Midbrain circuits that set locomotor speed and gait selection.

Locomotion is a fundamental motor function common to the animal kingdom. It is implemented episodically and adapted to behavioural needs, including exploration, which requires slow locomotion, and escape behaviour, which necessitates faster speeds. The control of these functions originates in brainstem structures, although the neuronal substrate(s) that support them have not yet been elucidated. Here we show in mice that speed and gait selection are controlled by glutamatergic excitatory neurons (GlutNs) segregated in two distinct midbrain nuclei: the cuneiform nucleus (CnF) and the pedunculopontine nucleus (PPN). GlutNs in both of these regions contribute to the control of slower, alternating-gait locomotion, whereas only GlutNs in the CnF are able to elicit high-speed, synchronous-gait locomotion. Additionally, both the activation dynamics and the input and output connectivity matrices of GlutNs in the PPN and the CnF support explorative and escape locomotion, respectively. Our results identify two regions in the midbrain that act in conjunction to select context-dependent locomotor behaviours.

Adenosine A2A receptors and basal ganglia physiology.

Adenosine A2A receptors are highly enriched in the basal ganglia system. They are predominantly expressed in enkephalin-expressing GABAergic striatopallidal neurons and therefore are highly relevant to the function of the indirect efferent pathway of the basal ganglia system. In these GABAergic enkephalinergic neurons, the A2A receptor tightly interacts structurally and functionally with the dopamine D2 receptor. Both by forming receptor heteromers and by targeting common intracellular signaling cascades, A2A and D2 receptors exhibit reciprocal antagonistic interactions that are central to the function of the indirect pathway and hence to basal ganglia control of movement, motor learning, motivation and reward. Consequently, this A2A/D2 receptors antagonistic interaction is also central to basal ganglia dysfunction in Parkinson's disease. However, recent evidence demonstrates that, in addition to this post-synaptic site of action, striatal A2A receptors are also expressed and have physiological relevance on pre-synaptic glutamatergic terminals of the cortico-limbic-striatal and thalamo-striatal pathways, where they form heteromeric receptor complexes with adenosine A1 receptors. Therefore, A2A receptors play an important fine-tuning role, boosting the efficiency of glutamatergic information flow in the indirect pathway by exerting control, either pre- and/or post-synaptically, over other key modulators of glutamatergic synapses, including D2 receptors, group I metabotropic mGlu5 glutamate receptors and cannabinoid CB1 receptors, and by triggering the cAMP-protein kinase A signaling cascade.

Expression of X-chromosome linked inhibitor of apoptosis protein in mature Purkinje cells and in retinal bipolar cells in transgenic mice induces neurodegeneration.

Transgenic mice with overexpression of the caspase-inhibitor, X-chromosome-linked inhibitor of apoptosis protein (XIAP) in Purkinje cell (PC) and in retinal bipolar cells (RBCs) were produced to study the regulation of cell death. Unexpectedly, an increased neurodegeneration was observed in the PCs in these L7-XIAP mice after the third postnatal week with the mice exhibiting severe ataxia. The loss of PCs was independent of Bax as shown by crossing the L7-XIAP mice with Bax gene-deleted mice. Electron microscopy revealed intact organelles in PCs but with the stacking of ER cisterns indicative of cell stress. Immunostaining for cell death proteins showed an increased phosphorylation of c-Jun in the PCs, suggesting an involvement in cell degeneration. Apart from PCs, the number of RBCs was decreased in adult retina in line with the expression pattern for the L7 promoter. The data show that overexpression of the anti-apoptotic protein XIAP in vulnerable neurons leads to enhanced cell death. The mechanisms underlying this neurodegeneration can be related to the effects of XIAP on cell stress and altered cell signaling.

The histamine H3 receptor antagonist thioperamide rescues circadian rhythm and memory function in experimental parkinsonism.

