Carol A. Murphy

Differential involvement of dopamine in the shell and core of the nucleus accumbens in the expression of latent inhibition to an aversively conditioned stimulus

Latent inhibition, the process whereby pre-exposure to a conditioned stimulus without consequence impairs subsequent learning of an association between the conditioned stimulus and an unconditioned stimulus, is reportedly disrupted in both amphetamine-treated rats and in acute schizophrenics. This has led to the suggestion that disruptions in latent inhibition model some of the cognitive impairments associated with hyperactive dopamine transmission as it is expressed in schizophrenic patients. Specifically, fluctuations in dopamine neurotransmission within the nucleus accumbens have been implicated in the mediation of latent inhibition; however, it has not been established whether these dopamine-mediated effects occur in the shell or core subregion of the nucleus. In the present study, 48h after conditioned stimulus-pre-exposed and non-pre-exposed animals experienced 10 pairings of tone and footshock, we measured extracellular levels of dopamine in the shell and core during the expression of latent inhibition to an aversively conditioned tone using in vivo microdialysis. Our results show that pre-exposure to the tone eliminated the conditioned release of dopamine in the shell of the nucleus accumbens and resulted in an attenuated conditioned freezing response to the tone conditioned stimulus. In contrast, dopamine release in the core was not affected by pre-exposure to the tone. These data suggest that it is specifically the shell of the nucleus accumbens in which alterations of dopaminergic tone, whether pharmacologically induced in rodents or the result of disease in humans, may act to disrupt latent inhibition.

Clozapine and Haloperidol Reinstate Latent Inhibition Following its Disruption during Amphetamine Withdrawal

Latent inhibition (LI) is a behavioral phenomenon whereby repeated exposure to a non-reinforced stimulus retards subsequent conditioning to that stimulus. Deficits in LI may reflect an inability to ignore irrelevant stimuli and are studied as a model of the cognitive/attentional abnormalities found in schizophrenia. We recently determined that pretreatment with escalating doses of the indirect dopamine agonist amphetamine (AMPH; 3 daily injections ip, 1-5 mg/kg, over 6 days) disrupts LI in rats tested in a 2-way active avoidance paradigm during withdrawal. In the present study, we evaluated the effects of the atypical neuroleptic clozapine and the typical neuroleptic haloperidol on the expression of LI on day 4 of AMPH withdrawal. Neuroleptic injections were given either 45 min prior to each of two tone preexposure sessions and a subsequent tone-shock avoidance test session, or only prior to the test session. As expected, saline-injected control groups showed LI during the test session, as reflected by significantly reduced avoidance in tone preexposed vs. non-preexposed rats. In contrast, animals pretreated with escalating doses of AMPH did not show LI, due to the improved avoidance of the preexposed animals. Both haloperidol (0.03 mg/kg) and clozapine (5 mg/kg) largely reversed the disruptive influence of AMPH on LI regardless of whether these drugs were administered prior to both preexposure and test sessions or only prior to the test session. These results provide pharmacological validation for an AMPH withdrawal model of schizophrenic symptoms.

Sensitized fos expression in subterritories of the rat medial prefrontal cortex and nucleus accumbens following amphetamine sensitization as revealed by stereology

Behavioral sensitization to the locomotor activating effects of amphetamine refers to the progressive, long lasting increase in locomotor activity that occurs with repeated injections. This phenomenon is thought to result from neuroadaptations occurring in the projection fields of mesocorticolimbic dopaminergic neurons. In the present study, we investigated the effects of amphetamine sensitization on Fos immunoreactivity (Fos-IR) in subterritories of the nucleus accumbens (core and shell) and medial prefrontal cortex (mPFC; dorsal and ventral) using stereology. Rats received five daily injections of amphetamine (1.5 mg/kg, i.p.) or saline. Behavioral sensitization was measured 48 h following the last injection, in response to a challenge injection of 1.5 mg/kg amphetamine. Sensitized rats showed a greater enhancement of locomotor activity upon drug challenge compared with their saline counterparts. Densities of Fos-positive nuclei were enhanced more in the dorsal than the ventral mPFC subterritory, whereas in the nucleus accumbens, densities of Fos-positive nuclei were increased more in the core than the shell of amphetamine-sensitized rats compared to controls. These results represent, to our knowledge, the first published report using stereological methods to quantify Fos-IR in the brain and suggest functional specialization of cortical and limbic regions in the expression of behavioral sensitization to amphetamine.

