GABA(B)-related activity involved in synaptic processing of somatosensory information in S1 cortex of the...

1. The possible role of GABA(B) receptor mechanisms in information processing in primary somatosensory (S1) cortex was assessed by use of extracellular recording combined with microiontophoretic methods from 161 neurones in anaesthetized, paralysed cats. 2. Baclofen-induced suppressions of cell responses were reversible and stereoselective, the (+)-isomer being inactive and the (-)-isomer having two to three times the apparent potency of gamma-aminobutyric acid (GABA). The responses measured were threshold to natural stimulation of receptive fields (RFs), responsiveness to thalamic electrical stimulation, change in RF size and magnitude of firing elicited by iontophoretic glutamate. 3. The action of GABA always was mimicked by muscimol or 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP) but not always by (-)-baclofen; in certain cases (-)-baclofen enhanced neuronal responses while the opposite occurred with GABA or with the other GABA(A) agonists. The elevation of response thresholds by (-)-baclofen was relatively stronger in peripheral than in central subregions of cutaneous RFs, by contrast with the action of muscimol which was relatively non-selective as to the area in which it was effective. 4. Glutamate-induced and thalamically-evoked cortical responses as well as spontaneous activity were differentially sensitive to the suppressant effects of muscimol and (-)-baclofen. 5. Bicuculline methiodide reversibly blocked THIP- and muscimol-induced suppressions of tactile- (air puffer)-induced S1 responses but spared those produced by (-)-baclofen. Phaclofen and delta-amino-n-valeric acid were essentially inactive as blockers of (-)-baclofen-induced effects and in fact often acted as (-)-baclofen-like agonists, phaclofen being considerably weaker than delta-amino-n-valeric acid in this respect. 6. The range of suppressant effects produced by GABA as well as by muscimol and THIP, considered in conjunction with the actions of bicuculline methiodide, suggest that the effects observed by ejected GABA are likely to be due principally to GABA(A) processes, those mediated by GABA(B) receptors largely being masked. However, GABA(B) mechanisms are extant and do appear to be active, probably presynaptically and probably at sites distal to the soma.