Parkinson's disease (PD) is a common neurodegenerative disorder, characterized by motor impairment and a wide range of non-motor symptoms, including sleep disorders and cognitive and affective deficits. In this study, we used a mouse model of PD based on 6-hydroxydopamine (6-OHDA) to examine the effect of thioperamide, a histamine H3 receptor antagonist, on circadian activity, recognition memory and anxiety. A partial, bilateral 6-OHDA lesion of the striatum reduces motor activity during the active phase of the 24 h cycle. In addition, the lesion disrupts the endogenous circadian rhythm observed when mice are maintained in constant darkness. Administration of thioperamide to 6-OHDA-lesion mice rescues the normal rest/activity cycle. Moreover, thioperamide counteracts the deficit of novel object recognition produced by 6-OHDA. Our experiments show that this memory impairment is accompanied by disrupted gamma oscillations in the hippocampus, which are also rescued by thioperamide. In contrast, we do not observe any modification of the anxiogenic effect of 6-OHDA in response to administration of thioperamide. Our results indicate that thioperamide may act as a multifunctional drug, able to counteract disruptions of circadian rhythm and cognitive deficits associated with PD.

Distribution, biochemistry and function of striatal adenosine A2A receptors.

It is well known that the nucleoside adenosine exerts a modulatory influence in the central nervous system by activating G protein coupled receptors. Adenosine A2A receptors, the subject of the present review, are predominantly expressed in striatum, the major area of the basal ganglia. Activation of A2A receptors interferes with effects mediated by most of the principal neurotransmitters in striatum. In particular, the inhibitory interactions between adenosine acting on A2A receptors and dopamine acting on D2 receptors have been well examined and there is much evidence that A2A receptors may be a possible target for future development of drugs for treatment of Parkinson's disease, schizophrenia and affective disorders. Our understanding of the role of striatal A2A receptors has increased dramatically over the last few years. New selective antibodies, antagonist radioligands and optimized in situ hybridization protocols have provided detailed information on the distribution of A2A receptors in rodent as well as primate striatum. Studies on the involvement of A2A receptors in the regulation of DARPP-32 and the expression of immediate early genes, such as nerve growth factor-induced clone A and c-fos, have pointed out an important role for these receptors in regulating striatopallidal neurotransmission. Moreover, by using novel selective antagonists for A2A receptors and transgenic mice lacking functional A2A receptors, crucial information on the behavioral role of striatal A2A receptors has been provided, especially concerning their involvement in the stimulatory action of caffeine and the anti-Parkinsonian properties of A2A receptor antagonists. In the present review, current knowledge on the distribution, biochemistry and function of striatal A2A receptors is summarized.

Galanin and galanin antagonists: molecular and biochemical perspectives.

The neuropeptide galanin potently inhibits insulin release, hippocampal acetylcholine release and firing of locus coeruleus cells, and stimulates feeding and release of growth hormone. Galanin regulates K+ channels, adenylyl cyclase and phospholipase C by acting at Gi/Go protein-coupled high-affinity receptors. Galanin receptor agonists such as the N-terminal fragment galanin1-16 act synergistically with morphine in the somatosensory system and have potential analgetic application. Galanin antagonists may be useful therapeutic agents in endocrinology, neurology and psychiatry. The enhancing effect of such agents on hippocampal cholinergic function would be useful in treatment of Alzheimer's disease. Recent synthesis of a series of high-affinity galanin antagonists, reviewed, along with galanin's actions, by Tamas Bartfai and colleagues, opens the possibility of examining the functions of endogenous galanin and test the pharmacological usefulness of antagonism of galanin function in the endocrine, somatosensory and central nervous systems.

Activation of dopamine D2 receptors decreases DARPP-32 phosphorylation in striatonigral and striatopallidal projection neurons via different mechanisms.

The vast majority of striatal neurons are GABAergic medium-sized spiny neurons. These cells receive glutamatergic input from the cortex, thalamus and limbic areas and dopaminergic input from the mesencephalon. Most relevant evidence indicates that dopamine D1 receptors are located on striatonigral projection neurons, and that adenosine A2A receptors and most dopamine D2 receptors are located on striatopallidal projection neurons (see, however, Refs I and 13). Here we have utilized regulation of the phosphorylation of dopamine- and cyclic AMP-regulated phosphoprotein of mol. wt 32,000 (DARPP-32) to study the possible interactions among nigrostriatal dopaminergic neurons and the two classes of dopaminoceptive target neurons. We show that, in striatal slices, the D2 receptor agonist, quinpirole, strongly inhibits the phosphorylation of DARPP-32 induced by either the D1 receptor agonist, SKF 81297, or the A2A receptor agonist, CGS 21680. Tetrodotoxin abolished the effect of quinpirole on the D1 agonist-induced but not the A2A agonist-induced phosphorylation of DARPP-32. These data indicate that: (i) adenosine A2A and dopamine D2 receptors interact within the same striatopallidal neurons, and (ii) D2 receptors present on the striatopallidal neurons modulate the effects of D1 receptors on the striatonigral neurons. Thus, a single neurotransmitter is capable of activating distinct classes of receptors on distinct populations of target neurons, which, in turn, interact with each other through intercellular communication.