Behavioural consequences of withdrawal from three different administration schedules of amphetamine.

Different administration schedules of amphetamine (AMPH) lead to different behavioural consequences, neurochemical changes and gene expression patterns in a variety of brain areas during drug withdrawal. However, direct comparisons of these different effects within the same experiment are rare in the literature. Accordingly, in this study, rats were tested in relevant behavioural paradigms during withdrawal from three different administration schedules of AMPH. The intermittent schedule (INT) consisted of one daily injection of 1.5 mg/kg AMPH for 6 days. The escalating administration schedule consisted of three daily injections for 6 days with increasing dosages from 1 to 5 mg/kg AMPH during the first five injections and 5 mg/kg for the following 13 injections (ESC-5). A second more severe escalating administration schedule (three injections per day for 3 days escalating from 1 to 9 mg/kg AMPH and a final 10 mg/kg AMPH injection on day 4, ESC-10) was also investigated. Control animals received saline injections according to the three administration schedules. Rats pretreated with AMPH according to the ESC-10 administration schedule exhibited a transient reduction of locomotor activity on day 1, but not day 5, of withdrawal, as well as a permanent disruption of prepulse inhibition (PPI) on days 4, 20, and 40 of withdrawal. There was no effect on anxiety measured by the elevated plus-maze on withdrawal day 2, and all the AMPH pretreated animals exhibited a similar magnitude of behavioural sensitization following an AMPH challenge irrespective of administration schedule on withdrawal day 42. These data suggest that, based on their persistent disruption of PPI, animals withdrawn from AMPH ESC-10 might model specific symptoms of schizophrenic patients.

Amphetamine withdrawal modulates FosB expression in mesolimbic dopaminergic target nuclei: effects of different schedules of administration

Different patterns of psychostimulant intake can elicit widely varying behavioral and neurochemical consequences. Accordingly, rats were studied during withdrawal from either of two schedules of amphetamine administration, one consisting of 6 days of low-dose (1.5 mg/kg, i.p.) daily intermittent (INT) amphetamine (AMPH) injections, and the other of 6 days of moderately high-dose (1-5 mg/kg, i.p.) escalating (ESC) AMPH injections, for the effects of these treatments on numbers of FosB-positive nuclei and monoamine utilization in dopaminergic target areas. Withdrawal from AMPH pretreatment according to the ESC schedule markedly increased FosB expression in the nucleus accumbens shell and basolateral amygdala. In contrast, withdrawal from INT-AMPH administration did not increase FosB expression in any of the regions examined. Post-mortem neurochemical analyses of these same brain regions did not reveal effects of withdrawal from either INT or ESC administration of AMPH. These results suggest that withdrawal from a moderately high-dose AMPH regimen modifies patterns of gene expression in mesocorticolimbic dopaminergic target nuclei without significantly affecting basal monoamine levels. The strength of these effects in the nucleus accumbens shell and basolateral nucleus of the amygdala are consistent with behavioral and clinical data indicating the importance of these areas in the neuroadaptive changes which characterize addiction and withdrawal states.

The use of stereological counting methods to assess immediate early gene immunoreactivity

The issue of whether profile and stereological counting methods are interchangeably accurate when assessing immediate early gene expression still needs to be resolved. To compare these two counting techniques, we quantified the expression of c-fos in the nucleus accumbens core and shell, and in the lateral septum as a control structure, of rats treated with neuroleptics. With the profile counting method, which relies on selective placement of a counting grid within a structure, we evaluated the density of c-fos labeled cells within a box of fixed dimension. With stereology, which applies random and systematic sampling methods, we used the optical fractionator method and counted the absolute number of c-fos labeled cells within the contours of each structure examined. Our results showed that the substantial increase in c-fos expression in the shell and core induced by haloperidol treatment was detected by both stereological and profile counting methods; in contrast, the weaker effect of clozapine on c-fos expression was detected differentially by the two methods. Whereas the profile counting method reported a reduction of c-fos in the core by clozapine, and an increase in c-fos in the lateral septum, these effects were not replicated using stereology. These findings suggest that stereological and profile counting methods do not always produce equivalent results. This may be particularly relevant when a measured effect is relatively small, and it is not distributed homogeneously within a structure. In this respect, the random and systematic sampling methods of stereology may yield more accurate and unbiased results than the profile counting method, and therefore may be preferred for a more accurate and thorough investigation of a treatment effect on immediate early gene expression in a specific brain region.