The synaptic connections of pyramidal neurones and interneurones in rat and cat neocortex
The layer 4 neurones of the mammalian primary sensory neocortex comprise diverse functional components for the first stage of cortical sensory processing. Dual intracellular recordings of synaptically connected pairs of neurones with biocytin-filling were used to study intra-laminar layer 4 connections in adult cat and rat slices. Interestingly, all excitatory cells involved in intralaminar layer 4 connections were regular spiking despite burst firing cells comprising 37% of the population recorded. Neuronal morphology and synaptic properties were similar in both species, as were the probabilities of finding connections; 1 in 43 for pyramid to pyramid, 1 in 21 for pyramid to interneurone and 1 in 12 for interneurone to pyramid (cat and rat data combined). Pyramid to pyramid connections generated EPSPs 1.33 0.9mV in amplitude (mean SD), with rise times of 1.71 0.83ms and half width 14.67 7.1ms. All EPSPs recorded in excitatory cells and in parvalbumin immuno-positive interneurones exhibited depression, the second and subsequent EPSPs in trains being smaller in amplitude than the first. Fluctuation analysis indicated that this depression was presynaptically mediated. The interneuronal EPSPs recorded in this study, were briefer (rise times 0.63 0.26ms and half width 5.25 2.85ms) than those recorded in pyramidal cells. Two interneurones that were immuno-negative for parvalbumin received EPSPs that were facilitating, second and subsequent EPSPs in trains being significantly larger than the first. Again, fluctuation analysis indicated that this facilitation was presynaptically mediated. Possible branch point failure of axonal conduction, feed-forward inhibition and post-tetanic potentiation were observed at some excitatory connections in layer 4. In addition, novel evidence for electrical gap junctions between adult pyramidal cells was obtained in one dual recording in which current injections into an impaled layer 3 pyramidal cell elicited full action potentials and 'spikelets', both of which elicited EPSPs in a layer 5 pyramidal cell.