Neuropeptide Y in submucosal ganglia: regional differences in the innervation of guinea-pig large intestine

Properties of cholinergic and non-cholinergic submucosal neurons along the mouse colon

Submucosal neurons are vital regulators of water and electrolyte secretion and local blood flow in the gut. Due to the availability of transgenic models for enteric neuropathies, the mouse has emerged as the research model of choice, but much is still unknown about the murine submucosal plexus. The progeny of choline acetyltransferase (ChAT)-Cre × ROSA26(YFP) reporter mice, ChAT-Cre;R26R-yellow fluorescent protein (YFP) mice, express YFP in every neuron that has ever expressed ChAT. With the aid of the robust YFP staining in these mice, we correlated the neurochemistry, morphology and electrophysiology of submucosal neurons in distal colon. We also examined whether there are differences in neurochemistry along the colon and in neurally mediated vectorial ion transport between the proximal and distal colon. All YFP(+) submucosal neurons also contained ChAT. Two main neurochemical but not electrophysiological groups of neurons were identified: cholinergic (containing ChAT) or non-cholinergic. The vast majority of neurons in the middle and distal colon were non-cholinergic but contained vasoactive intestinal peptide. In the distal colon, non-cholinergic neurons had one or two axons, whereas the cholinergic neurons examined had only one axon. All submucosal neurons exhibited S-type electrophysiology, shown by the lack of long after-hyperpolarizing potentials following their action potentials and fast excitatory postsynaptic potentials (EPSPs). Fast EPSPs were predominantly nicotinic, and somatic action potentials were mediated by tetrodotoxin-resistant voltage-gated channels. The size of submucosal ganglia decreased but the proportion of cholinergic neurons increased distally along the colon. The distal colon had a significantly larger nicotinic ion transport response than the proximal colon. This work shows that the properties of murine submucosal neurons and their control of epithelial ion transport differ between colonic regions. There are several key differences between the murine submucous plexus and that of other animals, including a lack of conventional intrinsic sensory neurons, which suggests there is an incomplete neuronal circuitry within the murine submucous plexus.

Distribution of P2Y2 receptors in the guinea pig enteric nervous system and its coexistence with P2X2 and P2X3 receptors, neuropeptide Y, nitric oxide synthase and calretinin

The distribution of P2Y2 receptor-immunoreactive (ir) neurons and fibers and coexistence of P2Y2 with P2X2 and P2X3 receptors, neuropeptide Y (NPY), calretinin (CR), calbindin (CB) and nitric oxide synthase (NOS) was investigated with immunostaining methods. The results showed that P2Y2-ir neurons and fibers were distributed widely in myenteric and submucous plexuses of the guinea pig stomach corpus, jejunum, ileum and colon. The typical morphology of P2Y2-ir neurons was a long process with strong positive staining on the same side of the cell body. The P2Y2-ir neurons could be Dogiel type 1. About 40-60% P2X3-ir neurons were immunoreactive for P2Y2 in the myenteric plexus and all the P2X3-ir neurons expressed the P2Y2 receptor in the submucosal plexus; almost all the NPY-ir neurons and the majority of CR-ir neurons were also immunoreactive for P2Y2, especially in the myenteric plexus of the small intestine; no P2Y2-ir neurons were immunoreactive for P2X2 receptors, CB and NOS. It is shown for the first time that S type/Dogiel type 1 neurons with fast P2X and slow P2Y receptor-mediated depolarizations could be those neurons expressing both P2Y2-ir and P2X3-ir and that they are widely distributed in myenteric and submucosal plexuses of guinea pig gut.

