Tracing autonomic innervation of the rat pineal gland using viral transneuronal tracing

We have used the neurotropic Bartha strain of pseudorabies virus (PRV) to characterise the pathway linking the endogenous circadian pacemaker of the suprachiasmatic nucleus (SCN) to the pineal gland. This low virulent strain of virus replicates within synaptically linked neurones and is ideally suited to visualise the multisynaptic pathways through which the SCN modulates the activity of the rat pineal gland. Using specific antibodies against PRV, we could follow the immunohistochemical pattern of the spatiotemporal passage of virus through the sympathetic trunk and the neuraxis. The time course of virus infection indicated that the most prominent pathway from the SCN to the pineal gland is via a final sympathetic innervation from the superior cervical ganglion (SCG). The pathway arises in the dorsomedial portion of the SCN from where neurones project to the dorsal parvicellular subdivision of the hypothalamic paraventricular nucleus (PVN) to form synaptic contact with neurones descending to the intermediolateral nucleus (IML) of the upper thoracic spinal cord. The neurones of the IML constitute the presynaptic sympathetic input synaptically connected to postsynaptic sympathetic neurones in the SCG which constitute the final input to the pineal gland. Removal of the superior cervical ganglion (SCGX) prior to viral infection completely abolished infection of neurones in this circuit. However, an additional parasympathetic projection from the superior salivatory nucleus via the sphenopalatine ganglion to the pineal gland was observed in SCGX animals.