A mechanism for the involvement of colocalized neuropeptides in the actions of antipsychotic drugs

Intercellular Communication in the Central Nervous System as Deduced by Chemical Neuroanatomy and Quantitative Analysis of Images: Impact on Neuropharmacology

In the last decades, new evidence on brain structure and function has been acquired by morphological investigations based on synergic interactions between biochemical anatomy approaches, new techniques in microscopy and brain imaging, and quantitative analysis of the obtained images. This effort produced an expanded view on brain architecture, illustrating the central nervous system as a huge network of cells and regions in which intercellular communication processes, involving not only neurons but also other cell populations, virtually determine all aspects of the integrative function performed by the system. The main features of these processes are described. They include the two basic modes of intercellular communication identified (i.e., wiring and volume transmission) and mechanisms modulating the intercellular signaling, such as cotransmission and allosteric receptor-receptor interactions. These features may also open new possibilities for the development of novel pharmacological approaches to address central nervous system diseases. This aspect, with a potential major impact on molecular medicine, will be also briefly discussed.

Cholecystokinin-Mediated Neuromodulation of Anxiety and Schizophrenia: A "Dimmer-Switch" Hypothesis

Cholecystokinin (CCK), the most abundant brain neuropeptide, is involved in relevant behavioral functions like memory, cognition, and reward through its interactions with the opioid and dopaminergic systems in the limbic system. CCK excites neurons by binding two receptors, CCK1 and CCK2, expressed at low and high levels in the brain, respectively. Historically, CCK2 receptors have been related to the induction of panic attacks in humans. Disturbances in brain CCK expression also underlie the physiopathology of schizophrenia, which is attributed to the modulation by CCK1 receptors of the dopamine flux in the basal striatum. Despite this evidence, neither CCK2 receptor antagonists ameliorate human anxiety nor CCK agonists have consistently shown neuroleptic effects in clinical trials. A neglected aspect of the function of brain CCK is its neuromodulatory role in mental disorders. Interestingly, CCK is expressed in pivotal inhibitory interneurons that sculpt cortical dynamics and the flux of nerve impulses across corticolimbic areas and the excitatory projections to mesolimbic pathways. At the basal striatum, CCK modulates the excitability of glutamate, the release of inhibitory GABA, and the discharge of dopamine. Here we focus on how CCK may reduce rather than trigger anxiety by regulating its cognitive component. Adequate levels of CCK release in the basal striatum may control the interplay between cognition and reward circuitry, which is critical in schizophrenia. Hence, it is proposed that disturbances in the excitatory/ inhibitory interplay modulated by CCK may contribute to the imbalanced interaction between corticolimbic and mesolimbic neural activity found in anxiety and schizophrenia.

Antipsychotic-induced gene regulation in multiple brain regions

The molecular mechanism of action of antipsychotic drugs is not well understood. Their complex receptor affinity profiles indicate that their action could extend beyond dopamine receptor blockade. Single gene expression studies and high-throughput gene profiling have shown the induction of genes from several molecular classes and functional categories. Using a focused microarray approach, we investigated gene regulation in rat striatum, frontal cortex, and hippocampus after chronic administration of haloperidol or olanzapine. Regulated genes were validated by in situ hybridization, real-time PCR, and immunohistochemistry. Only limited overlap was observed in genes regulated by haloperidol and olanzapine. Both drugs elicited maximal gene regulation in the striatum and least in the hippocampus. Striatal gene induction by haloperidol was predominantly in neurotransmitter signaling, G-protein coupled receptors, and transcription factors. Olanzapine prominently induced retinoic acid and trophic factor signaling genes in the frontal cortex. The data also revealed the induction of several genes that could be targeted in future drug development efforts. The study uncovered the induction of several novel genes, including somatostatin receptors and metabotropic glutamate receptors. The results demonstrating the regulation of multiple receptors and transcription factors suggests that both typical and atypical antipsychotics could possess a complex molecular mechanism of action.

