Research Synopsis:
Thalamic relay cells receive two types of afferents: drivers, considered responsible for the relay cells receptive field properties, and modulators, thought not to contribute significantly to the receptive field, but, rather, to modulate the membrane potential and response properties of relay cells. Thalamic drivers have two origins. Some relay cells receive their drivers from subcortical areas, including those in the lateral geniculate nucleus (LGN), the ventral posterior nucleus (VP), and the ventral portion of the medial geniculate body (MGBv). These have been called first order relays (FO) since this is the first time that a particular information type is relayed to cortex. Other relay cells receive drivers from layer 5 of cortex, including those in the lateral posterior nucleus (LP), the posterior medial nucleus (POm), and the dorsal portion of the medial geniculate body (MGBd). These are known as higher order relays (HO). Both FO and HO relays receive modulatory inputs, mainly from brainstem areas (e.g., cholinergic input from the parabrachial region, serotonergic input from the dorsal raphé nucleus, etc). The project’s goal has been to determine the effects of modulators in the two types of relays. In order to test this, we used current and voltage clamp recordings of rat thalamic cells in the whole-cell, patch-clamp configuration. We bath-applied general agonists for muscarinic and serotonergic receptors and determined their effects on relay cells of six sensory nuclei, three FO relays (LGN, VP, and MGBv) and three HO relays (LP, POm, and MGBd). We have recently shown that cholinergic input hyperpolarizes about 20% of the HO relay cells (Varela & Sherman, 2007), whereas it depolarizes all responsive FO relay cells. More recent results suggest that serotonergic inputs also have differential effects in FO and HO relays: A small but significant proportion of cells in HO are hyperpolarized by serotonin whereas virtually all cells from FO are depolarized.