Activation of adenosine A2A and dopamine D1 receptors stimulates cyclic AMP-dependent phosphorylation of DARPP-32 in distinct populations of striatal projection neurons.

In the striatum, adenosine A2A and dopamine D1 receptors are segregated in striatopallidal and striatonigral projection neurons, respectively. In this study, we have examined the effects of activating adenosine A2A and dopamine D1 receptors on the state of phosphorylation of DARPP-32 (dopamine- and cyclic AMP-regulated phosphoprotein of mol. wt 32,000), a potent endogenous regulator of protein phosphatase-1 that is highly expressed in striatal medium-sized spiny neurons. In rat striatal slices, the D1 receptor agonist SKF 81297 and the A2A receptor agonist CGS 21680 transiently increased the levels of phosphorylated DARPP-32 in a concentration-dependent manner. In the same preparation, the two agonists were also able to induce a significant increase in cyclic AMP formation. When striatal slices were incubated with a combination of CGS 21680 and SKF 81297, the effects of the two agonists on both DARPP-32 phosphorylation and cyclic AMP formation were additive. The maximal effects of SKF 81297 and CGS 21680 on DARPP-32 phosphorylation were of similar magnitude, and were completely abolished by the cyclic AMP-dependent protein kinase inhibitor, Rp-cAMPS. The present results show that DARPP-32 phosphorylation in the striatum is stimulated by adenosine, acting on A2A receptors, and dopamine, acting on D1 receptors, and that cyclic AMP is the mediator in both cases. Our data also suggest that dopamine and adenosine regulate the state of phosphorylation of DARPP-32 in distinct sub-populations of medium-sized spiny neurons expressing dopamine D1 and adenosine A2A receptors, respectively.

Modulation of phospholipid methylation in rat striatum by the corticostriatal pathway.

Surgical undercutting of cortical afferents and subsequent interruption of excitatory input increased phospholipid (PL) methylation (30-40%) in rat striatal synaptosomes as early as 3 days after lesion. The increase was still present 60 days after lesion, was protein-dependent and lesion-specific. The role of the dopaminergic agents on PL methylation in decorticated rats was investigated. In contrast with sham-lesioned rats, in which dopamine stimulated PL methylation, dopamine did not further stimulate phosphatidylethanolamine N-methyltransferase (PEMT) activity in decorticated rats. In addition, dopamine antagonists reversed the increase in PL methylation in lesioned rats at concentrations that had no effect in blocking DA-stimulated PL methylation in sham-lesioned rats. The specific antagonist of N-methyl-D-aspartate receptors, 2-amino-5-phosphonovaleric acid, increased PL methylation in non-lesioned striata, mimicking the effect of decortication, while N-methyl-D-aspartic acid counteracted the decortication-induced increase in PEMT activity. These data suggest a modulatory role of the cortex on PEMT activity in rat striatum.

Galanin inhibits the potassium-evoked release of acetylcholine and the muscarinic receptor-mediated stimulation of phosphoinositide turnover in slices of monkey hippocampus.

In the ventral hippocampus of Cynomologus monkey, galanin, a 29 amino acid long neuropeptide, reduced the potassium-evoked release of [3H]acetylcholine from slices preloaded with [3H]choline and diminished the carbachol-stimulated accumulation of [3H]inositol polyphosphates in hippocampal microprisms preincubated with myo-[2-3H]inositol. Using receptor autoradiography a strong, specific binding of iodinated galanin was observed in the molecular layer of the dentate gyrus. These may thus be the sites where galanin exerts its inhibitory effects on acetylcholine (ACh) release and phosphoinositide breakdown. These data provide evidence that galanin is a modulator of cholinergic function in septo-hippocampal neurons of primates.

