Effects of conantokins on L-3,4-dihydroxyphenylalanine-induced behavior and immediate early gene expression

Activation of mu opioid receptors in the striatum differentially augments methamphetamine-induced gene expression and enhances stereotypic behavior

Mu opioid receptors are densely expressed in the patch compartment of striatum and contribute to methamphetamine-induced patch-enhanced gene expression and stereotypy. To further elucidate the role of mu opioid receptor activation in these phenomena, we examined whether activation of mu opioid receptors would enhance methamphetamine-induced stereotypy and prodynorphin, c-fos, arc and zif/268 expression in the patch and/or matrix compartments of striatum, as well as the impact of mu opioid receptor activation on the relationship between patch-enhanced gene expression and stereotypy. Male Sprague-Dawley rats were intrastriatally infused with d-Ala(2)-N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO; 1 μg/μL), treated with methamphetamine (0.5 mg/kg) and killed at 45 min or 2 h later. DAMGO augmented methamphetamine-induced zif/268 mRNA expression in the patch and matrix compartments, while prodynorphin expression was increased in the dorsolateral patch compartment. DAMGO pre-treatment did not affect methamphetamine-induced arc and c-fos expression. DAMGO enhanced methamphetamine-induced stereotypy and resulted in greater patch versus matrix expression of prodynorphin in the dorsolateral striatum, leading to a negative correlation between the two. These findings indicate that mu opioid receptors contribute to methamphetamine-induced stereotypy, but can differentially influence the genomic responses to methamphetamine. These data also suggest that prodynorphin may offset the overstimulation of striatal neurons by methamphetamine.

Methamphetamine-induced stereotypy correlates negatively with patch-enhanced prodynorphin and arc mRNA expression in the rat caudate putamen: the role of mu opioid receptor activation

Amphetamines induce stereotypy, which correlates with patch-enhanced c-Fos expression the patch compartment of caudate putamen (CPu). Methamphetamine (METH) treatment also induces patch-enhanced expression of prodynorphin (PD), arc and zif/268 in the CPu. Whether patch-enhanced activation of any of these genes correlates with METH-induced stereotypy is unknown, and the factors that contribute to this pattern of expression are poorly understood. Activation of mu opioid receptors, which are expressed by the neurons of the patch compartment, may underlie METH-induced patch-enhanced gene expression and stereotypy. The current study examined whether striatal mu opioid receptor blockade altered METH-induced stereotypy and patch-enhanced gene expression, and if there was a correlation between the two responses. Animals were intrastriatally infused with the mu antagonist CTAP (10 microg/microl), treated with METH (7.5 mg/kg, s.c.), placed in activity chambers for 3h, and then sacrificed. CTAP pretreatment attenuated METH-induced increases in PD, arc and zif/268 mRNA expression and significantly reduced METH-induced stereotypy. Patch-enhanced PD and arc mRNA expression in the dorsolateral CPu correlated negatively with METH-induced stereotypy. These data indicate that mu opioid receptor activation contributes to METH-induced gene expression in the CPu and stereotypy, and that patch-enhanced PD and arc expression may be a homeostatic response to METH treatment.

Antagonism of metabotropic glutamate receptor type 5 attenuates l-DOPA-induced dyskinesia and its molecular and neurochemical correlates in a rat model of Parkinson's disease

Metabotropic glutamate receptor type 5 (mGluR5) modulates dopamine and glutamate neurotransmission at central synapses. In this study, we addressed the role of mGluR5 in l-DOPA-induced dyskinesia, a movement disorder that is due to abnormal activation of both dopamine and glutamate receptors in the basal ganglia. A selective and potent mGluR5 antagonist, 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl] pyridine, was tested for its ability to modulate molecular, behavioural and neurochemical correlates of dyskinesia in 6-hydroxydopamine-lesioned rats treated with l-DOPA. The compound significantly attenuated the induction of abnormal involuntary movements (AIMs) by chronic l-DOPA treatment at doses that did not interfere with the rat physiological motor activities. These effects were paralleled by an attenuation of molecular changes that are strongly associated with the dyskinesiogenic action of l-DOPA (i.e. up-regulation of prodynorphin mRNA in striatal neurons). Using in vivo microdialysis, we found a temporal correlation between the expression of l-DOPA-induced AIMs and an increased GABA outflow within the substantia nigra pars reticulata. When co-administered with l-DOPA, 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl] pyridine greatly attenuated both the increase in nigral GABA levels and the expression of AIMs. These data demonstrate that mGluR5 antagonism produces strong anti-dyskinetic effects in an animal model of Parkinson's disease through central inhibition of the molecular and neurochemical underpinnings of l-DOPA-induced dyskinesia.

Different responsiveness of striatonigral and striatopallidal neurons to L-DOPA after a subchronic intermittent L-DOPA treatment

Early gene induction by L-DOPA in the striatum of dopamine denervated rats represents a useful way to study long-term modifications produced by this drug. The effects of acute and subchronic L-DOPA administration on zif-268 mRNA expression were compared in 6-hydroxydopamine-lesioned rats. Rats received a subchronic intermittent L-DOPA (6 mg/kg) treatment, which produces behavioural sensitization, a correlate of dyskinetic movements. Three days after interruption of subchronic treatment, zif-268 mRNA was evaluated after an L-DOPA challenge. Zif-268 mRNA levels increased in the lesioned dorsolateral striatum after either acute or subchronic L-DOPA administration. Double labelling of striatal cells with zif-268 and enkephalin or dynorphin mRNA probes was performed to assess neuronal activation in the indirect and direct output pathway. Single acute L-DOPA significantly increased zif-268 in all striatal neurons reflecting a hyperresponsiveness of dopamine-depleted striatum. After subchronic L-DOPA, zif-268 mRNA labelling was still increased in the striatonigral pathway, limited to dynorphin(+) neurons, whereas in all other neurons it was similar to the control value. Results suggest that striatal neurons responding to acute L-DOPA differ from those responding to subchronic L-DOPA. L-DOPA-induced behavioural sensitization was associated to a down-regulation in the responsiveness of striatopallidal and striatonigral dynorphin(-) neurons, whereas in striatonigral neurons containing dynorphin a hyperresponsiveness to L-DOPA was observed. High levels of zif-268, together with a persistent hyperresponsiveness of striatonigral dymorphinergic neurons and hyporesponsiveness of striatopallidal neurons, by creating an unbalanced state of striatal efferent neurons, may be implicated in dyskinetic movements observed in Parkinson's disease (PD).

