Modulated long-term potentiation in the cat superior cervical ganglion in vivo

Abstract

In the sympathetic ganglion a high frequency conditioning train produces a post-train potentiation (PTrP) which decays with a temporal course that can be described by two components: an early faster component known as post-tetanic potentiation (PTP), and a long-lasting one which is known as long-term potentiation (LTP). The magnitude of PTP and LTP is thought to be a function of the subliminal fringe size. Moreover, under full activation of ganglion cells LTP does not appear due to the saturation conditions. LTP emerges only when the subliminal fringe is increased, and to achieve this, a low-level tetanization must be performed. However, another possibility, explored in this paper, is that in addition to the desaturation effect of the low-level conditions of tetanization, a different mechanism, responsible for the LTP appearence, may be introduced by these low-level conditions of tetanization. This possibility implies a variable LTP according to the input conditions; accordingly, the shape of LTP was compared in 4 experimental conditions in which the input to the ganglion cells was different: (1) supramaximal activation of the intact cervical sympathetic trunk (CST), (SPM); (2) submaximal activation of the intact CST, (SBM); (3) supramaximal activation of the partially transected CST, (TRN); and (4) supramaximal activation of the intact CST under partial nicotinic block with hexamethonium, (BLK). Even when PTrP was evoked with the same subliminal fringe obtained under the 3 low-level conditions of tetanization, the degree of potentiation and the proportion of PTP and LTP were different. After either SPM or SBM conditions LTP was negligible, though PTP developed to a certain degree. LTP emerged to a large degree in TRN experiments and particularly under BLK conditions, though PTP remained nearly constant under these and SPM, and SBM conditions. These different degrees and shapes of PTrP attained with similar subliminal fringe cannot be explained only by a desaturation effect, therefore a different mechanism must be involved. Furthermore, the fact that the changes were mainly at the expense of the growth of LTP, suggests the presence of a specific mechanism for this phenomenon, which it manifested particularly upon blockade conditions.