Merck, Sharp & Dohme Prize for Young Psychopharmacologists: A history of frontal and temporal lobe aspects of the neuropharmacology of schizophrenia

Different outcomes after acute and chronic treatment with nicotine in pre-pulse inhibition in Lister hooded rats

Excessive tobacco consumption by schizophrenic patients may be a form of self-medication, and nicotine in tobacco may alleviate deficits in information processing. We tested this hypothesis by determining whether nicotine (acute/chronic) would improve information processing in the rat using pre-pulse inhibition as a model. In study 1, rats were injected with nicotine 10 min prior to placement in startle chambers (0.001-0.1 or 0.03-0.3 mg/kg, s.c.). In study 2, rats were injected with either saline or nicotine (0.4 mg/kg, s.c.) for 21 consecutive days and assessed for locomotor activity, pre-pulse inhibition and changes in [3H]nicotine binding in whole brain. Acutely, nicotine had no effect on pre-pulse inhibition. By contrast, after chronic nicotine treatment, rats demonstrated a robust deficit in pre-pulse inhibition and significant increases in locomotor activity and [3H]nicotine binding. The deficit in pre-pulse inhibition after chronic treatment with nicotine may be the result of non-specific behavioural activation due to increased mesolimbic dopamine release or, possibly, nicotine may rapidly desensitize nicotinic receptors important for normal information processing.

Immunoautoradiographic evidence for a loss of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-preferring non-N-methyl-D-aspartate glutamate receptors within the medial temporal lobe in schizophrenia

Decreased expression of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)-preferring non-N-methyl-D-aspartate (non-NMDA) glutamate receptors (GluRs) occurs in the medial temporal lobe of schizophrenics in terms of reduced abundance of GluR1 and GluR2 subunit mRNAs. To investigate further these receptors in schizophrenia, we have performed a quantitative immunoautoradiographic study in medial temporal lobe sections of 11 schizophrenics and 10 well-matched controls. GluR1 and GluR2/3 were detected with polyclonal antisera coupled to 35S-labeled secondary antibodies. Both subunits were vulnerable to a prolonged postmortem interval and poor agonal state as indicated by brain pH. GluR1 also tended to decline with increasing age. These factors were therefore used as covariates. GluR1 abundance was reduced in schizophrenics in parahippocampal gyrus (p < .025), while GluR2/3 was lower in most subfields in the schizophrenics, significantly so in CA4 (p < .02). The present data extend the evidence for decreased expression of the AMPA subtype of non-NMDA receptors in the medial temporal lobe in schizophrenia, although the magnitude and spatial extent of the loss is smaller than that affecting the encoding mRNAs. Impaired AMPA receptor expression is consistent with a neurodevelopmental origin and with hypotheses of glutamatergic hypofunction in the disease; however, its true pathophysiological significance and relationship to the other neuropathological and pathochemical abnormalities in the medial temporal lobe in schizophrenia remain to be determined.

Deficient sensorimotor gating after 6-hydroxydopamine lesion of the rat medial prefrontal cortex is reversed by haloperidol

The present study sought to test the hypothesis that dopamine in the prefrontal cortex exerts an inhibitory influence on subcortical dopamsine systems and that depletion of prefrontal dopamine may affect behaviour via an increase in dopamine release in the basal ganglia. We used prepulse inhibition of the acoustic startle response, i.e. the inhibition of the acoustic startle response by a preceding non-startling stimulus, as the behavioural test, because this phenomenon of sensorimotor gating is modified in opposite directions by dopamine in the prefrontal cortex and in the basal ganglia. Rats were tested for prepulse inhibition before and after injections of the neurotoxin 6-hydroxydopamine into the medial prefrontal cortex. We attempted to differentiate the contributions of prefrontal dopamine and noradrenaline by pretreating the animals with desipramine (6-OHDAMI rats) or bupropion (6-OHDABUP rats), selective inhibitors of noradrenaline and dopamine reuptake respectively. 6-Hydroxydopamine lesion reduced prefrontal dopamine by 90% and noradrenaline by 80% in 6-OHDADMI rats, while prefrontal dopamine was reduced by 54% and noradrenaline by 95% in 6-OHDABUP rats. The ability of an acoustic prepulse (75 dB, 10 kHz) to inhibit the response to a startle pulse (100 dB noise burst) was maintained in sham-lesioned rats and in 6-OHDABUP rats. However, there was a marked reduction of prepulse inhibition (by 26%) in the 6-OHDADMI rats. Systemic administration of the dopamine antagonist haloperidol (0.05 mg/kg), which did not affect prepulse inhibition in sham-lesioned and in 6-OHDABUP rats, antagonized the lesion-induced deficit in prepulse inhibition in 6-OHDADMI rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Failure to find linkage between a functional polymorphism in the dopamine D4 receptor gene and schizophrenia

