Medullary taste modulation by nucleus accumbens shell 1 Descending projections from the nucleus accumbens shell excite 1 activity of taste - responsive neurons in the nucleus of the solitary 2 tract in the hamster 3 4 5

30 31 The nucleus of the solitary tract (NST) and the parabrachial nuclei (PbN) are the first 32 and second relays in the rodent central taste pathway. A series of electrophysiological 33 experiments revealed that spontaneous and taste-evoked activities of brainstem gustatory 34 neurons are altered by the descending input from multiple forebrain nuclei in the central taste 35 pathway. The nucleus accumbens shell (NAcSh) is a key neural substrate of reward circuitry, 36 but it has not been verified to be a classical gustatory nucleus. A recent in vivo 37 electrophysiological study demonstrated that the NAcSh modulates the spontaneous and 38 gustatory activities of hamster pontine taste neurons. In the present study, we investigated 39 whether activation of the NAcSh modulates gustatory responses of the NST neurons. 40 Extracellular single-unit activity was recorded from medullary neurons in urethane 41 anesthetized hamsters. After confirming taste response by delivering sucrose, NaCl, citric 42 acid, and quinine hydrochloride to the anterior tongue, the NAcSh was stimulated bilaterally 43 using concentric bipolar stimulating electrodes. Stimulation of the ipsilateral and contralateral 44 NAcSh induced firings from 54 and 37 out of 90 medullary taste neurons, respectively. Thirty 45 cells were affected bilaterally. No inhibitory responses or antidromic invasion were observed 46 following NAcSh activation. In the subset of taste cells tested, high frequency electrical 47 stimulation of the NAcSh during taste delivery enhanced taste-evoked neuronal firing. These 48 results demonstrate that two-thirds of the medullary gustatory neurons are under excitatory 49 descending influence from the NAcSh, which is a strong indication of communication 50 between the gustatory pathway and the mesolimbic reward pathway. 51 52