EVIDENCE FOR ENHANCEMENT OF GAP JUNCTIONAL COUPLING BETWEEN RAT ISLAND OF CALLEJA GRANULE CELLS IN-VITRO BY THE ACTIVATION OF DOPAMINE D-3 RECEPTORS

1. Using patch-clamp techniques, we hare studied actions of dopamine a nd related compounds on granule neurones within the islands of Calleja in vitro, in slices of similar to 200 mu m thickness or as groups of varying cell number following enzymic digestion. 2. Prior to agonist a pplication, island of Calleja granule cells displayed spontaneous step wise shifts in whole-cell conductance ranging from 104 to 632 pS. The reversal potentials of these conductance changes ranged widely and mat ched the distribution of the cells' membrane potentials. Reversal pote ntials and membrane potentials shifted equally when cells were uniform ly depolarized in 24 mM external K+. 3. Bath-applied dopamine elicited , after a delay of 4-9 min, an exaggerated form of the spontaneous beh aviour that frequently gave way to a sudden large (up to thirtyfold) c onductance change. At concentrations of 100-300 nM, a range of agonist s with increasing affinity for the D-3 receptor (apomorphine, quinpiro le, 7-OH DPAT and PD 128907) triggered the response. The actions were neither mimicked by SKF-38393 nor antagonized by SCH-23390 (a selectiv e D-1 agonist and antagonist, respectively). Haloperidol reversibly bl ocked responses elicited by the D-3/D-2 agonist quinpirole. The action of effective agonists was maintained when transmitter release was abo lished. Given the reported lack of D-2 receptors in the islands of Cal leja, these findings indicate a direct action of dopamine at the D-3 r eceptor. 4. The dopaminergic effects were not affected by Gd3+ or subs tantial replacement of external Na+ with TEA, Tris or choline, elimina ting stretch-activated channels but suggesting that if transmembrane c hannels were to be involved in this dopaminergic action they posseses a non-selective permeability to large cations. The reported presence o f 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 junct ional coupling the following experimental observations were made: (i) alkalinization of slices mimicked the effect of D-3 agonists; (ii) in cell groups, recording from pairs provided evidence of intercellular c oupling, and mechanical separation of recorded neurones from neighbour ing cells during the agonist-evoked response caused shutdown of the ad ditional conductance; (iii) when applied to slices, the gap junctional blocker, 18 alpha-glycyrrhetinic acid, whilst not preventing the full -blown dopamine response, significantly reduced both the variance of r ecorded granule cell input conductance and the cells' apparent capacit ance. 6. Taken together the results indicate a D-3 action in granule c ells, which is best explained by a dopaminergic promotion of intercell ular coupling. The physiological relevance of such a mechanism is disc ussed.