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Hyperexcitability in the medial entorhinal cortex-hippocampal (mEC-HC) circuit in the initial weeks after prolonged seizure activity may contribute to the epileptogenic process in animal models of temporal lobe epilepsy (TLE). The present study examined combined mEC-HC slices (400 microm) using field potential recordings 1-2 weeks following the multiple administration, low-dose kainic acid (KA) model of TLE [Hellier, J.L., Patrylo, P.R., Buckmaster, P.S., Dudek, F.E., 1998. Recurrent spontaneous motor seizures after repeated low-dose systemic treatment with kainate: assessment of a rat model of temporal lobe epilepsy. Epilepsy Res. 31, 73-84]. Field potential recordings in slices from KA-treated rats demonstrated hallmarks of hyperexcitability in the mEC and in the CA1 and CA3 cell body regions of the HC. Spontaneous burst (SB) activity was observed under baseline recording conditions in the mEC of several slices from KA-treated rats, but not in the slices from saline-treated control rats. Elevating ACSF [K(+)](o) (6mM) in the presence of picrotoxin (50 microM) increased SB rates in all slices tested. However, there was a significantly shorter latency to onset of bursting and prolonged evoked response durations in layer II of the mEC of slices from KA-treated rats versus those from controls. Neither carbamazepine (CBZ) nor phenytoin (PHT) abolished SB activity in slices from KA-treated rats; whereas, SB activity in slices from control rats was dose-dependently reduced at 100 microM CBZ. In contrast, the novel anticonvulsant retigabine (RGB) dramatically reduced SB frequency in both control and KA-treated groups. The hyperexcitability observed in combined mEC-HC brain slices from KA-treated rats suggests that the mEC, as well as the HC, may contribute to the epileptogenic process after KA-induced seizure activity. This model may provide an efficient, flexible in vitro paradigm for differentiating novel AEDs in a model of pharmacoresistant bursting.

Cortical afferents excite striatal efferent neurons through activation of N-methyl-D-aspartate (NMDA) receptors, which can be modulated by D2 dopamine receptors. It is suggested that activation of PKA by D2 receptor blockade leads to NMDA receptor phosphorylation in the dendrites or phosphorylation of transcription factors in the nucleus. Thus, the levels and cellular localization of activated PKA may determine if D2 antagonist-mediated gene expression is dependent on NMDA receptor activation. We have previously demonstrated that NMDA receptor antagonists block gene expression induced by a high dose of eticlopride in medial and central but not lateral striatum. Here, we examined the effects of NMDA receptor antagonists on striatal gene expression after administration of a low dose of eticlopride. The results showed that NMDA receptor antagonists blocked gene induction by eticlopride throughout striatum. Less PKA activation by the low dose of eticlopride might explain why the expression was more sensitive in the lateral striatum to NMDA receptor blockade than in our previous study. To increase levels of PKA activation to the extent that NMDA receptor blockade would have less effect on eticlopride-mediated gene induction in all regions of striatum, we administered the phosphodiesterase inhibitor IBMX to animals treated with eticlopride. The combined administration of IBMX and eticlopride induced gene expression that was only partially attenuated (c-fos) or unaffected (zif268) by NMDA receptor blockade. These data support the suggestion that the degree of second messenger activation by D2 receptor blockade determines whether D2 dopamine receptor antagonist-mediated gene expression is dependent on NMDA receptor activation.

Immediate early genes (IEGs) are induced by different signaling pathways. It has been proposed that D2 dopamine receptor blockade induces IEG expression through activation of protein kinase A (PKA), although few studies have examined this issue in vivo. We infused the PKA inhibitor H-89 into the striatum of male rats, followed 30 min later by systemic administration of eticlopride. Eticlopride-induced c-fos and zif268 mRNA expression in striatum was not blocked by H-89. In addition, eticlopride did not produce measurable levels of PKA activity in striatum, whereas the cAMP activator Sp-8-Br-cAMPs increased levels of activated PKA. Neither the adenosine A2a receptor agonist CGS 21680 nor the phosphodiesterase-4 inhibitor rolipram, each of which should increase PKA activation, potentiated eticlopride-induced IEG expression. To test whether other signaling pathways are involved in eticlopride-mediated gene induction, we also infused inhibitors of the mitogen-activated and calcium/calmodulin-dependent protein kinases into animals and then treated them with eticlopride. The data suggest that eticlopride-induced IEG expression is not solely dependent on these kinases either. These data suggest that PKA activation may not be necessary for induction of IEGs by D2 dopamine receptor antagonists and that other intracellular signaling pathways may be involved.

