Jeffrey N. Joyce

Distribution of dopamine D3 receptor expressing neurons in the human forebrain: comparison with D2 receptor expressing neurons.

The dopamine D2 and D3 receptors are members of the D2 subfamily that includes the D2, D3 and D4 receptor. In the rat, the D3 receptor exhibits a distribution restricted to mesolimbic regions with little overlap with the D2 receptor. Receptor binding and nonisotopic in situ hybridization were used to study the distribution of the D3 receptors and neurons positive for D3 mRNA in comparison to the D2 receptor/mRNA in subcortical regions of the human brain. D2 binding sites were detected in all brain areas studied, with the highest concentration found in the striatum followed by the nucleus accumbens, external segment of the globus pallidus, substantia nigra and ventral tegmental area, medial preoptic area and tuberomammillary nucleus of the hypothalamus. In most areas the presence of D2 receptor sites coincided with the presence of neurons positive for its mRNA. D3 binding sites and D3 mRNA positive neurons were most abundant in the limbic striatum and efferent structures, such as the nucleus accumbens, ventral striatum, substantia nigra, internal segment of the globus pallidus, anteroventral nucleus of the thalamus, and rostral pars reticulata of the substantia nigra. One important difference from the rat is that D3 receptors were virtually absent in the ventral tegmental area. D3 receptor and D3 mRNA positive neurons were observed in sensory, hormonal, and association regions such as the nucleus basalis, anteroventral, mediodorsal, and geniculate nuclei of the thalamus, mammillary nuclei, the basolateral, basomedial, and cortical nuclei of the amygdala. As revealed by simultaneous labeling for D3 and D2 mRNA, D3 mRNA was often expressed in D2 mRNA positive neurons. Neurons that solely expressed D2 mRNA were numerous and regionally widespread, whereas only occasional D3-positive-D2-negative cells were observed. The regions of relatively higher expression of the D3 receptor and its mRNA appeared linked through functional circuits, but co-expression of D2 and D3 mRNA suggests a functional convergence in many regions of the signals mediated by the two receptor subtypes.

Serotonin Uptake Sites and Serotonin Receptors Are Altered in the Limbic System of Schizophrenics

Serotonin (5-HT) uptake sites were mapped by antoradiographic means with [3H]cyano-imipramine [[3H)]CN-IML), the 5-HT1A receptor with [3H]8-hydroxy-2-[di-n-propyl-amino]tetralin ([3H]8-OH-DPAT), and the 5-HT2 receptor with both [3H]ketanserin and [125I]lysergic acid diethylamide ([125I]LSD) in eight unneurologic controls and 10 cases with a diagnosis of schizophrenia. In the striatum, there was a marked heterogeneous patterning of 5-HT uptake sites that corresponded to the striosomal/matrix compartmentalization of the striatum. This organization m not matched with an equally heterogeneous pattern of either 5-HT2 or 5-HT1A receptors. For the isocortex, a general organizational scheme was observed with the 5-HT1A receptor expression high in the external laminae and deep laminae, but 5-HT2 receptor expression was higher in the internal laminae. There was a laminar distribution of 5-HT uptake sites that approximated the combined distributions of the 5-HT1A receptor and the 5-HT2 receptor. In the parahippocampal gyrus and hippocampus, the distribution of 5-HT uptake sites was Complementary to the distribution of 5-HT1A and 5-HT2 receptors. In schizophrenic cases, there was a large increase in the number and altered striosomal/matrix organization of 5-HT uptake sites in the striatum. There was also an increase in the numbers of 5-HT2 receptors in the nucleus accumbens and ventral putamen of the schizophrenics. The number of 5-HT1A receptors was not modified. There was a marked reduction in 5-HT uptake sites in the external and middle laminae of the anterior Cingulate, frontal cortex, and posterior cingulate, and no changes were observed in the motor cortex, temporal cortex, or hippocampus. Increased numbers of 5-HT1A receptors were found in the posterior cingulate, motor cortex, and hippocampus. Serotonin2 receptors were substantially elevated in the posterior cingulate, temporal cortex, and hippocampus, but not in the frontal, anterior cingulate, or motor cortices. Examination of the temporal lobe and hippocampus of a group of nonschizophrenic suicides (n = 8) indicated the alterations in 5-HT system in the limbic regions of the striatum, the limbic cortex, and hippocampus of the schizophrenic cases may be disease specific.

