Electrophysiological and Morphological Analyses of Cortical Neurons Obtained from Children with Catastrophic Epilepsy: Dopamine Receptor Modulation of Glutamatergic Responses

Abstract

The present study examined the electrophysiological effects produced by activation of specific dopamine (DA) receptors and the distribution of DA receptor subtypes and glutamate receptor subunits [N-methyl-D-aspartate (NMDAR1) and GluR1] in cortical tissue samples obtained from children (ages 3 months to 16 years) undergoing epilepsy surgery. DA receptor activation produced differential effects depending on the receptor subtype that was activated. D1 receptor family agonists generally enhanced cortical excitability and favored the emergence of epileptogenic activity. In contrast, D2 receptor family agonists had more variable effects on cortical excitability and the expression of epileptiform discharges. Activation of D1 or D2 receptors decreased the amplitude of non-NMDA-mediated excitatory postsynaptic potentials. In contrast, DA and D1 agonists increased the amplitude of NMDA-mediated potentials. Immunohistochemical analysis showed that the DA receptor subtypes and glutamate receptor subunits examined were present in all cortical layers and areas throughout development. Whole-cell voltage clamp recordings of pyramidal neurons visualized with differential interference contrast optics and infrared videomicroscopy indicated that these neurons displayed a persistent Na⁺ current, followed by an outward current. DA reduced the outward current but had little effect on the persistent Na⁺ current. These results suggest a dual role for DA’s actions in the human cerebral cortex. Activation of D2 receptors or antagonism of D1 receptors may help control seizures in children.

Dopamine D1/D5 receptor modulation of excitatory synaptic inputs to layer V prefrontal cortex neurons

“…Likewise, the non-NMDA response was depressed by D2 receptor activation in some studies (27,39), not affected in others (38) and increased by D1 receptors and protein kinase A (PKA) in others (26). Although D1 receptor activation has been reported to consistently increase the NMDA response in both striatal and cortical neurons (11,12,(40)(41)(42), studies on dorsal striatal neurons suggest the increase is mediated by PKA (27), whereas, in ventral striatal neurons, it is mediated by protein kinase C (43). In neurons from the hippocampus or nearby cortices, dopamine has been reported to depress both NMDA and non-NMDA responses (44-47) through a PKAdependent process (44), whereas direct application of PKA has been reported to increase non-NMDA responses (48).…”

“…Glutamatergic afferents from the hippocampus and dopaminergic terminals are, moreover, in direct apposition to one another in the PFC, suggesting a presynaptic site of modulation (8). The few physiological studies available to date indicate that dopamine is capable of altering excitatory synaptic responses in the PFC, although the mode of action is not known (11)(12)(13)(14)(15).…”

Neurons Recorded from Pediatric Epilepsy Surgery Patients with Cortical Dysplasia

“…Brain slice preparation followed a previously described protocol (9)(10)(11). Cortical samples were placed in ice-cold oxygenated artificial cerebrospinal fluid (ACSF; pH 7.2 to 7.4) in the operating room and transported immediately to the laboratory.…”

“…The glutamate receptor agonists N-methyl-D-aspartate, kainate, or y-aminobutyric acid (GABA) were applied iontophoretically through a five-barreled pipette (tip diameter 6 to 8 pm) placed close to the recorded cell (15 to 50 pm) (11). The pipette contained N-methyl+-aspartate (0.1 mol/L, pH 8), kainate or a-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (0.1 molL, pH 8.5), or GABA (0.1 mol/L, pH 5 ) and saline for current balancing and control.…”

Santos et al. reply

“…Thus, in some patients, methylphenidate in the dose range we used apparently improved seizure control and this possibility should be kept in mind, particularly in light of recent data indicating that activation of dopamine D2 receptors may help control seizures in children. 3 Finally, we agree that we could have chosen a different term concerning the severity of the epilepsy. However, because even those patients who were on monotherapy or eventually achieved seizure control during AED adjustments had had a number of medication changes over the yearsand because we had the clinician in mind -we kept the term 'difficult-to-treat'.…”