After periods of high-frequency firing, the normal rhythmically active serotonin (5HT)-containing neurosecretory neurons of the lobster ventral nerve cord display a period of suppressed spike generation and reduced synaptic input that we refer to as ''autoinhibition.'' The duration of this autoinhibition is directly related to the magnitude and duration of the current injection triggering the high-frequency firing. More interesting, however, is that the autoinhibition is inversely related to the initial firing frequency of these cells within their normal range of firing (0.5-3 Hz). This allows more active 5HT neurons to resume firing after shorter durations of inhibition than cells that initially fired at slower rates. Although superfused 5HT inhibits the spontaneous firing of these cells, the persistence of autoinhibition in saline with no added calcium, in cadmium-containing saline, and in lobsters depleted of serotonin suggests that intrinsic membrane properties account for the autoinhibition. A similar autoinhibition is seen in spontaneously active octopamine neurons but is absent from spontaneously active ␥-aminobutyric acid cells. Thus, this might be a characteristic feature of amine-containing neurosecretory neurons. The 5HT cells of vertebrate brain nuclei share similarities in firing frequencies, spike shapes, and inhibition by 5HT with the lobster cells that were the focus of this study. However, the mechanism suggested to underlie autoinhibition in vertebrate neurons is that 5HT released from activated or neighboring cells acts back on inhibitory autoreceptors that are found on the dendrites and cell bodies of these neurons.Biogenic amines appear to play key roles in the regulation of a wide array of physiological processes in both vertebrate and invertebrate nervous systems. They have been implicated in physiological processes and behaviors such as feeding (1), sleep (2, 3), repetitive motor acts such as locomotion (1), nociception (4), depression (5), and aggressive behavior, including the establishment of social hierarchies (6-9).In vertebrates, spontaneously active serotonergic neurons of the midline raphe nuclei are implicated in these behaviors, and these neurons have been the subject of many studies (for reviews, see refs. 10-12). These cells fire at slow rates between 0.5 and 3 Hz, depending on the state of wakefulness of the animal (1, 13-15). They generate large, approximately 60-mV, action potentials with prominent afterhyperpolarizations, which are likely to be mediated by calcium-activated potassium currents (10, 16). Both a high density of inhibitory somatodendritic type 1A 5-hydroxytryptamine (serotonin; 5HT 1A ) autoreceptors and the afterhyperpolarization are suggested to be important mechanisms for regulating the pacemaker activity of these cells (10, 17). Periods of high-frequency firing, experimentally evoked by electrical stimulation, are followed by a ''postactivation inhibition'' during which no action potentials are generated. The duration of the period of suppressed sp...
The distribution of octopamine-immunoreactive neurons is described using whole-mount preparations of all central ganglia of the cricket, Gryllus bimaculatus. Up to 160 octopamine-immunoreactive somata were mapped per animal. Medial unpaired octopamine-immunoreactive neurons occur in all but the cerebral ganglia and show segment-specific differences in number. The position and form of these cells are in accordance with well-known, segmentally-organized clusters of large dorsal and ventral unpaired medial neurons demonstrated by other techniques. In addition, bilaterally arranged groups of immunoreactive somata have been labelled in the cerebral, suboesophageal and terminal ganglia. A detailed histological description of octopamine-immunoreactive elements in the prothoracic ganglion is given. Octopamine-immunoreactive somata and axons correspond to the different dorsal unpaired medial cell types identified by intracellular single-cell staining. In the prothoracic ganglion, all efferent neurons whose primary neurites are found in the fibre bundle of dorsal unpaired cells are immunoreactive. Intersegmental octopamine-immunoreactive neurons are also present. Collaterals originating from dorsal intersegmental fibres terminate in different neuropils and fibre tracts. Fine varicose fibres have been located in several fibre tracts, motor and sensory neuropils. Peripheral varicose octopamine-immunoreactive fibres found on several nerves are discussed in terms of possible neurohemal releasing sites for octopamine.
Responses to sensory stimuli and spike activity during walking were investigated in bilaterally symmetrical dorsal unpaired median (DUM) neurons of the cricket. Intracellular recordings within the prothoracic ganglion were made either in restrained animals or in stationary walking specimens whilst parameters of their intended locomotion were measured. Three types of DUM cells were distinguished morphologically and physiologically. DUMa neurons send axons through segmental nerves. They often generated spontaneously large action potentials with low frequencies. Most DUMa neurons showed multimodal sensitivity, preferentially to cercal wind puffs and 15 kHz sound. Mean latencies ranged from 25 to 349 ms. Their large intraindividual variability could be correlated with behavioral modes during walking. Generally, the spike frequency increased with increased forward speed, while it was not related to turning. DUMb neurons projected either through the anterior or posterior connectives, but seemed physiologically similar to DUMa neurons. DUMc neurons were H-shaped with axons in both pairs of connectives. No external stimulus led to discrete spikes, but the regular spontaneous activity was modulated following cercal wind puffs to a restrained animal. During wind evoked escape the spike activity of another DUMc cell was modulated in phase with the rhythmic running behavior. The possibly different functions of DUMa and DUMc neurons during walking are discussed.
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