Small molecule BDNF mimetics activate TrkB signaling and prevent neuronal degeneration in rodents

Brain-derived neurotrophic factor (BDNF) activates the receptor tropomyosin-related kinase B (TrkB) with high potency and specificity, promoting neuronal survival, differentiation, and synaptic function. Correlations between altered BDNF expression and/or function and mechanism(s) underlying numerous neurodegenerative conditions, including Alzheimer disease and traumatic brain injury, suggest that TrkB agonists might have therapeutic potential. Using in silico screening with a BDNF loop-domain pharmacophore, followed by low-throughput in vitro screening in mouse fetal hippocampal neurons, we have efficiently identified small molecules with nanomolar neurotrophic activity specific to TrkB versus other Trk family members. Neurotrophic activity was dependent on TrkB and its downstream targets, although compound-induced signaling activation kinetics differed from those triggered by BDNF. A selected prototype compound demonstrated binding specificity to the extracellular domain of TrkB. In in vitro models of neurodegenerative disease, it prevented neuronal degeneration with efficacy equal to that of BDNF, and when administered in vivo, it caused hippocampal and striatal TrkB activation in mice and improved motor learning after traumatic brain injury in rats. These studies demonstrate the utility of loop modeling in drug discovery and reveal what we believe to be the first reported small molecules derived from a targeted BDNF domain that specifically activate TrkB.We propose that these compounds constitute a novel group of tools for the study of TrkB signaling and may provide leads for developing new therapeutic agents for neurodegenerative diseases.

Dopamine D3 receptor antagonists as therapeutic agents.

The behavioral and pathophysiological role of the dopamine D(3) receptor, which was deduced from anatomical, lesion and drug treatment studies in the ten years following cloning of the receptor, indicated that its functions differed from those of the D(2) receptor. There is increasingly strong evidence that D(3) receptor antagonists will be effective antipsychotic agents. In this regard, an amelioration of the negative and cognitive symptoms of schizophrenia holds the most promise for D(3) receptor antagonists, a concept currently under clinical evaluation. In addition, D(3) receptors could be involved in behavioral sensitization and the potential application of D(3) receptor antagonists in the treatment of drug abuse is undergoing intensive experimental investigation.

Effects of estrogen on the basal ganglia

Recent research suggests that estrogen regulates the activity of dopamine-containing fibers originating in the midbrain and terminating in the basal ganglia, and/or dopamine-sensitive cells in the basal ganglia. The mechanism by which estrogen acts is not clear, since cells in neither of these regions concentrate estrogens. Nevertheless, estrogens clearly affect behaviors mediated by the basal ganglia, as illustrated in human patients suffering from extrapyramidal disorders. Both biochemical and behavioral research in animals has confirmed that estrogen modulates basal ganglia function, but there has not been agreement concerning either the locus, the direction, or the mechanism of its action. These topics are the focus of this review. The effects of estrogen on behaviors mediated by DA in the basal ganglia depend on the dose of estrogen administered, the time interval between estrogen treatment and testing, the behavior measured, and the part of the basal ganglia from which the behavior is elicited. A high dose of estrogen results in an initial suppression and later enhancement of DA-related behaviors elicited from the striatum. However, no later enhancement of these behaviors occurs if a low dose of estrogen is given. Even after low doses of estrogen, the latency to behavioral suppression varies depending upon the behavior measured. These varying latencies suggest that more than one mechanism is involved in the effects of estrogen on basal ganglia output. In addition, estrogen may also act on some regions in the mesolimbic DA system. While estrogen may act indirectly via the catechol estrogens and prolactin, it has been demonstrated that estrogen can act directly on the striatum. These findings are related to the effects of estrogen on human extrapyramidal disorders.

Dopamine D3 receptor as a therapeutic target for antipsychotic and antiparkinsonian drugs.

