Recordings were made from pairs of neurons in cat striate visual cortex in vitro to study the AMPA-channel-mediated components of intracortical excitatory synaptic connections between layer 4 spiny neurons and between layer 6 and layer 4 spiny neurons. Forty-six of the 72 cells recorded were identified morphologically. They consisted of spiny stellate and pyramidal cells in layer 4, and pyramidal cells in layer 6. Connections between layer 4 excitatory cells involve excitatory postsynaptic

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... tials (EPSPs) averaging 949 µV, with an average coefficient of variation of 0.21 (n = 30). The synapses operate at very high release probabilities (0.69-0.98). With repetitive stimulation these EPSPs show varying degrees of depression, largely mediated by presynaptic changes in release probability. Four pairs of layer 4 cells were reciprocally connected. The connections from layer 6 to layer 4 involve smaller, more variable EPSPs, with an average amplitude of 214 µV, and average coefficient of variation 0.72 (n = 7). These synapses operate at moderately high release probabilities (0.37-0.56). They show facilitation with repetitive stimulation, mediated largely by presynaptic changes in release probability. One excitatory connection from a layer 4 neuron to a layer 6 pyramidal cell was also detected. Thus, layer 4 spiny neurons receive effective excitation from two intracortical sources that have different synaptic dynamics and are likely to contribute significantly to the temporal properties of these cells in vivo. Materials and Methods Slice Preparation Recordings were made in slices of visual cortex taken from cats aged 12-16 weeks (1.0-1.6 kg). Anaesthesia was induced with pentobarbitone (Sagatal, Sigma, 60 mg/kg, i.p. or i.m.) and maintained with Saffan (Sigma, i.v., as needed). The visual cortex was accessed by craniotomy, and a block of cortex excised after removal of the dura. The cat was subsequently killed by an overdose of anaesthetic. The slice preparation and maintenance techniques were similar to those reported previously (Mason et al., 1991) . Slices 400 or 500 µm thick were cut on a vibrating microtome (Vibroslice, Campden Instruments) and maintained at 34-36°C in an interface-type recording chamber supplied with ACSF and warmed, humidified carbogen (95% O 2 /5% CO 2 ). Slices remained in the recording chamber for at least 2 h before intracellular impalements were attempted. In some cases, slices were held in an interface 'holding' chamber, and transferred to the recording chamber during the experiment. The composition of the ACSF used for slice preparation and recover y was (in mM) NaCl, 124; KCl, 2.3; MgSO 4 , 1.0; KH 2 PO 4 , 1.3; CaCl 2 , 2.5; NaHCO 3 , 26; glucose, 10 (pH 7.4). For recording, 50 µM DL-2-amino-5-phosphopentanoic acid (AP-5, Sigma) was added to the ACSF to block N-methyl-D-aspartate (NMDA)-mediated currents.