Electrophysiological characteristics distinguish three classes of neuron in submucosal ganglia of the guinea-pig distal colon

Intracellular recordings were made from neurons in the submucosal ganglia of the guinea-pig distal colon. The recording electrode contained the intracellular marker biocytin, which was injected into neurons so that their electrophysiological characteristics could be correlated with their shape. Correlations of electrophysiology and shape have not been reported previously for neurons in this region. Three types of neuron were identified on electrophysiological grounds. Neurons of the first type (S neurons) had tetrodotoxin-sensitive soma action potentials, and received fast and slow excitatory synaptic inputs. They had uniaxonal morphologies and may function as secretomotor or possibly vasomotor neurons. The second type (AH neurons) received only slow synaptic input, while the soma action potential had tetrodotoxin-sensitive and -insensitive components with a shoulder on the falling phase and a long-lasting afterhyperpolarisation of the membrane potential following a single action potential. Neurons of this type had multipolar morphologies and provided dense innervation of adjacent submucosal ganglia. These neurons are similar to the submucosal intrinsic primary afferent neurons of the guinea-pig small intestine. The final type of neuron [the low-threshold (LT) neuron] had electrophysiological characteristics that set it apart from those described previously within enteric plexuses. They expressed tetrodotoxin-insensitive voltage-gated soma currents, did not have long-lasting afterhyperpolarisations and received only slow synaptic input. In addition, these neurons were very excitable: they had large input resistances and low thresholds for action potential discharge, and often fired action potentials in the absence of stimulation. Neurons with these characteristics were uniaxonal and thus are likely to be secretomotor or possibly vasomotor neurons. This study has shown that submucosal neurons of the distal colon fall into three distinct types, which can be distinguished by a combination of electrophysiological and morphological criteria.

Inhibitory effects of NPY on ganglionic transmission in myenteric neurones of the guinea-pig descending colon

Intracellular recordings were made from myenteric neurones of the guinea-pig descending colon. Neuropeptide Y (NPY) and related pancreatic polypeptides were applied by superfusion and the effects upon the amplitude of fast excitatory synaptic potentials (ESPs) and the ratio of paired fast ESPs evoked by stimulation of internodal fibre tracts were noted. NPY produced a concentration-dependent inhibition in fast ESP amplitude in the majority of neurones (17/21) with a calculated IC50 value of 7 nM; in some neurones this inhibition was mediated via the local release of noradrenaline. Peptide YY (PYY) (eight out of 11 neurones; IC50 = 1 nM), NPY(3-36) (three out of three neurones) and [Leu31, Pro34]NPY (four out of five neurones) also decreased the amplitude of fast ESPs. The effects of two or more pancreatic polypeptides or analogues on fast synaptic transmission were compared directly in six neurones; the apparent relative potency of agonists suggested the involvement of Y2-receptors and at least one other Y-receptor type. In the absence of any direct postsynaptic effects of pancreatic polypeptides on the active or passive properties of myenteric neurones, or on their sensitivity to ionophoretically applied acetylcholine, inhibition of fast ganglionic transmission was presumed to be presynaptic in origin. It is concluded that, in addition to their previously described depressant actions on neuro-effector transmission to colonic smooth muscle, pancreatic polypeptides can exert powerful inhibitory effects on myenteric neurones of the descending colon.

Electrophysiology of neurochemically identified submucosal neurones of the guinea-pig intestine

A number of electrophysiological studies have shown that neurones in the submucous plexus are endowed with three major types of synaptic potentials in response to nerve stimulation: a fast EPSP, a slow IPSP, and a slow EPSP. Combined electrophysiological and immunohistochemical studies enabled analysis of the types of neurochemically identified neurones which receive each type of synaptic input. This short review briefly summarizes the results obtained from these studies.

Neuropeptide Y hyperpolarizes submucosal neurons of the guinea-pig descending colon

Effects of neuropeptide Y (NPY) on submucosal neurons of the guinea-pig descending colon were investigated electrophysiologically by means of intracellular electrophysiological recordings. NPY (100 nM) induced a marked and prolonged hyperpolarization, accompanied by a decrease in input resistance in most (90%) neurons. This NPY-induced hyperpolarization was diminished and augmented by membrane hyperpolarization and depolarization, respectively. The NPY-hyperpolarization was not affected by exposure to either calcium-free solutions or the alpha2-adrenoceptor antagonist, idazoxan (1 microM). When more than one peptide was applied to a neuron, NPY, PYY and Pro34-NPY were equipotent, whilst NPY13-36 was less potent. It was concluded that NPY hyperpolarized submucosal neurons of the guinea-pig descending colon, possibly via a direct action on postsynaptic Y1-receptor and increasing potassium conductance.