Cyclic adenosine-3',5'-monophosphate potentiates the synaptic potential mediated by NMDA receptors in the amygdala

Modulation of NMDA receptors in the cerebellum. II. Signaling pathways and physiological modulators regulating NMDA receptor function

NMDA receptors in cerebellum have specific characteristics that make their function and modulation different from those of NMDA receptors in other brain areas. The properties of the NMDA receptor that modulate its function: Subunit composition, post-translational modifications and synaptic localization are summarized in an accompanying article. In this review we summarize how different signaling molecules modulate the function of NMDA receptors. The function of the receptors is modulated by the co-agonists glycine and serine and this modulation is different in cerebellum than in other areas. The NMDA receptor also has binding sites for polyamines that regulate its function. Other signaling molecules that modulate NMDA receptors function are: cAMP, neurotrophic factors such as BDNF, FGF-2 or neuregulins. These and other molecules allow an interplay between NMDA receptors and other receptors for neurotransmitters that may in this way modulate NMDA receptor function. This has been reported, for example, for metabotropic glutamate receptors. The expression and function of NMDA receptor is also modulated by synaptic activity, allowing an adaptation of the receptors function to the external inputs. NMDA receptors modulate important cerebral processes. NMDA receptors in different brain areas seem to modulate different processes. Cerebellar NMDA receptors play a special role in the modulation of motor learning and coordination. This is also briefly reviewed.

Forskolin-induced LTP in the CA1 hippocampal region is NMDA receptor dependent

Chemically induced long-term potentiation (cLTP) could potentially work by directly stimulating the biochemical machinery that underlies synaptic plasticity, bypassing the need for synaptic activation. Previous reports suggested that agents that raise cAMP concentration might have this capability. We examined the cLTP induced in acute slices by application of Sp-cAMPS or a combination of the adenylyl cyclase activator, forskolin, and the phosphodiesterase inhibitor, rolipram. Under our conditions, cLTP was induced but only if inhibition was reduced. We found that this form of cLTP was blocked by a N-methyl-d-aspartate receptor (NMDAR) antagonist and required the low-frequency test stimulation typically used to monitor the strength of synapses. Interestingly, similar LTP could be induced by lowering the Mg(2+) concentration of the ACSF during forskolin/rolipram or Sp-cAMPS application or even by just lowering Mg(2+) concentration alone. This LTP was also NMDAR dependent and required only a few ( approximately 5) low-frequency stimuli for its induction. The finding that even low-frequency synaptic stimulation was sufficient for LTP induction indicates that a highly sensitized plasticity state was generated. The fact that some stimulation was required means that potentiation is probably restricted to the stimulated axons, limiting the usefulness of this form of cLTP. However, when similar experiments were conducted using slice cultures, potentiation occurred without test stimuli, probably because the CA3-CA1 connections are extensive and because presynaptic spontaneous activity is sufficient to fulfill the activity requirement. As in acute slices, the potentiation was blocked by an NMDAR antagonist. Our general conclusion is that the induction of LTP caused by elevating cAMP requires presynaptic activity and NMDA channel opening. The method of inducing cLTP in slice cultures will be useful when it is desirable to produce NMDAR-dependent LTP in a large fraction of synapses.

Early Parkinson's disease: what is the best approach to treatment

Early and correct diagnosis and treatment of Parkinson's disease (PD) are crucial for the patient's well being. At the first visit, it is important to deal with the patient's misconceptions of the disease and its course, to offer sources of information and to suggest exercises. To make a correct initial diagnosis of PD we need to assess the course of the initial levodopa responsiveness. The most frequent challenges in diagnosing PD are the conditions of essential tremor and multiple system atrophy. PD has 3 stages of development: (i) early--from the onset of symptoms to the appearance of motor fluctuations; (ii) middle--from motor fluctuations to the appearance of moderate-to-severe disability; and (iii) advanced--when moderate-to-severe disability is present. The medical treatment of early PD should be started when functional disability appears, which is a different threshold for each patient. For patients below 65 years old, or above 65 years old but with preserved mental function and with no severe comorbidity, initial monotherapy with a dopamine agonist is advisable. This approach appears to delay the appearance and reduce the amount of late motor complications with subsequent levodopa treatment. All dopamine agonists have similar efficacy, which is less than that of levodopa. It is important to consider the adverse effect profile when a choice for initial or adjunctive therapy is made. When levodopa therapy is started as an adjunct in younger patients or as initial monotherapy in older patients, sustained-release levodopa preparations are preferred. They have a longer half-life and possibly stimulate the dopamine receptors more continuously. Anticholinergic drugs are appropriate for younger patients with tremor-dominant PD. Amantadine is mainly used for dyskinesia control. Catechol-O-methyl-transferase inhibitors and neurosurgery are not treatments of choice for early PD but can be very effective for more advanced disease. The presence of presymptomatic markers of PD, such as changes in odour detection, handwriting, speech, movement time of self-initiated motor acts, personality traits, presence of antibodies against dopaminergic neurons, pattern of positron emission tomography results, appearance of mitochondrial DNA mutation profiles, etc., appear to be very important in the light of the emerging neuroprotective therapies. Neuroprotection is aimed at slowing the rate of disease progression. Selegiline has been shown to cause a mild delay in the need for levodopa, possibly suggesting some protection. However, this initial benefit was not sustained in long term studies. Currently, there is no neuroprotective drug for PD.

