Group I metabotropic glutamate receptors (mGluRs) are linked to the release of Ca²⁺ from intracellular stores, as well as affecting a number of membrane conductances. In the superficial superior colliculus (SC) presynaptic group I mGluRs are thought to modulate synaptic transmission (Cirone et al. 2002), but postsynaptic actions remain to be demonstrated. In this study we have investigated the actions of the selective group I mGluR agonist dihydrophenylglycine (DHPG) on Ca²⁺ signalling and membrane conductances in cultured SC neurones and SC slices.

Primary cultures of rat superficial SC neurones were loaded with the cell-permeant Ca²⁺ indicator fura-2-AM (2 µM). [Ca²⁺]_i was monitored using digital epifluorescence microscopy. SC slices (350 mm), obtained from 3- to 4-week-old rats were used for combined whole-cell voltage clamp recording and Ca²⁺ imaging. The Ca²⁺ indicator bis fura-2 (150 µM) was included in the pipette solution. Cultured cells or slices were perfused with standard media containing TTX (0.5 µM) at room temperature. All animals were killed humanely by cervical dislocation.

In 47 % of cultured SC neurones, exposure to DHPG (50-100 µM; 90 s) resulted in reproducible, transient rises in [Ca²⁺]_i. This reflected the release of Ca²⁺ from intracellular stores as it was blocked following prior incubation with thapsigargin (2 µM), a Ca²⁺ store depletory agent. The DHPG response was reduced both by the mGluR5 antagonist MPEP (3 µM) and the mGluR1 antagonist LY367385 (40 µM) (81.1 ± 5.7 % and 79.0 ± 9.2 %, respectively; n = 19 and 13; means ± S.E.M.; P < 0.01; Student’s unpaired t tests). Exposure of SC slices to DHPG (100 µM for 2 min at -60 mV) was also associated with a transient increase in [Ca²⁺]_i levels in 8/26 (32 %) neurones. Increasing Ca²⁺ store loading by depolarisation to between -35 and -20 mV enhanced DHPG responses in 6/8 neurones (7.4 ± 1.5 control, 19.7 ± 6.1 depolarised; total area of responses measured; P < 0.05). Under control conditions, DHPG exposure was often associated with an inward current (18.1 ± 4.3 pA; 12/26 cells), which was much more prolonged than the intracellular Ca²⁺ elevation. In 7/26 of cells held at -60 mV, an inward current was evoked in the absence of any Ca²⁺ response (18.6 ± 7.5 pA).

In conclusion, activation of Group I mGluRs in superficial SC neurones causes both the release of Ca²⁺ from intracellular stores and an inward current in SC neurones, which are likely to reflect different signalling mechanisms. In addition to the proposed presynaptic actions of mGluRs in the SC, postsynaptic effects may also exert a modulatory role on SC function.