Binding and orientation of conantokins in PL vesicles and aligned PL multilayers

The association of a ligand with its cognate cell surface receptor can be facilitated by interactions between the ligand and the lipid phase of the cell membrane. With respect to the N-methyl-D-aspartate receptor (NMDAR), we have previously established a low affinity, nonreceptor-mediated interaction of the peptidic conantokins with synaptic membranes in conjunction with a high affinity binding to the NMDARs present therein [Klein, R. C., Prorok, M., and Castellino, F. J. (2003) J. Pept. Res. 61, 307-317]. In the current study, several techniques including size-exclusion chromatography, circular dichroism, fluorescence, and NMR spectroscopies were used to investigate the binding, conformation, and orientation of conantokins and their variants to a variety of phospholipid (PL) vesicles and multilayers. We have found that conantokins bind to PLs and that the effectors Ca(2+) and spermine slightly increase this binding ability. The conantokins preserve a high degree of helical conformation when bound to vesicles in the presence of Ca(2+). In the absence of Ca(2+), only conantokin-G (con-G) manifests an increase in conantokin helicity with increasing vesicle concentration. In solution, the conantokins appear to be localized at the headgroup of vesicles and do not insert into the hydrophobic core of the bilayer. On aligned PL films, the helical axis of the conantokins can either reside normal to the membrane surface or partition in a parallel orientation, depending on the nature of the conantokins and the PLs used. These orientation preferences may be conjoined with the biological activities of the conantokins.

Evidence for functionally distinct synaptic NMDA receptors in ventromedial versus dorsolateral striatum

N-methyl-D-aspartate receptors (NMDARs) are comprised of different subunits. NR2 subunits confer different pharmacological and biophysical properties to NMDARs. Although NR2B subunit expression is uniform throughout striatum, NR2A subunit expression is greater laterally. Pharmacologically isolated NMDAR-mediated excitatory postsynaptic currents (NMDAR-EPSCs) were elicited using minimal local stimulation and recorded in the whole cell configuration to test the hypothesis that biophysical and pharmacological properties of NMDAR-EPSCs of striatal neurons would vary as a function of their location in adult rat striatum. We observed that the decay-time kinetics of NMDAR-EPSCs are significantly slower in neurons of ventromedial versus dorsolateral striatum. Whereas ifenprodil did not differentially affect NMDAR-EPSCs in these regions, application of either glycine or D-serine increased the peak current of NMDAR-EPSCs selectively in dorsolateral striatum. These data provide evidence for functionally distinct NMDARs in the same neuron type in the same brain region of the adult rodent brain. These data thus suggest that the nature of synaptic processing of excitatory input is different in the ventromedial and dorsolateral striatum of the adult rodent brain, regions differentially involved in limbic versus sensorimotor processes, respectively.

The amino acid residue at sequence position 5 in the conantokin peptides partially governs subunit-selective antagonism of recombinant N-methyl-D-aspartate receptors

Whole cell voltage clamp recordings were performed to assess the ability of conantokin-G (con-G), conantokin-T (con-T), and a 17-residue truncated form of conantokin-R (con-R[1-17]) to inhibit N-methyl-d-aspartate (NMDA)-evoked currents in human embryonic kidney 293 cells transiently expressing various combinations of NR1a, NR1b, NR2A, and NR2B receptor subunits. Con-T and con-R[1-17] attenuated ion currents in cells expressing NR1a/NR2A or NR1a/NR2B. Con-G did not affect NMDA-evoked ionic currents in cells expressing NR1a/NR2A, but it showed inhibitory activity in cells expressing NR1a/NR2B receptors and the triheteromeric combination of NR1a/NR2A/NR2B. An Ala-rich con-G analog, con-G[Q6G/gamma7K/N8A/gamma10A/gamma14A/K15A/S16A/N17A] (Ala/con-G, where gamma is Gla), in which all nonessential amino acids were altered to Ala residues, manifested subunit specificity similar to that of con-G, suggesting that the replaced residues are not responsible for selectivity in the con-G framework. A sarcosine-containing con-T truncation analog, con-T[1-9/G1Src/Q6G], inhibited currents in NR1a/NR2A and NR1a/NR2B receptors, eliminating residues 10-21 as mediators of the broad subunit selectivity of con-T. In contrast to the null effects of con-G and Ala/con-G at a NR1a/NR2A-containing receptor, some inhibition ( approximately 40%) of NMDA-evoked currents was effected by these peptides in cells expressing NR1b/NR2A. This finding suggests that the presence of exon 5 in NR1b plays a role in the activity of the conantokins. Analysis of various conantokin analogs demonstrated that Leu(5) of con-G is an important determinant of conantokin selectivity. Taken as a whole, these results suggest that the important molecular determinants on conantokins responsible for NMDA receptor activity and specificity are discretely housed in specific residues of these peptides, thus allowing molecular manipulation of the NMDA receptor inhibitory properties of the conantokins.