observed in Huntingtons disease (HD). YAC46 and YAC72 mice show early electrophysiological abnormalities, indicating cytoplasmic dysfunction prior to observed nuclear inclusions or neurodegeneration. By 12 months of age, YAC72 mice have a selective degeneration of medium spiny neurons in the lateral striatum associated with the translocation of N-terminal htt fragments to the nucleus. Neurodegeneration can be present in the absence of macro- or microaggregates, clearly showing that aggregates are not essential to initiation of neuronal death. These mice demonstrate that initial neuronal cytoplasmic toxicity is followed by cleavage of htt, nuclear translocation of htt N-terminal fragments, and selective neurodegeneration.

TARGETED DISRUPTION OF THE HUNTINGTON'S DISEASE GENE RESULTS IN EMBRYONIC LETHALITY AND BEHAVIORAL AND MORPHOLOGICAL CHANGES IN HETEROZYGOTES

Huntingtons disease (HD) is an incurable neuropsychiatric disease associated with CAG repeat expansion within a widely expressed gene that causes selective neuronal death. To understand its normal function, we have created a targeted disruption in exon 5 of Hdh (Hdhex5), the murine homolog of the HD gene. Homozygotes die before embryonic day 8.5, initiate gastrulation, but do not proceed to the formation of somites or to organogenesis. Mice heterozygous for the Hdhex5 mutation display increased motor activity and cognitive deficits. Neuropathological assessment of two heterozygous mice shows significant neuronal loss in the subthalamic nucleus. These studies show that the HD gene is essential for postimplantation development and that it may play an important role in normal functioning of the basal ganglia.

Dopaminergic substrates of amphetamine-induced place preference conditioning.

The conditioned place preference paradigm was used to study the reinforcing properties of D-amphetamine. Rats were injected (i.p.) with D-amphetamine sulphate (0.5, 1.0 or 5.0 mg/kg) and 10 min later confined for 30 min to one side of a shuttle box in which each of the two compartments had distinctive features. On alternate (control) days they received saline injections and were confined for 30 min to the opposite side. At all doses D-amphetamine produced place preference for the distinctive compartment that previously had been associated with the drug. Pretreatment with haloperidol (0.15 or 1.0 mg/kg) antagonized the place preference produced by amphetamine (1.5 mg/kg). By itself, haloperidol (0.15 or 1.0 mg/kg) did not produce place aversion. In separate experiments the D-amphetamine-induced place preference was examined in rats that had received 6-hydroxydopamine (6-OHDA) lesions of the nucleus accumbens. Animals with the greatest depletion of dopamine did not show preference for the compartment associated with D-amphetamine. Furthermore, the time spent on the amphetamine-reinforced side correlated significantly with the levels of dopamine remaining in the nucleus accumbens but not with the dopamine content in the striatum. Depletion of peripheral catecholamines by systemic injections of 6-OHDA did not affect D-amphetamine-induced place preference conditioning. Other groups of animals that received the dopamine receptor agonist, apomorphine, also developed a conditioned preference for the compartment that had been associated with the drug treatment. These findings support the view that the reinforcing effects of D-amphetamine are mediated by central dopamine-containing neurons, and in particular those of the mesolimbic system.

