Evidence for enhancement of gap junctional coupling between rat island of Calleja granule cells in vitro by the activation of dopamine D₃ receptors

Abstract

1. Using patch-clamp techniques, we have studied actions of dopamine and related compounds on granule neurones within the islands of Calleja in vitro, in slices of •200 ìm thickness or as groups of varying cell number following enzymic digestion. 2. Prior to agonist application, island of Calleja granule cells displayed spontaneous stepwise shifts in whole-cell conductance ranging from 104 to 632 pS. The reversal potentials of these conductance changes ranged widely and matched the distribution of the cells' membrane potentials. Reversal potentials and membrane potentials shifted equally when cells were uniformly depolarized in 24 mÒ external K¤. 3. Bath-applied dopamine elicited, after a delay of 4-9 min, an exaggerated form of the spontaneous behaviour that frequently gave way to a sudden large (up to thirtyfold) conductance change. At concentrations of 100-300 nÒ, a range of agonists with increasing affinity for the D× receptor (apomorphine, quinpirole, 7-OH DPAT and PD 128907) triggered the response. The actions were neither mimicked by SKF-38393 nor antagonized by SCH-23390 (a selective D1 agonist and antagonist, respectively). Haloperidol reversibly blocked responses elicited by the D×ÏDµ agonist quinpirole. The action of effective agonists was maintained when transmitter release was abolished. Given the reported lack of Dµ receptors in the islands of Calleja, these findings indicate a direct action of dopamine at the D× receptor. 4. The dopaminergic effects were not affected by GdŤ or substantial replacement of external Na¤ with TEA, Tris or choline, eliminating stretch-activated channels but suggesting that if transmembrane channels were to be involved in this dopaminergic action they posseses a non-selective permeability to large cations. The reported presence of gap junctions in the islands of Calleja offers the explanation that these effects derive from enhanced activity of such channels or their hemi-constituents. 5. In testing the possible involvement of gap junctional coupling the following experimental observations were made: (i) alkalinization of slices mimicked the effect of D× agonists; (ii) in cell groups, recording from pairs provided evidence of intercellular coupling, and mechanical separation of recorded neurones from neighbouring cells during the agonist-evoked response caused shutdown of the additional conductance; (iii) when applied to slices, the gap junctional blocker, 18á-glycyrrhetinic acid, whilst not preventing the full-blown dopamine response, significantly reduced both the variance of recorded granule cell input conductance and the cells' apparent capacitance. 6. Taken together the results indicate a D× action in granule cells, which is best explained by a dopaminergic promotion of intercellular coupling. The physiological relevance of such a mechanism is discussed.

Olfactory maps, circuits and computations

“…There are several poorly characterized interneuron types in the OT, most of which probably represent variants on the major neurons of the striatum including crescent cells (possibly cholinergic interneurons), and spine-poor and spindle cells (possibly GABAergic interneurons) [88]. Electrophysiological characterization of IC granule cells reveals that they are coupled via gap junctions and that this coupling is modulated by dopamine; however, the relationship of the ICs and the rest of the OT remains a mystery [106]. Future in vitro slice experiments targeting genetically identified subtypes of cells will be crucial in understanding the organization and function of microcircuits within the OT.…”

New neurons in the adult striatum: from rodents to humans

“…There is only limited knowledge of the functional properties of ICj granule cells as well [64]. The tightly apposed granule cells forming the ICj are connected via gap junctions [65]; there are no data about the existence of similar electrical connections between striatal CR+ interneurons, newborn or otherwise, though they appear partly clustered. The best clues to date about their functional profile may come from the analysis of similar postnatally-generated CR+ interneurons in deep cortical layers [23].…”

An overview and synopsis of techniques for directing stem cell differentiation in vitro

“…In an interesting study, forced expression of gap junction proteins Cx43 and Cx32, through DNA transfection within cell lines, led to an enhancement of intercellular coupling as assessed by dye-coupling assays (Koffler et al 2000). Other molecular signals that have been reported to enhance gap junction formation and subsequent intercellular coupling include low-density lipoprotein (Paulson et al 2000), inhibitors of glycosylation (Wang and Mehta 1995), dopamine (Halliwell and Horne 1998) and potassium conductance blockers, tetraethylammonium and 4-aminopyridine (Kannan and Daniel 1978).…”