The brain-stem parabrachial region controls mode of response to visual stimulation of neurons in the cat’s lateral geniculate nucleus

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Abstract
We recorded the responses of neurons from the cat’s lateral geniculate nucleus to drifting sine-wave grating stimuli both before and during electrical stimulation of the parabrachial region of the midbrain. The parabrachial region provides a mostly cholinergic input to the lateral geniculate nucleus, and our goal was to study its effect on responses of geniculate cells to visual stimulation. Geniculate neurons respond to visual stimuli in one of two modes. At relatively hyperpolarized membrane potentials, low threshold (LT) Ca2+spikes are activated, leading to high-frequency burst discharges (burstmode). At more depolarized levels, the low threshold Ca2+spike is inactivated, permitting a more tonic response (relayortonicmode). During our intracellular recordings of geniculate cells, we found that, at initially hyperpolarized membrane potentials, LT spiking in response to visual stimulation was pronounced, but that parabrachial activation abolished this LT spiking and associated burst discharges. Coupled with the elimination of LT spiking, parabrachial activation also led to a progressive increase in tonic responsiveness. Parabrachial activation thus effectively switched the responses to visual stimulation of geniculate neurons from the burst to relay mode. Accompanying this switch was a gradual depolarization of resting membrane potential by about 5–10 mV and a reduction in the hyperpolarization that normally occurs in response to the inhibitory phase of the visual stimulus. Presumably, the membrane depolarization was sufficient to inactivate the LT spikes. We were able to extend and confirm our intracellular observations on the effects of parabrachial activation to a sample of cells recorded extracellularly. This was made possible by adopting empirically determined criteria to distinguish LT bursts from tonic responses solely on the basis of the temporal pattern of action potentials. During parabrachial activation, every cell responded only in the relay mode, an effect that corresponds to our intracellular observations. We quantified the effects of parabrachial activation on various response measures. The fundamental Fourier response amplitude (Fl) was calculated separately for the total response, the tonic response component, and the LT burst component. Parabrachial activation resulted in an increased Fl amplitude for the total response. This increase was due to an increase in the tonic response component. For a subset of cells showing epochs of LT bursting, parabrachial activation concurrently reduced LT bursting and increased the amplitude of the tonic response. Parabrachial activation, by eliminating LT bursting, also caused cells to respond with more linearity. By keeping geniculate cells in the relay mode, the parabrachial region serves to maintain a more linear retinogeniculate transfer of information to cortex, and this may be important for detailed analysis of visual targets. However, when a geniculate neuron becomes hyperpolarized, as may occur during states of visual inattention, it would not respond well to visual stimuli without the sort of nonlinear amplification provided by the LT spike. Thus, the LT spike may permit hyperpolarized cells to relay to cortex the presence of a potentially salient or dangerous stimulus, but this is done at the expense of linearity. This may serve as a sort of “wake-up call” that redirects attention to a particular stimulus and eventually enhances activity of appropriate parabrachial inputs to switch the critical geniculate neurons into the relay mode.