Conantokins, peptides from Conus snails, are N-methyl-D-aspartate (NMDA) receptor antagonists. NMDA receptor antagonists potentiate L-3,4-dihydroxyphenylalanine (L-DOPA)-induced rotation in 6-hydroxydopamine-treated rodents, an index of anti-Parkinsonian potential. This study examined the effects of conantokin-G, conantokin-T(G), CGS 19755, and ifenprodil on L-DOPA-induced contralateral rotation and immediate early gene (IEG) expression in 6-hydroxydopamine-treated rats. Rats received unilateral infusions of 6-hydroxydopamine into the medial forebrain bundle. Three weeks later, rats were treated with an NMDA receptor antagonist, followed by an injection of L-DOPA. Contralateral rotations were recorded for 2 h. In addition, the expression of zif268 and c-fos were examined. Conantokin-G, conantokin-T(G), and CGS 19755 potentiated L-DOPA-induced rotation. Conantokin-G and ifenprodil had no effect on L-DOPA-induced IEG expression, whereas conantokin-T(G) and CGS 19755 attenuated expression. These data suggest that conantokins may be useful in treating Parkinson's disease. Furthermore, different NMDA receptor antagonists have distinct effects on striatal gene expression.

Animals are commonly given opioid analgetics such as buprenorphine for post-operative pain management. In this study, the effect of the analgetic buprenorphine, a partial mu receptor agonist and kappa receptor antagonist, on l -DOPA-induced contralateral rotation was measured in 6-hydroxydopamine (6-OHDA) treated rats. Male Sprague–Dawley rats received dopamine-depleting brain lesions by infusion of 6-OHDA into the medial forebrain bundle. After the procedure, buprenorphine was administered (430 μg/kg, s.c.) to 17 of 54 animals. Three weeks after 6-OHDA treatment, animals were given benserazide HCl (25 mg/kg, i.p.) and l -DOPA (4 mg/kg, i.p.). Contralateral rotations were monitored for 2 h. Animals receiving buprenorphine had significantly higher rates of rotation as compared with non-buprenorphine-treated animals ( P =0.023). The results suggest that buprenorphine sensitizes animals to the effects of l -DOPA.

The Synechococcus PCC7942 metallothionein repressor gene smtB has been cloned into a high expression vector and the protein purified to near homogeneity (>/=98%). Analytical ultracentrifugation studies demonstrate that the protein is predominantly dimeric in 0.1 M NaCl, pH 7.4, and 22 degrees C, exhibiting a monomer-dimer-tetramer equilibrium. The monomer-dimer (Ka(1,2)) and the dimer-tetramer (Ka(2,4)) association constants are 3.24 x 10(5) and 9.90 x 10(2) M-1, respectively. The repressor binds two Zn2+ ions per subunit with an overall Kd of 3.49 x 10(-6) M. In the presence of Zn2+, Ka(1, 2) increases by 2 orders of magnitude to 1.25 x 10(7) M-1 and the apparent weight-averaged sedimentation coefficient increases from 2. 00 to 2.22 S. The fact that the increase in sedimentation coefficient is greater than that predicted by increased dimerization is interpreted as caused by compaction of the structure in the presence of metal ions. At pH 6.0, 0.1 M NaCl, and 22 degrees C, the protein exhibits only a monomer-dimer equilibrium, with Ka(1,2) = 1.52 x 10(7) M-1 which is almost identical to that seen upon binding Zn2+ at pH 7.4. The compaction and conformational change in SmtB caused by Zn2+ is consistent with a role for this altered quaternary state in derepression of smtA in Synechococcus challenged with heavy metal ions.