Novel selective allosteric activator of the M1 muscarinic acetylcholine receptor regulates amyloid processing and produces antipsychotic-like activity in rats

Recent studies suggest that subtype-selective activators of M(1)/M(4) muscarinic acetylcholine receptors (mAChRs) may offer a novel approach for the treatment of psychotic symptoms associated with schizophrenia and Alzheimer's disease. Previously developed muscarinic agonists have provided clinical data in support of this hypothesis, but failed in clinical development because of a lack of true subtype specificity and adverse effects associated with activation of other mAChR subtypes. We now report characterization of a novel highly selective agonist for the M(1) receptor with no agonist activity at any of the other mAChR subtypes, termed TBPB [1-(1'-2-methylbenzyl)-1,4'-bipiperidin-4-yl)-1H-benzo[d]imidazol-2(3H)-one]. Mutagenesis and molecular pharmacology studies revealed that TBPB activates M(1) through an allosteric site rather than the orthosteric acetylcholine binding site, which is likely critical for its unprecedented selectivity. Whole-cell patch-clamp recordings demonstrated that activation of M(1) by TBPB potentiates NMDA receptor currents in hippocampal pyramidal cells but does not alter excitatory or inhibitory synaptic transmission, responses thought to be mediated by M(2) and M(4). TBPB was efficacious in models predictive of antipsychotic-like activity in rats at doses that did not produce catalepsy or peripheral adverse effects of other mAChR agonists. Finally, TBPB had effects on the processing of the amyloid precursor protein toward the non-amyloidogenic pathway and decreased Abeta production in vitro. Together, these data suggest that selective activation of M(1) may provide a novel approach for the treatment of symptoms associated with schizophrenia and Alzheimer's disease.

Chemosensitive serotonergic neurons are closely associated with large medullary arteries

We have previously shown that serotonergic neurons of the medulla are strongly stimulated by an increase in CO(2), suggesting that they are central respiratory chemoreceptors. Here we used confocal imaging and electron microscopy to show that neurons immunoreactive for tryptophan hydroxylase (TpOH) are tightly apposed to large arteries in the rat medulla. We used patch-clamp recordings from brain slices to confirm that neurons with this anatomical specialization are chemosensitive. Serotonergic neurons are ideally situated for sensing arterial blood CO(2), and may help maintain pH homeostasis via wide-ranging effects on brain function. The results reported here support a recent proposal that sudden infant death syndrome (SIDS) results from a developmental abnormality of medullary serotonergic neurons.

Chemosensitivity of serotonergic neurons in the rostral ventral medulla

The medullary raphé contains two subtypes of chemosensitive neuron: one that is stimulated by acidosis and another that is inhibited. Both types of neuron are putative chemoreceptors, proposed to act in opposite ways to modulate respiratory output and other pH sensitive brain functions. In this review, we will discuss the cellular properties of these chemosensitive raphé neurons when studied in vitro using brain slices and primary dissociated cell culture. Quantification of chemosensitivity of raphé neurons indicates that they are highly sensitive to small changes in extracellular pH (pH(o)) between 7.2 and 7.6. Stimulation by acidosis occurs only in the specific phenotypic subset of neurons within the raphé that are serotonergic. These serotonergic neurons also have other properties consistent with a specialized role in chemoreception. Homologous serotonergic neurons are present within the ventrolateral medulla (VLM), and may have contributed to localization of respiratory chemoreception to that region. Chemosensitivity of raphé neurons increases in the postnatal period in rats, in parallel with development of respiratory chemoreception in vivo. An abnormality of serotonergic neurons of the ventral medulla has been identified in victims of sudden infant death syndrome (SIDS). The cellular properties of serotonergic raphé neurons suggest that they play a role in the CNS response to hypercapnia, and that they may contribute to interactions between the sleep/wake cycle and respiratory control.

Activation of group I metabotropic glutamate receptors produces a direct excitation and disinhibition of GABAergic projection neurons in the substantia nigra pars reticulata

A pathological increase in excitatory glutamatergic input to substantia nigra pars reticulata (SNr) from the subthalamic nucleus (STN) is believed to play a key role in the pathophysiology of Parkinson's disease. We present an analysis of the physiological roles that group I metabotropic glutamate receptors (mGluRs) play in regulating SNr functions. Immunocytochemical analysis at the light and electron microscopic levels reveal that both mGuR1a and mGluR5 are localized postsynaptically in the SNr. Consistent with this, activation of group I mGluRs depolarizes SNr GABAergic neurons. Interestingly, although both group I mGluRs (mGluR1 and mGluR5) are expressed in these neurons, the effect is mediated solely by mGluR1. Light presynaptic staining for mGluR1a and mGluR5 was also observed in some terminals forming symmetric synapses and in small unmyelinated axons. Consistent with this, activation of presynaptic mGluR1a and mGluR5 decreases inhibitory transmission in the SNr. The combination of direct excitatory effects and disinhibition induced by activation of group I mGluRs could lead to a large excitation of SNr projection neurons. This suggests that group I mGluRs are likely to play an important role in the powerful excitatory control that the STN exerts on basal ganglia output neurons.

