Investigations into the physiological actions of nitric oxide in the nervous systems of the ring dove and the rat

Abstract

A wealth of physiological and pathophysiological roles have been ascribed to nitric oxide (NO) in the nervous system. Many of these seem diverse and unrelated. However, the widespread role NO has to play in modulation of the nervous system suggests an underlying common and fundamental role. The ongoing popularity of NO research has lead to a mass of literature on the subject yet great controversy still prevails as to the true physiological function of NO in the nervous system.
In this thesis, the existing literature has been critically evaluated. Histological, autoradiographic and electrophysiological research approaches have been used to investigate further the physiological role for NO in the nervous system in the hope of explaining some of the controversy that still prevails.
Due to an unforseen move, two different models have been used during the investigations, the brain and the pituitary gland of the ring dove (Streptopelia risoria) and the superior cervical ganglion (SCG) of the rat. The histological NADPH-diaphorase (NDP) technique was used to show the distribution of the NO synthesising enzyme nitric oxide synthase (NOS) for the first time in the ring dove brain. Previously, the NDP distribution has been mapped in the brain of other species and found to be distinct from all other known neurotransmitters. NADPH-diaphorase is a known marker of the mammalian pontine cholinergic reticular formation. However, findings presented here strongly suggest that the distribution of NOS is far more widespread in structures comprising the avian reticular formation. The distribution of the NO synthesising enzyme and the freely diffusible property of NO make it a highly attractive messenger molecule that might coordinate the spatially distinct structures of the reticular formation enabling them to act as one functional unit.
Nitric oxide is released following NMDA receptor activation. Both the presence and the distribution of this component of the NO-cGMP pathway have been shown here for the first time in the ring dove brain. This has been achieved using receptor autoradiography of the [3H]-MK-801, a ligand that binds to the NMDA receptor channel.
The ring dove is a popular model for the study of behaviour changes and the hormonal control of the avian breeding cycle. Here, a study was set up to compare regional [3H]-MK-801 ligand binding in the brains of non breeding birds (naive) and first time breeders. Quantitative receptor autoradiographic analysis of the ligand binding has shown that significant regional binding changes occur during the breeding cycle of the ring dove. This result implies that the NO-cGMP pathway might have a physiological role in the breeding cycle of the dove and that further investigation of this matter would be worth undertaking.
Here, histological and electrophysiological approaches using the rat SCG model have been employed to investigate the role of NO-cGMP pathway in modulation of synaptic transmission. Histological mapping of the pathway components has reconfirmed that NOS is present in the presynaptic fibres and terminals, but it also suggests that it is restricted to a subpopulation of the principal neurones in the ganglion. Immunocytochemistry of the NO-donor induced cGMP accumulations shows that the satellite cells surrounding the principal neurones are major target sites for NO. This finding is consistent with that seen in the dorsal root ganglion (DRG), and supports a glial target for NO in this tissue. Although, NO-donors depolarise the ganglion and enhance synaptic transmission when extracellularly recording the compound action potential (CAP), intracellular recording did not reveal such clear responses. This difference was attributed to the fact that a subpopulation of the principal neurones receives a NO input. These findings suggest that the SCG is not a particularly efficient model for intracellular investigation of the NO­cGMP pathway.
Both intracellular and extracellular investigations into involvement of NO-cGMP pathway in ganglionic L TP suggest that it does not have a role to play in long-term potentiation (L TP) of synaptic efficacy in this tissue. Here, intracellular and extracellular approaches agreed. Further to this, extracellular recording ofLTP of the CAP supported the involvement of the pathway in a short term enhanced synaptic efficacy by a mechanism distinct from L TP. Moreover, it has been shown here that L TP of the epsp can be reliably produced in the SCG. It is therefore suggested that the SCG is a suitable preparation for both intracellular and extracellular investigation of LTP at the peripheral synapse.