Effects of morphine injection into the parabrachial area on saccharin preference: modulation by lateral hypothalamic neurons.

The aim of the present study was to analyze the effects of morphine injected into the second relay station of the gustatory input pathways, the parabrachial area, on preference for saccharin over water. This study was carried out using both rats whose lateral hypothalamic neurons had been lesioned by ibotenic acid and sham-lesioned rats. As already shown, an 0.3 mM solution of the sweetener, which was clearly preferred over water by the sham-lesioned animals, was neutral for the lesioned rats. The injection of 50 ng of morphine into each parabrachial area transformed this neutral response of the lesioned rats to a clear preference for the sweetener, whereas the preference of sham-lesioned rats for the same solution was converted to an aversive response. Likewise, with a more palatable solution of saccharin (2.5 mM), the injection of 50 ng of morphine decreased the preference of the nonlesioned rats but increased the preference of the lesioned animals. Using the 2.5 mM solution of saccharin, the intraparabrachial injection of higher doses of morphine (100 and 500 ng) did not greatly modify the preference for the sweetener but induced a significant decrease in total fluid intake that was still observed 11 h after the injection of the opiate. These results are discussed: the morphine-induced aversion observed in the nonlesioned rats could be explained either by a specific influence on certain opioid receptors in the parabrachial area or, more probably, by the stimulation of pathways involved in taste or visceral aversive processes and relaying in the parabrachial area.(ABSTRACT TRUNCATED AT 250 WORDS)