Activation of Group III mGluRs Inhibits GABAergic and Glutamatergic Transmission in the Substantia Nigra Pars Reticulata

The GABAergic projection neurons of the substantia nigra pars reticulata (SNr) exert an important influence on the initiation and control of movement. The SNr is a primary output nucleus of the basal ganglia (BG) and is controlled by excitatory inputs from the subthalamic nucleus (STN) and inhibitory inputs from the striatum and globus pallidus. Changes in the output of the SNr are believed to be critically involved in the development of a variety of movement disorders. Anatomical studies reveal that metabotropic glutamate receptors (mGluRs) are highly expressed throughout the BG. Interestingly, mRNA for group III mGluRs are highly expressed in STN, striatum, and globus pallidus, and immunocytochemical studies have shown that the group III mGluR proteins are present in the SNr. Thus it is possible that group III mGluRs play a role in the modulation of synaptic transmission in this nucleus. We performed whole cell patch-clamp recordings from nondopaminergic SNr neurons to investigate the effect of group III mGluR activation on excitatory and inhibitory transmission in the SNr. We report that activation of group III mGluRs by the selective agonist l(+)-2-amino-4-phosphonobutyric acid (l-AP4, 100 μM) decreases inhibitory synaptic transmission in the SNr. Miniature inhibitory postsynaptic currents studies and paired-pulse studies reveal that this effect is mediated by a presynaptic mechanism. Furthermore we found that l-AP4 (500 μM) also reduces excitatory synaptic transmission at the STN-SNr synapse by action on presynaptically localized group III mGluRs. The finding that mGluRs modulate the major inputs to SNr neurons suggests that these receptors may play an important role in motor function and could provide new targets for the development of pharmacological treatments of movement disorders.

Acidosis-Stimulated Neurons of the Medullary Raphe Are Serotonergic

Neurons of the medullary raphe project widely to respiratory and autonomic nuclei and contain co-localized serotonin, thyrotropin-releasing hormone (TRH), and substance P, three neurotransmitters known to stimulate ventilation. Some medullary raphe neurons are highly sensitive to pH and CO2 and have been proposed to be central chemoreceptors. Here it was determined whether these chemosensitive neurons are serotonergic. Cells were microdissected from the rat medullary raphe and maintained in primary cell culture for 13–70 days. Immunoreactivity for serotonin, substance P, and TRH was present in these cultures. All acidosis-stimulated neurons ( n = 22) were immunoreactive for tryptophan hydroxylase (TpOH-IR), the rate-limiting enzyme for serotonin biosynthesis, whereas all acidosis-inhibited neurons ( n= 16) were TpOH-immunonegative. The majority of TpOH-IR medullary raphe neurons (73%) were stimulated by acidosis. The electrophysiological properties of TpOH-IR neurons in culture were similar to those previously reported for serotonergic neurons in vivo and in brain slices. These properties included wide action potentials (4.55 ± 0.5 ms) with a low variability of the interspike interval, a postspike afterhyperpolarization (AHP) that reversed 25 mV more positive than the Nernst potential for K+, prominent A current, spike frequency adaptation and a prolonged AHP after a depolarizing pulse. Thus the intrinsic cellular properties of serotonergic neurons were preserved in cell culture, indicating that the results obtained using this in vitro approach are relevant to serotonergic neurons in vivo. These results demonstrate that acidosis-stimulated neurons of the medullary raphe contain serotonin. We propose that serotonergic neurons initiate a homeostatic response to changes in blood CO2 that includes increased ventilation and modulation of autonomic function.

Distribution and roles of metabotropic glutamate receptors in the basal ganglia motor circuit: implications for treatment of Parkinson's disease and related disorders

