Septocingulate and septohippocampal cholinergic pathways: involvement in working/episodic memory.

The contribution of the septohippocampal cholinergic pathway to performance of a working/episodic memory task was compared to that of the septocingulate cholinergic path. The septocingulate and septohippocampal cholinergic pathways were selectively destroyed in male Sprague-Dawley rats using site-specific injections of the anti-neuronal immunotoxin 192-IgG saporin into either the hippocampus or the cingulate cortex. 192-IgG-saporin selectively destroys cholinergic neurons and terminals that express the p75 neurotrophin receptor. Following extensive pre-operative training, working memory was assessed using a delayed nonmatch to sample eight arm radial maze task, with delays of 1, 4 and 8 h. The group with lesions of the septohippocampal cholinergic pathway displayed performance deficits on this task which were not related to length of delay. In contrast, the group with lesions of the septocingulate cholinergic pathway did display delay-dependent deficits which were observed at the 4- and 8-h delays, but not at the 1-h delay. These data suggest that the septocingulate cholinergic pathway is critically involved in working/episodic memory but that the septohippocampal cholinergic pathway is either not contributing to working/episodic memory per se or it is involved only at shorter delays.

Evidence for afferent fiber innervation of parasympathetic neurons of the guinea-pig cardiac ganglion.

The present study was done to establish whether peptidergic afferent inputs can modulate parasympathetic neurons of the guinea-pig cardiac ganglion. Whole mount preparations from the guinea-pig heart were utilized to localize afferent terminals by immunohistochemistry and for intracellular recordings from individual neurons in situ. Action potentials could be elicited by both intracellular current injection and stimulation of interganglionic fiber bundles. Two types of neuron, phasic (95%) and tonic (5%) as defined by their firing properties, were observed. High frequency (5-10 Hz) interganglionic fiber stimulation produced a calcium-dependent, slow depolarization in many cells which was not blocked by 100 microM hexamethonium or 1 microM atropine. A prolonged depolarization was also produced by local application of capsaicin (1 mM), which releases substance P and CGRP from afferent nerve terminals. Microinjection of the mammalian tachykinins substance P, neurokinin A and neurokinin B (all at 100 microM), also produced a slow depolarization. Application of specific agonists for the tachykinin receptor subtypes indicated that these neurons express both NK2 and NK3 receptors. Individual cells were filled with neurobiotin to examine their morphology and the preparations were counter-stained for SP-like immunoreactivity. The results demonstrated that SP-positive fibers are found in close apposition to both phasic and tonic neurons. From these results, we suggest that the parasympathetic neurons of the guinea-pig cardiac ganglion receive inputs from peptidergic, afferent fibers and that this input provides a pathway for potential local reflex control of cardiac function.

Innervation of the pacemaker in guinea-pig sinoatrial node.

Heart rate is regulated by the autonomic nervous system but little is known about the pattern of innervation of the pacemaker in the sinoatrial node, or the subpopulations of nerves involved. Therefore in this study the pacemaker was located using electrophysiological methods and the pattern of innervation established by cholinesterase staining. In subsequent experiments, subpopulations of sympathetic, sensory and parasympathetic nerves were identified. Sympathetic nerves were labelled by glyoxylic acid-induced catecholamine fluorescence or an antiserum raised against tyrosine hydroxylase (TH). These experiments showed that the entire sinoatrial node was densely innervated by sympathetic axons, the majority of which were immunoreactive for neuropeptide Y (NPY). There were a few axons which were only immunoreactive for TH. Sensory nerves which were immunoreactive for both substance P (SP) and calcitonin gene-related peptide (CGRP) were also found throughout the sinoatrial node. In the absence of a selective marker for parasympathetic neurons, hearts were extrinsically denervated by placing them in organotypic culture to allow degeneration of extrinsic axons. In this way intrinsic parasympathetic neurons could be characterised. These experiments revealed several distinct populations of parasympathetic nerves which innervated only a small, discrete part of the sinoatrial node. These populations were immunoreactive for NPY, somatostatin (SOM) or vasoactive intestinal peptide (VIP) alone, or SOM combined with NPY, SOM with dynorphin B, and SOM with SP. These results highlight a remarkable difference in the pattern of innervation of the sinoatrial node by the sympathetic and parasympathetic nervous systems. Furthermore the presence of several distinct populations of autonomic cardiac neurons indicates a further complexity in neuronal regulation of heart rate.