The cloning of the gene for the D3 receptor and subsequent identification of its distribution in brain and pharmacology allowed for serious consideration of the possibility that it might be a target for drugs used to treat schizophrenia and Parkinsons disease (PD). That is because it is highly expressed in limbic regions of the brain, exhibits low expression in motor divisions, and has pharmacologic similarity to the D2 receptor. Thus, antipsychotics that were presumed to block D2 receptors also had high affinity for the D3 receptor. Dopamine agonists used to treat the clinical symptoms of PD also have high affinity for the D3 receptor, and two D3 receptor-preferring agonists were found to be effective for treatment of PD. Many compounds achieving high potency and selectivity are now available, but few have reached clinical testing. Recent findings with respect to the anatomy of this receptor in human brain, altered expression in schizophrenia and PD, and biological models to study its function support the proposal that it is a target for development of drugs to alleviate symptoms in neuropsychiatric and neurologic disorders. Because of distinct aspects of regulation of the D3 receptor, it represents a unique target for therapeutic intervention in schizophrenia without high potential for unintended side effects such as tardive dyskinesia. It may also be that D3 receptor agonists can provide neuroprotective effects in PD and can modify clinical symptoms that D2 receptor-preferring agonists cannot provide.

Alterations in the cortical serotonergic system in schizophrenia: A postmortem study

Previous studies have suggested a disturbance in the cortical serotonergic (5-HT) system in schizophrenia; however, these studies have been confounded by suicide in the patients groups, which in itself is associated with alterations in the 5-HT system. In this study we characterized various components of the 5-HT system in 14 areas of the frontal and parietal cortex in tissue obtained at postmortem from aged chronically hospitalized nonsuicidal schizophrenics compared to age-matched controls. We found no differences between control and schizophrenic subjects in the density of 5-HT uptake sites or other markers of 5-HT innervation. In Brodmann areas 24 and 6 the concentration of 5-HT2A,C receptors was decreased in all schizophrenics regardless of their antipsychotic treatment history. In all other areas examined 5-HT2A,C receptor concentrations were dramatically decreased in schizophrenics patients on drugs at time of death, whereas those off drugs at death showed the same values as control subjects. The density of 5-HT1A receptors was increased in areas 24, 9a (caudal part of area 9), 44, and 6 in subjects with schizophrenia. Antipsychotic treatment did not appear to have a significant effect. Thus, the specific pattern of alterations in the 5-HT system in schizophrenia may depend on the patient population and on antemortem antipsychotic treatment. These data also provide evidence that regulation of the 5-HT2 receptor may be involved in antipsychotic action.

Neuroinflammation in Alzheimer's disease and Parkinson's disease: are microglia pathogenic in either disorder?

Microglial activation similar to that which occurs in peripheral macrophages during inflammatory attack was first demonstrated in the Alzheimers disease (AD) brain two decades ago. Localization to pathologically vulnerable regions of AD cortex, localization to sites of specific AD pathology such as amyloid-beta peptide (Abeta) deposits, and the ability of activated microglia to release toxic inflammatory factors suggested that the activation of microglia in AD might play a pathogenic role. However, proving this hypothesis in a disease in which so many profound pathologies occur (e.g., Abeta deposition, neurofibrillary tangle formation, inflammation, neuronal loss, neuritic loss, synaptic loss, neuronal dysfunction, vascular alterations) has proven difficult. Although investigations of microglia in Parkinsons disease (PD) are more recent and therefore less extensive, demonstration of a pathogenic role for microglial activation may actually be much simpler in PD than AD because the root pathological event in PD, loss of dopamine (DA)-secreting substantia nigra neurons, is already well established. Indeed, indirect but converging evidence of a pathogenic role for activated microglia in PD has already begun to emerge. The nigra reportedly has the highest density of microglia in brain, and, in PD, nigral microglia are not only highly activated but also highly clustered around dystrophic DA neurons. 6-OHDA and MPTP models of PD in rodents induce substantia nigra microglial activation. More cogent, injections of the classic microglial/macrophage activator lipopolysaccharide into or near the rodent nigra cause a specific loss of DA neurons there. Culture models with human microglia and human cellular targets replicate this phenomenon. Notably, nearly all the proposed etiologies of PD, including brain bacterial and viral exposure, pesticides, drug contaminants, and repeated head trauma, are known to cause brain inflammation. A mechanism by which activated microglia might specifically target DA neurons remains a critical missing link in the proof of a pathogenic role for activated microglia in PD. If such a link could be established, however, clinical intervention trials with agents that dampen microglial activation might be warranted in PD.