The CCK-A receptor gene possibly associated with auditory hallucinations in schizophrenia

In this study, a PstI polymorphic site with two individual alleles, namely A1 and A2, was identified withinthe boundary between intron 1 and exon 2 of the cholecystokinin (CCK) type A receptor gene. The PstI polymorphic site was used as a genetic marker to study its association with psychotic symptoms in schizophrenia. A significant difference in allelic frequency was found between schizophrenic patients with and without auditory hallucinations(chi(2) = 6.26, df = 1, P = 0.012), and the odds ratio for the allelic association was 2.21 (95% CI 1.18-4.15) with an attributable fraction of 0.1. The frequency of A1-A1 and A1-A2 genotypes showed a significant excess in schizophrenic patients with auditory hallucinations as compared to those without such symptoms (chi(2) = 5.45, df = 1, P = 0.02), and the odds ratio for the genotypic association was 2.27 (95% CI 1. 13-4.57) with an attributable fraction of 0.177. The haplotype-based haplotype relative risk (HHRR) test revealed a significant difference between transmitted and non-transmitted alleles in nuclear families of schizophrenic patients with auditory hallucinations (chi(2) = 4.54, df = 1,P = 0.033) but not in those of schizophrenic patients without them. The present study suggests that the CCK-A receptor gene may be associated with auditory hallucinations in schizophrenia.

Structure-activity and conformational studies of a series of modified C-terminal hexapeptide neurotensin analogues

Neurotensin (NT), is a linear tetradecapeptide (pGlu1-Leu2-Tyr3-Glu4-Asn5- Lys6-Pro7-Arg8-Arg9-Pro10-Tyr11-Ile12-Leu13) that has been found in the central nervous system and peripheral tissues and appears to have a variety of physiological properties. A C-terminal hexapeptide analogue [N alpha Me-Arg-Lys-Pro-Trp-Tle-Leu, (1) Tle = tert-leucine] has recently been reported to have high affinity for the NT receptor and appears to possess central activity after systemic administration. In an effort to probe the structure-activity and conformational properties of the dipeptide, Pro-Trp for binding and functional activity, these residues have been substituted with several natural and unnatural amino acids. Some of these analogues have binding affinities similar to compound 1, while in other cases, such as D-amino acid substitutions, the peptides had negligible binding affinity. In general, the Pro10 position seems more tolerant of substitution by amino acids that favor a reverse turn, rather than those that favor an extended conformation. The Trp11 position accepted extra steric bulk more readily than conformational constraints.

Impulse flow dependency of galanin release in vivo in the rat ventral hippocampus

Using microdialysis and a sensitive RIA, we have studied the in vivo release of the neuropeptide galanin (GAL) from the ventral hippocampus of freely moving rats. The spontaneous outflow of GAL-like immunoreactivity (GAL-LI) (1.8 +/- 0.3 fmol per ml per 20 min) was dependent on the presence of extracellular Ca2+ and was inhibited by tetrodotoxin. Evoked release induced by infusion of KCl (60 mM) or veratridine (148 microM) was also Ca(2+)-dependent and sensitive to tetrodotoxin. Electrical stimulation of the ventral limb of the diagonal band nuclei induced a frequency-dependent (50-200 Hz) and tetrodotoxin-sensitive overflow of GAL-LI in the hippocampus. In vitro GAL-LI release (1.0 +/- 0.02 fmol per ml per 5 min), studied in slices of rat ventral hippocampus, was also Ca(2+)-dependent and was increased in a concentration-dependent manner by KCl depolarization. This study demonstrates the release of the neuropeptide GAL in the rat central nervous system. The in vivo release is related to the activity of the cholinergic GAL-LI-containing cells in the septal diagonal band nuclei. The results are discussed in relation to the coexistence of GAL and acetylcholine within the septal/diagonal band complex.

