Neuroscience
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To clarify the circuitry through which opioid compounds modulate spinal and trigeminal nociceptive transmission, we have examined the synaptic associations formed by leucine-enkephalin-containing (enkephalin) neurons in the superficial dorsal horn of the cat. As described previously, punctate enkephalin immunoreactivity is concentrated in the marginal layer (lamina I) and in both the outer and inner layers of the substantia gelatinosa (lamina IIo and IIi). In colchicine treated cats, enkephalin perikarya are most numerous in lamina I and at the border between laminae I and II. ⋯ These include inputs which may derive from primary afferent axons. Enkephalin neurons, in turn, influence nociceptive transmission predominantly through postsynaptic mechanisms. Finally, while we did not observe enkephalin terminals presynaptic in an axoaxonic relationship, the possibility that enkephalin neurons modulate the excitability of fine fiber nociceptive and nonnociceptive afferents via "nonsynaptic interactions" is discussed.
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Comparative Study
Intracellular and extracellular electrophysiology of nigral dopaminergic neurons--1. Identification and characterization.
Intracellular recordings were obtained from directly identified rat nigral dopamine cells in vivo. This identification was based on an increase in glyoxylic acid-induced catecholamine fluorescence in the impaled dopamine neurons. One of three compounds was injected intracellularly into each cell to produce the heightened fluorescence: (1) L-DOPA, to increase the intracellular dopamine content by precursor loading; (2) tetrahydrobiopterin, a cofactor for tyrosine hydroxylase, to increase intracellular dopamine concentration through activation of the rate-limiting enzyme for dopamine synthesis and (3) colchicine, to arrest intraneuronal transport and thus allow the build-up of dopamine synthesizing enzymes and dopamine in the soma. ⋯ Intravenously administered apomorphine demonstrated the same inhibitory effect on cell firing that was previously reported to occur when recording extracellularly from identified dopaminergic neurons. The determination of the electrophysiological characteristics of a population of cells directly identified as containing a specific neurotransmitter (in this case, dopamine) may allow one to construct better models of a system's functioning. Thus, the high input resistance and long time constant of dopamine-containing cells, combined with their burst/pause firing mode, may be important functionally with respect to a possible modulatory effect of dopamine in postsynaptic target areas.