Noradrenergic modulation of dendrodendritic inhibition in the olfactory bulb

Noradrenaline containing fibres in the central nervous system (CNS) have been characterized in detail by histochemical techniques^1-3. A particularly striking feature of this neurotransmitter system is its diffuse pattern of innervation. Although noradrenaline generally inhibits spontaneous activity of neurones^4-7, in a number of systems^5,8-10 it can actually enhance orthodromic excitatory responses. Enhancement of excitatory transmission through sensory relay neurones is consistent with the view that noradrenergic systems are intimately involved in such behaviours as arousal and attention¹¹. However, the basis for this enhancement is unclear. Two proposed hypotheses are a facilitation of excitatory transmitter action ⁵ and a disinhibitory action in which noradrenaline inhibits inhibitory inter-neurones⁸. The olfactory bulb is ideally suited for studying the cellular action of noradrenaline because the neuronal circuitry is relatively simple. Using intracellular recording techniques we have now found that noradrenaline attenuates the inhibitory feedback onto relay neurones, thus facilitating their firing, while glutamate and aspartate augment the inhibitory feedback.