Nicholas M. Barnes

A review of central 5-HT receptors and their function

It is now nearly 5 years since the last of the currently recognised 5-HT receptors was identified in terms of its cDNA sequence. Over this period, much effort has been directed towards understanding the function attributable to individual 5-HT receptors in the brain. This has been helped, in part, by the synthesis of a number of compounds that selectively interact with individual 5-HT receptor subtypes--although some 5-HT receptors still lack any selective ligands (e.g. 5-ht1E, 5-ht5A and 5-ht5B receptors). The present review provides background information for each 5-HT receptor subtype and subsequently reviews in more detail the functional responses attributed to each receptor in the brain. Clearly this latter area has moved forward in recent years and this progression is likely to continue given the level of interest associated with the actions of 5-HT. This interest is stimulated by the belief that pharmacological manipulation of the central 5-HT system will have therapeutic potential. In support of which, a number of 5-HT receptor ligands are currently utilised, or are in clinical development, to reduce the symptoms of CNS dysfunction.

Cytokine function in medication-naive first episode psychosis: a systematic review and meta-analysis.

This systematic review sets out to give a comprehensive overview of the cytokine profile at the onset of psychosis un-confounded by medication. We aim to provide insight into the early pathophysiological process of psychosis and areas for future research of potential biomarkers able to chart the extent of illness or effectiveness of treatment. Following PRISMA guidelines, a systematic primary search identified 4638 citations, 4651 studies were retrieved and screened, and 23 studies met the inclusion criteria (published in English before June 2013, patients with neuroleptic naive first episode psychosis, and assessed circulating cytokines). These reported 570 patients, 683 healthy control subjects, and 20 cytokine/cytokine receptors. Papers that contained sufficient stratified data were included in a random-effects pooled effect size meta-analysis. Highly significant effect sizes were found for elevated IL-1β, sIL-2r, IL-6, and TNF-α. Non-significant effect size estimates were obtained for IL-2, IL-4, and IFN-γ. Thus, we found significant elevation in pro-inflammatory cytokine levels in the serum of patients with medication-naive first episode psychosis. This adds to the evidence of a pro-inflammatory immune deregulation in schizophrenia and suggests these cytokines should be the focus for further research in biomarkers of progress and extent of illness. Future studies should focus on the medication-naive group at the early stages of illness with numbers large enough to allow for the control of other potential confounding factors.

The 5-HT3 receptor--the relationship between structure and function.

The 5-hydroxytryptamine type-3 (5-HT3) receptor is a cation-selective ion channel of the Cys-loop superfamily. 5-HT3 receptor activation in the central and peripheral nervous systems evokes neuronal excitation and neurotransmitter release. Here, we review the relationship between the structure and the function of the 5-HT3 receptor. 5-HT3A and 5-HT3B subunits are well established components of 5-HT3 receptors but additional HTR3C, HTR3D and HTR3E genes expand the potential for molecular diversity within the family. Studies upon the relationship between subunit structure and the ionic selectivity and single channel conductances of 5-HT3 receptors have identified a novel domain (the intracellular MA-stretch) that contributes to ion permeation and selectivity. Conventional and unnatural amino acid mutagenesis of the extracellular domain of the receptor has revealed residues, within the principle (A-C) and complementary (D-F) loops, which are crucial to ligand binding. An area requiring much further investigation is the subunit composition of 5-HT3 receptors that are endogenous to neurones, and their regional expression within the central nervous system. We conclude by describing recent studies that have identified numerous HTR3A and HTR3B gene polymorphisms that impact upon 5-HT3 receptor function, or expression, and consider their relevance to (patho)physiology.

