PREPROENKEPHALIN MESSENGER RNA-EXPRESSING NEURONS IN THE RAT PARABRACHIAL NUCLEUS - SUBNUCLEAR ORGANIZATION AND PROJECTIONS TO THE INTRALAMINAR THALAMUS

The pontine parabrachial nucleus, which is a key structure in the cent ral processing of autonomic, nociceptive and gustatory information, is rich in a variety of neuropeptides. In this study we have analysed th e distribution of parabrachial neurons that express preproenkephalin m essenger RNA, which encodes for the precursor protein for enkephalin o pioids. Using an in situ hybridization method, we found that preproenk ephalin messenger RNA-expressing neurons were present in large numbers in four major areas of the parabrachial nucleus: the Kolliker-Fuse nu cleus, the external lateral subnucleus, the ventral lateral subnucleus , and in and near the internal lateral subnucleus. Many preproenkephal in messenger RNA-expressing neurons were also seen in the central late ral subnucleus, and in the medial and external medial subnuclei. Few l abeled neurons were found in the dorsal and superior lateral subnuclei . Injection of the retrograde tracer substance cholera toxin subunit B into the midline and intralaminar thalamus demonstrated that the enke phalinergic neurons in and near the internal lateral subnucleus were t halamic-projecting neurons. Taken together with the results of previou s tract-tracing studies, the present findings show that many of the en kephalinergic cell groups in the parabrachial nucleus are located with in the terminal zones of the ascending projections that originate from nociresponsive neurons in the medullary dorsal horn and spinal cord, as well as from viscerosensory neurons within the nucleus of the solit ary tract. The enkephalinergic neurons in the parabrachial nucleus may thus transmit noci- and visceroceptive-related information to their e fferent targets. On the basis of the present and previous observations , we conclude that these targets include the intralaminar and midline thalamus, the ventrolateral medulla and the spinal cord. Through these connections, nociceptive and visceroceptive stimuli may influence sev eral functions, such as arousal, respiration and antinociception.