RELEASE OF THE NITRIC-OXIDE PRECURSOR, ARGININE, FROM THE THALAMUS UPON SENSORY AFFERENT STIMULATION, AND ITS EFFECT ON THALAMIC NEURONS IN-VIVO

The neurophysiology and neuroanatomy of the thalamus have been extensi vely studied in a variety of species and sensory systems.(9) The ident ity of the neurotransmitter(s) which mediate the excitation from ascen ding sensory afferents on to thalamic relay neurons is, however, still unclear, although it appears to be a substance which is a ligand for excitatory amino acid receptors,(16,18) as the responses of ventrobasa l thalamus neurons to natural stimulation of somatosensory afferents a rising from the mustachial vibrissae of the rat are mediated by ionotr opic excitatory amino acid receptors, when stimulation is performed us ing an air-jet directed at the vibrissa receptor field.(16,18) In an e ffort to determine the transmitter of these sensory afferents, we have attempted to detect the release of amino acids in the ventrobasal tha lamus in vivo upon such stimuli. We have thus used a similar natural s timulation protocol, together with push-pull perfusion and recording i n the ventrobasal thalamus, and we describe the release of the amino a cid, arginine, in this brain area following physiological stimulation of afferents. Furthermore, we show that application of L-arginine on t o thalamic relay neurons can facilitate sensory synaptic transmission, possibily via the synthesis of the diffusable messenger, free radical gas, nitric oxide. This may represent a novel, local positive-feedbac k, modulatory system which could enhance the responsiveness of thalami c neurons to sensory input.