The basal ganglia (BG) are a set of interconnected subcortical structures that play a critical role in motor control. The BG are thought to control movements by a delicate balance of transmission through two BG circuits that connect the input and output nuclei: the direct and the indirect pathways. The BG are also involved in a number of movement disorders. Most notably, the primary pathophysiological change that gives rise to the motor symptoms of Parkinson's Disease (PD) is the loss of dopaminergic neurons of the substantia nigra pars compacta (SNc) that are involved in modulating function of the striatum and other BG structures. This ultimately results in an increase in activity of the indirect pathway relative to the direct pathway and the hallmark PD symptoms of rigidity, bradykinesia, and akinesia. A great deal of effort has been dedicated to finding treatments for this disease. The current pharmacotherapies are aimed at replacing the missing dopamine, while the current surgical treatments are aimed at reducing transmission through the indirect pathway. Dopamine replacement therapy has proven to be helpful, but is associated with severe side effects that limit treatment and a loss of efficacy with progression of the disease. Recently developed surgical therapies have been highly effective, but are highly invasive, expensive, and assessable to a small minority of patients. For these reasons, new effort has been dedicated to finding pharmacological treatment options that will be effective in reducing transmission through the indirect pathway. Members of the metabotropic glutamate receptor (mGluR) family have emerged as interesting and promising targets for such a treatment. This review will explore the most recent advances in the understanding of mGluR localization and function in the BG motor circuit and the implications of those findings for the potential therapeutic role of mGluR-targeted compounds for PD.

Activation of group II metabotropic glutamate receptors inhibits synaptic excitation of the substantia Nigra pars reticulata

Loss of nigrostriatal dopaminergic neurons in Parkinson's disease (PD) leads to increased activity of glutamatergic neurons in the subthalamic nucleus (STN). Recent studies reveal that the resultant increase in STN-induced excitation of basal ganglia output nuclei is responsible for the disabling motor impairment characteristic of PD. On the basis of this, it is possible that any manipulation that reduces activity at excitatory STN synapses onto basal ganglia output nuclei could be useful in the treatment of PD. We now report that group II metabotropic glutamate receptors (mGluRs) are presynaptically localized on STN terminals and that activation of these receptors inhibits excitatory transmission at STN synapses. In agreement with the hypothesis that this could provide a therapeutic benefit in PD, a selective agonist of group II mGluRs induces a dramatic reversal of catalepsy in a rat model of PD. These results raise the exciting possibility that selective agonists of group II mGluRs could provide an entirely new approach to the treatment of PD. These novel therapeutic agents would provide a noninvasive pharmacological treatment that does not involve the manipulation of dopaminergic systems, thus avoiding the problems associated with current therapies.

Immunohistochemical localization of subtype 4a metabotropic glutamate receptors in the rat and mouse basal ganglia

Recent studies suggest that metabotropic glutamate receptors (mGluRs) may play a significant role in regulating basal ganglia functions. In this study, we investigated the localization of mGluR4a protein in the mouse and rat basal ganglia. Polyclonal antibodies that specifically react with the metabotropic glutamate receptor subtype mGluR4a were produced and characterized by Western blot analysis. These antibodies recognized a native protein in wild-type mouse brain with a molecular weight similar to the molecular weight of the band from a mGluR4a-transfected cell line. The immunoreactivity was absent in brains of knockout mice deficient in mGluR4. mGluR4a immunoreactivity was most intense in the molecular layer of the cerebellum. We also found a striking mGluR4a immunoreactivity in globus pallidus, and moderate staining in substantia nigra pars reticulata and entopeduncular nucleus. Moderate to low mGluR4a immunoreactivity was present in striatum and other brain regions, including hippocampus, neocortex, and thalamus. Double labeling with mGluR4a antibodies and antibodies to either a dendritic marker or a marker of presynaptic terminals suggest a localization of mGluR4a on presynaptic terminals. Immunocytochemistry at electron microscopy level confirmed these results, revealing that in the globus pallidus, mGluR4a is mainly localized in presynaptic sites in axonal elements. Finally, quinolinic acid lesion of striatal projection neurons decreased mGluR4a immunoreactivity in globus pallidus, suggesting a localization of mGluR4a on striatopallidal terminals. These data support the hypothesis that mGluR4a serves as a presynaptic heteroreceptor in the globus pallidus, where it may play an important role in regulating g-amino-n-butyric acid (GABA) release from striatopallidal terminals.

Heterogeneity of metabotropic glutamate receptors in autonomic cell groups of the medulla oblongata of the rat

