TRANSNEURONAL LABELING OF A NOCICEPTIVE PATHWAY, THE SPINO(TRIGEMINO-)PARABRACHIO-AMYGDALOID, IN THE RAT

Transneuronal tracing of a nociceptive pathway, the spino(trigemino)-p arabrachio-amygdaloid pathway, was performed using an alpha-herpes vir us, the Bartha strain of pseudorabies virus (PRV). Microinjection of P RV into the central nucleus of the amygdala (Ce) resulted in progressi ve retrograde and transneuronal infection of a multisynaptic circuit i nvolving neurons in the brainstem and spinal cord as detected immunocy tochemically. At short survival (26 hr), retrogradely labeled neurons were concentrated in the external lateral nucleus of the parabrachial complex (eIPB) but were absent from both the trigeminal nucleus caudal is (TNC) and the spinal cord. At longer survivals (52 hr), labeled cel ls were present in lamina I of both the TNC and spinal dorsal horn. Re trograde labeling from the Ce with Fluorogold demonstrated that eIPB n eurons have long dendrites extending laterally into the terminal field of spinal and trigeminal afferents, where transneuronal passage of PR V to these afferents could occur. Even longer survivals (76 hr) result ed in a columnar pattern of cell labeling in the TNC and spinal dorsal horn that extended from lamina I into lamina II. At this longest surv ival, primary sensory neurons became infected. Bilateral excitotoxic l esions of the eIPB blocked almost all viral passage from the Ce to sup erficial laminae of the TNC and spinal dorsal horn. These results demo nstrate that nociceptive input to the amygdala is relayed from neurons in lamina I through the eIPB. We propose that this modular arrangemen t of lamina I and II neurons may provide the basis for spinal processi ng of peripheral input to the amygdala.