EFFECTS OF BRAIN-STEM PARABRACHIAL ACTIVATION ON RECEPTIVE-FIELD PROPERTIES OF CELLS IN THE CATS LATERAL GENICULATE-NUCLEUS

1. The lateral geniculate nucleus is the primary thalamic relay for th e transfer of retinal signals to the visual cortex. Geniculate cells a re heavily innervated from nonretinal sources, and these modify retino geniculate transmission. A major ascending projection to the lateral g eniculate nucleus arises from cholinergic cells in the parabrachial re gion of the brain stem. This is an important pathway in the ascending control of arousal. In an in vivo preparation, we used extracellular r ecordings to study the effects of electrical activation of the parabra chial region on the spontaneous activity and visual responses of X and Y cells in the lateral geniculate nucleus of the cat. 2. We studied t he effects of two patterns of parabrachial activation on the spontaneo us activity of geniculate cells. Burst stimulation consisted of a shor t pulse at high frequency (16 ms at 250 Hz). Train stimulation was of longer duration at lower frequency (e.g., 1 s at 50 Hz). The firing ra te of almost all geniculate cells was enhanced by either pattern of st imulation. However, the burst pattern of stimulation elicited a short, modulated response with excitatory and inhibitory epochs. We found th at the different epochs could differentially modulate the visual respo nses to drifting gratings. Thus the temporal alignment of the brain st em and visual stimuli was critical with burst stimulation, and varied alignments could dramatically confound the results. In comparison, the train pattern of stimulation consistently produced a relatively flat plateau of increased firing, after a short initial period of more vari able effects. We used the less confounding pattern of train stimuli to study the effects of parabrachial activation on visual responses. 3. Our main emphasis was to examine the parabrachial effects on the visua l responses of geniculate cells. For most visual stimuli, we used drif ting sine wave gatings that varied in spatial frequency; these evoked modulated responses from the geniculate cells. Parabrachial activation enhanced the Visual responses of almost all geniculate cells, and thi s enhancement included both increased depth of modulation and greater response rates. 4. Our results were incorporated quantitatively into a difference-of-Gaussians model of visual receptive fields in order to study the parabrachial effects on the spatial structure of the recepti ve field. This model fit our data well and provided measures of the re sponse amplitude and radius of the receptive field center (K-c and R(c ), respectively) and the response amplitude and radius of the receptiv e field surround (K-s and R(s), respectively). Parabrachial activation produced a fairly consistent elevation of K-c, and R(c) was Little af fected, leading to an increase in the strength of the receptive field center (proportional to K-c . R(c)(2)). The effects on K-s and R(s) we re more variable. Despite this variability, increases in K-s more than offset decreases in R(s) (and vice versa), such that parabrachial act ivation also consistently increased the strength of the receptive fiel d surround (proportional to K-s . R(s)(2)). In some cells, surround an d center strength increased proportionally, resulting in a proportiona te increase at all spatial frequencies. In other cases, surround stren gth increased more than center strength, causing the cells to behave m ore like high-pass filters. The reverse was found for other cells. 5. By most measures, geniculate X and Y cells were similarly affected by parabrachial activation. One notable exception is that K-c in X cells was increased significantly more than in Y cells. We suggest that this may relate to a morphological difference in retinogeniculate circuitr y between cell types. Most retinal inputs to X cells are strongly affe cted by interneuron terminals, with which they form triadic contacts; retinal inputs to Y cells tend to be simpler and nontriadic. There is considerable parabrachial input to triads, affording the parabrachial region with a potentially powerful means of gating retinogeniculate tr ansmission for X cells. 6. We confirm that parabrachial activation enh ances the transmission of ascending Visual information through the lat eral geniculate nucleus. For almost all cells, the increase was observ ed at all spatial frequencies. The enhanced transmission of higher-fre quency stimuli will better convey information about the details of a v isual scene. In addition, the increase in the strength of the receptiv e field surround will maintain the lateral inhibitory mechanisms that are crucial for the enhancement of visual contrast edges. These combin ed effects will result in the transmission of a sharper visual image. This is what one would expect under conditions of increased alertness, and it is consistent with the idea that the parabrachial region is in volved in arousal.