Neuroleptics with differential affinities at dopamine D2 receptors and sigma receptors affect differently the N-methyl-D-aspartate-induced increase in intracellular calcium concentration: involvement of protein kinase

This study examined the effect of chronic antipsychotic treatment on the NMDA-elicited changes in intracellular free Ca2+ concentration ([Ca2+]i) in the primary culture of rat frontal cortical neurons. Antipsychotics used in the study were chosen for their differential affinities at dopamine D2 receptors and sigma receptors. The potential involvement of protein kinases in this action of antipsychotics were also examined. Chronic treatment of cells with antipsychotics (sulpiride, clozapine, and chlorpromazine) which are known to be potent dopamine D2 receptor ligands, whereas possessing low or no appreciable affinity for sigma receptors, caused a dose-dependent potentiation of the NMDA-induced increase in [Ca2+]i. On the contrary, haloperidol, which is as potent a sigma receptor ligand as a dopamine D2 receptor ligand, did not affect the NMDA-elicited increase in [Ca2+]i. Sulpiride increased the maximum effect afforded by different concentrations of NMDA and shifted the dose-response curve of NMDA to the left (EC50 value from 12.5 microM to 1.39 microM). Consistent with sulpiride's affinity at dopamine D2 receptors, this action of sulpiride was stereoselective: S(-)-sulpiride was active whereas R(+)-sulpiride was inactive. Treatment of cells with dopamine (3 microM) tends to decrease the NMDA-induced increase in [Ca2+]i. Sulpiride at 1 microM totally abolished this action of dopamine and restored its potentiating action on the NMDA-induced increase in [Ca2+]i. Haloperidol, a potent dopamine D2 and sigma receptor ligand, did not affect the sulpiride's potentiating action on the NMDA-induced responses. On the other hand, chronic treatment of cells with a sigma receptor agonist, DTG, at a concentration producing no effect of its own (10 nM), led to an enhancement of the potentiating effect of sulpiride on NMDA-induced increase in [Ca2+]i. This action of DTG was abolished by haloperidol. Further, chronic, but not acute, treatment of cells with either a protein kinase inhibitor H-7 or a cAMP-dependent protein kinase (PKA) inhibitor H-89 abolished this effect of sulpiride on the NMDA-induced [Ca2+]i changes. These results indicate that the action of NMDA in the primary cortical neurons are regulated differently by ligands with differential affinities at dopamine D2 and sigma receptors. The results with protein kinase inhibitors indicate that the potentiation of NMDA responses by sulpiride involves intracellular biochemical events.

Gene therapy for Parkinson's disease: an approach to the prevention or palliation of levodopa-associated motor complications

Gene therapy holds considerable potential for the treatment of central nervous system disease. The introduction of functional genes into the brain of patients with Parkinson's disease may, for example, prove useful as a means to replace a defective gene, introduce a potentially neuroprotective or neurorestorative protein, or permit the physiological delivery of a deficient neurotransmitter. Recent observations suggest that the oral administration of currently available dopaminomimetics to relatively advanced parkinsonian patients leads to nonphysiologic intermittent stimulation of striatal neurons that express dopamine receptors. Resultant activation of signal transduction pathways from these dopaminergic receptors on medium-sized GABAergic neurons apparently induces long-term potentiation of adjacent glutamatergic receptors of the N-methyl-D-aspartate subtype. The effects of dopaminergic drugs thus become modified in ways that favor the clinical appearance of response fluctuations and peak-dose dyskinesias. In parkinsonian models was well as in patients with Parkinson's disease, continuous dopaminergic replacement tends to prevent or alleviate these adverse effects. By continuously maintaining appropriate cerebral dopamine concentrations, molecular techniques which stimulate an increase in the intrastriatal activity of tyrosine hydroxylase, the rate-limiting enzyme for dopamine synthesis, might be expected to palliate parkinsonian symptoms with less risk of the disabling consequences of current therapy. Clinical study of these approaches could also serve as initial, relatively simple, proof-of-principle evaluations of the safety and efficacy of genetic approaches to the treatment of basic disease processes in Parkinson's disease and related neurodegenerative disorders.