Infant rats: hypothalamic unit activity.

Single-unit recordings were made from lateral hypothalamic area (LHA) neurons of infant (8 through 21 days of age) male and female albino rats. Basal (or spontaneous) firing rates of LHA neurons were recorded for 20 min from rats 8 through 21 days of age, and mean basal firing rates did not differ across ages. Basal firing rates of LHA neurons in infant rats ranged from less than 1 spike/sec up to 27 spikes/sec, with 56% of the basal rates less than 1 spike/sec. Infant rats also received hypertonic saline injections (and control injections) to determine osmosensitivity characteristics of LHA neurons. Across ages, 56%--88% of LHA neurons were osmosensitive, with the highest percentages of osmosensitive units found in rats 14--21 days of age. Infant rats showed LHA unit electrophysiological characteristics that were quite similar to those obtained from LHA neurons in adult rats with respect to basal firing rates and osmosensitivity, however, LHA neuron activity in infants was not modulated by sensory stimuli as is frequently reported for adults. The results are discussed in terms of functional development of the LHA.