Dopamine functions in appetitive and defensive behaviours

The data reviewed here are compatible with the hypothesis that telencephalic dopamine activity is elicited by motivationally significant stimuli which in turn creates a neural state in which animals are more prepared to respond to significant stimuli in the environment. This analysis may be viewed as extensions of both the sensorimotor hypothesis, which depicts dopamine as potentiating the ability of stimuli to elicit responses (Clody and Carlton, 1980; Marshall et al., 1974; White, 1986) and of the incentive motivational hypothesis, which emphasizes the importance of dopamine in responding to stimuli that serve as signals of biologically significant events (Blackburn et al., 1989a; Crow, 1973; Mogenson and Phillips, 1976). In addition, we have sought to emphasize that not all responses are equally dependent upon the integrity of forebrain dopamine activity. Some responses, such as ingestion of standard foods by hungry animals, copulation, and escape, are relatively impervious to dopamine disruption. Further, once other behaviours, such as avoidance or appetitive operant responses, have been acquired, they can be maintained at an initially high rate despite perturbation of dopamine systems, although performance deteriorates with repeated testing. This analysis has emerged from the joint consideration of how both appetitive and defensive behaviours are influenced by dopamine antagonists, along with an examination of dopamine release during sequences of behaviour. The data reviewed suggest that dopamine is involved in fundamental psychological processes through which environmental stimuli come to exert control over certain aspects of behaviour. In the future, as knowledge in this field advances, there will have to be an integration of the literature on dopamine and motivation with the literature on dopamine and motor systems. We expect that dopamine release will be seen as a mechanism by which important environmental cues, of innate or learned significance, lead to a general enhancement of motor skeletal responses directed towards distal cues. We conclude with a caveat: Caution must be exercised when attempting to infer a general role of any neurotransmitter in motivated behaviour based on the study of a limited number of motivational systems. Although neurotransmitter pathways may figure prominently in the control of certain behaviours, it is incorrect to think of neurotransmitters as having a single role in behaviour. However, when comparative analyses reveal a common thread among different motivational systems, as is becoming apparent for the general role of mesotelencephalic dopamine pathways in behaviour, then the goal of generating coherent and comprehensive theory concerning a neurotransmitters function in behaviour will begin to be realised.(ABSTRACT TRUNCATED AT 400 WORDS)

Sexual behavior increases dopamine transmission in the nucleus accumbens and striatum of male rats: comparison with novelty and locomotion.

Extracellular concentrations of dopamine (DA) and its metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were examined concurrently, using in vivo microdialysis, in the nucleus accumbens and dorsal striatum of sexually active male rats during tests of locomotor activity, exposure to a novel chamber, exposure to sex odors, the presentation of a sexually receptive female, and copulation. DA increased significantly in the nucleus accumbens when the males were presented with a sexually receptive female behind a screen and increased further during copulation. Although DA also increased significantly in the dorsal striatum during copulation, the magnitude of the effect was significantly lower than that observed in the nucleus accumbens. In contrast, forced locomotion on a rotating drum, exposure to a novel chamber, and exposure to sex odors did not increase DA significantly in either region, although both DOPAC and HVA increased significantly in both regions during the locomotion test. These results indicate that novelty or locomotor activity alone cannot account for the increased extracellular DA concentrations observed in the nucleus accumbens of male rats during the presentation of a sexually receptive female behind a screen, nor can they account for the increased DA concentrations observed in both the nucleus accumbens and dorsal striatum of male rats during copulation. The preferential increase in DA transmission in the nucleus accumbens, compared with that in the striatum, suggests that anticipatory and consummatory aspects of sexual activity may belong to a class of naturally occurring events with reward values that are mediated by DA release in the nucleus accumbens. Much of the evidence linking central dopamine (DA) systems to the control of mammalian sexual behavior comes from pharmacological analyses in rats. Systemic administration of DA receptor agonists stimulates anticipatory and

Sexual behavior enhances central dopamine transmission In the male rat

Central dopamine transmission was examined in the nucleus accumbens and striatum of sexually experienced male rats during mating behaviour using in vivo brain microdialysis. Dopamine release increased significantly in the nucleus accumbens when males were placed in a novel mating chamber and when a receptive female was introduced behind a screen partitioning this chamber. Subsequently, during copulation dopamine transmission increased sharply, this being followed by a gradual decrease after the female was removed. In contrast, striatal dopamine transmission increased significantly only during copulation. These data provide a neurochemical basis for the well-known interactions between dopaminergic drugs and male sexual behaviour and demonstrate the feasibility of using brain microdialysis to elucidate the neurochemical correlates of motivated behaviour.

Reinforcing Effects of Morphine Microinjection into the Ventral Tegmental Area

A neural substrate for the reinforcing property of an opiate drug was identified in the ventral tegmental area (VTA) by establishing conditioned reinforcement to salient environmental stimuli paired with intracerebral microinjections of morphine. Bilateral microinjections of morphine into the VTA in doses of 0.2 microgram and 1.0 microgram produced a subsequent change in place preference to a distinctive compartment previously associated with the stimulant effects of morphine. Microinjection of 1.0 microgram morphine at sites 2.5 mm dorsal to the VTA had no effect. Pretreatment with naloxone (2 mg/kg) antagonized the reinforcing effects of 1.0 microgram morphine as this group showed no significant change in place preference. Nor did control groups receiving microinjections of sterile physiological saline. Taken together, these data suggest that opiate receptors, located in the ventral tegmental area, play an important role in mediating the reinforcing effects of morphine. The possible involvement of dopaminergic neurons in these effects is discussed.