Characterisation and autoradiographic localisation of 5-HT3 receptor recognition sites identified with [3H]-(S)-zacopride in the forebrain of the rat

The pharmacological characterisation and topographical distribution of [3H]-(S)-zacopride recognition sites in the forebrain of the rat was studied using homogenate and autoradiographic radioligand binding techniques. [3H]-(S)-Zacopride labelled a single, saturable, specific binding site (defined by 10.0 microM granisetron) in homogenates prepared from the entorhinal cortex of the rat (pKD = 9.51 +/- 0.08; Bmax = 104 +/- 7 fmol mg-1 protein; mean +/- SEM, n = 8). Pharmacological characterisation of the recognition site, within the entorhinal cortex, suggested that [3H]-(S)-zacopride selectively labelled the recognition site of the 5-HT3 receptor. Specific binding of [3H]-(S)-zacopride (defined by 1.0 microM granisetron) was differentially distributed throughout the forebrain of the rat; highest densities were located within sub-nuclei of the amygdala (cortical amygdaloid nucleus, amygdalohippocampal area, posterior medial cortical amygdaloid nucleus, posterior lateral amygdaloid nucleus), cortical areas (primary olfactory cortex, entorhinal cortex) and hippocampus. Non-specific binding was distributed homogeneously, although lower in myelinated structures. It is concluded that [3H]-(S)-zacopride selectively labels 5-HT3 receptor recognition sites within the forebrain of the rat; the topographical distribution of these sites, within the limbic nuclei, is consistent with the behavioural actions in animal models of the selective 5-HT3 receptor antagonists.

Angiofensin converting enzyme density is increased in temporal cortex from patients with Alzheimer's disease

The present study assesses the binding density of the selective angiotensin converting enzyme (ACE) radioligand [3H]ceranapril in brain tissue homogenates derived from patients with Alzheimers disease and those from age-, sex- and post-mortem delay-matched neurologically normal patients. Saturation studies with [3H]ceranapril identified that the specific binding (defined by captopril, 10 μM) was homogenous and of high affinity. ACE inhibitor recognition site density was higher by some 70% in the temporal cortex (Brodmann area 22) from Alzheimers patients whereas densities were similar in frontal cortex and cerebellum when compared to control tissue. It is unknown whether this apparently selective alteration in ACE density is directly related to. or a compensatory effect of the disease, but it provides additional support for the development of compounds which interact with the central angiotensin system as novel therapies for cognitive dysfunction.

[3H]zacopride: ligand for the identification of 5-HT3 recognition sites.

Abstract— [3H]Zacopride displayed saturable binding to homogenates of the rat entorhinal cortex as measured by the inclusion of the 5‐HT3 receptor antagonist BRL 43694 in the incubation media. Scatchard analysis indicated a single high affinity binding site (KD 0.76 ± 0.08 nM, Bmax 77.5 ±6.5 fmol (mg protein)−1) with a Hill slope close to unity. Other 5‐HT3 receptor antagonists (zacopride, ICS 205–930, GR38032F, GR65630, metoclopramide and cocaine) also competed for the binding site displacing 60% of the total [3H]zacopride binding. 5‐HT and 2‐methyl‐5‐HT also were competitive antagonists for [3H]zacopride binding whereas 5‐HT1/5‐HT2 agonists and antagonists, and agents acting on other neurotransmitter receptors had Ki values greater than 10−5 M. It is concluded that [3H]zacopride may prove a useful ligand for the study of 5‐HT3 recognition sites.

Identification and characterisation of 5-hydroxytryptamine 3 recognition sites in human brain tissue.

Abstract: [3H]Zacopride displayed regional saturable specific binding to homogenates of human brain tissues, as defined by the inclusion of BRL43694 [endo‐N‐(9‐methyl‐9‐azabicyclo[3.3.1]non‐3‐yl)‐l‐methylindazole‐3‐carboxamide] in the incubation media. Scatchard analysis of the saturation data obtained from amygdaloid and hippocampal tissues identified the binding as being of high affinity and to a homogeneous population of binding sites (KD= 2.64 ± 0.75 and 2.93 ± 0.41 nmol/L and Bmax= 55 ± 7 and 44 ± 9 fmol/ mg of protein in the amygdala and hippocampus, respectively). 5‐Hydroxytryptamine3 (5‐HT3) receptor agonists and antagonists competed for the [3H]zacopride binding site, competing with up to 40% of total binding with a similar rank order of affinity in both tissues; agents acting on various other neurotransmitter receptors failed to inhibit binding. Kinetic data revealed a fast association that was fully reversible (k+1= 6.61 × 105 and 7.65 × 105/mol/L/s and k‐1= 3.68 × 10−3 and 3.45 × 10−3/s in the amygdala and hippocampus, respectively). It is concluded that [3H]zacopride selectively labels with high affinity 5‐HT3 recognition sites in human amygdala and hippocampus and, if these binding domains represent 5‐HT3 receptors, may provide the opportunity for 5‐HT3 receptor antagonists to modify 5‐HT function in the human brain