The 5-HT1A receptor selective ligands, (R)-8-OH-DPAT and (S)-UH-301, differentially affect the activity of midbrain dopamine neurons

The effects of the selective 5-HT1A receptor agonist (R)-8-hydroxy-2(di-n-propylamino)tetralin [(R)-8-OH-DPAT] and the novel 5-HT1A antagonist (S)-5-fluoro-8-hydroxy-2-(dipropylamino)-tetralin [(S)-UH-301] were studied with regard to the firing pattern of single mesencephalic dopamine (DA) neurons with extracellular recording techniques in chloral hydrate anesthetized male rats. Neuronal activity was studied with respect to firing rate, burst firing and regularity of firing. In the ventral tegmental area (VTA) low doses of (R)-8-OH-DPAT (2-32 micrograms/kg i.v.) caused an increase in all three parameters. The effect on firing rate of DA neurons was more pronounced in the parabrachial pigmentosus nucleus than in the paranigral nucleus, the two major subdivisions of VTA. In the substantia nigra zona compacta (SN-ZC), (R)-8-OH-DPAT (2-256 micrograms/kg i.v.) had no effect on firing rate and regularity of firing and only slightly increased burst firing. High doses of (R)-8-OH-DPAT (512-1024 micrograms/kg i.v.) decreased the activity of DA cells in both areas, an effect that was prevented by pretreatment with the selective DA D2 receptor antagonist raclopride. (S)-UH-301 (100-800 micrograms/kg i.v.) decreased both firing rate and burst firing without affecting regularity of DA neurons in the VTA. In the SN-ZC, (S)-UH-301 decreased the firing rate but failed to affect burst firing and regularity of firing. These effects of (S)-UH-301 were blocked by raclopride pretreatment. Local application by pneumatic ejection of 8-OH-DPAT excited the DA cells in both the VTA and the SN-ZC, whereas (S)-UH-301 inhibited these cells when given locally. These results show that 5-HT1A receptor related compounds differentially affect the electrophysiological activity of central DA neurons. The DA receptor agonistic properties of these compound appear to contribute to the inhibitory effects of high doses of (R)-8-OH-DPAT and (S)-UH-301 on DA neuronal activity. Given the potential use of 5-HT1A receptor selective compounds in the treatment of anxiety and depression their effects on central DA systems involved in mood regulation and reward related processes are of considerable importance.

Effects of frequency and pattern of medial forebrain bundle stimulation on caudate dialysate dopamine and serotonin

In vivo microdialysis was employed to detect changes in extracellular dopamine and serotonin in the rat caudate in response to electrical stimulation of the medial forebrain bundle. Extracellular dopamine concentrations increased linearly as a function of the frequency (4-33 Hz) of evenly spaced stimuli in both the presence and absence of cocaine added to the dialysate. Because dopamine neurons are known to fire in single-spike and burst patterns, stimulation pulses were also delivered in a bursting pattern. The response of extracellular dopamine was augmented in both the presence and absence of cocaine when the same number of stimuli were delivered in bursts as compared to an evenly spaced pattern. Serotonin, which was only assessed in the presence of cocaine, similarly increased linearly with frequency, but, in contrast to the dopamine response, levels of serotonin were not augmented by stimuli presented in bursts. These results suggest that microdialysis can be used to detect physiological changes in synaptic transmitter concentrations.

Further indication that distinct dopaminergic subsets project to the rat cerebral cortex: lack of colocalization with neurotensin in the superficial dopaminergic fields of the anterior cingulate, motor, retrosplenial and visual cortices

The extent of neurotensin (NT) colocalization in the different dopamine (DA) terminal fields of the rat cerebral cortex has been investigated and compared to previous data obtained in man (Gaspar et al., J. Comp. Neurol., 279 (1989) 249-271). Both innervations were revealed with single- or double-labeling immunocytochemical methods. Tyrosine hydroxylase (TH) was used as a specific marker of DA fibers after lesioning the noradrenergic system either with 6-hydroxydopamine (6-OHDA) at birth or DSP4 in adulthood. Three classes of afferents were observed which had a different regional and laminar distribution. First, a dense meshwork of finely dotted NT-positive varicosities occupied restricted areas of the limbic system: the granular retrosplenial and the deep entorhinal cortices and the subicular complex. These NT projections contained no double-labeled fibers and did not correspond to a mixed NT/TH pathway. Secondly, the mixed NT/DA projections identified previously in the prefrontal cortex (Studler et al., Neuropeptides, 11 (1988) 95-100), extended in fact rostrocaudally in layer VI of the whole cerebral cortex and formed small cluster-like groupings in layers II-III of the medial and lateral entorhinal cortex. In all these areas, the mixed NT/TH projections constituted approximately half of the DA terminals. Finally, the DA projections to the superficial layers of the anterior cingulate, motor, retrosplenial and visual cortices, were not colocalized with NT. The DA innervation of layers I-III of the rat anterior cingulate cortex displays striking similarities with that observed in the cingulate, primary motor, premotor and supplementary motor cortices in man: highest regional and laminar density of DA afferents and lack of colocalization with NT. It might thus represent a valuable model for understanding the pharmacology of the DA system besides the mixed DA/NT pathway which does not seem to have a counterpart in the human cerebral cortex. By contrast, that part of the NT innervation of the limbic system which is not colocalized with DA in rat, appears to represent the major fraction of the cortical NT innervation in man.

