Periaqueductal gray matter projections to midline and intralaminar thalamic nuclei of the rat

The periaqueductal gray matter (PAG) projections to the intralaminar and mi dline thalamic nuclei were examined in rats. Phaseolus vulgaris-leucoagglut inin (PHA-L) was injected in discrete regions of the PAG, and axonal labeli ng was examined in the thalamus. PHA-L was also placed into the dorsal raph e nuclei or nucleus of Darkschewitsch and interstitial nucleus of Cajal as controls. In a separate group of rats, the retrograde tracer cholera toxin beta-subunit (CTb) was injected into one of the intralaminar thalamic nucle i-lateral parafascicular, medial parafascicular, central lateral (CL), para central (PC), or central medial nucleus-or one of the midline thalamic nucl ei-paraventricular (PVT), intermediodorsal (IMD), mediodorsal, paratenial, rhomboid (Rh), reuniens (Re), or caudal ventral medial (VMc) nucleus. The d istribution of CTb labeled neurons in the PAG was then mapped. All PAG regi ons (the four columns of the caudal two-thirds of the PAG plus rostral PAG) and the precommissural nucleus projected to the rostral PVT, IMD, and CL. The ventrolateral, lateral, and rostral PAG provided additional inputs to m ost of the other intralaminar and midline thalamic nuclei. PAG inputs to th e VMc originated from the rostral and ventrolateral PAG areas. In addition, the lateral and rostral PAG projected to the zona incerta. No evidence was found for a PAG input to the ventroposterior lateral parvicellular, ventro posterior medial parvicellular, caudal PC, oval paracentral, and reticular thalamic nuclei. PAG --> thalamic circuits may modulate autonomic-, nocicep tive-, and behavior-related forebrain circuits associated with defense and emotional responses. J. Comp. Neurol. 424:111-141, 2000. (C) 2000 Wiley-Lis s, Inc.