Interneuron

Antagonists of muscarinic acetylcholine receptors had widely been used since 1860 for the treatment of Parkinson s disease, prior to the discovery of l-DOPA. They block receptors that mediate the response to striatal cholinergic interneurons. The antiparkinsonian effects of drugs like benzatropine, trihexyphenidyl and biper-iden are moderate the resting tremor may sometimes respond in a favorable manner. The adverse effects, e.g., constipation, urinary retention, and mental confusion, may be troublesome, especially in the elderly. 

GABA is the most prominent inhibitory neurotransmitter in the mammalian nervous system and acts via GABA receptors. Activation of GABAb receptors by GABA released from local spinal interneurons (Fig. 1) negatively modulates nociceptive transmission in the spinal cord. Agonists at GABAb receptors... 

The substantia gelatinosa is part of the dorsal horn of the spinal cord, also called lamina II . The substantia gelatinosa is made up almost exclusively of interneurons (both excitatory and inhibitory), some of which respond only to nociceptive inputs, while others respond also to non-noxious stimuli. 

Carta M, Ariwodola OJ, Weiner J L, et al Alcohol potently inhibits the kainate receptor-dependent excitatory drive of hippocampal interneurons. Proc Natl Acad Sci U SA 100 6813-6818, 2003... 

Figure 1.3 Some possible basic neurotransmitter-synaptic arrangements for the excitation and inhibition of different neurons, (a) The single NT activates neuron B and inhibits neuron C by being able to activate both excitatory and inhibitory receptors or, more probably, acting on one receptor linked to both events. There is potential, however, for the NT to activate any inhibitory receptors that may be on B or excitatory receptors on C. (b) The same NT is used as in (a) but the excitatory receptors are now only on dendrites and separated from the inhibitory receptors only on the soma. There is less chance of unwanted mixed effects, (c) Neuron A releases distinct excitatory and inhibitory NTs from its two terminals each acting on specific and morphologically separated receptors. But this depends on a neuron being able to release two NTs. (d) Neuron A releases the same NT from both terminals. It directly excites B but inhibits C through activating an inhibitory interneuron (I) which releases an inhibitory NT onto specific receptors on C. This last scheme (d) is clearly more functional and is widely used...

Fortunately there is another way in which one neuron can excite and inhibit different neurons using just one NT. Neuron A could excite B and inhibit C by the introduction of an inhibitory interneuron the activation of which by A, using the same excitatory NT as at B, automatically inhibits C (Fig. 1.3(d)). This form of inhibition is quite common in the CNS and in fact much inhibition is mediated by these so-called short-axon interneurons and a neuron may inhibit itself through feedback via an axon collateral synapsing onto an adjacent inhibitory short-axon interneuron (Fig. 1.2). 

Lesions in conjunction with concentration studies can also be useful. Section of dorsal roots and degeneration of afferent fibres produces a reduction in glutamate and substance P which can then be associated with sensory inputs. Temporary reduction of the blood supply to the cord causes preferential destruction of interneurons and a greater loss of asparate and glycine, compared with other amino acids and so links... 

These approaches are, in any case, only suitable for classical neurotransmitters. Those with slow background effects will probably not be released in large amounts. For such substances we require a measure of their utilisation, or turnover, over a much longer period of time. With NTs released from short-axon interneurons there are no pathways to stimulate and it becomes necessary to activate the neurons intrinsically by field stimulation, which is of necessity not specific to the terminals of the interneurons. 

Since ACh is the transmitter at the skeletal neuromuscular junction one might also expect it to be released from any axon collaterals arising from the motor nerve to it. Such collaterals innervate (drive) an interneuron (the Renshaw cell) in the ventral horn of the spinal cord, which provides an inhibitory feedback onto the motoneuron. Not... 

Gupta, A, Wang, Y and Markram, H (2000) Organizing principles for a diversity of GABAergic interneurons and synapses in the neocortex. Science 287 213-218. 

ACh is released from the large non-spiny striatal interneurons (Fig. 15.10) which only represent some 5% of total striatal neuron number. Since ACh is excitatory and DA inhibitory on striatal neurons, various schemes have been proposed to balance their antagonistic action but the role of ACh in striatal function (and PD) appears to be relatively minor. 

Kawaguchi, Y, Wilson, CJ, Augood, ST and Emson, PC (1995) Striatal interneurones chemical physiological and morphological characterization. Trends Neurosci. 18 527-535. 

Lamina II is also known as the substantia gelatinosa (SG) and can be divided into two layers, the outer layer (IIo) and the inner layer (Ili). This layer is densely packed with small neurons and lacks myelinated axons. Neurons with cell bodies in Hi receive inputs from low-threshold mechanoreceptive primary afferents, while those in IIo respond to inputs from high-threshold and thermoreceptive afferents. The intrinsic cells which comprise the SG are predominantly stalk and islet cells. Stalk cells are found located in lamina IIo, particularly on the border of lamina I, and most of their axons have ramifications in lamina I although some also project to deeper layers. These cells are thought to predominantly relay excitatory transmission. Islet cells, on the other hand, are located in Hi and have been demonstrated to contain the inhibitory neurotransmitters, y-aminobutyric acid (GABA), glycine and enkephalins in their dendrites. Hence these cells have been proposed to be inhibitory interneurons. 

The cells comprising lamina V are more diverse than those of lamina IV and their dendrites extend vertically toward the superficial layers. Cell bodies in lamina V contribute to three projection pathways, the SCT, PSDC and STT. However, the STT cells appear to be predominant in this lamina. Lamina V plays an important role in nociception since it receives both A - and C-fibre inputs. Some cells in lamina V also respond to cutaneous low- and high-threshold mechanical stimuli and receive nociceptive inputs from the viscerae. Many of these neurons also project onto mononeurons and so act as interneurons in the polysynaptic withdrawal reflex to noxious stimuli. 

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