Mechanism of the galanin induced increase in acetylcholine release in vivo from striata of freely moving rats.

Galanin (GAL) administered intracerebroventricularly (i.c.v.) induced a strong and long-lasting increase in the basal acetylcholine (ACh) release from striata of freely moving rats only when the excitatory corticostriatal input was removed, while its effect was transient in striata of sham-operated rats. This effect was dose-dependent (0.78, 1.56 and 3.12 nmol) and was completely prevented by the GAL receptor antagonist, galantide. GAL injected locally (3.12 nmol) in deafferented striata also induced a persistent increase in ACh release although to a lower extent. The impairment of monoaminergic neurotransmission caused by alpha-methylparatyrosine or p-chlorophenylalanine, respectively inhibitors of catecholamine and serotonin synthesis, completely prevented the rise in ACh output from deafferented striata while the muscarinic antagonist, scopolamine (0.5 mg/kg, s.c.), failed to do it. The data suggest that GAL in the deafferented striatum facilitates basal ACh release through an indirect mechanism. The effect seems to be at least partly mediated by an action of GAL on specific receptors in the striata.

N-terminal galanin fragments inhibit the hippocampal release of acetylcholine in vivo.

Using synthetic N-terminal fragments of galanin, galanin (1-7), galanin (1-9), galanin (1-12) and galanin (1-16), we have shown that the minimal sequence required for inhibition of acetylcholine release in vivo from rat ventral hippocampus corresponds to galanin (1-12). The fragment (1-9) displays activity in vivo but only at a very high concentration of 6.23 nmol while galanin (1-7) and C-terminal fragment (17-29) are without effect. Binding studies showed that galanin (1-16) and galanin (1-12) bind with submicromolar IC50 values to rat hippocampal galanin receptors. Galanin (1-9) has substantially lower affinity towards rat ventral hippocampal galanin receptor.

The N-terminal 1-16, but not C-terminal 17-29, galanin fragment affects the flexor reflex in rats.

The biological activity of two galanin (GAL) fragments, GAL-(1-16) and GAL-(17-29), was tested in vivo by using a spinal nociceptive flexor reflex model in the rat. Intrathecal (i.t.) GAL-(1-16) had a similar biphasic effect on the flexor reflex, with facilitation at lower doses and facilitation followed by depression at higher doses, as the full length peptide GAL-(1-29). GAL-(1-16) also effectively depressed the facilitation of the flexor reflex caused by i.t. substance P (SP) or C-fiber conditioning stimulation (CS) and potentiated the depressive effect of i.t. morphine on the reflex, both actions that have been reported earlier with GAL-(1-29). In contrast, i.t. GAL-(17-29), even at high doses, did not induce changes in the amplitude of the flexor reflex, nor did it interact with the effects of i.t. SP, morphine or C-fiber CS. It is concluded that the N-terminal portion of GAL-(1-29) is critical for the biological activity of the intact peptide in the dorsal horn of the rat spinal cord. The similarity between the effects of GAL-(1-16) and GAL-(1-29) indicates that they probably act on the same GAL receptor.

Galanin-induced inhibition of insulin secretion from rat islets: effects of rat and pig galanin and galanin fragments and analogues.

The 29-amino acid neuropeptide galanin occurs in intrapancreatic nerves and inhibits insulin secretion. To study the structure-activity relations of galanin, we examined the effects of pig and rat galanin, three galanin fragments (galanin-(1-11), galanin-(1-16) and rat galanin-(17-29) and four galanin analogues ([Ala2]pig galanin, [Ala2]rat galanin, [D-Trp2]rat galanin and [D-Trp2]galanin-(1-16] on glucose-stimulated insulin secretion from isolated rat islets. Pig and rat galanin and galanin-(1-11) equipotently inhibited glucose-stimulated (8.3 mM) insulin secretion at and above 10(-7) M (P less than 0.05), whereas galanin-(1-16), inhibited insulin secretion at 10(-6) M (P less than 0.01). In contrast, the C-terminal rat galanin-(17-29) and the galanin analogues did not influence insulin secretion. Thus, rat and pig galanin are equipotent in inhibiting glucose-stimulated insulin secretion from rat islets. The active site resides in the N-terminal part of the molecule. Furthermore, the binding of galanin to its receptor depends on structural characteristics governed by the N-terminal position and in particular by the Trp2 residue.