Quantitative autoradiography of dopamine D2 sites in rat caudate-putamen: Localization to intrinsic neurons and not to neocortical afferents

Dopamine D2 receptors, labeled with [3H]spiroperidol or [3H]sulpiride, show a lateral-to-medial gradient in the caudate-putamen, with a more than two-fold greater density laterally than medially. It has been thought that D2 receptors are located on at least two neuronal elements of the caudate-putamen, neurons intrinsic to this structure and axons whose cell bodies reside in the cortex. As a first step in establishing what neuronal elements underlie this heterogeneous organization of D2 receptors, we took advantage of quantitative autoradiography to examine the association of these receptors with those elements. The present findings show that the D2 sites are almost exclusively located on neurons whose somata reside in the caudate-putamen and are not located on terminals of corticostriatal axons. A detailed comparison of the distribution of histochemically identified acetylcholinesterase neurons with that of D2 receptors in serially adjoining sections suggests a common organizational pattern. The density of [3H]spiroperidol sites in rat caudate-putamen was determined after unilateral injection of the neurotoxin quinolinic acid into this structure or after ablation of neocortical regions. Quantification of the tissue damage was achieved by acetylcholinesterase histochemistry (following diisopropylfluorophosphate treatment), as well as by thionin and luxol fast staining of sections adjacent to those used for [3H]spiroperidol autoradiography. In identically treated animals, biochemical determination of the extent of tissue damage was made utilizing assays for high-affinity [3H]choline and [3H]glutamate uptake in the caudate-putamen. In quinolinic acid-injected rats, the density of D2 sites was decreased by 90-95% at the site of complete loss of large acetylcholinesterase-positive neurons. Other animals, given ablations of specific neocortical fields (medial prefrontal, motor, somatosensory) or of the entire parietal-frontal cortex of one hemisphere, showed no loss of caudate-putamen D2 sites unless the cortical ablation caused accompanying damage of the caudate-putamen. In the caudate-putamen of all animals there was a close correspondence between the D2 sites and the striatal neurons (and processes) that show strong acetylcholinesterase reactivity. We suggest that the caudate-putamen topography of D2 sites is based largely on the internal organization of this structure and may preferentially involve acetylcholine-containing intrinsic neurons.

Ontogeny of dopamine D1 and D2 receptor subtypes in rat basal ganglia: a quantitative autoradiographic study

The ontogeny of D1 and D2 dopamine (DA) receptors in rat basal ganglia was examined by quantitative autoradiography using the iodinated ligands [125I]SCH 23982 and [125I]iodobenzamide [( 125I]IBZM), respectively. Temporal and spatial differences in the development of the receptor subtypes were observed. Scatchard transformation of saturation isotherms conducted at postnatal day 10 (P10) and P60, showed that there was no age-related change in the affinity of [125I]SCH 23982 binding to D1 receptors (Kd = 2.6 nM) but there was a significant increase in the Bmax (771 compared to 2032 fmol/mg protein, P = 0.002). A statistically significant difference in Kd was noted between ages P10 and P60 for [125I]IBZM labelling of D2 receptors (0.62 vs 1.00 nM, respectively, P less than 0.01). A significant increase in the Bmax (211 and 721 fmol/mg protein, P less than 0.01) was also observed. D1 receptors were visible as distinct patches at P1. The highest density was found in the ventrolateral caudate-putamen (CPu). By P5 the patches were found in all subregions of the CPu and nucleus accumbens. Between P7 and P10 the binding became distinctly less patchy due to a marked increase in the density of D1 receptors in non-patch (matrix) regions. Adult levels of receptor were seen by P30. The concentration of DA (measured by HPLC) and binding of [3H]mazindol to DA uptake sites in whole striatum showed similar and nonlinear increases with age. The age-related change in the topography of binding sites for [3H]mazindol was similar to that of D1 receptors at the same ages. Both D2 receptors and [3H]hemicholinium-3 (HC-3) binding to high affinity transport sites for choline developed initially in the dorso-lateral CPu. Their topography was largely overlapping but distinct from that of the D1 receptor. D2 receptors were not consistently observed until P3 in the CPu, and zones of enriched binding were aligned with zones of low density for D1 receptors. The density of D2 receptors reached adult levels by P30. The differential development of the DA receptors was also evident in the substantia nigra (SN) and globus pallidus (GP). D1 receptors were found in SN prior to the appearance of D2 receptors and throughout development the density was greater in pars reticulata than in pars compacta, whereas the density of D2 receptors was higher in the pars compacta. At all ages the density of D1 receptors was greater than the density of D2 receptors in the GP and reached adult levels before reaching it in the CPu or SN.(ABSTRACT TRUNCATED AT 400 WORDS)