We report the results of a linkage study in 24 families multiply affected with schizophrenia using a polymorphic DNA sequence encoding the third cytoplasmic loop of the dopamine D4 receptor. Two-point LOD score analyses with a range of single gene models ranging from near dominant to near recessive revealed no evidence for linkage. In addition, we examined the data by non-parametric sib-pair analysis and found no excess sharing of alleles between affected sib-pairs. We therefore conclude that mutations within the dopamine D4 receptor gene do not have a major aetiological role in schizophrenia in our collection of pedigrees.

New perspectives on the clinical neurobiology of treatment response in schizophrenia

A structured approach is used to review the rapid progress in neurobiology related to treatment response in schizophrenia. Findings are presented and discussed according to the molecular, cellular and regional levels of brain organization. The genotype-phenotype structure is used to consider potential interactions between genes, clinical manifestations of the illness and treatment response. The integration of neurobiological research into the future development of new treatment strategies for schizophrenia holds significant promise.

Prepulse inhibition of the acoustic startle response of rats is reduced by 6-hydroxydopamine lesions of the medial prefrontal cortex

Prepulse inhibition (PPI) of the acoustic startle response (ASR) is impaired by dopamine (DA) overactivity in the nucleus accumbens and anteromedial striatum. Since there is evidence that DA in the medial prefrontal cortex exerts an inhibitory control on striatal DA systems, it was investigated whether depletion of prefrontal DA reduces PPI. Rats were tested for PPI both before and after injections (2 x 1 microliter per side) of vehicle, a low (3.0 microgram/microliter) or a high (6.0 microgram/microliter) dose of 6-hydroxydopamine hydrobromide (6-OHDA) into the prefrontal cortex. Only the high dose of 6-OHDA, leading to an 87% depletion of prefrontal DA, impaired PPI. The ability of an acoustic prepulse (75 dB, 10 kHz) to reduce the response to a startle pulse (100 dB noise burst) was maintained in sham lesioned rats, but was significantly disturbed in rats lesioned with the high dose of 6-OHDA. The 6-OHDA treatment did not affect the ASR amplitude in the absence of a prepulse. The reduction of PPI in lesioned rats correlated with the extent of DA depletion. These results suggest that the DA innervation of the prefrontal cortex is involved in the modulation of the ASR and they provide further evidence for opposite actions of prefrontal and subcortical DA systems in the control of behaviour. The present findings are discussed with regard to the potential role of prefrontal DA in schizophrenia.

Clozapine, single photon emission tomography, and the D2 dopamine receptor blockade hypothesis of schizophrenia

According to the dopamine hypothesis of schizophrenia, D2 receptor blockade is essential for a drug to have antipsychotic potency, and antipsychotic potency and D2 blockade are linearly related in vitro. To test this assumption in vivo, we have compared clinical response with central D2 dopamine receptor availability measured by 123I-iodobenzamide single photon emission tomography in two groups of schizophrenic patients. 6 patients were on typical antipsychotic drugs and 10 were on the atypical antipsychotic clozapine, including 2 patients from the first group. The patients on typical antipsychotics showed poor therapeutic response despite D2 receptor blockade. Significant clinical improvement occurred in all patients on clozapine, but at a lower level of D2 blockade by the drug. These findings suggest a more complex relation between D2 blockade and clinical efficacy than was previously thought.