Correlation of electrophysiology, shape and synaptic properties of myenteric AH neurons of the guinea pig distal colon

Well-defined correlations between morphology, electrophysiological properties and the types of synaptic inputs received are established for myenteric neurons in the guinea pig ileum. However, in the distal colon, the correlations between AH electrophysiological properties, presence of fast excitatory post-synaptic potentials (EPSPs) and neuronal shape have been inadequately resolved and it is unknown whether any colon neurons receive synaptic inputs that generate sustained excitation. In this work, we have used intracellular recording, dye filling via the recording electrode, and immunohistochemistry to classify distal colon neurons. Neurons (24 of 168) had Dogiel type II morphology and 42% of these were dendritic type II neurons, compared to about 10% in the ileum. All Dogiel type II neurons had AH electrophysiological properties, including a prolonged post-spike after-hyperpolarization (AHP). None of these received fast excitatory post-synaptic potentials, 11 of 22 tested exhibited sustained slow post-synaptic excitation (SSPE) in response to 1 Hz pre-synaptic stimulation and 13 of 15 tested were immunoreactive for calbindin. Neurons (127) had Dogiel type I, filamentous or other uniaxonal cell shape and S type electrophysiology. Neurons of this group had fast excitatory post-synaptic responses to stimulation of synaptic inputs, but did not exhibit a prolonged post-spike after-hyperpolarization or sustained slow post-synaptic excitation. Another group of neurons (17) had both AH electrophysiological characteristics and fast excitatory post-synaptic potentials. These neurons had Dogiel type I, filamentous or other uniaxonal shapes, but none had Dogiel type II morphology and none showed sustained slow post-synaptic excitation. It is concluded that Dogiel type II neurons are all AH neurons and are probably intrinsic sensory neurons that could be involved in long-term changes in excitability in the colon. All other neurons are monoaxonal; these are motor neurons and interneurons, and most are S neurons, electrophysiologically. A small number of monoaxonal neurons display AH electrophysiology and also receive fast excitatory synaptic inputs. These include motor and interneurons, but not sensory neurons.