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The effects of a specific GABA B receptor antagonist, p -(3-aminopropyl)- p -diethoxymethyl-phosphonic acid (CGP 35348), on pyramidal tract neuron responses to transcallosal stimulation were investigated in the cat motor cortex in vivo. The paired-pulse method was used to obtain more insight into the role of GABA B receptors. At a 200-ms inter-stimulus interval the spike response was inhibited in 75% of the neurons. There was an approximately 40% depression of the mean spike value in the control. CGP 35348 reduced paired-pulse inhibition, while (−)-baclofen increased it. Stronger drug effects on the second stimulation-induced response possibly indicate their presynaptic action on GABA B receptors.

The effects of a specific GABAB receptor antagonist, p-(3-aminopropyl)-p-diethoxymethyl-phosphonic acid (CGP 35348), on pyramidal tract neuron responses to transcallosal stimulation were investigated in the cat motor cortex in vivo. Iontophoretic application of CGP 35348 significantly increased the number of spikes from 10.3 +/- 4.4 (control; n = 27; mean +/- S.D.) to 16.7 +/- 7.2 (CGP 35348) for 20 transcallosal stimulation trials, while the latency of neuronal activity was significantly shortened from 4.4 +/- 2.1 ms (control; n = 27; mean +/- S.D.) to 3.8 +/- 1.7 ms (CGP 35348). In conclusion, CGP 35348 facilitated transcallosal synaptic transmission between pyramidal tract neurons by removal of GABAB inhibition.

The topography of the commissural fibers in the corpus callosum (CC) of the cat was systematically investigated using the WGA-HRP method. WGA-HRP was injected into various parts of the cerebral cortex and locations of WGA-HRP-stained commissural fibers in the CC were examined. Commissural fibers were arranged in a topological fashion in the CC. Cortical areas rostral to the cruciate sulcus (CrS), corresponding to motor or premotor cortices, projected fibers into the genu of the CC, while fibers from the cortex caudal to the CrS passed through the CC slightly caudal to the genu. When WGA-HRP was injected into the lateral gyrus (LG), it was observed that fibers from the anterior LG passed through the anterior one-third of the CC, whereas those from the posterior LG passed through or near the splenium, and fibers from the middle LG passed between those from the anterior and posterior LG. Similarly, the suprasylvian gyrus (SSG) projected commissural fibers in the CC in a rostrocaudal topological manner. Fibers from the anterior SSG passed through the anterior one-third of the CC, and those from the middle SSG through the middle one-third of the CC and upper part of the splenium. Injection into the most posterior part of the middle SSG revealed fibers passing through the caudal end of the splenium. Callosal fibers from the anterior SSG were focused on in this study, because this area (area 2v) is considered one of the vestibular projection cortices and is an area of special interest to the authors. Callosal fibers from the anterior SSG were observed to pass through the anterior one-third of the body of the CC. When WGA-HRP was injected into auditory areas, fibers from the anterior and middle ectosylvian gyri (ESG) were observed to pass through the posterior one-third of the body of the CC or through the splenium, while fibers from the posterior ESG passed through the splenium. WGA-HRP was also injected into the cingulate gyrus (CiG). Fibers from the anterior CiG (area 24) passed through the anterior portion of the CC while those from the posterior CiG (area 23) passed through the posterior portion of the CC.

GABAergic characteristics of transcallosal activity of cat pyramidal tract neurons (PTNs) and non-PTNs (nPTNs) were studied with stressing on GABAB receptors. PTNs and nPTNs were further classified into group 1 (10 ms) and group 2 (10 ms) based on the latency upon transcallosal stimulation. However, mainly the results of group 1 neurons were presented here, due to the small number of group 2 neurons. GABA, bicuculline, CGP 35348 and phaclofen were iontophoretically applied. The spike number to 20 trials of transcallosal stimulation was 8.9 +/- 4.3 (mean +/- S.D.) for group 1 PTNs (n = 14) and 10.4 +/- 4.5 for group 1 nPTNs (n = 38) under the control conditions. CGP 35348, phaclofen and bicuculline significantly increased the spike numbers in both cases. The increase was greater for nPTNs than for PTNs. GABA decreased them. The transcallosal latency was 3.9 +/- 1.1 ms for PTNs under the control conditions. CGP 35348, phaclofen and bicuculline significantly shortened the latency, and GABA elongated it. The transcallosal latency for nPTNs under the control conditions was 2.7 +/- 1.2 ms. This was significantly shortened by application of CGP 35348, phaclofen or bicuculline. GABA restored it. In conclusion, CGP 35348, phaclofen and bicuculline increased spike discharge and shortened the latency upon transcallosal stimulation for both group 1 PTNs and nPTNs.