Qualitative differences in the effects of adenosine analogs on the cholinergic systems of rat striatum and hippocampus.

The effect of the purinergic agonist, 2-chloroadenosine (2-CADO), on central cholinergic parameters was studied in the rat. The drug (20 micrograms, i.c.v.) increased acetylcholine (ACh) content (approximately 30%) and inhibited sodium dependent high affinity choline uptake (30%) in the hippocampus. In striatum, the increase of ACh content was less marked (approximately 15%) and was not associated with inhibition of choline uptake. In both areas, ACh accumulation was prevented by theophylline but not by atropine or oxotremorine pretreatments. Differences were noted in the purinergic control of cholinergic function in the hippocampus and striatum. In hippocampus, the selective degeneration of noradrenergic, serotonergic and glutamatergic afferent pathways or the destructions of intrinsic neurons did not prevent the rise in ACh content induced by 2-CADO. Differently, in striatum, the action of 2-CADO was potentiated both by raphe deafferentation and by inhibition of serotonin synthesis and was completely prevented by chronic unilateral decortication. The cholinergic effect of 2-CADO was unchanged after impairment of the noradrenergic or dopaminergic systems. In addition, the D- and L-isomers of phenylisopropyladenosine, which have different affinities for A1 purinergic receptors but equal affinity for the A2 purinergic subtype, differed in their ability to affect acetylcholine content in these two brain regions, suggesting that A1 purinergic receptor activation mediates the effect of 2-CADO in the hippocampus and A2 receptor activation mediates the drug's action in the striatum.

Role of the hippocampus in the sex-dependent regulation of eating behavior: studies with kainic acid.

Marked hyperphagia with an increase in the rate of body weight gain was noted in adult female rats 4 days after injections of 2 nmoles of kainic acid into the dorsal and ventral parts of hippocampus. The effect was still present 70 days later. At this time the increase in daily food intake and body weight gain amounted, respectively, to 39% and 93% over the control value. There was no change in water intake. The injection of kainic acid into only one part of the hippocampus--either dorsal or ventral--did not induce hyperphagia. Male rats with kainic acid lesion did not show changes in food intake or body weight gain as compared to vehicle-treated controls. In both sexes the degeneration of hippocampal perikarya induced by kainic acid was associated with a 50-60% decrease in glutamic acid decarboxylase activity and [3H]glutamate uptake, as well as with a small decrease in [3H]glutamate uptake in the hypothalamus, an area that receives glutamatergic fibers from the hippocampus. The results show that the hippocampus appears to play an important role in appetite motivation control by a mechanism which is sex-related.

Group III and subtype 4 metabotropic glutamate receptor agonists: discovery and pathophysiological applications in Parkinson's disease.

Restoring the balance between excitatory and inhibitory circuits in the basal ganglia, following the loss of dopaminergic (DA) neurons of the substantia nigra pars compacta, represents a major challenge to treat patients affected by Parkinson's disease (PD). The imbalanced situation in favor of excitation in the disease state may also accelerate excitotoxic processes, thereby representing a potential target for neuroprotective therapies. Reducing the excitatory action of glutamate, the major excitatory neurotransmitter in the basal ganglia, should lead to symptomatic improvement for PD patients and may promote the survival of DA neurons. Recent studies have focused on the modulatory action of metabotropic glutamate (mGlu) receptors on neurodegenerative diseases including PD. Group III mGlu receptors, including subtypes 4, 7 and 8, are largely expressed in the basal ganglia. Recent studies highlight the use of selective mGlu4 receptor positive allosteric modulators (PAMs) for the treatment of PD. Here we review the effects of newly-designed group-III orthosteric agonists on neuroprotection, neurorestoration and reduction of l-DOPA induced dyskinesia in animal models of PD. The combination of orthosteric mGlu4 receptor selective agonists with PAMs may open new avenues for the symptomatic treatment of PD. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.