Postnatal development of the noradrenergic system in the dorsal lateral geniculate nucleus of the rat

Methylphenidate and atomoxetine enhance sensory-evoked neuronal activity in the visual thalamus of male rats

Attention deficits and inappropriate regulation of sensory signal processing are hallmarks of many neuropsychiatric conditions, including attention deficit hyperactivity, for which methylphenidate (MPH) and atomoxetine (ATX) are commonly prescribed therapeutic treatments. Despite their widespread use and known mechanism of blocking reuptake of catecholamine transmitters in the brain, the resultant actions on individual neuron and neural circuit function that lead to therapeutic efficacy are poorly understood. Given the ability of MPH and ATX to improve cognitive performance in humans and rodent assays of attention, we were interested in their influence on early sensory processing in the dorsal lateral geniculate nucleus (dLGN), the primary thalamic relay for visual information from the retina to the visual cortex. In male rats, dLGN neuronal responses to light stimuli were altered in multiple ways after doses of MPH or ATX observed to enhance performance in visually guided assays of attention (MPH = 2 mg/kg; ATX = 0.5 mg/kg). Latencies to response onset and to the peak of the primary response were decreased, while the peak intensity and area of the primary response were increased. In addition, some cells that were unresponsive to light stimuli prior to drug treatment displayed a "gating effect," wherein prominent responses to light stimuli were evident after drug administration. Our results begin to reveal unique effects of MPH and ATX in enhancing sensory signal transmission through visual circuitry, and may yield new insights for understanding the pathophysiology of certain cognitive disorders and inform development of improved therapeutic treatments for these conditions.

Excitatory actions of synaptically released catecholamines in the rat lateral geniculate nucleus

The gating properties of thalamic relay neurons are influenced by the actions of a variety of neuromodulators in concert with the intrinsic properties of these relay neurons. In this study, we have investigated the consequences of synaptically released catecholamines on the excitability of neurons in the rat dorsal lateral geniculate nucleus. Tetanic stimulation of the optic tract, in which catecholamine fibers also course near or through, produced a strong depolarization that consisted of a fast and slow component. The fast excitatory postsynaptic potential was attenuated by ionotropic glutamate receptor antagonists and further unmasked the slow excitatory postsynaptic potential. The amplitude of the slow excitatory postsynaptic potential was dependent on the frequency and intensity of the tetanic stimulation. The alpha1-adrenergic receptor antagonist, prazosin, and the D1-like dopamine receptor antagonist, SCH23390, attenuated the slow excitatory postsynaptic potential; however, the slow excitatory postsynaptic potential was unaltered by metabotropic glutamate, cholinergic, alpha2-adrenergic, and beta-adrenergic receptor antagonists. On the other hand, tetanic stimulation of the optic radiations (corticothalamic axons) evoked a slow excitatory postsynaptic potential that was completely attenuated by metabotropic glutamate receptor antagonists. Our results suggest that tetanic stimulation of catecholamine fibers within the optic tract produces synaptic release of norepinephrine and dopamine that in turn activates both alpha(1)-adrenergic and D1-like dopamine receptors leading to a robust membrane depolarization. By altering the excitability of relay neurons, ascending activating systems may modulate the efficacy of information transfer through the thalamus.