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The principal neurons of the striatum, GABAergic medium spiny neurons (MSNs), are interconnected by local recurrent axon collateral synapses. Although critical to many striatal models, it is not clear whether these connections are random or whether they preferentially link functionally related groups of MSNs. To address this issue, dual whole patch-clamp recordings were made from striatal MSNs in brain slices taken from transgenic mice in which D1 or D2 dopamine receptor expression was reported with EGFP (enhanced green fluorescent protein). These studies revealed that unidirectional connections were common between both D1 receptor-expressing MSN (D1 MSN) pairs (26%) and D2 receptor-expressing MSN (D2 MSN) pairs (36%). D2 MSNs also commonly formed synapses on D1 MSNs (27% of pairs). Conversely, only 6% of the D1 MSNs formed detectable connections with D2 MSNs. Furthermore, synaptic connections formed by D1 MSNs were weaker than those formed by D2 MSNs, a difference that was attributable to fewer GABAA receptors at D1 MSN synapses. The strength of detectable recurrent connections was dramatically reduced in Parkinson's disease models. The studies demonstrate that recurrent collateral connections between MSNs are not random but rather differentially couple D1 and D2 MSNs. Moreover, this recurrent collateral network appears to be disrupted in Parkinson's disease models, potentially contributing to pathological alterations in MSN activity patterns and psychomotor symptoms. [ABSTRACT FROM AUTHOR]

Among the electrophysiological properties differentiating stratum oriens horizontal interneurons from pyramidal neurons of the CA1 hippocampal subfield are the more depolarized resting potential and the higher input resistance; additionally, these interneurons are also less sensitive to ischemic damage than pyramidal cells. A differential expression of pH-sensitive leakage potassium channels (TASK) could contribute to all of these differences. To test this hypothesis, we studied the expression and properties of TASK channels in the two cell types. Electrophysiological recordings from acute slices showed that barium- and bupivacaine-sensitive TASK currents were detectable in pyramidal cells but not in interneurons and that extracellular acidification caused amuchstronger depolarization in pyramidal cells than in interneurons. This pyramidal cell depolarization was paralleled by an increase of the input resistance, suggesting the blockade of a background conductance. Single-cell reverse transcription-PCR experiments showed that the expression profile of TASK channels differ between the two cell types and suggested that these channels mediate an important share of the leakage current of pyramidal cells. We suggest that the different expression of TASK channels in these cell types contribute to their electrophysiological differences and may result in cell-specific sensitivity to extracellular acidification in conditions such as epilepsy and ischemia. [ABSTRACT FROM AUTHOR]