Efferent projections of ProTRH neurons in the ventrolateral periaqueductalgray

Our previous study has shown that prothyrotropin-releasing hormone (proTRH) gene expression is increased in the ventrolateral periaqueductal gray (PAG ) neurons following precipitated morphine withdrawal and continues to be ac tivated even 24 h after withdrawal. We have hypothesized that peptide produ cts of proTRH may participate in the recovery from morphine withdrawal. To identify neuroanatomical substrates of the proposed action of proTRH-derive d peptides originating from the ventrolateral PAG proTRH neurons, projectio ns of these neurons were investigated by a series of anterograde and retrog rade tract-tracing experiments. First, Phaseolus vulgaris-leucoagglutinin ( PHA-L) was injected in the ventrolateral PAG in Sprague-Dawley rats. Follow ing transport of the tracer, simultaneous immunolabeling for PHA-L and proT RH peptides was performed and mapped in discrete brain regions. PHA-L-immun oreactive (IR) fibers showing preterminal and terminal-like arborization th at contained proTRH were identified in the dorsolateral and lateral PAG, de ep layer of superior colliculus (CS), parafascicular nucleus (PF), ventrome dial zona incerta (ZI) and at the border of the locus coeruleus (LC) and Ba rrington's nucleus. Scattered double-labeled fibers were present in the lat eral septal nucleus, ventromedial preoptic nucleus, lateral hypothalamus, p erifornical area and in the periventricular region at the diencephalon/midb rain junction. The retrogradely transported marker, cholera toxin beta -sub unit (CTb) was then injected in the dorsolateral PAG, CS, PF, ZI and medial to the LC. Double-labeled perikarya for both CTb and proTRH in the ventrol ateral PAG were found for each region injected with CTb, corroborating the findings by the anterograde tracing experiment. These studies demonstrate t hat proTRH neurons in the ventrolateral PAG project to several regions of t he brain that are involved in autonomic and behavioral regulation and there by, may function as an integrating center to coordinate responses to opiate withdrawal. (C) 2001 Elsevier Science B.V. All rights reserved.