GABA and mGluRs : their role in signalling and plasticity in the rat superficial superior colliculus
The roles of γ-amino butyric acid (GABA) and metabotropic glutamate receptors (mGluRs) in signalling and plasticity in the rat superficial superior colliculus (SSC) were investigated using electrophysiological techniques in the slice, and both confocal and calcium imaging methods in SSC cultures. A number of novel actions of GABA and group I mGluRs were observed and characterised. An unusual action of GABA, GABA-induced long-term potentiation (LTPG), which has previously been demonstrated in the guinea pig SSC, was observed in the rat SSC slice. Superfusion of GABA led to an enhancement and an enduring potentiation of synaptic transmission. The ionic mechanisms underlying this action were characterised and dependencies on the bicarbonate, chloride and calcium equilibria but not the potassium equilibrium were uncovered. A developmental study was performed to compare postnatal maturation of synaptic transmission of LTPG in the SSC of both rats and guinea-pigs (altricial and non-altricial species, respectively), and two different rat strains (albino and pigmented) to compare maturation in animals with or without a visual deficiency. Guinea pigs did not show any obvious differences with respect to synaptic transmission and synaptic plasticity at both ages studied (at birth and one month old), indicating that maturation must have occurred prenatally. Rats, however, underwent synaptic maturation and refinement to produce stronger synaptic transmission and a more robust level of synaptic plasticity one month after birth compared to the conditions at eye-opening. The state of pigmentation was found to have a critical influence, with albino rats showing less enhancement of the strength of synaptic transmission in the SC. Whole-cell path clamping was used to perform physiological and morphological characterisation of SSC neurones, followed by an assessment of GABA and other amino acid messenger mediated actions. Five main cell types were identified morphologically, however no cell type-specific current induced or synaptic physiological properties could be identified in current clamp recordings. GABA and GABA-related messengers had a strong inhibitory influence on cells, and caused hyperpolarisation of the membrane potential and a decrease in input resistance.