Metabotropic glutamate receptors (mGluRs) in the medulla oblongata have been suggested to be involved in the regulation of autonomic function. The aim of the present study was to examine the localization and expression of four types of mGluRs: mGluRla, mGluR2/3, mGluR5, and mGluR7 in the dorsal and ventral autonomic nuclei of the medulla of the rat. The four mGluR subtypes studied were differentially distributed in distinct subnuclei in the nucleus of the solitary tract (NTS). mGluRla immunoreactivity was identified in cell bodies, dendrites, and axonal processes in the intermediate, dorsal lateral, and interstitial subnuclei of the NTS. No mGluRla immunoreactivity was observed in the commissural or medial NTS subnuclei. Immunoreactivity for mGluR2/3 and mGluR5 as observed in fibers and putative axonal processes in the interstitial, intermediate, and dorsolateral subnuclei of the NTS. In contrast, mGluR7 was expressed primarily in fibers and terminals in the central and commissural NTS subnuclei. Expression of mGluR2/3 was clearly evident in cell bodies, dendrites, and axonal processes within the area postrema. The vagal outflow nuclei were also studied. The dorsal motor nucleus of the vagus (DMN) contained mGluRla cell bodies, dendrites, and axonal fibers and light mGluR2/3 processes. Throughout the rostral-caudal extent of the compact and semicompact formation nucleus ambiguus, mGluRla was found in cell bodies and fibers. Within the caudal and rostral regions of the ventral lateral medulla, mGluRla was observed in cell bodies and fibers. Cell bodies containing mGluRla were found adjacent to cells staining positive for tyrosine hydroxylase (TH) in these regions but were not colocalized with the TH staining. However, mGluRla-expressing neurons in the ventral lateral medulla did appear to receive innervation from TH-containing fibers. These results suggest that the mGluRla-expressing neurons within the ventral lateral medulla are predominantly not catecholaminergic but may be innervated by catecholamine-containing fibers. These data are the first to provide a mapping of the different mGluR subtypes within the medulla and may facilitate predictions regarding the function of L-glutamate neurotransmission in these regions.

Distribution and developmental regulation of metabotropic glutamate receptor 7a in rat brain

To determine the regional and cellular distribution of the metabotropic glutamate receptor mGluR7a, we used rabbit anti-peptide polyclonal-targeted antibodies against the C-terminal domain of mGluR7a. Here we report that immunocytochemistry at the light-microscopic level revealed that mGluR7a is widely distributed throughout the adult rat brain, with a high level of expression in sensory areas, such as piriform cortex, superior colliculus, and dorsal cochlear nucleus. In most brain structures, mGluR7a immunoreactivity is characterized by staining of puncta and fibers. However, in some regions, including the locus ceruleus, cerebellum, and thalamic nuclei, both cell bodies and fibers are immunopositive. The changes in levels of mGluR7a during development were investigated with immunoblotting and immunocytochemical analysis. Immunoblot analysis revealed that the levels of mGluR7a are differentially regulated across brain regions during postnatal development. In cortical regions (hippocampus, neocortex, and olfactory cortex), mGluR7a levels were highest at postnatal day 7 (P7) and P14, then declined in older rats. In contrast, mGluR7a levels were highest at P7 in pons/medulla and cerebellum and decreased markedly between P7 and P14. In these regions, mGluR7a immunoreactivity was at similar low levels at P14 and P21 and in adults. Immunocytochemical analysis revealed that staining for mGluR7a was exceptionally high in fiber tracts in P7 animals relative to adults. Furthermore, the pattern of mGluR7a immunoreactivity in certain brain structures, including cerebellum, piriform cortex, and hippocampus, was significantly different in P7 and adult animals. In summary, these data suggest that mGluR7a is widely distributed throughout the rat brain and that this receptor undergoes a dynamic, regionally specific regulation during postnatal development.

Immunocytochemical localization of group III metabotropic glutamate receptors in the hippocampus with subtype-specific antibodies

The metabotropic glutamate receptors (mGluRs) mGluR4 and mGluR7 have been postulated to serve as presynaptic autoreceptors in the hippocampal formation. The cellular and synaptic localization of these proteins in the hippocampus, however, is not known. Polyclonal antibodies that specifically react with mGluR4a or mGluR7 were produced and used for immunocytochemical localization of these receptor proteins. Immunoblot analysis with mGluRs expressed in stably transfected baby hamster kidney cells suggests that each of the antibodies reacts specifically with the appropriate mGluR subtype but not with other mGluRs. Both antibodies recognized native proteins in rat brain with molecular weights similar to the molecular weights of the bands in mGluR-transfected cell lines. The distribution of mGluR4a immunoreactivity was fairly uniform in all brain regions. Immunoreactivity for mGluR7 was variable in different brain regions and closely paralleled the previously reported distribution of mGluR7 mRNA. Immunocytochemistry with light and electron microscopy (EM) revealed that immunoreactivities for mGluR4a and mGluR7 are widely but differentially distributed throughout the hippocampal formation. Staining with antibodies directed against mGluR4a was intense in cell bodies and dendrites of pyramidal cells, granule cells, and scattered interneurons. Analysis at the EM level revealed postsynaptic mGluR4a localization at asymmetrical (presumably glutamatergic) synapses and presynaptic localization at both asymmetrical and symmetrical synapses. Immunoreactivity for mGluR7 revealed largely presynaptic localization of this receptor at asymmetrical but not symmetrical synapses. These data are consistent with a largely presynaptic role of mGluR7 in the hippocampus and suggest that mGluR4 may have both presynaptic and postsynaptic functions.