Baclofen, a gamma-aminobutyric acid (GABA)/B receptor agonist, was bath applied while recording the responses of second- and third-order neurons in the mudpuppy retina. Baclofen receptors were largely restricted to amacrine and ganglion cells. 2. Baclofen hyperpolarized the membrane potential of many, but not all, third-order neurons. This involved an increase in input conductance, probably associated with an opening of potassium channels. 3. The maximal increase in input conductance associated with the activation of GABA/B receptors was approximately one-third of that produced by activation of GABA/A receptors. 4. Baclofen suppressed sustained responses but enhanced transient responses. The net effect was that responses throughout the inner retina became more transient in the presence of baclofen. 5. In sustained cells baclofen not only suppressed the sustained responses but also revealed large transient responses. Thus baclofen converted the light responses of these cells from sustained to transient. This suggests that sustained cells receive significant transient excitation which is normally masked by the sustained inputs. 6. The role of the GABA/B receptor in controlling response characteristics and information content of amacrine and ganglion cells is discussed.
