How are insect circadian rhythms controlled?

Unequal coupling between locomotor pacemakers of the German cockroach, Blattella germanica (L.)

Male adult German cockroaches, Blattella germanica (L.), expressed robust locomotor circadian rhythmicity under 28 degrees C and constant darkness (DD) conditions. By surgically severing the connections between the optic lobes and midbrain and their subsequent regeneration, we demonstrated that the locomotor circadian pacemaker was located in the optic lobes and that it controlled the locomotor circadian rhythm through neural pathways. From the results that unilaterally optic tract severed males still showed locomotor circadian rhythmicity (93.1%, n=29) without significantly changing the circadian period (tau) or level of motor activity, we concluded that the right and left optic lobes each contain a circadian pacemaker competent to drive the locomotor circadian rhythm. These two pacemakers were strongly coupled since only one rhythm was expressed when the pacemakers were independently exposed to opposite lighting conditions (DD or LL) at the same time. However, an unequal contribution of each pacemaker in determining the overt circadian period was found under constant dim light (10 lux) conditions, revealing a major-minor coupling relationship between these two pacemakers, so that the unilaterally blinded male expressed either a LL-rhythm with a circadian period of 24.27+/-0.21 h (41.7%, n=24) or a DD-rhythm with a circadian period of 23.43+/-0.19 h (58.3%, n=24). However, higher intensity of photic information (200-300 lux) could overpower this relationship and cause the minor pacemaker to lead the rhythm.

Parallel control mechanisms underlying locomotor activity and sexual receptivity of the female German cockroach, Blattella germanica (L.)

We have studied the effects of ovaries, juvenile hormone (JH) and mating on locomotor activity and sexual receptivity of female German cockroaches. Our results indicate that locomotor activity and sexual receptivity are under the same control mechanisms. The ovary served as a negative masking factor for the locomotor circadian rhythm, but did not affect the frequency of locomotor activity. We conclude that JH controls the locomotor activity of females from the following evidence: (1) increasing locomotion of virgin females coincided with an increasing volume of the corpora allata; (2) allatectomy reduced female locomotion significantly; (3) after absorbing the JH analogue (fenoxycarb) through their tarsi, allatectomized females regained their high level of locomotor activity. Since the daily locomotor activity of allatectomized and ovari-allatectomized females changed cyclically with continuous (non-cyclic) contact of fenoxycarb, an unidentified factor which was independent of ovarian development is proposed to regulate cyclic locomotor activity. In addition to controlling the frequency of locomotor activity, JH was essential for the expression of the locomotor circadian rhythm because allatectomy abolished the circadian rhythm expressed in ovariectomized females. Mating significantly decreased the frequency of locomotor activity and the degree of sexual receptivity. The inhibitory effect of mating resulted from the transmission of a mating signal through the ventral nerve cord when sperm was transferred successfully. The mating experiments with allatectomized and ovariectomized females showed that JH was the major factor in regulating the expression of sexual receptivity.

Localization of the circadian pacemakers of Hemideina thoracica (Orthoptera; Stenopelmatidae)

The location of the circadian pacemakers of the orthopteran Hemideina thoracica (White) has been investigated through observation of the effects of surgical removal of brain tissues (principally optic lobes and tracts) on free-running and entrained locomotor rhythms. Bilobectomy and severance of optic tracts invariably resulted in arrhythmicity, whereas rhythmicity was sustained following unilateral lobectomy, generally with increases in the free-running period (FRP) and decreases in both the active-phase lengths and activity-to-rest ratios of the rhythm. Bilobectomized subjects could be entrained by temperature cycles, but exhibited no transients or residual rhythmicity, indicating that temperature brought about a direct response or masking effect. These results support the hypothesis that the circadian locomotor pacemakers of Hemideina are located within each optic lobe, and that there are no extraoptic centers for the control of the timing of locomotor activity. Although confirmation of the pacemaker role of the optic lobes requires transplantation of the tissues, the conclusion may be drawn by inference from other studies (e.g., Leucophaea maderae--Page, 1983; Gryllus bimaculatus--Tomioka and Chiba, 1986). Light entrainment continued after surgical binding and blackening of the compound eyes and ocelli, supporting the view that direct illumination of neural tissue through the cuticle may be one possible pathway for light entrainment.

