Deep brain stimulation (DBS) requires a precise localization, which is especially difficult at the hypothalamus, because it is usually performed in anesthetized patients. We aimed to characterize the neurophysiological properties posteromedial hypothalamus (PMH), identified by the best neurophysiological response to electrical stimulation. We obtained microelectrode recordings from four patients with intractable aggressivity operated under general anesthesia. We pooled data from 1.5 mm at PMH, 1.5 mm upper (uPMH) and 1.5 mm lower (lPMH). We analyzed 178 units, characterized by the mean action potential (mAP). Only 11% were negative. We identified the next types of units: P1N1 (30.9%), N1P1N2 (29.8%), P1P2N1 (16.3%), N1P1 and N1N2P1 (6.2%) and P1N1P2 (5.0%). Besides, atypical action potentials (amAP) were recorded in 11.8%. PMH was highly different in cell composition from uPMH and lPMH, exhibiting also a higher percentage of amAP. Different kinds of cells shared similar features for the three hypothalamic regions. Although features for discharge pattern did not show region specificity, the probability mass function of inter-spike interval were different for all the three regions. Comparison of the same kind of mAP with thalamic neurons previously published demonstrate that most of cells are different for derivatives, amplitude and/or duration of repolarization and depolarization phases and also for the first phase, demonstrating a highly specificity for both brain centers. Therefore, the different properties described for PMH can be used to positively refine targeting, even under general anesthesia. Besides, we describe by first time the presence of atypical extracellular action potentials.

Background: We obtained microelectrode recordings from four patients with intractable aggressivity who underwent surgery at posteromedial hypothalamus under general anaesthesia. We described two general types of extracellular action potentials (EAPs): typical/canonical and atypical. Methods: We analysed 337 units and 67 traces, which were characterized by the mean action potential (mAP). For the first phase, depolarization and repolarization, we computed amplitudes (VFP, VDep and VRep) and durations (dFP, dDep and dRep), maximum and minimum values of the first derivative (dVmax, dVmin), and amplitude and duration ratios. Results: Most of the canonical mAPs were positive (81.1%). EAPs with atypical mean action potentials (amAPs) were recorded in 42/337 cases. Only 35.6% of mAPs showed 2 phases. We identified the following types: N1P1N2 (38.3%), P1N1 (35.9%), amAP (12.5%), P1P2N1 (12.2%), N1P1 (4.7%), P1N1P2 (4.1%) and N1N2P1 (3.2%). We can define the properties of canonical forms as those units with (i) at least two opposite phases; (ii) VDep∈[1.2,2.7]×|VRep| and strongly related by this function VRep=-0.56⁢(±0.01)⁢VDep-1.83⁢(±0.79); (iii) a very strong relationship between dVmax and dVmin, given by the equation d⁢Vmin=-0.91⁢(±0.03)⁢d⁢Vmax-0.37⁢(±0.12), both of which were included in the depolarization phase; (iv) related with VDep by the equation d⁢Vmax=0.08⁢(±0.001)⁢VDep-0.28⁢(±0.14); and (v) dDep⁢~⁢0.38⁢dRep. However, the first phase does not pertain to the same dynamic process responsible for depolarization and repolarization. Conclusions: Atypical units are described here for the first time and are true EAPs that differ strikingly from canonical forms. To date, they have been observed only in the hypothalamus, but future research is needed to assess their existence in other brain structures.