Mechanisms of action of atypical antipsychotic drugs. Implications for novel therapeutic strategies for schizophrenia

The mechanisms which contribute to the actions of atypical antipsychotic drugs, such as clozapine and the putative atypical agents remoxipride and raclopride, are reviewed. Examination of available preclinical and clinical data leads to two hypotheses concerning the mode of action of atypical antipsychotic drugs. The first hypothesis is that antagonism of the dopamine D2 receptor is both necessary and sufficient for the atypical profile, but that interaction with subtypes of the D2 receptor differentiates typical from atypical antipsychotic drugs. The second hypothesis has been previously advanced, and suggests that a relatively high ratio of serotonin 5-HT2:dopamine D2 receptor antagonism may subserve the atypical profile. It seems likely that the atypical antipsychotic drug profile may be achieved in more than one way.

Dopaminergic innervation of the cerebral cortex: unexpected differences between rodents and primates

Until recently, views on the organization and role of the mesotelencephalic dopaminergic (DA) systems were mostly based on studies of rodents, and it was assumed that homology existed across mammalian species. However, recent studies of both human and non-human primates indicate that this might not be so. The mesocortical DA system in primates, which is directly involved in the pathophysiology of severe illnesses such as Parkinson's disease and psychoses, shows substantial differences from that of rodents. These differences include much larger, re-organized terminal fields, a different phenotype for the co-localization of neuropeptides and a very early prenatal development.

The electrophysiological actions of neurotensin in the central nervous system

The endogenous neuropeptide, neurotensin (NT) alters the firing frequencies of certain neurons in the central nervous system (CNS). This is one of the findings that support the hypothesis that NT is a neurotransmitter substance. The direct application of NT on CNS neurons causes predominantly excitatory effects. These effects occur in a dose-related fashion via a calcium-dependent postsynaptic mechanism. The C-terminal hexapeptide fragment, NT 8-13 exerts similar electrophysiological effects to NT, while the N-terminal octapeptide fragment, NT 1-8 is devoid of such activity. NT produces a significant increase in the firing rates of individual neurons in the substantia nigra (SN), ventral tegmental area (VTA), medial prefrontal cortex (MPF), hypothalamus, and periaqueductal grey (PAG). This excitation occurs with a rapid onset and is readily reversible after cessation of NT application. In contrast, NT has no effect or weak inhibitory effects on the firing rates of neurons in the locus coeruleus (LC) and cerebellum. These electrophysiological actions of NT appear to be unique and not shared by other neurotransmitter and neuropeptide receptor antagonists and agonists that have been studied via direct co-application. NT attenuates dopamine (DA)-induced inhibition associated with direct application onto neurons in the SN and VTA both in vivo and in vitro. Intracellular recordings suggest that direct application of higher concentrations of NT appears to produce 'depolarization block' on individual neurons in the SN, VTA, MPF, and hypothalamus. The electrophysiological consequences of NT application not only show similarities to clinically efficacious antipsychotic medications, but also demonstrate the ability of NT to modulate the activity of dopamine (DA) neurons at the cellular level via specific NT binding sites. These findings further underscore the possibility that NT may play a pre-eminent role in the pathogenesis of, and psychopharmacological management of neurological and psychiatric disorders purportedly related to perturbation of CNS DA systems including schizophrenia.