Substance P-like immunoreactivity is present in the central nervous system of Limulus polyphemus

The distribution of substance P-like immunoreactivity (substance P-li) in the central nervous system of Limulus polyphemus was studied by using indirect immunocytochemical techniques. Six bilaterally symmetrical pairs of cell clusters in the circumesophageal connectives and the subesophageal mass contain substance P-li. Two of those pairs are the source of a system of efferent fibers that is involved in the expression of circadian rhythms of photosensitivity by the lateral eye. Substance P-li-containing cells were also observed scattered along the length of the circumesophageal connectives, which contain abundant stained fibers and some terminals. Substance P-li fibers leave through the ventral and dorsal nerves of the posterior circumesophageal ring. The neuropil of the subesophageal mass contains an abundance of stained terminals. Immunoreactive fibers can be seen throughout the length of the two longitudinal connectives of the ventral cord, in discrete fiber tracts in the lateral edges of the interganglionic connectives, and in the dorsal and ventral nerves of abdominal ganglia 1-4. Each of these ganglia contains three pairs of substance P-immunoreactive cell body clusters: an anterolateral, a medial longitudinal, and a medial posterior cluster. Substance P-li fibers entering through the ventral (posterior) nerves form very distinctive fascicles in each side of the ganglia, giving off fibers throughout their length. The neuropil is filled with immunoreactive terminals distributed homogeneously. The anterolateral clusters of the abdominal ganglia may be involved in cardioregulation. The six pairs of clusters in the posterior circumesophageal ring, and perhaps some of those in the abdominal ganglia, are believed to constitute a neurosecretory system, projecting to multiple targets throughout the organism. This system is postulated to modulate various sensory inputs and motor activity, and could be driven by a circadian clock, as well as by other systems responsible for integrated organismic responses.

Analysis of sleep-wakefulness rhythms in male rats after suprachiasmatic nucleus lesions and ocular enucleation

To determine quantitatively characteristics of sleep-wakefulness rhythms in male albino rats, computer analysis of long term polygraphic records (24 h/day) of cortical EEG activity, neck EMG and EOG taken from 23 rats under 12:12 light-dark schedule was performed. After bilateral suprachiasmatic nucleus (SCN) lesions, the circadian rhythm in sleep-wakefulness was completely eliminated, although no attenuation or even slight enhancement of the ultradian rhythms with 2-4 h pweiodicity was observed. After enucleation of both eyes, the circadian rhythm was free-running with a phase shift in the range from --12 to +22 min/day in 6 rats. A gradual decrease of the spectral value of the circadian rhythm and inverse enhancement of the ultradian rhythms with 4--7 h periodicity (predominantly 6 h in 4 out of 6 rats) were also shown. In the spectral diagram, the appearance of paradoxical sleep (PS) paralleled slow-wave sleep (SWS), in the cases of the circadian rhythm and ultradian rhythms with 4--7 h periodicity. Behaviorally blind rats with bilateral primary optic tract (POT) lesions maintained the circadian rhythm in sleep-wakefulness entrained to the environmental light-dark cycle. Power spectral analysis showed no characteristic difference from normal rats. Based on these data, the role of the SCN as a pacemaker of endogenous circadian rhythm in sleep-wakefulness is discussed.

Circadian rhythm: population of interacting neurons

The circadian rhythm in the requency of compound action potentials recorded from the isolated eye of Aplysia is a consequence of interactions among the cells of the retinal population. As the population number is reduced to a critical 20 percent, progressively shorter circadian periods and ranges are expressed. Below the critical number, the population oscillates at ultradian frequencies.

Neuroendocrine control of ecdysis in silkmoths

An adult moth sheds its pupal skin only during a specific period of the day. The brain is necessary for the synchronization of this behavior with the environmental photoperiod. This function is fully preserved when all the brain's nervous connections are severed or when a "loose" brain is transplanted into the tip of the abdomen. By appropriate experiments it was possible to show that the entire mechanism is brain-centered. The components include a photoreceptor mechanism, a clock, and a neuroendocrine output. The clock-controlled release of the hormone acts on the central nervous system to trigger a species-specific behavior pattern which culminates in ecdysis.