Cognitive enhancing actions of PD123177 detected in a mouse habituation paradigm

The anxiolytic-like and cognitive enhancing potential of PD123177, a non-peptide with selectivity for the angiotensin II-2 receptor recognition site, was assessed in a mouse light/dark aversion test and habituation test, respectively. PD123177 (0.01 ng kg-1 to 1.0 mg kg-1 i.p.) failed to alter the behavioural repertoire of animals in the light/dark aversion test. In contrast, daily administration of PD123177 (10.0 ng kg-1 i.p. b.d.) enhanced the performance of mice in a habituation test. In addition, PD123177 overcame the cognitive impairment induced by the administration of scopolamine (0.25 mg kg-1 i.p.). Such findings indicate for the first time a functional role for the angiotension II-2 receptor and further implicate the angiotensin system in the modulation of cognitive processes.

Alterations in angiotensin AT1 and AT2 receptor subtype levels in brain regions from patients with neurodegenerative disorders

The present studies assessed the levels of [125I][Sar1,ILE8]angiotensin II-labelled angiotensin AT1 and AT2 receptor recognition sites in homogenates of various brain areas (including caudate nucleus, putamen, substantia nigra, hippocampus, frontal cortex, temporal cortex and cerebellum) from patients with clinically diagnosed Parkinsons disease, Huntingtons disease and Alzheimers disease and those from age-, sex- and post-mortem delay-matched neurologically and psychiatrically normal patients. Radiolabelled angiotensin AT1 receptor recognition site levels were significantly decreased by approximately 70%, 70% and 90% in the caudate nucleus, putamen and substantia nigra, respectively, from patients with Parkinsons disease relative to matched controls. Furthermore, radiolabelled angiotensin AT2 receptor levels were decreased by some 60% in the caudate nucleus of patients with Parkinsons disease relative to control patients. In brain tissue homogenates from patients with Huntingtons disease, the angiotensin AT1 receptor recognition site levels were decreased by approximately 30% in putamen relative to the control patients whilst angiotensin AT2 receptor levels were increased by some 90% in the caudate nucleus relative to the control patients. In brain tissue homogenates from patients with Alzheimer disease, the angiotensin AT2 receptor recognition site levels were significantly increased by approximately 200% in the temporal cortex relative to the control patients. The present results indicate that the reduction of angiotensin AT1 and/or AT2 receptor recognition site levels in the caudate nucleus, putamen and substantia nigra correlates with the principal neuropathology associated with Parkinsons disease and as such indicates that at least a significant population of angiotensin AT1 and AT2 receptors are located on the human dopaminergic nigrostriatal pathway. In addition, the marked increase in the levels of angiotensin AT2 receptor recognition sites in temporal cortex from patients with Alzheimers disease correlates with some other markers associated with the renin-angiotensin system previously investigated in tissue from patients with this neurological disease.

Angiotensin II inhibits the release of [3H]acetylcholine from rat entorhinal cortex in vitro

The effects of angiotensin I and II on basal potassium-induced release of [3H]acetylcholine were investigated in slices of rat entorhinal cortex. Potassium (10-25 mM) produced a concentration-dependent increase in the release of [3H]acetylcholine in the presence of extracellular calcium. Angiotensin II (10(-9)-10(-5) M) (but not angiotensin I) reduced the potassium-induced release of [3H]acetylcholine in a concentration-related manner to 60% of control levels, but did not effect basal tritium release. The effect of angiotensin II was antagonised by [1-sarcosine, 8-threonine] angiotensin II, an angiotensin II receptor antagonist, but not by agents acting on alpha- and beta-adrenoceptors, muscarinic, nicotinic, histamine or 5-hydroxytryptamine receptors nor by the angiotensin converting enzyme (ACE) inhibitor SQ 29852. The results indicate that angiotensin II acting via an angiotensin II receptor can inhibit the release of [3H]acetylcholine in slices of the rat entorhinal cortex. It is hypothesised that the ability of ACE inhibitors to facilitate cognitive processes may be related to a reduced availability of angiotensin II.