NO-dependent mechanisms of amygdalocortical influence

Abstract

The brain amygdala complex (AC) participates in awide spectrum of motivational and emotional behav-ioral responses, which are accompanied by changes inthe visceral and endocrine systems [1]. AC projectionsto the anterior limbic cortex are an important pathwayof AC effects on the integrative brain functionsinvolved in visceral regulation [2–4].Numerous neuroanatomical data indicate that ACprojections run from the parvicellular region of the ACbasal nucleus to deeper layers of the prelimbic andinfralimbic cortices [5]. However, the neurophysiolog-ical mechanisms of the amygdalocortical effects medi-ated by this connections remain unclear., To date, theinhibitory and excitatory AC effects on cells of the ante-rior limbic cortex have been described in a few reports[3, 6–8], though nothing is known about the neurotrans-mitters of these processes.At the same time, nitric oxide (NO) is generallyknown to be one of the most important neurotransmit-ters and/or neuromodulators in the central nervous sys-tem [9, 10]. NO is synthesized by some cortical cells[11, 12] and plays an important role in the regulation ofcortical processes; in particular, it modulates the releaseof γ-aminobutyric acid and acetylcholine [10, 13].The nitroergic activity of cortical neurons may bealso involved in the mechanisms underlying the ACeffects on the anterior limbic cortex. Nitroergic activitymodulation may change the level of neuronal excitabil-ity in the prelimbic and infralimbic regions and, hence,the sensitivity of these regions to the regulatory influ-ences of other brain structures, including the ACnucleus.We verified this hypothesis in a series of histochem-ical and neurophysiological experiments performed onWistar male rats weighing 250–360 g; the animals wereanesthetized with urethane (1.5 g/kg body weight;intraperitoneal administration). In response to electricalstimulation of the parvicellular part of the AC basalnucleus, the NO-synthase activity in various regions ofthe anterior limbic cortex was studied histochemically(detection of intracellular nicotinamide dinucleotidephosphate diaphorase (NADP-d). In neurophysiologi-cal experiments, the effects of the same AC nucleusstimulation on the neuronal activity of the anterior lim-bic cortex were studied before and after the administra-tion of 7-nitroindasol, a blocker of the neuronalNO-synthase, into the left lateral brain ventricle. Two groups of animals, each containing four rats(the control one without stimulation and the group ofelectrically stimulated animals) were used in his-tochemical studies. The surgical preparation was thesame in both groups. The parvicellular part of the basalAC nucleus was subjected to a monopolar electricalstimulation in a region with the following coordinates:3.3 mm caudal of bregma, 5.0 mm lateral of the medianline, and about 7.0 mm away from the dorsal brain sur-face; tungsten electrodes in varnish insulation wereused (tip diameter, 50–60 µm; resistance, 80–100 kΩ).Three series of 0.5-ms rectangular pulses were used(current, 30–50 µA). The pulse frequency in a serieswas 50 pulses per second, the series length was 10 s,and the interval between series was 20 s.On completion of the stimulation, the animals weretranscardially perfused with 4% paraformaldehydesolution; 24 h later, 30-µm frontal sections across theanterior limbic cortex were made using a freezingmicrotome. The sections were treated histochemicallyto detect NADP-d according to a modified method ofScherer-Singler et al. [14]. NADP-d-positive neuronswere located, and their number and optical densitywere determined on the cortical sections using animage-analyzing device, which was connected with apersonal computer; the program packages Video TestMaster 4.0 and Image Tool were used. The neuronaloptical density was expressed in arbitrary units(arb. un.) within a range from 0 (a white object) to 1.55(a black object).Neurophysiological experiments were performed on34 rats. In this experimental series, the parvicellularpart of the AC basal nucleus was stimulated by solitaryrectangular pulses with the same parameters. The neu-ronal responses from the anterior limbic cortex were