Functional Differences Between the Prelimbic and Anterior Cingulate Regions of the Rat Prefrontal Cortex

The effects of reversible lidocaine-induced lesions of 2 subregions of the rat medial prefrontal cortex (mPFC) were examined on a series of cognitively based foraging behaviors on a radial-arm maze. Lesions of the prelimbic (PL) or anterior cingulate (AC) cortex prior to the retention phase of a delayed-foraging task disrupted performance differentially; rats with PL lesions visited arms in a random manner, whereas rats with AC lesions revisited previously baited arms preferentially. Rats with AC lesions were also impaired on a single-trial foraging task; they made numerous revisits to previously baited arms. PL lesions had no effect on performance of this task in well-trained rats. However, rats trained on the 2-phase task did not adapt to a new foraging strategy after a PL lesions, when they were switched unexpectedly to the single-trial foraging task. These data demonstrate functional heterogeneity within the rat mPFC and suggest that the PL is involved in processes through which recently acquired information is used to organize and modify foraging behavior, whereas the AC may play an important role in response flexibility.

Attenuation of heroin reward in rats by disruption of the mesolimbic dopamine system

The rewarding propertics of systemically administered heroin were investigated with a conditioned place-preference paradigm. Reinforcing effects were observed with all doses of heroin tested (0.5, 1.0, 2.0 mg/kg) as indicated by a significant increase in preference for the place paired with drug injections. No similar change in preference was observed following saline injections. The rewarding effect of heroin (2 mg/kg) was attenuated by pretreatment with haloperidol (0.2 mg/kg), but haloperidol alone did not produce a place aversion. Lesions induced by 6-hydroxydopamine (6-OHDA) of the mesolimbic dopaminergic pathway at the level of the nucleus accumbens also attenuated the heroin-induced place preference. In contrast, depletion of central and peripheral noradrenaline induced by 6-OHDA had no significant effect on heroin-induced place preference. These data suggest an important role for the mesolimbic DA system in mediating the reinforcing effects of heroin.

Role of dopamine in anticipatory and consummatory aspects of sexual behavior in the male rat.

The ability of dopamine (DA) receptor antagonists to disrupt anticipatory and consummatory measures of sexual behavior displayed by male rats in bilevel chambers was investigated. In Experiment 1, systemic administration of haloperidol, pimozide, and the D1 antagonist SCH 23390 reduced the number of anticipatory level changes (LC) displayed during a 5-min period before the introduction of a sexually receptive female, increased the mount and intromission latencies (ML and IL), and decreased the number of intromissions before ejaculation (NI) and the total number of ejaculations (NE). The dosages of these drugs required to reduce the LC were lower than those required to increase the ML or IL. Clozapine and the D2 antagonist sulpiride reduced the LC and increased the IL at comparable dosages, although neither drug affected the NI or NE. High dosages of haloperidol, pimozide, and clozapine delayed or abolished level changing and the initiation of copulation. In Experiment 2, bilateral infusions of haloperidol into the nucleus accumbens reduced the LC but did not affect consummatory measures of copulation, whereas bilateral infusions into the dorsal striatum increased the NE. Midline infusions of haloperidol to the medial preoptic area (MPOA) produced nearly all the effects of systemic administration, including a reduced LC, increased ML and IL, a decreased NI, and a decreased NE. These results indicate that both anticipatory and consummatory measures of sexual behavior were disrupted by DA receptor antagonists; however, the measure of anticipatory sexual behavior was more sensitive to disruption than consummatory measures of copulation. DA in the nucleus accumbens and MPOA may be involved in the control of anticipatory sexual behavior, whereas in the MPOA it may also be involved in the initiation of copulation and copulatory rate.