The effects of temperature and synaptic blockade in vitro on neurons of the horizontal limb of the diagonal band of Broca in the rat basal forebrain.

We studied the thermosensitivity of neurons in the rat horizontal limb of the diagonal band of Broca (HDB) in vitro under normal conditions and under conditions of a low calcium/high magnesium synaptic blockade (SB). Of 52 HDB neurons tested, 34 neurons (65%) were warm-sensitive (WS), three neurons (6%) were cold-sensitive (CS), 11 neurons (21%) were temperature-insensitive (TI) and four additional neurons (8%) were both warm- and cold-sensitive (WS/CS). Of 34 neurons tested for thermosensitivity under SB, 11 were WS, 4 were CS and 19 were TI. Nearly half (48%) of the WS neurons maintained warm sensitivity under SB, 43% became TI and 9% became CS. Baseline firing rates of neurons significantly decreased during SB and then increased during recovery from SB. In addition, a distinct anatomical distribution of thermosensitive neurons was found in the HDB. The most ventral aspect of the HDB (interaural +0.9-1.3 mm) had proportionally fewer temperature sensitive neurons (65% vs. 88%) than areas more dorsal (interaural +1.3-1.7 mm), and only one of seven ventral HDB neurons (14%) remained thermosensitive during SB. In the dorsal HDB, 65% of the neurons maintained thermosensitivity during SB. These results demonstrate that the HDB contains inherently thermosensitive neurons, and that a difference in thermal characteristics exists between the ventral and dorsal HDB neurons.