Research Highlights

Research Highlights from the Resource

Ionotropic L-Glutamate receptors in Aplysia

aplysia graph

Expression levels of the twelve ionotropic L-Glutamate receptor subunits and 1 splice variant found in Aplysia ganglia, determined by quantitative real-time PCR, with absolute copy number calculated via standard curves. A. Comparisons of total expression of each iGluR subtype in different Aplysia ganglia, with copy number on the y-axis. B. Each subunit’s contribution to the total expression of its subtype. Reprinted from Greer et al. 2017.

L-Glutamate is the primary excitatory neurotransmitter in the nervous systems of all organisms, where it activates 3 different receptor subtypes. A recent study from the Resource (Greer et al. 2017) demonstrated that ancestral genes for both NMDA- and AMPA-receptor subtypes were present in the common bilaterian ancestor of both vertebrates and invertebrates. This suggests a conserved mechanism of neuroplasticity and learning that involve these subtypes, and validates the Aplysia neural model for understanding the function of the nervous system.

Studying Aging in Aplysia

Studying Aging in Aplysia

Aplysia californica, California sea hares, are annual animals that undergo rapid decline & death at approximately one year of age in the wild or in the laboratory. We have defined stages of aging in sexually mature animals as Mature, Aged I, or Aged II based on behavioral, electrophysiological, and molecular criteria. Aplysia’s well known nervous system combined with this predictable aging process makes Aplysia an excellent model for studying the effects of aging on the nervous system.

 Sensitization in the tail withdrawal reflex (TWR) of mature and advanced-aged sibling animals. Following sensitizing tail shocks, TWR amplitude increased significantly in mature, but not in aged II Aplysia (top). Aged II sensory neuron excitability cannot be increased by the shocks (bottom), one of 3 events that contribute to lack of sensitization in the reflex. * and ** denote increase compared to baseline at p≤0.05 and p≤ 0.01, via Tukey’s posthoc tests following repeated measures ANOVA (n = 18). Modified from Kempsell and Fieber 2015.

Rearing in Exercise

Studying Aging in Aplysia

Sensitization of a reflex occurs when a noxious stimulus such as electric shock produces a very large response to a later stimulus for the reflex, such as touching the tail to cause it to withdraw. In a past study from the Resource (Kempsell and Fieber 2015), we demonstrated that short-term sensitization in the tail withdrawal reflex (TWR) declined during aging. TWR could not be sensitized in aged Aplysia, due to 3 important events in this reflex. First, the excitability of aged sensory neurons was significantly impaired following sensitizing tail shocks (see bottom of Fig). Second, the motoneurons receiving aged sensory neuron input produced smaller amplitude potentials than those connected to mature sensory neurons, even when the aged sensory neuron was electrically driven to increase its output. Last, the tail muscle of aged Aplysia responded to the weak signal from motoneurons with a small tail withdrawal movement (see top of Fig). These results demonstrate the usefulness of the small nervous system of Aplysia: since so few neurons are involved in a reflex, it is a straightforward process to assign an aging deficiency to specific participants in a behavioral reflex. In this case, both sensory neurons and motoneurons exhibited aged physiological characteristics.

Aplysia Aplysia Aplysia

Top: turbulence regimes in which sibling Aplysia were reared for 2 months: still water controls, pumps, dumpbuckets. Bottom: fail speed in flume as a function of animal mass for age 6-month hatchery Aplysia after 2 months in turbulence regimes, compared to wild animals. No relationships of flume performance with mass or with any turbulence treatment were noted. Modified from Fieber et al. (2018).

In a recent study from the Resource (Fieber et al. 2018), Aplysia were hatchery-reared in still water or two turbulence protocols (see top of Fig.) to imitate the intermittent turbulence of the native habitat and exercise the foot muscle. The objective was to learn if the turbulence-reared phenotype mimicked the wild phenotype in reflex behavior, swim tunnel performance (see bottom of Fig), and resting oxygen consumption (MO2). No group exhibited different MO2. Turbulence-induced exercise did not affect the righting reflex or the tail withdrawal reflex, standard behavioral tests that involve the foot muscle exercised in turbulence. Wild individuals did not perform better in flume tests, nor did any turbulence regime improve flume performance compared to siblings reared in still water. These results suggests that the intertidal environment may select for individuals that shelter from intertidal wave energy to remain near optimal feeding areas.